Metal Casting

Virtually nothing moves, turns, rolls, or flies without the benefit of cast metal products. The metal
casting industry plays a key role in all the major sectors of our economy. There are castings in
locomotives, cars trucks, aircraft, office buildings, factories, schools, and homes. The following Fig
1. shows some metal cast parts.

Metal Casting is one of the oldest materials shaping methods known. Casting means pouring molten
metal into a mold with a cavity of the shape to be made, and allowing it to solidify. When solidified,
the desired metal object is taken out from the mold either by breaking the mold or taking the mold
apart. The solidified object is called the casting. By this process, intricate parts can be given strength
and rigidity frequently not obtainable by any other manufacturing process. The mold, into which
the metal is poured, is made of some heat resisting material. Sand is most often used as it resists
the high temperature of the molten metal. Permanent molds of metal can also be used to cast
products.

Fig. 1: Metal Cast parts

Advantages
The metal casting process is extensively used in manufacturing because of its many advantages.
1. Molten material can flow into very small sections so that intricate shapes can be made by
this process. As a result, many other operations, such as machining, forging, and welding,
can be minimized or eliminated.
2. It is possible to cast practically any material that is ferrous or non-ferrous.
3. As the metal can be placed exactly where it is required, large saving in weight can be
achieved.
4. The necessary tools required for casting molds are very simple and inexpensive. As a result,
for production of a small lot, it is the ideal process.
5. There are certain parts made from metals and alloys that can only be processed this way.
6. Size and weight of the product is not a limitation for the casting process.
Limitations
1. Dimensional accuracy and surface finish of the castings made by sand casting processes are
a limitation to this technique. Many new casting processes have been developed which can
take into consideration the aspects of dimensional accuracy and surface finish. Some of
these processes are die casting process, investment casting process, vacuum-sealed
molding process, and shell molding process.
2. The metal casting process is a labor intensive process.

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 Components Used for making a Mould Cavity

The details can also be seen in the Fig. 2 below:

 Initially a suitable size of molding box for creating suitable wall thickness is selected for a two
piece pattern. Sufficient care should also be taken in such that sense that the molding box must
adjust mold cavity, riser and the gating system (sprue, runner and gates etc.).
 Next, place the drag portion of the pattern with the parting surface down on the bottom (ram-
up) board as shown in Fig. 2 (a).
 The facing sand is then sprinkled carefully all around the pattern so that the pattern does not
stick with molding sand during withdrawn of the pattern.
 The drag is then filled with loose prepared molding sand and ramming of the molding sand is done
uniformly in the molding box around the pattern. Fill the molding sand once again and then
perform ramming. Repeat the process three four times,
 The excess amount of sand is then removed using strike off bar to bring molding sand at the
same level of the molding flask height to completes the drag.
 The drag is then rolled over and the parting sand is sprinkled over on the top of the drag [Fig.
2(b)].
 Now the cope pattern is placed on the drag pattern and alignment is done using dowel pins.
 Then cope (flask) is placed over the rammed drag and the parting sand is sprinkled all around
the cope pattern.
 Sprue and riser pins are placed in vertically position at suitable locations using support of
molding sand. It will help to form suitable sized cavities for pouring molten metal etc. [Fig. 2
(c)].
 The gaggers in the cope are set at suitable locations if necessary. They should not be located
too close to the pattern or mold cavity otherwise they may chill the casting and fill the cope
with molding sand and ram uniformly.
 Strike off the excess sand from the top of the cope.
 Remove sprue and riser pins and create vent holes in the cope with a vent wire. The basic
purpose of vent creating vent holes in cope is to permit the escape of gases generated during
pouring and solidification of the casting.
 Sprinkle parting sand over the top of the cope surface and roll over the cope on the
bottom board.
 Rap and remove both the cope and drag patterns and repair the mold suitably if needed
and dressing is applied
 The gate is then cut connecting the lower base of sprue basin with runner and then the
mold cavity.
 Apply mold coating with a swab and bake the mold in case of a dry sand mold.
 Set the cores in the mold, if needed and close the mold by inverting cope over drag.
 The cope is then clamped with drag and the mold is ready for pouring, [Fig. 2 (d)].

1. Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed.
Depending upon the position of the flask in the molding structure, it is referred to by various
names such as drag – lower molding flask, cope – upper molding flask, cheek – intermediate
molding flask used in three piece molding.

2. Pattern: It is the replica of the final object to be made. The mold cavity is made with the
help of pattern.

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3. Parting line: This is the dividing line between the two molding flasks that makes up the
mold.

4. Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It
is a mixture of silica sand, clay, and moisture in appropriate proportions.

5. Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of
the mold cavity to give a better surface finish to the castings.

6. Core: A separate part of the mold, made of sand and generally baked, which is used to
create openings and various shaped cavities in the castings.

7. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten
metal is poured.

8. Sprue: The passage through which the molten metal, from the pouring basin, reaches the
mold cavity. In many cases it controls the flow of metal into the mold.

9. Runner: The channel through which the molten metal is carried from the sprue to the gate.

10. Gate: A channel through which the molten metal enters the mold cavity.

11. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its
own weight and overcome the metallostatic force.

12. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and
solidifies. Also known as “feed head”.

13. Vent: Small opening in the mold to facilitate escape of air and gases.

Fig. 2: Steps for making a mould cavity

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 PATTERN

A pattern is a model or the replica of the object (to be casted). It is embedded in molding sand and
suitable ramming of molding sand around the pattern is made. The pattern is then withdrawn for
generating cavity (known as mold) in molding sand. Thus it is a mould forming tool. Pattern can be
said as a model or the replica of the object to be cast except for the various al1owances a pattern
exactly resembles the casting to be made. It may be defined as a model or form around which sand is
packed to give rise to a cavity known as mold cavity in which when molten metal is poured, the result
is the cast object. When this mould/cavity is filled with molten metal, molten metal solidifies and
produces a casting (product). So the pattern is the replica of the casting.

OBJECTIVES OF A PATTERN

1. Pattern prepares a mould cavity for the purpose of making a casting.

2. Pattern possesses core prints which produces seats in form of extra recess for core
placement in the mould.
3. It establishes the parting line and parting surfaces in the mould.
4. Runner, gates and riser may form a part of the pattern.
5. Properly constructed patterns minimize overall cost of the casting.
6. Pattern may help in establishing locating pins on the mould and therefore on the casting
with a purpose to check the casting dimensions.
7. Properly made pattern having finished and smooth surface reduce casting defects.
Patterns are generally made in pattern making shop. Proper construction of pattern and its
material may reduce overal1 cost of the castings.

COMMON PATTERN MATERIALS

The common materials used for making patterns are wood, metal, plastic, plaster, wax or
mercury. The some important pattern materials are discussed as under.

1. Wood
Wood is the most popular and commonly used material for pattern making. It is cheap, easily available
in abundance, repairable and easily fabricated in various forms using resin and glues. It is very light
and can produce highly smooth surface. Wood can preserve its surface by application of a shellac
coating for longer life of the pattern. But, in spite of its above qualities, it is susceptible to shrinkage
and warpage and its life is short because of the reasons that it is highly affected by moisture of the
molding sand. After some use it warps and wears out quickly as it is having less resistance to sand
abrasion.
It can not withstand rough handily andis weak in comparison to metal. In the light of above qualities,
wooden patterns are preferred only when the numbers of castings to be produced are less. The main
varieties of woods used in pattern-making are shisham, kail, deodar, teak and mahogany.
Shisham
It is dark brown in color having golden and dark brown stripes. It is very hard to work and blunts the
cutting tool very soon during cutting. It is very strong and durable. Besides making pattern, it is also
used for making good variety of furniture, tool handles, beds, cabinets, bridge piles, plywood etc.
Kail
It has too many knots. It is available in Himalayas and yields a close grained, moderately hard and
durable wood. It can be very well painted. Besides making pattern, it is also utilized for making
wooden doors, packing case, cheap furniture etc.

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Deodar
It is white in color when soft but when hard, its color turns toward light yellow. It is strong and durable.
It gives fragrance when smelled. It has some quantity of oil and therefore it is not easily attacked by
insects. It is available in Himalayas at a height from 1500 to 3000 meters. It is used for making pattern,
manufacturing of doors, furniture, patterns, railway sleepers etc. It is a soft wood having a close grain
structure unlikely to warp. It is easily workable and its cost is also low. It is preferred for making
pattern for production of small size castings in small quantities.

Teak Wood
It is hard, very costly and available in golden yellow or dark brown color. Special stripes on it add to its
beauty. In India, it is found in M.P. It is very strong and durable and has wide applications. It can
maintain good polish. Besides making pattern, it is used for making good quality furniture, plywood,
ships etc. It is a straight-grained light wood. It is easily workable and has little tendency to warp. Its
cost is moderate.
Mahogany
This is a hard and strong wood. Patterns made of this wood are more durable than those of above
mentioned woods and they are less likely to warp. It has got a uniform straight grain structure and it
can be easily fabricated in various shapes. It is costlier than teak and pine wood, It is generally not
preferred for high accuracy for making complicated pattern. It is also preferred for production of small
size castings in small quantities. The other Indian woods which may also be used for pattern making
are deodar, walnllt, kail, maple, birch, cherry and shisham.

Advantages of wooden patterns

1 Wood can be easily worked.
2 It is light in weight.
3 It is easily available.
4 It is very cheap.
5 It is easy to join.
6 It is easy to obtain good surface finish.
7 Wooden laminated patterns are strong.
8 It can be easily repaired.

Disadvantages
1 It is susceptible to moisture.
2 It tends to warp.
3 It wears out quickly due to sand abrasion.
4 It is weaker than metallic patterns.

2. Metal
Metallic patterns are preferred when the number of castings required is large enough to justify their
use. These patterns are not much affected by moisture as wooden pattern. The wear and tear of this
pattern is very less and hence posses longer life. Moreover, metal is easier to shape the pattern with
good precision, surface finish and intricacy in shapes. It can withstand against corrosion and handling
for longer period. It possesses excellent strength to weight ratio. The main disadvantages of metallic
patterns are higher cost, higher weight and tendency of rusting. It is preferred for production of
castings in large quantities with same pattern. The metals commonly used for pattern making are

DEPARTMENT OF MECHANICAL ENGINEERING

 cast iron, brass and bronzes and aluminum alloys.

Cast Iron
It is cheaper, stronger, tough, and durable and can produce a smooth surface finish. It also
possesses good resistance to sand abrasion. The drawbacks of cast iron patterns are that they are
hard, heavy, brittle and get rusted easily in presence of moisture.
Advantages
1. It is cheap
2. It is easy to file and fit
3. It is strong
4. It has good resistance against sand abrasion
5. Good surface finish

Disadvantages
1 It is heavy
2 It is brittle and hence it can be easily broken
3 It may rust

Brasses and Bronzes

These are heavier and expensive than cast iron and hence are preferred for manufacturing small
castings. They possess good strength, machinability and resistance to corrosion and wear. They can
produce a better surface finish. Brass and bronze pattern is finding application in making match plate
pattern
Advantages
1. Better surface finish than cast iron.
2. Very thin sections can be easily casted.

Disadvantages
1. It is costly
2. It is heavier than cast iron.

Aluminum Alloys
Aluminum alloy patterns are more popular and best among all the metallic patterns because of
their high light ness, good surface finish, low melting point and good strength. They also possesses
good resistance to corrosion and abrasion by sand and there by enhancing longer life of pattern. These
materials do not withstand against rough handling. These have poor repair ability and are preferred
for making large castings.
Advantages
1. Aluminum alloys pattern does not rust.
2. They are easy to cast.
3. They are light in weight.
4. They can be easily machined.

Disadvantages
1. They can be damaged by sharp edges.
2. They are softer than brass and cast iron.
3. Their storing and transportation needs proper care.

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White Metal (Alloy of Antimony, Copper and Lead)

Advantages
1. It is best material for lining and stripping plates.
2. It has low melting point around 260°C
3. It can be cast into narrow cavities.
Disadvantages
1. It is too soft.
2. Its storing and transportation needs proper care
3. It wears away by sand or sharp edges.

3. Plastic
Plastics are getting more popularity now a days because the patterns made of these materials are
lighter, stronger, moisture and wear resistant, non sticky to molding sand, durable and they are not
affected by the moisture of the molding sand. Moreover they impart very smooth surface finish on
the pattern surface. These materials are somewhat fragile, less resistant to sudden loading and their
section may need metal reinforcement. The plastics used for this purpose are thermosetting resins.
Phenolic resin plastics are commonly used. These are originally in liquid form and get solidified when
heated to a specified temperature. To prepare a plastic pattern, a mould in two halves is prepared in
plaster of paris with the help of a wooden pattern known as a master pattern. The phenolic resin is
poured into the mould and the mould is subjected to heat. The resin solidifies giving the plastic
pattern. Recently a new material has stepped into the field of plastic which is known as foam plastic.
Foam plastic is now being produced in several forms and the most common is the expandable
polystyrene plastic category. It is made from benzene and ethyl benzene.

4. Plaster
This material belongs to gypsum family which can be easily cast and worked with wooden tools and
preferable for producing highly intricate casting. The main advantages of plaster are that it has high
compressive strength and is of high expansion setting type which compensate for the shrinkage
allowance of the casting metal. Plaster of paris pattern can be prepared either by directly pouring
the slurry of plaster and water in moulds prepared earlier from a master pattern or by sweeping it
into desired shape or form by the sweep and strickle method. It is also preferred for production of
small size intricate castings and making core boxes.

5. Wax
Patterns made from wax are excellent for investment casting process. The materials used are blends
of several types of waxes, and other additives which act as polymerizing agents, stabilizers, etc. The
commonly used waxes are paraffin wax, shellac wax, bees-wax, cerasin wax, and micro-crystalline
wax. The properties desired in a good wax pattern include low ash content up to 0.05 per cent,
resistant to the primary coat material used for investment, high tensile strength and hardness, and
substantial weld strength. The general practice of making wax pattern is to inject liquid or semi-
liquid wax into a split die. Solid injection is also used to avoid shrinkage and for better strength.
Waxes use helps in imparting a high degree of surface finish and dimensional accuracy castings. Wax
patterns are prepared by pouring heated wax into split moulds or a pair of dies. The dies after having
been cooled down are parted off. Now the wax pattern is taken out and used for molding. Such
patterns need not to be drawn out solid from the mould. After the mould is ready, the wax is poured
out by heating the mould and keeping it upside down. Such patterns are generally used in the
process of investment casting where accuracy is linked with intricacy of the cast object.

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 Advantages of wooden patterns
1 Wood can be easily worked.
2 It is light in weight.
3 It is easily available.
4 It is very cheap.
5 It is easy to join.
6 It is easy to obtain good surface finish.
7 Wooden laminated patterns are strong.
8 It can be easily repaired.

Disadvantages
5 It is susceptible to moisture.
6 It tends to warp.
7 It wears out quickly due to sand abrasion.
8 It is weaker than metallic patterns.

TYPES OF PATTERN

The types of the pattern and the description of each are given as under.
1. One piece or solid pattern
2. Two piece or split pattern
3. Cope and drag pattern
4. Three-piece or multi- piece pattern
5. Loose piece pattern
6. Match plate pattern
7. Follow board pattern
8. Gated pattern
9. Sweep pattern
10. Skeleton pattern
11. Segmental or part pattern

Single-piece or solid pattern

Solid pattern is made of single piece without joints, partings lines or loose pieces. It is the simplest
form of the pattern. Typical single piece pattern is shown in Fig. 3.

Fig. 3: Single piece pattern

Two-piece or split pattern

When solid pattern is difficult for withdrawal from the mold cavity, then solid pattern is splited in two
parts. Split pattern is made in two pieces which are joined at the parting line by means of dowel pins.
The splitting at the parting line is done to facilitate the withdrawal of the pattern. A typical example
is shown in Fig. 4.

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 Fig. 4: Two-piece or split pattern

Cope and drag pattern

In this case, cope and drag part of the mould are prepared separately. This is done when the
complete mould is too heavy to be handled by one operator. The pattern is made up of two halves,
which are mounted on different plates. A typical example of match plate pattern is shown in Fig. 5.

Fig. 5: Cope and drag pattern

Three-piece or multi-piece pattern

Some patterns are of complicated kind in shape and hence cannot be made in one or two pieces
because of difficulty in withdrawing the pattern. Therefore these patterns are made in either three
pieces or in multi- pieces. Multi molding flasks are needed to make mold from these patterns. The
pattern can also be seen from the Fig. 6.

Fig. 6: Three-piece or multi-piece pattern

Loose-piece pattern
A single piece are made to have loose piece in easy to allow withdrawal from the mold the molding
process are completed, after the main pattern is withdrawn leaving from that piece in the sand. After
the withdrawal of piece from mold, it cavity separately formed by the pattern. It loose piece pattern
is highly skilled job and expensive. The pattern can also be seen from the Fig. 7.

Fig. 7: Loose-piece pattern

Match plate pattern
The match plate pattern types is having two parts, one for one side and another one for another side
of pattern. It is called match plate pattern. The sand casting pattern making in two pieces. It also
having gates and runner attached with pattern. The molding process completed after that match plate

DEPARTMENT OF MECHANICAL ENGINEERING

removed together, the gating is obtained for joining the cope and drag. It pattern is mainly used for
casting of metal, usually aluminum are machined in this method with light weight and machinability.
It should be possible for mass production of small casting with high dimensional accuracy. They are
also used for machine molding. The cost will be high of molding but it is easily compensated by high
rate of production and more accuracy. The pattern can also be seen from the Fig. 8.

Fig. 8: Match plate pattern

Follow board type pattern
In casting process some portions are structurally weak. It is not supported properly and may be break
under the force of ramming. In this stage the special pattern to allow the mold may be such as wooden
material. The pattern can also be seen from the Fig. 9.

Fig. 9: Follow board type pattern

Gated Pattern
In the mass production of casings, multi cavity moulds are used. Such moulds are formed by joining a
number of patterns and gates and providing a common runner for the molten metal, as shown in Fig.
10. These patterns are made of metals, and metallic pieces to form gates and runners are attached to
the pattern.

Fig. 10: Gated Pattern

Sweep Pattern
Sweep patterns are used for forming large circular moulds of symmetric kind by revolving a sweep
attached to a spindle as shown in Fig. 11. Actually a sweep is a template of wood or metal and is
attached to the spindle at one edge and the other edge has a contour depending upon the desired
shape of the mould. The pivot end is attached to a stake of metal in the center of the mould.

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 Fig. 11: Sweep Pattern

Skeleton Pattern

When only a small number of large and heavy castings are to be made, it is not economical to make a
solid pattern. In such cases, however, a skeleton pattern may be used. This is a ribbed construction of
wood which forms an outline of the pattern to be made. This frame work is filled with loam sand and
rammed. The surplus sand is removed by strickle board. For round shapes, the pattern is made in two
halves which are joined with glue or by means of screws etc. A typical skeleton pattern is shown in Fig.
12.

Fig. 12: Skeleton Pattern

Segmental pattern
The segmental pattern is used to prepare the mold of larger circular casting to avoid the use of solid
pattern of exact size. It is similar to sweep pattern, but the difference from Sweep pattern, the sweep
pattern is give a continuous revolve motion to generate the part, the segmental pattern itself and
mold is prepared. In this segmental pattern construction should be save the material for pattern make
and easy carried. The segmental pattern is mounted on the central pivot and mold in one position for
after prepare of mold the segment is moved for next position. That is repeat together the complete
mold is done. A typical segmental pattern is shown in Fig. 13.

Fig. 13: Segmental pattern

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Pattern Allowances

Pattern may be made from wood or metal and its color may not be same as that of the casting. The
material of the pattern is not necessarily same as that of the casting. Pattern carries an additional
allowance to compensate for metal shrinkage. It carries additional allowance for machining. It
carries the necessary draft to enable its easy removal from the sand mass. It carries distortions
allowance also. Due to distortion allowance, the shape of casting is opposite to pattern. Pattern may
carry additional projections, called core prints to produce seats or extra recess in mold for setting
or adjustment or location for cores in mold cavity. It may be in pieces (more than one piece) whereas
casting is in one piece. Sharp changes are not provided on the patterns. These are provided on the
casting with the help of machining. Surface finish may not be same as that of casting.

The size of a pattern is never kept the same as that of the desired casting because of the fact that
during cooling the casting is subjected to various effects and hence to compensate for these effects,
corresponding allowances are given in the pattern. These various allowances given to pattern can
be enumerated as, allowance for shrinkage, allowance for machining, allowance for draft, allowance
for rapping or shake, allowance for distortion and allowance for mould wall movement. These
allowances are discussed as under.

Shrinkage Allowance
In practice it is found that all common cast metals shrink a significant amount when they are cooled
from the molten state. The total contraction in volume is divided into the following parts:
1. Liquid contraction, i.e. the contraction during the period in which the temperature of the liquid
metal or alloy falls from the pouring temperature to the liquidus temperature.
2. Contraction on cooling from the liquidus to the solidus temperature, i.e. solidifying contraction.
3. Contraction that results there after until the temperature reaches the room temperature.
This is known as solid contraction.
The first two of the above are taken care of by proper gating and risering. Only the last one, i.e. the
solid contraction is taken care by the pattern makers by giving a positive shrinkage allowance. This
contraction allowance is different for different metals. The contraction allowances for different
metals and alloys such as Cast Iron 10 mm/mt.. Brass 16 mm/mt., Aluminium Alloys. 15 mm/mt.,
Steel 21 mm/mt., Lead 24 mm/mt. In fact, there is a special rule known as the pattern marks
contraction rule in which the shrinkage of the casting metals is added. It is similar in shape as that
of a common rule but is slightly bigger than the latter depending upon the metal for which it is
intended. A typical shrinkage allowance can be shown in the Fig. 14.

Fig. 14: Shrinkage Allowance

Machining Allowance
It is a positive allowance given to compensate for the amount of material that is lost in machining or
finishing the casting. If this allowance is not given, the casting will become undersize after machining.

DEPARTMENT OF MECHANICAL ENGINEERING

The amount of this allowance depends on the size of casting, methods of machining and the degree
of finish. In general, however, the value varies from 3 mm. to 18 mm. A typical machining allowance
can be shown in the Fig. 15.

Fig. 15: Machining Allowance

Draft or Taper Allowance
Taper allowance (Fig. 1.1.11) is also a positive allowance and is given on all the vertical surfaces of
pattern so that its withdrawal becomes easier. The normal amount of taper on the external surfaces
varies from 10 mm to 20 mm/mt. On interior holes and recesses which are smaller in size, the taper
should be around 60 mm/mt. These values are greatly affected by the size of the pattern and the
molding method. In machine molding its, value varies from 10 mm to 50 mm/mt. A typical taper
allowance can be shown in the Fig. 16.

Fig. 16: Machining Allowance

Rapping or Shake Allowance

Before withdrawing the pattern it is rapped and thereby the size of the mould cavity increases.
Actually by rapping, the external sections move outwards increasing the size and internal sections
move inwards decreasing the size. This movement may be insignificant in the case of small and
medium size castings, but it is significant in the case of large castings. This allowance is kept negative
and hence the pattern is made slightly smaller in dimensions 0.5-1.0 mm.

Distortion Allowance
This allowance is applied to the castings which have the tendency to distort during cooling due to
thermal stresses developed. For example a casting in the form of U shape will contract at the closed
end on cooling, while the open end will remain fixed in position. Therefore, to avoid the distortion,
the legs of U pattern must converge slightly so that the sides will remain parallel after cooling.

Mold wall Movement Allowance

Mold wall movement in sand moulds occurs as a result of heat and static pressure on the surface layer
of sand at the mold metal interface. In ferrous castings, it is also due to expansion due to
graphitisation. This enlargement in the mold cavity depends upon the mold density and mould
composition. This effect becomes more pronounced with increase in moisture content and
temperature.

Gating System

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 Gating system refers to all those elements which are connected with the flow of molten metal from
the ladle to the mould cavity. The various elements that are connected with the gating system are:
Pouring Basin
 Sprue
 Sprue- base
 Well
Runner
 Runner
 Extension In-gate
 Riser
Pouring Basin: In order to avoid mould erosion, molten metal is poured into a pouring basin, which
acts as a reservoir from which it moves smoothly into the sprue. The pouring basin is also able to
stop the slag from entering the mould cavity by means of a skimmer or skim core.

Sprue: It is the channel through which the molten metal is brought into the parting plane, where it
enters the runners and gates to ultimately reach the mould cavity. If the sprue were to be straight-
cylindrical then the meatl flow would not be full at the bottom to avoid this problem the sprue is
designed tapper.

Sprue Base Well: This is a reservoir for metal at the bottom of the sprue, to reduce the momentum of
the molten metal.

Runner: The runner takes the molten metal from sprue to the casting. Ingate: This is the final stage
where the molten metal moves from the runner to the mold cavity.

Riser: Riser is a source of extra metal which flows from riser to mold cavity to compensate for
shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the
casting will have shrinkage defects, either on the surface or internally.

Types of Gating Systems:

The gating system also depends on the direction of the parting plane, which contains the sprue,
runner and the ingate. They are as follows:
Horizontal Gating System : This is used most widely. This type is normally applied in ferrous metal's
sand casting and gravity die-casting of non-ferrous metals. They are used for flat casting, which are
filled under gravity.
Vertical Gating System : This is applied in tall castings were high-pressure sand mold, shell mold and
die-casting processes are done. Top Gating System : this is applied in places where the hot metal is
poured form the top of the casting. It helps directional solidification of the casting from top to
bottom. It suits only flat castings to limit the damage of the metal during the initial filling.
Bottom Gating System : it is used in tall castings where the molten metal enters the casting through
the bottom.
Middle Gating System : It has the characteristics of both the top and bottom.

DEPARTMENT OF MECHANICAL ENGINEERING

 Fig. 17: Temperature as a function of time for the solidification of pure metals. Note that the
freezing takes place at a constant temperature. (b) Density as a function of time.

In order to provide defect-free casting the gating system should make certain provisions while
designing the gating system.
1. The mould should be completely filled in the smallest time possible without having to raise the
metal temperature or use high metal heads.
2. The metal should flow smoothly into the mould without any turbulence. A turbulence metal flow
tends to form dross in the mould.
3. Unwanted material such as slag, dross and other mould material should not be allowed to enter
the mould cavity
4. The metal entry into the mould cavity should be properly controlled in such a way that aspiration
of the atmospheric air is prevented.
5. A proper thermal gradient be maintained so that the casting is cooled without any shrinkage
cavities or distortions.
6. Metal flow should be maintained in such a way that no gating or mould erosion takes place.
7. The gating system should ensure that enough molten metal reaches the mould cavity
8. The gating system design should be economical and easy to implement and remove after casting
solidification.
9. Ultimately, the casting yield should be maximized.

Solidification of Metals
After pouring molten metal into a mold, a series of events takes place during the solidification of the
metal and cooling to room temperature. These events greatly influence the size, shape uniformity,
and chemical composition of the grains formed throughout the casting, which in turn influence its
over all properties.

Solidification of Pure Metals: A pure metal solidifies at a constant temperature. It has a clearly
defined melting (or freezing) point (see table above and Fig. 17). After the temperature of the molten
metal drops to its freezing point, its temperature remains constant while the latent heat of fusion is
given off. The solidification front (solid-liquid interface) moves through the molten metal, solidifying
from the mold walls in toward the center.

The grain structure of a pure metal cast in a square mold is shown in Fig. 17 a: 9At the mold walls
(usually at room temp), the metal cools rapidly and produces a solidified skin (or shell) of fine

DEPARTMENT OF MECHANICAL ENGINEERING

equiaxed grains (approx. equal dims. in all dirs.) 9The grains grow in a direction opposite to that of
the heat transfer out through the mold. Those grains that have favorable orientations grow
preferentially away from the surface of the mold producing columnar grains (Fig. 10.3). 9As the
driving force of the heat transfer is reduced away from the mold walls, the grains become equiaxed
and coarse. Those grains that have substantially different orientations are blocked from further
growth. Such grains development is known as homogeneous nucleation, meaning that the grains
grow upon themselves, starting at the mold wall.
When the heat is abstracted rapidly, however, solidification it leads to fine structures due to a
decrease in diffusion rates.

Fig. 18: Schematic illustration of three cast structures of metals solidified in a square mold: (a)
Pure metals (b) Solid-solutions alloys and (C) Structure obtained by using nucleating agents.

Solidification of Alloys

Fig. 19: Schematic illustration of alloy solidification and temperature distribution in the solidifying
metal. Note the formation of dendrites in the mushy zone.

DEPARTMENT OF MECHANICAL ENGINEERING

Solidification begins when the temperature drops below the liquidus, TL, and is complete when it
reaches the solidus, TS (Fig.19). Within this temperature range, the alloy is in a mushy or pasty state
with columnar dendrites (close to tree). Note the liquid metal present between the dendrite arms.

Dendrites have 3-D arms and branches (secondary arms) which eventually interlock, as can be seen
in Fig.20.
(L & S) is an important factor during solidification. It is described by the freezing range as: 5
Freezing range = TL - TS (Fig. 17) .

It can be seen in Figure 10.1 that pure metals have no freezing range, and that the solidification front
moves as a plane front without forming a mushy zone. In alloys with a nearly symmetrical phase
diagram, the structure is generally lamellar, with two or more solid phases present, depending on the
alloy system. When the volume fraction of the minor phase of the alloy is less than about 25%, the
structure generally becomes fibrous. These conditions are particularly important for cast irons. For
alloys, a short freezing range generally involves a temperature difference < 50o C, and a long freezing
range > 110o C.

Ferrous castings generally have narrow mushy zones, whereas aluminum and magnesium alloys have
wide mushy zones.

Fig. 20: (a) Solidification patterns for grey cast iron in a 180mm square casting. Note that after 11
minutes of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes
about two hours for this casting to solidify completely. (b)Solidification of carbon steels in sand
and chill (metal) molds. Note the difference in solidification patterns as the carbon content
increases.

Riser
Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage
which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting
will have shrinkage defects, either on the surface or internally.
Risers are known by different names as metal reservoir, feeders, or headers. Shrinkage
in a mold, from the time of pouring to final casting, occurs in three stages.
1. during the liquid state
2. during the transformation from liquid to solid
3. during the solid state
First type of shrinkage is being compensated by the feeders or the gating system. For the second
type of shrinkage risers are required. Risers are normally placed at that portion of the casting which

DEPARTMENT OF MECHANICAL ENGINEERING

is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to
feed the casting during this time.

Functions of Risers
 Provide extra metal to compensate for the volumetric shrinkage
 Allow mold gases to escape
 Provide extra metal pressure on the solidifying mold to reproduce mold details more exact

Design Requirements of Risers

1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface
area)2 of the riser must be greater than that of the casting. However, when this condition
does not meet the metal in the riser can be kept in liquid state by heating it externally or
using exothermic materials in the risers.
2. Riser placement: the spacing of risers in the casting must be considered by effectively
calculating the feeding distance of the risers.
3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers,
although considers as best, are difficult to cast. To increase volume/surface area ratio the
bottom of the riser can be shaped as hemisphere.

Riser Design
The riser is a reservoir in the mold that serves as a source of liquid metal for the casting to
compensate for shrinkage during solidification. The riser must be designed to freeze after the main
casting in order to satisfy its function Riser Function As described earlier, a riser is used in a sand-
casting mold to feed liquid metal to the casting during freezing in order to compensate for
solidification shrinkage. To function, the riser must remain molten until after the casting solidifies.
Chvorinov’s rule can be used to compute the size of a riser that will satisfy this requirement. The
following example illustrates the calculation. The riser represents waste metal that will be separated
from the cast part and re-melted to make subsequent castings. It is desirable for the volume of
metal in the riser to be a minimum. Since the geometry of the riser is normally selected to maximize
the V/A ratio, this tends to reduce the riser volume as much as possible Risers can be designed in
different forms. The design shown in Figure below is a side riser. It is attached to the side of the
casting by means of a small channel. A top riser is one that is connected to the top surface of the
casting. Risers can be open or blind. An open riser is exposed to the outside at the top surface of
the cope. This has the disadvantage of allowing more heat to escape, promoting faster solidification.
A blind riser is entirely enclosed within the mold, as in Figure below.

This process was patent in 20 century to make higher standards hollow castings. The first centrifugal

DEPARTMENT OF MECHANICAL ENGINEERING

 casting machine was invented by a British, A.G. Eckhardt in 1807. This process is widely used for
casting hollow pipes, tubes and other symmetrical parts.

Core and Core Box:

Cores are compact mass of core sand that when placed in mould cavity at required location
with proper alignment does not allow the molten metal to occupy space for solidification in
that portion and hence help to produce hollowness in the casting. The environment in which
the core is placed is much different from that of the mold. In fact the core (Fig. 22) has to
withstand the severe action of hot metal which completely surrounds it. Cores are classified
according to shape and position in the mold. There are various types of cores such as horizontal
core, vertical core, balanced core, drop core and hanging core are shown in the Fig. 22 below.

Fig. 22: Types of Core

There are various functions of cores which are given below

Core is used to produce hollowness in castings in form of internal cavities.

It may form a part of green sand mold

1. It may be deployed to improve mold surface.

2. It may provide external undercut features in casting.
3. It may be used to strengthen the mold.
4. It may be used to form gating system of large size mold
5. It may be inserted to achieve deep recesses in the casting

DEPARTMENT OF MECHANICAL ENGINEERING

Special Casting processes:
This process was patent in 20 century to make higher standards hollow castings. The first centrifugal
casting machine was invented by a British, A.G. Eckhardt in 1807. This process is widely used for
casting hollow pipes, tubes and other symmetrical parts.

Centrifugal Casting:

Working Principle: It works on basic principle of centrifugal force on a rotating Component. In this
process, a mould is rotated about its central axis when the molten metal is poured into it. A centrifugal
force acts on molten metal due to this rotation, which forces the metal at outer wall of mould. The
mould rotates until the whole casting solidifies. The slag oxide and other inclusion being lighter, gets
separated from metal and segregate towards the center.

Types
True Centrifugal Casting:

True centrifugal casting is sometime known as centrifugal casting is a process of making symmetrical
round hollow sections. This process uses no cores and the symmetrical hollow section is created by
pure centrifugal action. In this process, the mould rotates about horizontal or vertical axis. Mostly
the mould is rotated about horizontal axis and the molten metal introduce from an external source.
The centrifugal force acts on the molten metal which forces it at the outer wall of mould. The mould
rotates until the whole casting solidifies. The slag particles are lighter than metal thus separated at
the central part of the casting and removed by machining or other suitable process. This process

DEPARTMENT OF MECHANICAL ENGINEERING

 used to make hollow pipes, tubes, hollow bushes etc. which are axi symmetrical with a concentric
hole.
Semi Centrifugal Casting:
This process is used to cast large size axi symmetrical object. In this process mould is placed
horizontally and rotated along the vertical axis. A core is inserted at the center which is used to cast
hollow section. When the mould rotates, the outer portion of the mould fill by purely centrifugal
action and as the liquid metal approaches toward the center, the centrifugal component decreases

and gravity component increase. Thus a core is inserted at center to make hollow cavity at the center
without centrifugal force. In this process centrifugal force is used for uniform filling of axi symmetrical
parts. Gear blanks, flywheel etc. are made by this process.
Centrifuging:
In this process there are several mould cavities connected with a central sprue with radial gates. This
process uses higher metal pressure during solidification. It is used to cast shapes which are not axi
symmetrical. This is only suitable for small objects.

Application:
It is widely used in aircraft industries to cast rings, flanges and compressor casting.
It is used for cast Steam turbine bearing shell.
Roller for steel rolling mill is another example of centrifugal casting.
It is used in automobile industries to cast gear blank, cylindrical liners, piston rings etc.
It is used to cast bearings.
This process used to cast switch gear components used in electronic industries.

Advantages and Disadvantages:

It provides dense metal and high mechanical properties.
Unidirectional solidification can obtain up to a certain thickness.
It can use for mass production.
No cores are required for cast hollow shapes like tubes etc.
Gating system and runner are totally eliminated.

DEPARTMENT OF MECHANICAL ENGINEERING

 All the impurity like oxide or other slag particles, segregated at center from where it can easily
remove.
It required lower pouring temperature thus save energy.
Lower casting defects due to uniform solidification. Disadvantages:
Limited design can be cast. It can cast only symmetrical shapes.
High equipment or setup cost.
It is not suitable for every metal.
Higher maintenance required.
High skill operator required.
In this casting process, solidification time and temperature distribution is difficult to
determine.

Investment Casting Process

The root of the investment casting process, the cire perdue or “lost wax” method dates back to at
least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the
rudiments of the investment casting process to create intricately detailed jewelry, pectorals and idols.
The investment casting process alos called lost wax process begins with the production of wax
replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be
produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A number of
patterns are attached to a central wax sprue to form a assembly. The mold is prepared by
surrounding the pattern with refractory slurry that can set at room temperature. The mold is then
heated so that pattern melts and flows out, leaving a clean cavity behind. The mould is further
hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified,
the mold is broken and the casting taken out.

The basic steps of the investment casting process are ( Figure 11 ) :

1. Production of heat-disposable wax, plastic, or polystyrene patterns
2. Assembly of these patterns onto a gating system
3. “Investing,” or covering the pattern assembly with refractory slurry
4. Melting the pattern assembly to remove the pattern material
5. Firing the mold to remove the last traces of the pattern material
6. Pouring
7. Knockout, cutoff and finishing.

The basic Steps of Investment Casting

Advantages
 Formation of hollow interiors in cylinders without cores
 Less material required for gate
 Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities
and porosity

DEPARTMENT OF MECHANICAL ENGINEERING

 Disadvantages
 More segregation of alloy component during pouring under the forces of rotation
 Contamination of internal surface of castings with non-metallic inclusions
 Inaccurate internal diameter

Ceramic Shell Investment Casting Process

The basic difference in investment casting is that in the investment casting the wax pattern is
immersed in a refractory aggregate before dewaxing whereas, in ceramic shell investment casting a
ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry (refractory
material such as zircon with binder). After each dipping and stuccoing is completed, the assembly is
allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the
assembly. The thickness of this shell is dependent on the size of the castings and temperature of the
metal to be poured.

After the ceramic shell is completed, the entire assembly is placed into an autoclave or flash fire
furnace at a high temperature. The shell is heated to about 982 o C to burn out any residual wax and
to develop a high-temperature bond in the shell. The shell molds can then be stored for future use or
molten metal can be poured into them immediately. If the shell molds are stored, they have to be
preheated before molten metal is poured into them.

Advantages
 excellent surface finish
 tight dimensional tolerances
 machining can be reduced or completely eliminated

The Process
The Mold: Like in all permanent mold manufacturing processes, the first step in die casting is the
production of the mold. The mold must be accurately created as two halves that can be opened and
closed for removal of the metal casting, similar to the basic permanent mold casting process. The
mold for die casting is commonly machined from steel and contains all the components of the gating
system. Multi-cavity die are employed in manufacturing industry to produce several castings with each
cycle. Unit dies which are a combination of smaller dies are also used to manufacture metal castings
in industry.

In a die casting production setup, the mold, (or die), is designed so that its mass is far greater than that
of the casting. Typically the mold will have 1000 times the mass of the metal casting. So a 2 pound
part will require a mold weighing a ton! Due to the extreme pressures and the continuous exposure
to thermal gradients from the molten metal, wearing of the die can be a problem. However in a well
maintained manufacturing process, a die can last hundreds of thousands of cycles before needing to
be replaced.

Die Casting Machines

In addition to the opening and closing of the mold to prepare for and remove castings, it is very
important that there is enough force that can be applied to hold the two halves of the mold together
during the injection of the molten metal. Flow of molten metal under such pressures will create a
tremendous force acting to separate the die halves during the process. Die casting machines are large
and strong, designed to hold the mold together against such forces.

DEPARTMENT OF MECHANICAL ENGINEERING

 In manufacturing industry, die casting machines are rated on the force with which they can hold the
mold closed. Clamping forces for these machines vary from around 25 to 3000 tons.

Melting Practices
Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can
be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends
on the type of metal to be melted. Some of the furnaces used in metal casting are as following:
 Crucible furnaces
 Cupola
 Induction furnace
 Reverberatory furnace

The crucible furnace as you see in the image below is how a crucible furnace looks. So, in this basically
you have a crucible, which is normally made of clay or graphite and then this crucible will be kept and
there will be heating source, and through this heating source this crucible is heated and the molten
metal which is kept in the crucible normally that is melted. So, normally it is used for very you know
smaller quantity of material can be normally is a held, but you can have the larger crucible, you have
the crucible numbers, sometimes you have different numbers and that is basically specifies by amount
of copper which can be melted into that particular number of crucible.

So, kgs of copper which can be melted like that so, if this is a large crucible certainly you need a large
and that it has to be put in you will have the refractory, which is rammed from the other sides and
then you will have the heat source, and then using that heat source you can heat the liquid metal
which will be melted. So, as you see you will have a ladle you have the fire brick lining and then this is
a chimney this is the covers, you can take the cover out and this is a air blower. So, that will be blown
and then this way the heating will be done, and due to the heat the metal is melted here.

So, it is normally convenient for a smaller foundries. So, that you know for handling these smaller
quantities you have to take this crucible out. So, for a smaller foundries when you have to melt in
small quantities, this crucible furnace are basically important.

Now there may be basically coke fired or the oil or gas fired. So, basically when you have to use these
normally for nonferrous melting, nonferrous metal these coke fired furnaces are used, and because
you have the low installation cost in that low fuel cost and ease in operation. So, because of these
reasons these coke fired furnaces are used for nonferrous metal. Now you have also the oil and gas

DEPARTMENT OF MECHANICAL ENGINEERING

fired furnace. So, as the name indicts you have oil or gas, which are used as the heating source in such
furnaces. So, basically they are cylindrical in shape and then the flame is produced by heating of this
atomised fuel.

So, they will combine with air and then they will be heated, and then they will be sweeping around
the crucible. So, that way it will have the enveloped in enveloped in the crucible and then uniformly
heat the crucible. So, this way you have the combustion products will be coming into the contact with
the charge, then they will be heating it and then they that way you can melt them by tilting you can
take the you know metal out and pour it into the mould. So, you have certain advantages of this oil
or gas fired furnace like you do not have any wastage of fuel. So, that is one of the. So, here if this is
the oil or gas fired furnace you do not have much of the wastage of the fuel. So, you have more
thermal efficiency in the case of oil or you know gas fired furnace, you have better temperature
control by controlling the flow of this gases by controlling through a knob, you can have accurate
control of the temperature, air contamination will be less in the case of this oil or gas fired furnaces.
So, it will also save the floor space and you have also the low labour cost because here you need one
person just simply to regulate these burners or so. So, this way you have these crucible furnaces and
they are the different types normally you will have this used by the smaller foundries.

Cupola Furnace:
Cupola Furnace is a melting device used to melt cast iron, Ni-resist iron, and some bronzes and It is
used in Foundries. The cupola can be made of any size and the size of the cupola is measured in
diameters which range from 1.5 to 13 feet. The shape of the cupola is cylindrical and the equipment
is arranged in the vertical fitted with doors which swings down and Out to drop bottom.

The top is open or fitted with a cap to escapes gases or rain entering. The cupola may be fitted with a
cap to control emission of gases and to Pull the gases into the device to cool the gases and remove all
the Particulate Matter.

The cupola shell is made of steel and has a refractory brick and plastic refractory Patching material
lining. The clay and sand mixture is used as a bottom line and the lining is temporary. The coal can be
mixed with the clay lining so when it heats up the coal decomposes and the bond becomes friable.
This makes opening up of the two holes easy. The bottom of the cupola lining is compressed against
the bottom doors. Cooling jackets are also fitted with some of the cupola’s to keep the sides cool and
with oxygen injection to make the coke fire burn hotter.

Principal:
The cupola furnace works on a simple principal that combustion of coke generates carbon dioxide and
heat and this causes the iron to melt. The iron drains downward when get melted. Afterwards, the
carbon dioxide is reduced partly, reduced again by consuming energy and coke with carbon monoxide,
carbon dioxide and supplied coke is present in the reaction equilibrium so it is possible to show a
defined combustion ratio for the utilization of thermal energy for the coke combustion.

DEPARTMENT OF MECHANICAL ENGINEERING

Finally, high concentration of carbon monoxide is present in the exhaust gas and it can be extracted
from the furnace.

Construction:
Cupola furnace constructed in the form of a hollow cylindrical vertical steel shell and it is lined from
inside with a refractory material. This furnace is generally supported on four cast iron lags mounted
on a concrete base.

Cupola Furnace

The bottom of the furnace is closed by two cast iron doors hinged to the bed plate of the furnace. A
wind box cast iron encircles to the outside of the furnace bottom. This box is connected to the furnace
blower pipe known as the blast pipe. Air which supplies the oxygen necessary to burn the fuels forced
through the cupola by a blower. The top of the furnace is shielded by a mesh screen and topped with
a cone-shaped spark arrester, which permits the free vent of the waste gas and deflects spark and
dust back into the furnace.

Working:

Basically, the operation of cupola furnace consists of following steps:

After building the cupola make sure it is dried completely before getting it to fire. Any slag around the
tuyeres from previous runs needs to be cleaned properly.

Also, A broken part is repaired with the mixture of the silica sand and fire clay. Over the Brunt area, a
layer of refractory material is applied To about thickness 6 inches or more is rammed on the bottom
sloping toward the tap hole to ensure better flow of molten metal.

DEPARTMENT OF MECHANICAL ENGINEERING

A hole opening of about 30 mm diameter and a tap hole of about 25 mm diameter is being provided
there. A fire of wood is ignited. When the wood burns well coke is dumped on the bed well from the
top. Make sure that the coke gets burned too. A bed of coke about 40 inches is placed next to the
sand.

Firstly The air blast is turned on At a lower blowing rate than as normal for provoking the coke. A
measuring rod is also used which indicates the height of the coke bed. For about 3 hours firing is done
before the molten metal required.

Now the charge is fed into the cupola. Many factors like charge composition, affect the final structure
of the gray cast iron obtained. It composed of 10% steel,50% grey cast iron scrap, and 3% limestone
as a flux.

Alternate layers are formed by these constituents. Besides limestone, fluorspar and soda ash are also
used as flux material. The main function of flux is to remove the impurities in the iron and protect the
iron from oxidation.

After the fully charged furnace, it is allowed to remain as such for about 1 hour. As this process goes
in charge slowly gets heated up as the air blast is kept shut this time and because of this, the iron gets
soaked up.

At the end of the soaking period, the air blast is opened. The topmost opening is kept closed till the
metal melts. The sufficient amount of metal is collected. The contents of the charge move downwards
as the melting proceeds.

The rate of charging is equal to the rate of melting. The furnace is kept full throughout the heat.
Closing feeding of charge and air blast is stopped when no more melting is required. The bottom plate
swings to open when the prop is removed. The deposited slag is being removed. The cupola runs
continuously and the melting period does not exceed 4 hours in most of the time. But can be operated
for more than 10 hours.

Advantages:
 Low cost of construction.
 Low cost of maintenance.
 Low cost of operation.
 Very skilled operators are not required.
 Simple in construction
 Simple in operations.
 Melting composition can be controlled.
 Small floor area is required.

Disadvantages:
 With a long list of advantages, cupola furnace also comes with few of limitations or
disadvantages and they are listed below:
 It is sometimes difficult to maintain the temperature in a cupola furnace.
 Metal elements are converted to their oxides which are not suitable for casting.

DEPARTMENT OF MECHANICAL ENGINEERING

Application:
Cupola furnace is a used widely as a melting unit for cast iron. Some of the characteristic that makes
the cupola furnace a primary method used in melting irons in the foundries. Some of them are:
 Cupola furnace is the only method which is continuous while operations.
 The melting rate of cupola furnace is high.

It is easy to operate. Operating costs in use of cupola furnace is comparability very low to other
methods for this purpose.

Advanced Casting Processes:

Continuous Casting:

In this process, the molten metal is continuously supplied to the mold. The mold has an indeterminate
length. When the molten metal is cast through a mold, it keeps travelling downward increasing in its
length as the time passes by. The molten metal is continuously passed through the mold, at the same
rate to match the solidifying casting. This results in casting of long strands of metal. The whole process
of continuous casting is a precisely deliberated process that can produce astounding results.

Benefits of continuous casting

Unlike other processes of casting, the time line of steps in continuous casting is entirely different.
While in other casting processes, each step of casting heating of the metal, poring of the molten liquid
into casts, solidification and cast removal are a sequential process, in continuous casting all steps
occur congruently and hence it saves a lot of processing time.

Continuous Casting of Steel

The process
Continuous casting has several advantages but it is also a process that needs distinct resources. This
is the reason why this process is employed only in industries that require high yield of steel cast. The
metal is first liquefied and poured into a tundish, which is a container that leads to the mold that will
cast the steel. The tundish is placed about 80-90 feet above the ground level and the whole process
of casting sues gravity to operate. The tundish is constantly supplied with molten steel to keep the
process going. The whole process is controlled to ensure there is smooth flow of molten steel through
tundish. Further, the impurities and slag are filtered in tundish before they move into the mold. The
entrance of the mold is filled with inert gases to prevent reaction of molten steel with the gases in the
environment like oxygen. The molten metal moves swiftly through the mold and it does not
completely solidify in it.The entire mold is cooled with water that flows along the outer surface.
Typically, steel casting solidifies along the walls of the casting and then gradually moves to the interior
of the steel casting. The metal casting moves outside the mold with the help of different sets of rollers.

DEPARTMENT OF MECHANICAL ENGINEERING

While one set of rollers bend the metal cast, another set will straighten it. This helps to change the
direction of flow of the steel slab from vertical to horizontal.

Squeeze Casting
Squeeze casting is a combination of casting and forging process. The process can result in the highest
mechanical properties attainable in a cast product. The development of squeeze casting process, can
usher in tremendous possibility for manufacturing of components of aluminium alloys, which are not
properly commercialized as yet. It can also be effective in for import substitution of critical
components.

The process starts when the molten metal is poured into the bottom half of a pre-heated die. As soon
as the metal starts solidifying, the upper half of the die closes and starts applying pressure during the
solidification process. The extent of pressure applied is significantly less than that in forging. Parts of
great detail can be produced. Coring can be used in tandem with the process to form holes and
recesses. The high pressure and the close contact of molten alloy with the metal die surface results in
minimum porosity and improvised mechanical properties. This process can be used for both ferrous
and non-ferrous metals. This technique is very much suited for making fiber-reinforced castings from
fiber cake preform.

Squeeze Casting Process (or squeeze forming) are of two types:

Direct (liquid metal forging)
This is done in equipment which closely resemble the forging process. Liquid metal is poured into
lower die segment, contained in a hydraulic press. Upper die segment is closed. A very high pressure
of 100 Mpa or more is applied to the whole cavity until the part gets solidified.

Indirect Squeeze Casting

This process is very much similar to die casting. It takes place in a die casting equipment. This
equipment van be vertical or horizontal. The melt which is cleaned and grain -refined is poured in to
the shot sleeve of a horizontal or vertical casting machine. The melt is then injected into the die
through relatively large gates. This is accomplished through relatively slow velocity (less than
0.5m/sec). The melt in the die cavity is then solidified under pressures, ranging from 55MPa to
300MPa. In this process the parts displays good tensile strength.

Squeeze Casting

Application of Squeeze Casting:

Squeeze casting is an economical, simple and convenient process. It has found extensive application
in automotive industry in producing aluminium front steering knuckles, chassis frames, brackets or
nodes. High capacity propellers for boat-engine.

DEPARTMENT OF MECHANICAL ENGINEERING

Vacuum Mould Casting:
V-process or vacuum molding which was developed by Japanese using unbonded sand and vacuum is
a perfect substitute for permanent mold and die casting process. Now the process is employed
worldwide as an effective method to cast quality products in start up and low to medium job. The
most highlighted feature of vacuum molding is that the flow of molten metal can be controlled.

Process: Patterns are mounted on plates and boards, which are perforated, and each board is
connected with a vacuum chamber. Unbonded sand is used for the molding purpose. Permeability is
not a concern in this casting process, therefore sand of the finest structure can be used. The vented,
plated pattern is coated with a layer of flexible plastic, which expand when the vacuum is applied in
the mold. Enabling, the pattern to be stripped easily from the mold. Patterns should be perfectly
smooth since in vacuum molding, every small intricate design gets imprinted on the cast. The pattern
are not damaged during the process so they can be uses repeatedly.

In the vacuum molding process the mold are made is two parts (cope & drag) with each parts attached
with its vacuum chamber. The pattern is kept and a metal or wooden flask place around it. Unbonded
sand is poured over the molding box, and the tables is shaken vibrantly, by which the sand particle
become tight and compact. Another layer of plastic sheet is draped over the molding box. The two
halves are joined. Now the vacuum is formed through the patter. The vacuum makes the sand strong
and the pattern coating expands, which makes it easy to strip the pattern from the mold.

The mold in kept in a housing and placed above a furnace of molten metal. Using sprue or gating the
mold is connected inside the molten metal. When the vacuum from the mold is evacuated the molten
metal gets forces into the mold, because of the difference in pressure that is created between the
outer atmosphere and the mold. The plastic sheet melts and the mold is filled with the molten metal.
After the metal solidifies and cools, the vacuum is released. The sand mold starts to fall apart as the
solidification process completes. This sand can be cooled and reused for further casting process.

The power of vacuum: In mid 1600,Otto von Guericke a German mayor and scientist conducted the
first experiment to prove the power of vacuum. He joined two large copper hemispheres and
evacuated the air out of it. Now, eight horses were hooked on opposite side of the hemispheres. The
horse pulled the hemisphere is two different direction, but the ball could not be torn apart. Guericke
then let in air and the hemisphere came apart. In this way he proved the power and possibilities of
vacuum.

Application: Vacuum molding process can be used to cast industrial components from both ferrous
and non-ferrous metals.

Application:
 Casted products have high dimensional accuracy and surface finish.
 The process is economical, environment friendly and clean
 No moisture related defects for the castings
 Provides consistent thickness for wall that give the casting an aesthetic appeal
 Low cost operations.

DEPARTMENT OF MECHANICAL ENGINEERING

Evaporative casting
Consumable or eva-foam casting is a sand casting process where the foam pattern evaporates into the
sand mold. A process similar to investment casting, this expendable casting process is predicted to be
used for 29% of aluminum and 14% of ferrous casting in 2010. There are two main evaporative
casting process lost-foam casting and full moldcasting which are widely used because intricate design
can be cast with relative ease and with reasonable expense. The main difference between the two is
that in the lost-foam casting unbonded sand is used and in the full-mold casting green sand or bonded
sand is used.

Process: In the first step of evaporative casting, a foam pattern is shaped using material like
polystyrene. The pattern is attached with sprues, and gates using adhesives and brushed with
refractory substances so that the molds are strong and resistant to high temperature. Refractory
covered pattern assembly is then surrounded by a sand mixture to form a mold. In some instances the
pattern assembly is mixed in ceramic slurry which forms a shell round the pattern when it dries.

In both cases, the mold in kept at a specific temperature to allow the metal to flow smoothly and enter
into every designs and cuts made by the pattern. Molten metal is poured into the mold and the
pattern-forming material disappears into the mold. The molten metal takes the shape of the mold and
solidifies. When the metal solidifies it is removed from the mold to form the casting.

Unlike in the traditional sand casting method, in evaporative sand casting, the pattern does not have
to be removed from the mold which reduces the need for draft provisions. Some of the parameter
that are used to determine the quality of a eva-foam casting are grain fineness number, time of
vibration, degree of vacuum and pouring temperature on surface roughness etc.

Applications: Evaporative castings is used for steel-casting cast iron parts like water pipe and pump
parts, aluminum castings etc.

Advantages:
 High dimensional accuracy and superior casting surface smoothness
 Reduced work process unlike other casting methods
 Light weight casting are be done
 Casting have improved heat resistance and also abrasion resistance and other cast steel
properties.
 Complicated shapes can be cast without using cores or drafts.

Ceramic Shell Casting:

Introduction: A process that can be fully automated, ceramic shell molding is the most rapidly used
technique for mold and core making. Also known a croning process, this casting technique was
invented and patented by J.Croning during World War II. Also known as the process, shell molding
technique is used for making thin sections and for acquiring surface finish and dimensional accuracy.

Process: In the first stage of ceramic shell molding, a metal pattern is made which is resistant to high
temperature and can withstand abrasion due to contact with sand. The sand and resin mixture for the
shell mold is brought in contact with the pattern. The mold is placed in an oven where the resin is
cured. This process causes the formation of a thin shell around the pattern. The thickness of the mold
can be 10-20mm as compared to the heavy mold made for sand castings. When fully cured the skin is
removed from the pattern, which is the shell mold.

DEPARTMENT OF MECHANICAL ENGINEERING

For each ceramic shell molds there are two halves know as the cope and drag section. The two
sections are joined by resin to form a complete shell mold. If an interior design is required, the cores
are placed inside the mold before sealing the two parts.

For heavy castings, ceramic shell molds are held together by metals or other materials. Now, the
molten metal is poured into the mold, and once it solidifies, the shell is broken to remove the casting.
This process is highly useful for near net shape castings. Another advantage is that shell molding can
be automated.
Automated Ceramic Shell Molding Machines and Robots: Shell molding machines like the cold shell
molding machines helps in making castings with little molding material. In a cold shell molding
machine the molds are made using cold binding materials. In it patterns made of wood, metal or
plaster can be used. And the greatest benefit is that the mold can be kept horizontally or vertically.

Robotizing: Using robots for ceramic shell molding is a milestone for the old molding technology.
Robots which are multi functional and re programmable are used in some foundries. Robots are used
for a number of activities like robotic gate and sprue removal, robotic cutting of wedges for gate
valves, robotic core setting, etc. The robots are reliable, consistent, more productive, provides better
surface finish, and less machining etc.

Applications: A sizable amount of the casting in the steel industry are made by shell molding process,
that ensures better profitability. Carbon steel, alloy steel, stainless steel, low alloys, aluminum alloys,
copper, are all cast using shell molding process. Casting that require thin section and excellent
dimensional accuracy are cast using this process. Body panes, truck hoods, small size boats, bath tubs,
shells of drums, connecting rods, gear housings, lever arms, etc. are cast using croning process.

Advantages:
 Thin sections, complex parts and intricate designs can be cast
 Excellent surface finish and good size tolerances
 Less machining required for the castings
 Near net shape castings, almost 'as cast' quality
 Simplified process that can be handled by semi skilled operators
 Full mechanized and automated casting process
 Less foundry space required.

Casting Defects:
The following are the major defects, which are likely to occur in sand castings
 Gas defects
 Shrinkage cavities
 Molding material defects
 Pouring metal defects
 Mold shift

Gas Defects
A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases
evolved during the pouring of the casting. The defects in this category can be classified into blowholes
and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the
surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which
gets entrapped during heating of molten metal.

DEPARTMENT OF MECHANICAL ENGINEERING

Causes
The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability
of the mold or improper design of the casting. The lower permeability is caused by finer grain size of
the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.

 Metal contains gas

 Mold is too hot
 Poor mold burnout

Shrinkage Cavities
These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate
for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate
places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing
shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.

Molding Material Defects

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

Cut and washes

These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by
the flowing metal. This is caused by the molding sand not having enough strength and the molten
metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand
and the latter can be overcome by the proper design of the gating system.

Metal penetration
When molten metal enters into the gaps between sand grains, the result is a rough casting surface.
This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The
coarser the sand grains more the metal penetration.

Fusion
This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy
appearance on the casting surface. The main reason for this is that the clay or the sand particles are
of lower refractoriness or that the pouring temperature is too high.

Swell
Under the influence of metallostatic forces, the mold wall may move back causing a swell in the
dimension of the casting. A proper ramming of the mold will correct this defect.

Inclusions
Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during
pouring solidification. The provision of choke in the gating system and the pouring basin at the top of
the mold can prevent this defect.

Pouring Metal Defects

The likely defects in this category are
 Mis-runs and
 Cold shuts.
A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled
cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before

DEPARTMENT OF MECHANICAL ENGINEERING

freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together
properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold
cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one
solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the
part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the
poured metal and thick enough walls in the casting design.

The mis-run and cold shut defects are caused either by a lower fluidity of the mold or when the section
thickness of the casting is very small. Fluidity can be improved by changing the composition of the
metal and by increasing the pouring temperature of the metal.

Mold Shift
The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned.

Casting Defects

DEPARTMENT OF MECHANICAL ENGINEERING

 UNIT II
WELDING
Unit - II
Welding which is the process of joining two metallic components for the desired purpose, can be defined
as the process of joining two similar or dissimilar metallic components with the application of heat, with or
without the application of pressure and with or without the use of filler metal. Heat may be obtained by
chemical reaction, electric arc, electrical resistance, frictional heat, sound and light energy. If no filter
metal is used during welding then it is termed as ‘Autogenous Welding Process'.

During ‘Bronze Age' parts were joined by forge welding to produce tools, weapons and ornaments etc,
however, present day welding processes have been developed within a period of about a century. First
application of welding with carbon electrode was developed in 1885 while metal arc welding with bare
electrode was patented in 1890. However, these developments were more of experimental value and
applicable only for repair welding but proved to be the important base for present day manual metal arc
(MMAW) welding and other arc welding processes.

In the mean time resistance butt welding was invented in USA in the year 1886. Other resistance welding
processes such as spot and flash welding with manual application of load were developed around 1905.
With the production of cheap oxygen in 1902, oxy – acetylene welding became feasible in Europe in 1903.

When the coated electrodes were developed in 1907, the manual metal arc welding process become viable
for production/fabrication of components and assemblies in the industries on large scale.
Subsequently other developments are as follows:
• Thermit Welding (1903)
• Cellulosic Electrodes (1918)
• Arc Stud Welding (1918)
• Seam Welding of Tubes (1922)
• Mechanical Flash Welder for Joining Rails (1924)
• Extruded Coating for MMAW Electrodes (1926)
• Submerged Arc Welding (1935)
• Air Arc Gouging (1939)
• Inert Gas Tungsten Arc (TIG) Welding (1941)
• Iron Powder Electrodes with High Recovery (1944)
• Inert Gas Metal Arc (MIG) Welding (1948)
• Electro Slag Welding (1951)
• Flux Cored Wire with CO 2 Shielding (1954)
• Electron Beam Welding (1954)
• Constricted Arc (Plasma) for Cutting (1955)
• Friction Welding (1956)
• Plasma Arc Welding (1957)
• Electro Gas Welding (1957)
• Short Circuit Transfer for Low Current, Low Voltage Welding with CO 2 Shielding (1957)
• Vacuum Diffusion Welding (1959)
• Explosive Welding (1960)
• Laser Beam Welding (1961)
• High Power CO 2 Laser Beam Welding (1964)
All welded ‘Liberty ' ships failure in 1942, gave a big jolt to application of welding. However, it had drawn
attention to fracture problem in welded structures.
Applications:
Although most of the welding processes at the time of their developments could not get their place in the
production except for repair welding, however, at the later stage these found proper place in
manufacturing/production. Presently welding is widely being used in fabrication of pressure vessels,
bridges, building structures, aircraft and space crafts, railway coaches and general applications. It is also
being used in shipbuilding, automobile, electrical, electronic and defense industries, laying of pipe lines
and railway tracks and nuclear installations etc.
General Applications:
Welding is vastly being used for construction of transport tankers for transporting oil, water, milk and
fabrication of welded tubes and pipes, chains, LPG cylinders and other items. Steel furniture, gates, doors
and door frames, body and other parts of white goods items such as refrigerators, washing machines,
microwave ovens and many other items of general applications are fabricated by welding.
Pressure Vessels:
One of the first major use of welding was in the fabrication of pressure vessels. Welding made
considerable increases in the operating temperatures and pressures possible as compared to riveted
pressure vessels.
Bridges:
Early use of welding in bridge construction took place in Australia. This was due to problems in
transporting complete riveted spans or heavy riveting machines necessary for fabrication on site to
remote areas. The first all welded bridge was erected in UK in 1934. Since then all welded bridges are
erected very commonly and successfully.
Ship Building:
Ships were produced earlier by riveting. Over ten million rivets were used in ‘Queen Mary' ship which
required skills and massive organization for riveting but welding would have allowed the semiskilled/
unskilled labor and the principle of pre-fabrication. Welding found its place in ship building around 1920
and presently all welded ships are widely used. Similarly submarines are also produced by welding.
Building Structures:
Arc welding is used for construction of steel building leading to considerable savings in steel and money.
In addition to building, huge structures such as steel towers etc also require welding for fabrication.
Aircraft and Spacecraft:
Similar to ships, aircrafts were produced by riveting in early days but with the introduction of jet engines
welding is widely used for aircraft structure and for joining of skin sheet to body.
Space vehicles which have to encounter frictional heat as well as low temperatures require outer skin and
other parts of special materials. These materials are welded with full success achieving safety and
reliability.
Railways:
Railways use welding extensively for fabrication of coaches and wagons, wheel tyres laying of new railway
tracks by mobile flash butt welding machines and repair of cracked/damaged tracks by thermit welding.
Automobiles:
Production of automobile components like chassis, body and its structure, fuel tanks and joining of door
hinges require welding.
Electrical Industry:
Starting from generation to distribution and utilization of electrical energy, welding plays important role.
Components of both hydro and steam power generation system, such as penstocks, water control gates,
condensers, electrical transmission towers and distribution system equipment are fabricated by welding.
Turbine blades and cooling fins are also joined by welding.
Electronic Industry:
Electronic industry uses welding to limited extent such as for joining leads of special transistors but other
joining processes such as brazing and soldering are widely being used. Soldering is used for joining
electronic components to printed circuit boards. Robotic soldering is very common for joining of parts to
printed circuit boards of computers, television, communication equipment and other control equipment
etc.
Nuclear Installations:
Spheres for nuclear reactor, pipe line bends joining two pipes carrying heavy water and other components
require welding for safe and reliable operations.
Defence Industry:
Defence industry requires welding for joining of many components of war equipment. Tank bodies
fabrication, joining of turret mounting to main body of tanks are typical examples of applications of
welding.
Micro-Joining:
It employs the processes such as micro-plasma, ultrasonic, laser and electron beam welding, for joining of
thin wire to wire, foil to foil and foil to wire, such as producing junctions of thermocouples, strain gauges
to wire leads etc.
Apart from above applications welding is also used for joining of pipes, during laying of crude oil and gas
pipelines, construction of tankers for their storage and transportation. Offshore structures, dockyards,
loading and unloading cranes are also produced by welding.
Classification of Welding Processes:

Welding processes can be classified based on following criteria;

1. Welding with or without filler material.

2. Source of energy of welding.
3. Arc and Non-arc welding.
4. Fusion and Pressure welding.

1. Welding can be carried out with or without the application of filler material. Earlier only gas
welding was the fusion process in which joining could be achieved with or without filler material.
When welding was done without filler material it was called ‘autogenous welding'. However, with
the development of TIG, electron beam and other welding processes such classification created
confusion as many processes shall be falling in both the categories.
2. Various sources of energies are used such as chemical, electrical, light, sound, mechanical
energies, but except for chemical energy all other forms of energies are generated from electrical
energy for welding. So this criterion does not justify proper classification.
3. Arc and Non-arc welding processes classification embraces all the arc welding processes in one
class and all other processes in other class. In such classification it is difficult to assign either of
the class to processes such as electroslag welding and flash butt welding, as in electroslag welding
the process starts with arcing and with the melting of sufficient flux the arc extinguishes while in
flash butt welding tiny arcs i.e. sparks are established during the process and then components
are pressed against each other. Therefore, such classification is also not perfect.
4. Fusion welding and pressure welding is most widely used classification as it covers all processes
in both the categories irrespective of heat source and welding with or without filler material. In
fusion welding all those processes are included where molten metal solidifies freely while in
pressure welding molten metal if any is retained in confined space under pressure (as may be in
case of resistance spot welding or arc stud welding) solidifies under pressure or semisolid metal
cools under pressure. This type of classification poses no problems so it is considered as the best
criterion.
Processes falling under the categories of fusion and pressure welding are shown in Figures 2.1 and 2.2.

Need of welding symbols It is important to communicate information about welding procedure without
any ambiguity to all those who are involved in various steps of fabrication of successful weld joints ranging
from edge preparation to final inspection and testing of welds. To assist in this regard, standard symbols
and methodology for representing the welding procedure and other conditions have been developed.
Symbols used for showing the type of weld to be made are called weld symbols. Some common weld
symbols are shown below.
Basic Weld Symbols
Symbols which are used to show not only the type of weld but all relevant aspects related with welding
like size & location of weld, welding process, edge preparation, bead geometry and weld inspection
process and location of the weld to be fabricated and method of weld testing etc. are called welding
symbols. Following sections present standard terminologies and joints used in field of welding engineering.
22.7 Types of weld Joints the classification of weld joints is based on the orientation of plates/members
to be welded. Common types of weld joints and their schematics are shown in Fig. 22.2 (a-e). Butt joint:
plates are in same horizontal plane and aligned with maximum deviation of 50. Lap joint: plates
overlapping each other and the overlap can just one side or both the sides of plates being welded Corner
joint: joint is made by melting corners of two plates being welded and therefore plates are approximately
perpendicular (750 - 900) to each other at one side of the plates being welded Edge joint: joint is made by
melting the edges of two plates to be welded and therefore the plates are almost parallel (00 - 50)
T joint: one plate is approximately perpendicular to another plate (850 - 900).

Schematic of different types of weld joints a) Butt b) Lap c) Corner d) Edge and e) T- Joint

Types of weld:
This classification in based on the combined factors like “how weld is made” and “orientation of plates”
to be welded. Common types of weld joints and their schematics are shown in Figure shown below.
 Schematic of different types of welds a) Groove b) Fillet c) Plug and d) Bead on Plate

Gas Metal Arc Welding

Gas metal arc welding (GMAW) is the process in which arc is struck between bare wire electrode and
workpiece. The arc is shielded by a shielding gas and if this is inert gas such as argon or helium then it is
termed as metal inert gas (MIG) and if shielding gas is active gas such as CO2 or mixture of inert and active
gases then process is termed as metal active gas (MAG) welding. Figure 9.1 illustrates the process of GMA
welding.

Schematic Diagram of GMA Welding

Direct current flat characteristic power source is the requirement of GMAW process. The electrode wire
passing through the contact tube is to be connected to positive terminal of power source so that stable
arc is achieved. If the electrode wire is connected to negative terminal then it shall result into unstable
spattery arc leading to poor weld bead. Flat characteristic leads to self adjusting or self regulating arc
leading to constant arc length due to relatively thinner electrode wires.

GMA welding requires consumables such as filler wire electrode and shielding gas. Solid filler electrode
wires are normally employed and are available in sizes 0.8, 1.0, 1.2 and 1.6 mm diameter. Similar to
submerged arc welding electrode wires of mild steel and low alloyed steel, are coated with copper to
avoid atmospheric corrosion, increase current carrying capacity and for smooth movement through
contact tube. The electrode wire feeding system is shown in Figure 9.2.

Electrode Wire Feeding System

Pressure adjusting screw is used to apply required pressure on the electrode wire during its feeding to avoid
any slip. Depending on the size and material of the wire, different pressures are required for the smooth
feeding of wire with minimum deformation of the wire. Further, wire feeding rolls have grooves of
different sizes and are to be changed for a particular wire size.
The range of welding current and voltage vary and is dependent on material to be welded, electrode size
and mode of metal transfer i.e. mode of molten drop formed at the tip of electrode and its transfer to the
weld pool. This process exhibits most of the metal transfer modes depending on welding parameters.
The range of current and voltage for a particular size of electrode wire, shall change if material of electrode
wire is changed. With lower currents normally lower voltages are employed while higher voltages are
associated with higher currents during welding. Thin sheets and plates in all positions or root runs in
medium plates are welded with low currents while medium and heavy plates in flat position are welded
with high currents and high voltages. Welding of medium thickness plates in horizontal and vertical
positions are welded with medium current and voltage levels.

Table 9.1 gives the total range of currents and voltages for different sizes of structural steel i.e. mild steel
electrodes of different sizes.

Table 9.1: Welding Current and Voltage Ranges for Mild Steel Electrodes
Electrode Wire Diameter Current Range (A) Voltage Range (V)
(mm)
0.8 50-180 14-24
1.0 70-250 16-26
1.2 120-320 17-30
 1.6 150-380 18-34

Both inert gases like argon and helium and active gases like CO2 and N2 are being used for shielding
depending upon the metal to be welded. Mixtures of inert and active gases like CO2 and O2 are also being
used in GMA welding process. For mild steel carbon dioxide is normally used which gives high quality, low
current out of position welding i.e. also in welding positions other than flat position. Low alloyed and
stainless steels require argon plus oxygen mixtures for better fluidity of molten metal and improved arc
stability. The percentage of oxygen varies from 1-5% and remaining is argon in argon and oxygen mixtures.
However, low alloy steels are also welded with 80% argon and 20% CO2 mixture.

Nickel, monel, inconel, aluminum alloys, magnesium, titanium, aluminum bronze and silicon bronze are
welded with pure argon. Nickel and nickel alloys may sometimes be welded with mixture of argon and
hydrogen (upto 5%). Copper and aluminum are also welded with 75% helium and 25% argon mixture to
encounter their thermal conductivity. Nitrogen may be used for welding of copper and some of its alloys,
but nitrogen and argon mixtures are preferred over pure nitrogen for relatively improved arc stability.

The process is extremely versatile over a wide range of thicknesses and all welding positions for both
ferrous and nonferrous metals, provided suitable welding parameters and shielding gases are selected.
High quality welds are produced without the problem of slag removal. The process can be easily
mechanized / automated as continuous welding is possible. However, process is costly and less portable
than manual metal arc welding. Further, arc shall be disturbed and poor quality of weld shall be produced
if air draught exists in working area.

GMA welding has high deposition rate and is indispensable for welding of ferrous and specially for
nonferrous metals like aluminum and copper based alloys in shipbuilding, chemical plants, automobile
and electrical industries. It is also used for building structures.

This chapter presents the basic principle of arc welding processes with focus on shielded metal arc welding.
Further, the influence of welding parameters on performance of weld joint and the role of coating on
electrode have been described. Keywords: Arc welding, shielded metal arc welding, shielding in SMAW,
electrode coating, welding current, electrode size

Arc Welding Process All arc welding processes apply heat generated by an electric arc for melting the
faying surfaces of the base metal to develop a weld joint (Fig. 11.1). Common arc welding processes are
manual metal or shielded metal arc welding (MMA or SMA), metal inert gas arc (MIG), tungsten inert gas
(TIG), submerged arc (SA), plasma arc (PA), carbon arc (CA) selding etc.
Schematic diagram showing various elements of SMA welding system
 Arc Welding

Forge Welding
Principle
As we discussed, forge welding is a solid state welding process in which both the plates are heated quite
below its melting temperature. This heating deforms the work pieces plastically. Now a repeated
hammering or high pressurize load is applied on these plates together. Due to this high pressure and
temperature, inter-molecular diffusion takes place at the interface surface of the plates which make a
strong weld joint. This is basic principle of forge welding. One of the basic requirement of this types
of welding, is clean interface surface which should be free from oxide or other contaminant particles. To
prevent the welding surface from oxidation, flux is used which mixes with the oxide and lower down its
melting temperature and viscosity. This allow to flow out the oxide layer during heating and hammering
process.
Working
Forge welding was one of the most applied welding method in ancient time. This is a fundamental
welding process of all solid state welding. Its working can be summarized as follow.

 First both the work plates heated together. The heating temperature is about 50 to 90% of its
melting temperature. Both the plates are coated with flux.
 Now manual hammering is done by a blacksmith hammer for making a joint. This process is repeated
until a proper joint is created.
 For welding large work pieces, mechanical hammering is used which is either driven by electric
motor or by using hydraulic mean. Sometime dies are used which provides finished surface.

Application
 It is used to join steel or iron.
 It is used to manufacture gates, prison cells etc.
 It is widely used in cookware.
 It was used to join boiler plates before introduction of other welding process.
 It was used to weld weapon like sword etc.
 Used to weld shotgun barrels.
Advantages
 It is simple and easy.
 It does not require any costly equipment for weld small pieces.
 It can weld both similar and dissimilar metals.
 Properties of weld joint is similar to base material.
 No filler material required.
Disadvantages
 Only small objects can be weld. Larger objects required large press and heating furnaces, which
are not economical.
 High skill required because excessive hammering can damage the welding plates.
 High Welding defects involve.
 It cannot use as mass production.
 Mostly suitable for iron and steel.
 It is a slow welding process.

Resistance Welding
Resistance welding processes are pressure welding processes in which heavy current is passed for short
time through the area of interface of metals to be joined. These processes differ from other welding
processes in the respect that no fluxes are used, and filler metal rarely used. All resistance welding
operations are automatic and, therefore, all process variables are preset and maintained constant. Heat
is generated in localized area which is enough to heat the metal to sufficient temperature, so that the
parts can be joined with the application of pressure. Pressure is applied through the electrodes.

The heat generated during resistance welding is given by following expression:

H=I2RT
Where, H is heat generated
I is current in amperes
R is resistance of area being welded
T is time for the flow of current
The process employs currents of the order of few KA, voltages range from 2 to 12 volts and times vary from
few ms to few seconds. Force is normally applied before, during and after the flow of current to avoid
arcing between the surfaces and to forge the weld metal during post heating. The necessary pressure shall
vary from 30 to 60 N mm-2 depending upon material to be welded and other welding conditions. For good
quality welds these parameters may be properly selected which shall depend mainly on material of
components, their thicknesses, type and size of electrodes. Apart from proper setting of welding
parameters, component should be properly cleaned so that surfaces to be welded are free from rust, dust,
oil and grease. For this purpose components may be given pickling treatment i.e. dipping in diluted acid
bath and then washing in hot water bath and then in the cold water bath. After that components may be
dried through the jet of compressed air. If surfaces are rust free then pickling is not required but surface
cleaning can be done through some solvent such as acetone to remove oil and grease.

The current may be obtained from a single phase step down transformer supplying alternating current.
However, when high amperage is required then three phase rectifier may be used to obtain DC supply and
to balance the load on three phase power lines.
The material of electrode should have higher electrical and thermal conductivities with sufficient strength
to sustain high pressure at elevated temperatures. Commonly used electrode materials are pure copper
and copper base alloys. Copper base alloys may consist of copper as base and alloying elements such as
cadmium or silver or chromium or nickel or beryllium or cobalt or zirconium or tungsten. Pure tungsten
or tungsten-silver or tungsten-copper or pure molybdenum may also be used as electrode material. To
reduce wear, tear and deformation of electrodes, cooling through water circulation is required. Figure
11.1 shows the water cooling system of electrodes.

Water Cooling Electrodes a)Spot Welding b) Seam Welding

Commonly used resistance welding processes are spot, seam and projection welding which produce lap
joints except in case of production of welded tubes by seam welding where edges are in butting position.
In butt and flash welding, components are in butting position and butt joints are produced.

Spot Welding
In resistance spot welding, two or more sheets of metal are held between electrodes through which
welding current is supplied for a definite time and also force is exerted on work pieces. The principle is
illustrated in Figure 11.2.

Spot Welding
The welding cycle starts with the upper electrode moving and contacting the work pieces resting on lower
electrode which is stationary. The work pieces are held under pressure and only then heavy current is
passed between the electrodes for preset time. The area of metals in contact shall be rapidly raised to
welding temperature, due to the flow of current through the contacting surfaces of work pieces. The
pressure between electrodes, squeezes the hot metal together thus completing the weld. The weld nugget
formed is allowed to cool under pressure and then pressure is released. This total cycle is known as
resistance spot welding cycle and illustrated in Figure 11.3

Spot welding electrodes of different shapes are used. Pointed tip or truncated cones with an angle of 120°
- 140° are used for ferrous metal but with continuous use they may wear at the tip. Domed electrodes are
capable of withstanding heavier loads and severe heating without damage and are normally useful for
welding of nonferrous metals. The radius of dome generally varies from 50- 100 mm. A flat tip electrode
is used where minimum indentation or invisible welds are desired.

Electrode Shapes for Spot Welding

Most of the industrial metal can be welded by spot welding, however, it is applicable only for limited
thickness of components. Ease of mechanism, high speed of operation and dissimilar metal combination
welding, has made is widely applicable and acceptable process. It is widely being used in electronic,
electrical, aircraft, automobile and home appliances industries.

Seam Welding:
In seam welding overlapping sheets are gripped between two wheels or roller disc electrodes and current
is passed to obtain either the continuous seam i.e. overlapping weld nuggets or intermittent seam i.e. weld
nuggets are equally spaced. Welding current may be continuous or in pulses. The process of welding is
illustrated in Figure 11.5.

Process of Seam Welding

Types of Seam Welds

Electrodes shapes of Seam Welding

Overlapping of weld nuggets may vary from 10 to 50 %. When it is approaching around 50 % then it is
termed as continuous weld. Overlap welds are used for air or water tightness.
It is the method of welding which is completely mechanized and used for making petrol tanks for
automobiles, seam welded tubes, drums and other components of domestic applications. Seam welding
is relatively fast method of welding producing quality welds. However, equipment is costly and
maintenance is expensive. Further, the process is limited to components of thickness less than 3 mm.

Projection Welding:
Projections are little projected raised points which offer resistance during passage of current and thus
generating heat at those points. These projections collapse under heated conditions and pressure leading
to the welding of two parts on cooling. The operation is performed on a press welding machine and
components are put between water cooled copper platens under pressure. Figures 11.8 and 11.9 illustrate
the principle of resistance projection welding.
 Resistance Projection welding Machine

These projections can be generated by press working or machining on one part or by putting some external
member between two parts. Members such as wire, wire ring, washer or nut can be put between two
parts to generate natural projection. Insert electrodes are used on copper platen so that with continuous
use only insert electrodes are damaged and copper platen is safe. Relatively cheaper electrode inserts can
be easily replaced whenever these are damaged.
Formation of Welds from Projections on Components

Projection welding may be carried out with one projection or more than one projections simultaneously.
No consumables are required in projection welding. It is widely being used for fastening attachments like
brackets and nuts etc to sheet metal which may be required in electronic, electrical and domestic
equipment.

Production of seam welded Tubes:

Welded tubes are produced by resistance seam welding. Tubes are produced from strips which are
wrapped on spool with trimmed edges. The width of strip should be slightly bigger than the periphery of

Thermit Welding Process

The tube to be produced to take care for the loss of metal in flashout. The strip is fed through set of
forming rollers to form first the shape of the tube and then it is passed under the seam welding rolls.
Under seam welding rolls the edges are butt welded with some flash out on the joint. This flash out is
trimmed and then tubes are cut to required size. The process is shown in Figures 11.10 & 11.11.
Forming of Tube from Strip

Seam Welding of Tube

Thermit Welding:
After reading this article you will learn about:- 1. Process of Thermit Welding 2. Operation of Thermit
Welding 3. Application and Uses 4. Advantages 5. Disadvantages.
Process of Thermit Welding:
Thermit welding is a chemical welding process in which an exothermic chemical reaction is used to supply
the essential heat energy. That reaction involves the burning of Thermit, which is a mixture of fine
aluminum powder and iron oxide in the ratio of about 1:3 by weight.
Although a temperature of 3000°C may be attained as a result of the reaction, preheating of the Thermit
mixture up to about 1300°C is essential in order to start the reaction.
The mixture reacts according to the chemical reaction:
8 Al + 3 Fe3O4 → 9 Fe + 4 Al2O3 + heat (3000˚C, 35 kJ/kg of mixture).

Aluminum has greater affinity to react with oxygen; it reacts with ferric oxide to liberate pure iron and slag
of aluminum oxide. Aluminum oxide floats on top of molten metal pool in the form of slag and pure iron
(steel) settled below, because of large difference in densities.

Operation of Thermit Welding:

ADVERTISEMENTS:
Thermit welding process is essentially a casting and foundry process, where the metal obtained by the
Thermit reaction is poured into the refractory cavity made around the joint.

The various steps involved in Thermit welding are:

 1. The two pieces of metal to be joined are properly cleaned and the edge is prepared.
2. Then the wax is poured into the joint so that a wax pattern is formed where the weld is to be
obtained.
3. A moulding box is kept around the joint and refractory sand is packed carefully around the wax
pattern as shown in Fig. 7.40, providing the necessary pouring basin, sprue, and riser and gating
system.
4. A bottom opening is provided to run off the molten wax. The wax is melted through this opening
which is also used to preheat the joint. This makes it ready for welding.
5. The Thermit is mixed in a crucible which is made of refractory material that can withstand the
extreme high heat and pressure, produced during the chemical reaction.
6. The igniter (normally barium peroxide or magnesium) is placed on top of the mixture and is lighted
with a red hot metal rod or magnesium ribbon.
7. The reaction takes about 30 seconds and highly super-heated molten iron is allowed to flow into
the prepared mould cavity around the part to be welded.
8. The super-heated molten metal fuses the parent metal and solidifies into a strong homogeneous
weld.
9. The weld joint is allowed to cool slowly.

There are different Thermit mixtures available for welding different metals, such as copper and chromium.
They use different metal oxides in place of ferrous oxide. Some typical Thermit mixture reactions with
their temperature obtained are given below:
3 CuO + 2 Al → 3Cu + Al2O3 + Heat (4860°C, 275 Kcal) Cr2O3 + 2Al
→ 2Cr + Al2O3 + Heat (3000°C, 540 Kcal)

Application and Uses of Thermit Welding:

Thermit welding is a very old process and now-a-days, in most cases, it is replaced by electro-slag
welding. However, this process is still in use.

Some applications are:

 Thermit welding is traditionally used for the welding of very thick and heavy plates.
 Thermit welding is used in joining rail roads, pipes and thick steel sections.
 Thermit welding is also used in repairing heavy castings and gears.
 Thermit welding is suitable to weld large sections such as locomotive rails, ship hulls etc.
 Thermit welding is used for welding cables made of copper.

Advantages of Thermit Welding:

 Thermit welding is a simple and fast process of joining similar or dissimilar metals.
 This process is cheap, as no costly power supply is required.
 This process can be used at the places where power supply is not available.

Disadvantages of Thermit Welding:

 Thermit welding is essentially used for ferrous metal parts of heavy sections.
 It is uneconomical for welding cheap metals and light parts.
Plasma Arc Welding
Introduction The plasma arc welding (PAW) can be considered as an advanced version of TIG welding. Like
TIGW, PAW also uses the tungsten electrode and inert gases for shielding of the molten metal. Low
velocity plasma and diffused arc is generated in the TIG welding while in case of PAW very high velocity
and coherent plasma is generated. Large surface area of the arc exposed to ambient air and base metal in
case of TIG welding causes greater heat losses than PAW and lowers the energy density. Therefore, TIG arc
burns at temperature lower than plasma arc.

Principle of PAW
In plasma arc welding, arc is forced to pass through nozzle (water cooled copper) which causes the
constriction of the arc (Fig. 16.5). Constriction of arc results in (a) reduction in cross-sectional area of arc,
(b) increases (d) increases energy density and (c) increases to velocity of plasma approaching to the sound
velocity and temperature to about 25000 0 C. these factors together make PAW, a high energy density and
low heat input welding process therefore; it poses fewer which in turn reduces problems associated with
weld thermal cycle.

Constriction of arc increases the penetration and reduces the width of weld bead. Energy associated with
plasma depends on plasma current, size of nozzle, plasma gas (Fig. 16.6). A coherent, calumniated and
stiff plasma is formed due to constriction therefore it doesn’t get deflected and diffused. Hence, heat is
transferred to the base metal over a very small area which in turns results in high energy density and deep
of penetration and small width of the weld pool / key hole / cut. Further, stiff and coherent plasma makes
it possible to work having stable arc with very low current levels (<15A) which inturn has led to micro-
plasma system.

Energy density and penetration capability of plasma jet is determined by the various process parameters
namely plasma current, nozzle orifice diameter and shape, plasma forming gas (Air, He, Ar) and flow rate
of plasma carrying. Increasing plasma current, flow rate, thermal conductivity of plasma forming gas and
reducing nozzle orifice diameter increases together result in the energy density and penetration capability
of plasma jet. In general, the plasma cutting uses high energy density in combination with high plasma
velocity and high flow rate of high thermal conductivity plasma forming gas. A combination of such
characteristics for plasma cutting is achieved by controlling above process parameters. Further, thermal
conductivity of plasma forming gas must be high enough for cutting operation so that heat can be
effectively transferred rapidly to the base metal. Plasma welding needs comparatively low energy density
and low velocity plasma to avoid melt through or blowing away tendency of molten metal.

Schematic of plasma arc welding system showing important components

Schematic of constriction of arc in PAW

High energy density associated with plasma arc produces a temperature of order of 25,000 0 C. This
process uses the heat transferred by plasma (high temperature charged gas column) produced by a gas
(Ar, Ar-H2 mixture) passing through an electric arc, for melting of faying surfaces. Inert gas (Ar, He) is used
to protect the molten weld pool from the atmospheric gases. Charged particles (electrons and ions) formed
as a result of ionization of plasma gas tends to reunite when they strike to the surface of work piece.
Recombination of charged particles liberates heat which is also used in melting of base metal. Electric arc
can be produced between nonconsumable electrode and work-piece or non-consumable electrode and
nozzle. As discussed above, plasma arc welding uses two types of gases one is called plasma gas and other
is inert gas primarily for shielding the weld pool from the contamination by atmospheric gases. Plasma gas
is primarily used to develop plasma by passing through arc zone and transfer the heat to the weld pool.

PAW uses the constant current type power source with DCEN polarity. The DCEN polarity is invariable
used in PAW because tungsten electrode is used for developing the arc through which plasma forming gas
is passed. Tungsten electrode has good electron emitting capability therefore it is made cathode. Further,
DCEN polarity causes less thermal damage to the electrode during welding as about one third of total heat
is generated at the cathode and balance two-third of arc heat is generated at the anode side i.e. work-
piece. DCEP polarity does not help the process in either way. Current can vary from 2-200 A.
The plasma arc in PAW is not initiated by the conventional touch start method but it heavily depend on
use of high frequency unit. Plasma is generated using two cycles approach a) producing very small high-
intensity spark (pilot arc) within the torch body by imposing pulses of high voltage, high frequency and
low current about 50A (from HF unit) between the electrode and nozzle which in turn generates a small
pocket of plasma gas and then as soon as torch approaches the work-piece main current starts flowing
between electrode and job leading to the ignition of the transferred arc. At this stage pilot is extinguished
and taken off the circuit.

Types of PAW Plasma generated due to the arc between the non-consumable electrode and workpiece is
called transferred plasma whereas that due to arc between non-consumable electrode and nozzle is called
non-transferred plasma. Non-transferred plasma system to a large extent becomes independent of nozzle
to work piece distance.
Transferred plasma offers higher energy density than non-transferred plasma and therefore it is preferred
for welding and cutting of high speed steel, ceramic, aluminium etc. Non-transferred plasma is usually
applied for welding and thermal spray application of steel and other common metals. Depending upon
the current, plasma gas flow rate, and the orifice diameter following variants of PAW has been developed
such as:
 Micro-
 Melt-in mode (15–400 Amperes) plas
 Keyhole mode (>400 Amperes) plasma arc

Micro-plasma welding systems work with very low plasma forming current (generally lower than 15 A)
which in turn results in comparatively low energy density and low plasma velocity. These conditions
become good enough to melt thin sheet for plasma welding.

Plasma for melt-in mode uses somewhat higher current and greater plasma velocity than micro- plasma
system for welding applications. This is generally used up to 2.4 mm thickness sheet. For thickness of sheet
greater than 2.5 mm normally welding is performed using key-hole technique. The key hole technique
uses high current and high pressure plasma gas to ensure key-hole formation. High energy density of
plasma melts the faying surfaces of base metal and high pressure plasma jet pushes the molten metal
against vertical wall created by melting of base metal and developing key-hole. Plasma velocity should be
such that it doesn’t push molten metal out of the hole. The key is formed under certain combination of
plasma current, orifice gas flow rate and velocity of plasma welding torch and any disturbance to above
parameters will cause loss of key-hole. For key-holing, flow rate is very crucial and therefore is controlled
accurately + 0.14 liter/min. Nozzles are specified with current and flow rate.

Advantage of PAW
With regard to energy density, PAW stands between GTAW/GMAW and EBW/LBW accordingly it can be
used using melt-in mode and key-hole mode. Melt-in mode results in greater heat input and higher width
to depth of weld ratio than key-hole mode. Higher energy density associated with PAW than GTAW
produces narrow heat affected zone and lowers residual stress and distortion related problems. High
depth to width ratio of weld produced by PAW reduces the angular distortion. It generally uses about one
tenth of welding current as compared to GTAW for same thickness therefore it can be effectively applied
for joining of the thin sheets. Further, non- transferred plasma offers flexibility of variation in standoff
distance between nozzle and work- piece without extinction of the arc.

Limitation of PAW
Infrared and ultra-violet rays generated during the PA welding are found harmful to human being. High
noise (100dB) associated with PAW is another undesirable factor. PAW is a more complex, costlier, difficult
to operate than GTAW besides generating high noise level during welding. Narrow width of the PAW weld
can be problematic from alignment and fit-up point of view. Productivity of the PAW in respect of welding
speed is found lower than LBW.

Oxy-Fuel Gas Cutting:

This is the most frequently employed thermal cutting process used for low carbon and low alloy steel
plates and often referred to as ‘flame cutting’ or ‘gas cutting’. It can be used to cut steel upto 2 m thick.
Oxy-fuel gas process involves preheating a small zone, wherefrom the cut is to be started, to the kindling
temperature of the material. Compressed oxygen is then made to impinge upon the hot metal resulting
in very high rate of oxidation which is often accompanied by evolution of heat due to exothermic nature
of the reaction.
The fuel gas employed is generally acetylene but propane, LPG (liquefied petroleum gas), natural gas, or
methylacetylene propadiene stabilised (MAPP or MPS) may also be employed depending upon availability
and cost considerations.
The torch employed for oxy-acetylene cutting is shown in Fig. 19.2. It has a mixing chamber for oxygen
and acetylene as in a welding torch. But after mixing the gas mixture flows out of the torch nozzle through
a number of small holes placed in a circle around the central hole through which a stream of high pressure
pure oxygen can be made to flow by pressing a lever on the torch handle. The diameter of these holes
vary and increases with increase in thickness of the material to be cut.

When the material to be cut is raised to its kindling temperature* (which is 870 to 950°C for low carbon
steels, depending upon the carbon content) and high pressure pure oxygen reacts with it, the following
reactions are possible in the case of ferrous materials.
1. Fe + O → FeO + heat (267 KJ)… ................................(1)
2. 2Fe +1.5O2 →Fe2O3+heat (825 KJ)… ......................(2)
3. 3Fe + 2O2 →Fe3O4 + heat (1120KJ)… .....................(3)

Mainly third reaction takes place with tremendous release of heat. Second reaction occurs to some extent
in cutting of heavier sections only. Theoretically 0.29 m3 of O2 will oxidise 1 kg of iron to form Fe3O4.
However, in practice the consumption of oxygen is higher than this value for plate thickness less than 40
mm and it is lower for higher thicknesses, being the least for the thickness range of 100 to 125 mm.
The exothermic reaction between O2 and Fe generates enough heat to continue the thermal cutting
process without the use of preheating flame using only oxygen but in practice it is not possible because a
lot of heat is used up in burning dirt, paint, scale, etc., and a considerable amount is lost by radiation. Also,
the high speed jet impinging upon the surface causes cooling action which needs to be compensated by
preheating.
The chemical reaction between ferrous and oxygen is rarely complete and the analysis of the blown out
material (or slag) often indicates that 30% to 40% of the slag is parent material.

Steel and some other metals can be cut by oxy-acetylene flame if they fulfill the following conditions:
 The melting point of the metal should be higher than its kindling temperature.
 The metal oxide formed by reaction with oxygen should have lower melting point than the melting
point of the parent material and it should be fluid in molten state so as to blow out easily.
 It should have low thermal conductivity so that the material can be rapidly raised to its kindling
temperature.

When a workpiece is cut by a thermal cutting process, the width of the cut is referred to as KERF, which in
oxy-fuel gas process is a function of oxygen hole size in the nozzle tip, flowrate of oxygen and preheating
gases, speed of cutting and the nature of the material being cut.

Cutting of Ferrous and Non-Ferrous Metals:

Metal Powder Cutting:
It is an oxygen cutting process in which metal powder (iron or aluminum) is employed to facilitate cutting.
This process is used for cutting cast iron, chromium-nickel, stainless steel and some high alloy steels. The
working principle of powder cutting is lite injection of metal powder into the oxygen stream well before
it strikes the metal to be cut.

The powder is heated by its passage through the oxy-acetylene preheat flames and almost immediately
ignites in the stream of cutting oxygen. The powder from a powder dispenser is carried to the lip of the
cutting torch by the use of compressed air or nitrogen as shown in Fig. 19.7.
The ignited powder provides much higher temperature in the stream and that helps in culling the metal
in almost the same manner as cutting of low carbon steel. Preheating is not essential for powder cutting.

Cutting speeds and cutting oxygen pressures are similar to those for cutting mild steel; however for cutting
material thicker than 25 mm a nozzle one size larger should be used. Flow rates are generally kept at 010
to 0-25 kg of iron powder per minute of cutting. Powder cutting usually leaves a scale on the cut surface
which can be easily removed on cooling.

Metal powder culling was initially introduced for cutting stainless steel but has been successfully used for
cutting alloy steels, cast iron, bronze, nickel, aluminium, steel mill ladle spills, certain refractories, and
concrete. The same basic process can also be used for gouging and scarfing to condition billets, blooms,
and slabs in steel mills.

Powder cutting is also useful for stack cutting wherein preheat from an ordinary flame culling is not
sufficient on the lower plate(s) either due to large depth or separation between plates. By means of the
metal powder and its reaction in the oxygen the cut is completed even across separations. However,
powder cutting generates quite a bit of smoke that needs to be removed to safeguard the health of the
operator and to avoid interference with other operations in the area.

Process # 3. Chemical Flux Cutting:

In the oxygen-cutting process a chemical flux is injected into the oxygen stream as metal powder is
injected in powder cutting. The flux combines with the refractory oxides and makes them a soluble
compound. The chemical fluxes may be salts of sodium such as sodium carbonate.

Fig. 19.8 shows one of the setups used for flux cutting. In this method oxygen sucks flux from a hopper at
the rate of 0 06 to 0-30 kg per minute and flows through the jet of cutting oxygen.

The procedure for flux cutting involves heating the initiating point of cut to white heat, the cutting oxygen
valve is then opened half-turn and the flux in oxygen stream is led to the torch. As the molten metal
reaches the lower edge of the work, the torch is made to move along the line of the cut and the cutting
oxygen valve is fully opened. To halt the operation first flux-supply valve is closed and then the other torch
valves are shut-off.

It is advisable to position the flux-supply 10 m away from the cutting area. It should also be ensured that the
hoses through which the flux-oxygen mixture is passed have no sharp bends otherwise it may lead to
clogging.

This process can be used for cutting cast iron, chromium-steel, chromium-nickel steel, copper, brass and
bronze. However, it is not recommended for cutting steels of high-nickel type, for example, 15 Cr 35Ni
steel. Chemical flux cutting, however, is slowly losing its industrial importance because of the
development of more efficient methods like plasma cutting.

Metal Inert Gas (MIG) welding (also known as Gas Metal Arc Welding [GMAW])
MIG is an arc welding technique in which a consumable electrode is used to weld two or more work pieces.
A diagrammatic representation of metal inert gas welding is shown below:

Components used in Metal Inert Gas Welding (MIG Welding):

Metal Inert Gas Welding (MIG Welding) makes use of the following components:
1. Consumable Electrode
2. Inert Gas Supply
3. Welding Head
4. A.C or D.C Power Supply
5. Electrode Feeding Mechanism Working:
The workpiece to be welded and the consumable electrode (in the form of wire) are connected to the
Power Supply (D.C or A.C). Whenever the consumable electrode is brought near the workpiece (with a small
air gap), an arc is produced. This arc melts the electrode. The melted electrode fills uniformly over the
required regions of the workpiece.
An inert gas supply is provided around the electrode (hence the name ‘Metal Inert Gas Welding’) during
the welding process. It forms a gas shield around the arc and the weld (See the diagram above). This is
intended to protect the weld from the external atmosphere. The type of electrode used and the shielding
gas used primary depends on the material to be welded. In many cases the shielding gas used is a mixture
of many gases.

If many workpieces are to be welded continuously an electrode spool (in the form of coil) is used.
Consumable electrode is continuously supplied from this spool by a suitable feeding mechanism.
Commonly, servo mechanisms are used for feeding long electrodes. In MIG Welding, consumable
electrode itself acts as filler metal. So, no seperate filler rod or filler wire is needed.

Advantages of Metal Inert Gas Welding (MIG Welding):

1. Consumable electrodes are easy to feed.
2. No filler rod is needed.
3. Welding is simple.
4. Inert gas shield protects the weld automatically.

Disadvantages of Metal Inert Gas Welding (MIG Welding):

1. Improper welding may lead to the floating of solid impurities over the liquid weld.
2. If not handled properly, weld may become porous.
3. MIG Welding exposes welders to hazardous gases.
4. Care must be taken to avoid the formation of less ductile welds.
5. Work pieces and Electrodes should be kept clean before welding.

TIG Welding
Tungsten Inert Gas (TIG) or Gas Tungsten Arc (GTA) welding is the arc welding process in which arc is
generated between non-consumable tungsten electrode and work piece. The tungsten electrode and the
weld pool are shielded by an inert gas normally argon and helium. Figures 10.1 & 10.2 show the principle
of tungsten inert gas welding process.
 Principle of TIG Welding.

Schematic Diagram of TIG Welding System.

The tungsten arc process is being employed widely for the precision joining of critical components which
require controlled heat input. The small intense heat source provided by the tungsten arc is ideally suited
to the controlled melting of the material. Since the electrode is not consumed during the process, as with
the MIG or MMA welding processes, welding without filler material can be done without the need for
continual compromise between the heat input from the arc and the melting of the filler metal. As the filler
metal, when required, can be added directly to the weld pool from a separate wire feed system or
manually, all aspects of the process can be precisely and independently controlled i.e. the degree of
melting of the parent metal is determined by the welding current with respect to the welding speed,
whilst the degree of weld bead reinforcement is determined by the rate at which the filler wire is added
to the weld pool.

In TIG torch the electrode is extended beyond the shielding gas nozzle. The arc is ignited by high voltage,
high frequency (HF) pulses, or by touching the electrode to the workpiece and withdrawing to initiate
the arc at a preset level of current. Selection of electrode composition and size is not completely
independent and must be considered in relation to the operating mode and the current level. Electrodes
for DC welding are pure tungsten or tungsten with 1 or 2% thoria, the thoria being added to improve
electron emission which facilitates easy arc ignition. In AC welding, where the electrode must operate at
a higher temperature, a pure tungsten or tungsten-zirconia electrode is preferred as the rate of tungsten
loss is somewhat lesser than with thoriated electrodes and the zirconia aids retention of the ‘balled' tip.
Table below indicates the chemical composition of tungsten electrodes as per American Welding Society
(AWS) classification.

Chemical Composition of TIG Electrodes

AWS Tungsten, min. Thoria, percent Zirconia, Total other
Classification percent percent elements, max.
percent
EWP 99.5 - - 0.5
EWTh-1 98.5 0.8 to 1.2 - 0.5
EWTh-2 97.5 1.7 to 2.2 - 0.5
EWZr 99.2 - 0.15 to 0.40 0.5

Tungsten electrodes are commonly available from 0.5 mm to 6.4 mm diameter and 150 - 200 mm length.
The current carrying capacity of each size of electrode depends on whether it is connected to negative or
positive terminal of DC power source. AC is used only in case of welding of aluminum and magnesium
and their alloys. Table 10.2 gives typical current ranges for TIG electrodes when electrode is connected
to negative terminal (DCEN) or to positive terminal (DCEP).

Typical Current Ranges for TIG Electrodes

Electrode DCEN DCEP
Dia. (mm) Pure and Thoriated Pure and Thoriated
Tungsten Tungsten
0.5 5-20 -
1.0 15-80 -
1.6 70-150 10-20
2.4 150-250 15-30
3.2 250-400 25-40
4.0 400-500 40-55
4.8 500-750 55-80
6.4 750-1000 80-125

The power source required to maintain the TIG arc has a drooping or constant current characteristic
which provides an essentially constant current output when the arc length is varied over several
millimeters. Hence, the natural variations in the arc length which occur in manual welding have little
effect on welding current. The capacity to limit the current to the set value is equally crucial when the
electrode is short circuited to the workpiece, otherwise excessively high current shall flow, damaging the
electrode. Open circuit voltage of power source ranges from 60 to 80 V. Argon or helium may be used
successfully for most applications, with the possible exception of the welding of extremely thin material
for which argon is essential. Argon generally provides an arc which operates more smoothly and quietly,
is handled more easily and is less penetrating than the arc obtained by the use of helium. For these
reasons argon is usually preferred for most applications, except where the higher heat and penetration
characteristic of helium is required for welding metals of high heat conductivity in larger thicknesses.
Aluminum and copper are metals of high heat conductivity and are examples of the type of material for
which helium is advantageous in welding relatively thick sections.

Pure argon can be used for welding of structural steels, low alloyed steels, stainless steels, aluminum,
copper, titanium and magnesium. Argon hydrogen mixture is used for welding of some grades of stainless
steels and nickel alloys. Pure helium may be used for aluminum and copper. Helium argon mixtures may
be used for low alloy steels, aluminum and copper.

TIG welding can be used in all positions. It is normally used for root pass(es) during welding of thick pipes
but is widely being used for welding of thin walled pipes and tubes. This process can be easily mechanised
i.e. movement of torch and feeding of filler wire, so it can be used for precision welding in nuclear,
aircraft, chemical, petroleum, automobile and space craft industries. Aircraft frames and its skin, rocket
body and engine casing are few examples where TIG welding is very popular.

Friction Welding:
Friction welding works on basic principle of friction. In this welding process, the friction is used to generate
heat at the interference surface. This heat is further used to join two work pieces by applying external
pressure at the surface of work piece. In this welding process, the friction is applied until the plastic
forming temperature is achieved. It is normally 900-1300 degree centigrade for steel. After this heating
phase, a uniformly increasing pressure force applied until the both metal work pieces makes a permanent
joint. This joint is created due to thermo mechanical treatment at the contact surface.

Working:
There are many types of friction welding processes which works differently. But all different these
processes involves common a working principle which can be summarize as follow.

Friction Welding
 First both the work pieces are prepared for smooth square surface. One of them is mounted on a
rotor driven chuck and other one remains stationary.
 The rotor allows rotating at high speed thus it makes rotate mounted work piece. A little pressure
force is applied on the stationary work piece which permits cleaning the surface by burnishing action.
 Now a high pressure force applied to the stationary work piece which forces it toward rotating work
piece and generates a high friction force. This friction generates heat at the contact surface. It is
applied until the plastic forming temperature is achieved.
 When the temperature is reached the desire limit, the rotor is stopped and the pressure force is
applied increasingly until the whole weld is formed.
This welding is used to weld those metals and alloys which cannot be welded by other method

Types:
Continuous induce friction welding:
This welding is same as we discussed above. In this welding process, the rotor is connected with a band
brake. When the friction crosses the limit of plastic temperature, the band brake comes into action
which stops the rotor but the pressure applied on the work piece increasingly until the weld is formed.

Inertia friction welding:

In this type of friction welding the band brake is replaced by the engine flywheel and shaft flywheel.
These flywheels connect chuck to the motor. In the starting of the welding, both flywheels are connected
with one another. When the speed or friction reaches its limit, the engine flywheel separated from the
shaft flywheel. Shaft flywheel has low moment of inertia which stops without brake. The pressure force
is continuously applied to the work piece until the weld is formed.

Application:
 For welding tubes and shafts.
 It is mostly used in aerospace, automobile, marine and oil industries.
 Gears, axle tube, valves, drive line etc. components are friction welded.
 It is used to replace forging or casting assembly.
 Hydraulic piston rod, truck rollers bushes etc. are join by friction welding.
 Used in electrical industries for welding copper and aluminum equipment’s.
 Used in pump for welding pump shaft (stainless steel to carbon steels).
 Gear levers, drill bits, connecting rod etc. are welded by friction welding.
Advantages and Disadvantages:
Advantages:
 It is environment friendly process without generation smoke etc.
 Narrow heat affected zone so no change in properties of heat sensitive material.
 No filler metal required.
 Welding strength is strong in most cases.
 Easily automated.
 High welding speed.
 High efficiency of weld.
 Wide variety of metal can be weld by this process.
Disadvantages:
 This is mostly used only for round bars of same cross section.
 Non-forgeable material cannot be weld.
 Preparation of work piece is more critical
 High setup cost.
 Joint design is limited.
 This is all about friction welding principle, working, types, application, advantages and disadvantages. If
you have any query regarding this article, ask by commenting. If you like this article, don’t forget to share
it on your social networks. Subscribe our website for more interesting articles.

Explosive Welding:
Principle:
This welding process works on basic principle of metallurgical bonding. In this process, a controlled
detonation of explosive is used on the welding surface. This explosion generates a high pressure force, which
deform the work plates plastically at the interface. This deformation forms a metallurgical bond between
these plates. This metallurgical bond is stronger than the parent materials. The detonation process occurs
for a very short period of time which cannot damage the parent material. This is basic principle of explosion
welding. This welding is highly depend on welding parameters like standoff distance, velocity of detonation,
surface preparation, explosive etc. This welding is capable to join large area due to high energy available in
explosive.

Basic terminology:
Base Plate: This is one of the welding plate which is kept stationary on a avail. It involves a backer which
supports the base plate and minimizes the distortion during the explosion.

Flyer Plate: This is another welding plate which is going to be weld on base plate. It has lowest density and
tensile yield strength compare to base plate. It is situated parallel or at an angle on the base plate.

Buffer Plate: Buffer plate is situated on the flyer plate. This plate is used to minimize the effect or explosion
on upper surface of flyer plate. This protects the flyer plate from any damage due to explosion.

Standoff distance: Stand-off distance plays a vital role in explosion welding. It is distance between flyer
plate and base plate. Generally it is taken double of thickness of flyer plate for thin plates and equal to
thickness of flyer plate for thick plates.

Explosive: Explosive is placed over the flyer plate. This explosive is situated in a box structure. This box
placed on the flyer plate. Mostly RDX, TNT, Lead azide, PETN etc. used as explosive.
Velocity of detonation: It is the rate at which the explosive detonate. This velocity should be kept less
than 120% of sonic velocity. It is directly proportional to explosive type and its density.

Types:
This welding can be classified into two types according to the setup configuration.
Oblique Explosion Welding:
In this type of welding process base plate is fixed on an anvil and filler plate makes an angle with the base
plate. This welding configuration is used to join thin and small plates.

Parallel Explosion Welding:

As the name implies, in this welding configuration filler plate is parallel to the base plate. There is some
standoff distance between base plate and flyer plate. This configuration is used to weld thick and large
plates.

Working:
We have discussed about working principle of explosion welding. Its working can be summarized as
follow.
 First both the flyer plate and the base plate interface surface are cleaned and prepared for good
weld.
 Now the base plate fixed on the avail and the flyer plate place at the top surface of it at a pre- define
distance (stand-off distance). The flyer plate may be inclined or parallel according to the welding
configuration.
 The buffer plate is set over the flyer plate. This plate protects the upper surface of flyer place from
damage due to high impact force of explosion.
 The prepared explosive is place into a box of same size of welding surface. This box is placed over
buffer plate. There is a detonator at one side of the explosive. This is used to start explosion.
 Now the detonator ignited the explosive which create a high pressure wave. These waves deforms
the interface surface plastically and form a metallurgical bond between base plate and flyer plate.
This bond is stronger than parent material.
Application:
 Used to weld large structure sheets of aluminum to stainless steel.
 It is used to weld cylindrical component like pipe, concentric cylinder, tube etc.
  Weld clad sheet with steel in a heat exchanger.
 Join dissimilar metals which cannot be weld by other welding process.
 For joining cooling fan etc.

Advantages and Disadvantages:

Advantages:
 It can join both similar and dissimilar material.
 Simple in operation and handling.
 Large surface can be weld in single pass.
 High metal joining rate. Mostly time is used in preparation of the welding.
 It does not effect on properties of welding material.
 It is solid state process so does not involve any filler material, flux etc.

Disadvantages:
 It can weld only ductile metal with high toughness.
 It creates a large noise which produces noise Pollution.
 Welding is highly depends on process parameters.
 Higher safety precautions involved due to explosive.
 Designs of joints are limited.
This is all about explosion welding principle, working, types, application, advantages and disadvantages.
If you have any query regarding this article, ask by commenting. If you like this article, don’t forget to share
it on your social networks. Subscribe our website for more interesting articles.

Welding Defects
The defects in the weld can be defined as irregularities in the weld metal produced due to incorrect welding
parameters or wrong welding procedures or wrong combination of filler metal and parent metal. Weld
defect may be in the form of variations from the intended weld bead shape, size and desired quality. Defects
may be on the surface or inside the weld metal. Certain defects such as cracks are never tolerated but other
defects may be acceptable within permissible limits. Welding defects may result into the failure of
components under service condition, leading to serious accidents and causing the loss of property and
sometimes also life. Various welding defects can be classified into groups such as cracks, porosity, solid
inclusions, lack of fusion and inadequate penetration, imperfect shape and miscellaneous defects.

Cracks
Cracks may be of micro or macro size and may appear in the weld metal or base metal or base metal and
weld metal boundary. Different categories of cracks are longitudinal cracks, transverse cracks or
radiating/star cracks and cracks in the weld crater. Cracks occur when localized stresses exceed the ultimate
tensile strength of material. These stresses are developed due to shrinkage during solidification of weld
metal.
 Various Types of Cracks in Welds

Cracks may be developed due to poor ductility of base metal, high sulpher and carbon contents, high arc
travel speeds i.e. fast cooling rates, too concave or convex weld bead and high hydrogen contents in the
weld metal.

Porosity
Porosity results when the gases are entrapped in the solidifying weld metal. These gases are generated
from the flux or coating constituents of the electrode or shielding gases used during welding or from
absorbed moisture in the coating. Rust, dust, oil and grease present on the surface of work pieces or on
electrodes are also source of gases during welding. Porosity may be easily prevented if work pieces are
properly cleaned from rust, dust, oil and grease.Futher, porosity can also be controlled if excessively high
welding currents, faster welding speeds and long arc lengths are avoided flux and coated electrodes are
properly baked.

Different Forms of Porosities

 Solid Inclusion
Solid inclusions may be in the form of slag or any other nonmetallic material entrapped in the weld
metal as these may not able to float on the surface of the solidifying weld metal. During arc welding flux
either in the form of granules or coating after melting, reacts with the molten weld metal removing
oxides and other impurities in the form of slag and it floats on the surface of weld metal due to its low
density. However, if the molten weld metal has high viscosity or too low temperature or cools rapidly
then the slag may not be released from the weld pool and may cause inclusion.
Slag inclusion can be prevented if proper groove is selected, all the slag from the previously
deposited bead is removed, too high or too low welding currents and long arcs are avoided.

Slag Inclusion in Weldments

Lack of Fusion and Inadequate or incomplete penetration:

Lack of fusion is the failure to fuse together either the base metal and weld metal or subsequent beads
in multipass welding because of failure to raise the temperature of base metal or previously deposited
weld layer to melting point during welding. Lack of fusion can be avoided by properly cleaning of
surfaces to be welded, selecting proper current, proper welding technique and correct size of electrode.

Types of Lack of Fusion

Incomplete penetration means that the weld depth is not upto the desired level or root faces have not
reached to melting point in a groove joint. If either low currents or larger arc lengths or large root face
or small root gap or too narrow groove angles are used then it results into poor penetration.

Examples of Inadequate Penetration

Imperfect Shape
Imperfect shape means the variation from the desired shape and size of the weld bead.
During undercutting a notch is formed either on one side of the weld bead or both sides in which stresses
tend to concentrate and it can result in the early failure of the joint. Main reasons for undercutting are
the excessive welding currents, long arc lengths and fast travel speeds.
Underfilling may be due to low currents, fast travel speeds and small size of electrodes. Overlap may
occur due to low currents, longer arc lengths and slower welding speeds.

Various Imperfect Shapes of Welds

Excessive reinforcement is formed if high currents, low voltages, slow travel speeds and large size
electrodes are used. Excessive root penetration and sag occur if excessive high currents and slow travel
speeds are used for relatively thinner members. Distortion is caused because of shrinkage occurring due
to large heat input during welding.

Miscellaneous Defects
Various miscellaneous defects may be multiple arc strikes i.e. several arc strikes are one behind the other,
spatter, grinding and chipping marks, tack weld defects, oxidized surface in the region of weld,
unremoved slag and misalignment of weld beads if welded from both sides in butt welds.