Introduction to mechanical Engineering
Module 3
Metal Joining Process
Introduction
Some products cannot be manufactured as a single piece. The desired shape and size of such
products can be obtained by joining two parts of same or different materials. These parts are
manufactured individually and are joined together to obtain the desired product. For example,
air craft and ship bodies, welded machine frames, furniture, computers, bridges and the
transmission or electric towers etc., are all fabricated by joining several different parts.
Based on the type of joint produced joining processes can be classified as
1. Temporary Joint – Joints can be easily dismantled separating the original parts
without any damage to them. Eg: Bolt and Nuts, Screws.
2. Semi-Permanent Joint - Joint can be separated only by destroying the rivet without
affecting the parent elements. Eg: Rivets.
3. Permanent Joint - An attempt to separate the parts already joined will result in the
damage of the parts. Eg: Welding, Soldering, Brazing, Adhesive etc.
If a product is in use for a long time and there is wear and tear, the parts need to be dismantled
for maintenance, repair or replacement. A temporary joint can be easily dismantled separating
the original parts without any damage to them. In case it is a permanent joint, an attempt to
separate the parts already joined will result in the damage of the parts. In a permanent joint, the
joint is made such that it has properties similar to the base metal of the two parts. The joined
parts become one piece. These parts cannot be separated into their original shape, size and
surface finish. Based on the process used for making the joint, the joining processes can be
further classified as
1. Soldering.
2. Brazing.
3. Welding.
4. Mechanical Fasteners like bolts, nuts, rivets, screws etc.
5. Adhesive bonding.
Dr. Kiran M D, Asst. Professor, Dept. of Mechanical Engg, BMSIT&M, Bangalore Page 1
Introduction to mechanical Engineering
Mechanical fasteners are most widely used for temporary joints. Joints obtained by bolts and
screws are temporary in nature and can be dismantled easily whenever necessary. Rivets are
semi-permanent fastening devices and the joint can be separated only by destroying the rivet
without affecting the parent elements. Adhesive bonding has generally less strength than the
mechanical fasteners. But adhesive bonding is used to join odd shaped parts or thin sheets
which may not lend themselves to mechanical fastening. Brazing and soldering are considered
to form permanent joints, but for repair or replacement these joints can be dismantled by
heating. Welding is one of the most extensively used fabrication method. The joint strength
obtained in welding is being equal to or some times more than that of the parent metal.
Welding is not only used for making structures, but also for repair work such as the joining of
broken castings. The choice of a particular joining process depends on several factors such as
application, nature of loads or stresses, joint design, materials involved and size and shape of
the components.
Soldering
Soldering is a method of joining similar or dissimilar metals by the application of heat and using
a filler metal or alloy called solder, whose liquidus temperature is below 4500 C. The molten
filler metal is made to flow between the two closely placed adjacent surfaces by the capillary
action.
Though soldering obtains a good joint between the two plates, the strength of the joint is limited
by the strength of the filler metal used. Soldering is used for obtaining a leak proof joint or a
low resistance electrical joint. The soldered joints are not suitable for high temperature
applications because of the low melting temperatures of the filler metals used.
The purpose of using the flux is to prevent the formation of oxides on the metal surface when
the same is heated. The fluxes are available in the form of powder, paste, liquid or in the form
of core in the solder metal. It is necessary that the flux should remain in the liquid form at the
soldering temperature and be reactive to be of proper use. The filler metals used are essentially
alloys of lead and tin. The composition of solder used for different purposes are as given below
Soft solder - lead 37% tin 63%
Medium solder - lead 50% tin 50%
Plumber’s solder - lead 70% tin 30%
Electrician’s solder - lead 58% tin 42%
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Introduction to mechanical Engineering
Soldering is classified into soft soldering and hard soldering.
Soft soldering is used extensively in sheet metal work for joining parts that are not exposed to
the action of high temperatures and are not subjected to excessive loads and forces or vibrations.
Soft soldering is also employed for joining wires and small parts. The solder is mostly
composed of lead and tin. In soft soldering, Zinc chloride and ammonium chloride are the most
common soldering fluxes used which are quick acting and produce efficient joints. But because
of their corrosive nature the joint should thoroughly cleaned of the entire flux residue from the
joint. These are to be used only for non-electrical soldering work. Rosin and rosin plus alcohol-
based fluxes are least active type and are generally used for electrical soldering work.
Hard soldering employs solder which melts at higher temperatures (3500 C to 9000 C) is
stronger than used in soft soldering. Hard solder is an alloy of copper and zinc to which silver is
added some times. German silver, used as a hard solder for steel is an alloy of copper, zinc and
nickel.
Sequence of operations:
The following operations are required to be performed sequentially for making soldered joints.
1. Shaping and fitting of metal parts together: The two parts to be joined are shaped to fit
closely so that the space between them is extremely small and filled completely with
solder by capillary action. If a large gap is present, capillary action will not take place
and the joint will not be strong.
2. Cleaning of surfaces: In order to obtain a sound joint, the surfaces to be soldered are
cleaned to remove dirt grease or any other foreign material.
3. Application of flux: The flux is applied when the parts are ready for joining.
4. Application of heat and solder: The parts are held in a vice or with special work holding
devices so that parts do not move while soldering.
Advantages:
• Simple and economical process.
• Relatively low temperature process, there is no metallurgical damage to base
metal.
• The soft soldered joints can easily be dismantled by simple heating.
Disadvantage:
• The strength of joint relatively low.
• Flux must be thoroughly cleaned off after soldering, as it is often corrosive.
Dr. Kiran M D, Asst. Professor, Dept. of Mechanical Engg, BMSIT&M, Bangalore Page 3
Introduction to mechanical Engineering
WELDING
Welding is a process of metallurgically joining two pieces of metals by the application of heat
with or without the application of pressure and addition of filler metal. The joint formed is a
permanent joint. Modern methods of welding may be classified under two broad headings.
a. Plastic welding process
b. Fusion welding process
In plastic welding process the pieces of metal to be joined are heated to a plastic state and then
forced together by external pressure. This procedure is used in forge welding, resistance
welding, spot welding in which pressure is required.
In the fusion welding, the material at the joint is heated to a molten state and allowed to solidify.
This includes gas welding arc welding and Thermit welding.
The surfaces of the metal which are to be joined by any of the welding processes must be
sufficiently clean to permit clean metallic surfaces to come in to contact. In some operations,
materials known as fluxes are applied to the parts being welded to dissolve the oxides or to
prevent the formation of oxides. Fluxes are different for different metals. For ferrous materials
borax, sodium carbonate etc, have been found to give excellent results.
Types of Joints:
The welding joints are classified as Butt, Lap, Tee, Corner joints and edge joints. The choice of
the type of joint is governed by the kind of metal to be welded, its thickness and technique of
welding. Fig. 3.1 shows the different types of joints used in welding.
Fig. 3.1. Different Types of Joints
Dr. Kiran M D, Asst. Professor, Dept. of Mechanical Engg, BMSIT&M, Bangalore Page 4
Introduction to mechanical Engineering
Arc Welding:
Arc welding is a method of joining metals with heat produced by an electrical arc. In this
process the heat necessary to melt the edges of the metal to be joined is obtained from an
electric are struck between the electrode (filler rod) and the work, producing a temperature of
40000C, in the welding zone. The heat of the arc melts the base metal or edges of the parts
fusing them together. Filler metal, usually added melts and mixes with molten base metal to
form the weld metal. The weld metal cools and solidifies to form the weld. In most cases, the
composition of the filler material, known as welding rod, needed to provide extra metal to the
weld, is same as that of the material being welded.
A typical arc welding setup is shown in Fig. 3.2.
1. An arc welding circuit consists of a power
supply to furnish electric power.
2. An electrode to conduct the electricity to the
arc.
3. Cables which connect the power supply to the
electrode and workpiece to complete the
welding circuit.
4. The arc itself provides the heat for welding.
5. The workpiece to weld is kept on a metallic table. Fig. 3.2. Arc Welding Setup
The arc must be shielded because; as it hardens the molten metal combines with oxygen and
nitrogen to form impurities that weaken the weld. Shielding can be obtained by adding a paste,
powder or fibrous flux to the arc. The electrodes are usually coated with a flux. This coating
forms a gaseous cloud that shields the molten metal from the atmosphere. The coating also
forms a protective slag. The slag floats on the molten pool and hardens as the weld cools. This
keeps impurities out of the weld. The process is shown in Fig. 3.3.
Fig 3.3. Arc Welding Process
Dr. Kiran M D, Asst. Professor, Dept. of Mechanical Engg, BMSIT&M, Bangalore Page 5
Introduction to mechanical Engineering
Advantages:
1. As a manual process it is applicable to an infinite variety of work and can be executed in
any position.
2. There is less buckling and warping of the work.
3. It produces strong sound and ductile welds.
4. Satisfactory welds can be produced in heavy as well as in light sections.
5. Low cost process.
6. Excellent joint properties can be obtained in mild, low alloy and stainless steels, nickel
and copper-base alloys.
Disadvantages:
1. Basically, a manual process requiring adequate operator skill for good results.
2. Electrodes require frequent changing.
3. Multi run welds necessary on thick plate-slag chipping necessary after each run.
4. The principal disadvantage has been the high heat of the metal arc which makes it
unsuitable for use on materials less than 1.55 mm thick.
5. High initial cost of welding equipment.
Difference between soldering and welding
Soldering Welding
Soldering using the filler metal having the Welding using the filler metal having the
melting point less than 450ºC melting point nearly equal to the base metal,
above 3500ºC
Capillary action is also present in soldering No capillary action is present. Joint takes
between the base metal and filler metal. place due to fusion.
Base metal does not melt. Base metal melts in welding
Filler metal is having the melting point less than In welding filler metal is not having the
the base metal. melting point less than the base metal.
Filler metal is uniformly distributed because of Filler metal melts and gets mixed with the
capillary action base metal
Joints are weaker than welding Joints are stronger as compared to soldering
It uses the filler metal which contains lead and It uses the filler metal mostly having the same
tin. composition as that of base metal.
Flux used is Zinc-Chloride Flux used is Borax ash
Power consumption Very less Power consumption Less
Dr. Kiran M D, Asst. Professor, Dept. of Mechanical Engg, BMSIT&M, Bangalore Page 6
Introduction to mechanical Engineering
Tungsten inert gas (TIG) welding
Fig 3.4. TIG Welding Process
➢ In this operation, the work pieces to be joined are cleaned to remove dirt, grease and
other oxides chemically or mechanically to obtain a sound weld.
➢ The welding current and inert gas supply are turned ON.
➢ An arc is struck by touching the tip of the tungsten electrode with the work piece, and
instantaneously the electrode is separated from the work piece by a small distance of
1.5-3 mm such that the arc still remains between the electrode and the work piece.
➢ The high intensity of the arc melts the work piece metal forming a small molten metal
pool. filler metal in the form of a rod is added manually to the front end of the weld
pool.
➢ The deposited filler metal fills and bonds the joint to form a single piece of metal.
➢ The arc is extinguished by widening the gap between the workpiece and the electrode.
➢ The shielding gas is allowed to imping on the solidifying weld pool for a few seconds
even after the arc is extinguished.
➢ This will avoid atmospheric contamination of the solidifying metal thereby increasing
the strength of the joint.
➢ Fig. 3.4. represents the TIG welding process and components.
Advantages
➢ Suitable for the thin metals
➢ Clear visibility of the arc provides the operator to have a greater control over the
weld
➢ Strong and high quality joints are obtained
➢ No flux is used. Hence, no slag formation. This results in clean weld joints.
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Introduction to mechanical Engineering
Disadvantages
➢ TIG is the most difficult process compared to all the other welding processes.
➢ Skilled operator is required
➢ Process is slower
➢ Not suitable for thick metals.
Metal inert gas (MIG) welding
Fig 3.5. MIG Welding Process
➢ In this operation, the workpieces to be joined are cleaned to remove dust, gease and
other oxides, chemically or mechanically to obtain a sound weld. The tip of the electrode
is also cleaned with a wire brush.
➢ The control switch provided in the welding torch is switched ON to initiate the electric
power, shielding gas and the wire (electrode) feed.
➢ An arc is struck by touching the tip of the electrode with the work piece, and
instantaneously the electrode is separated from the work piece by a small distance of
1.5-3 mm such that the arc still remains between the electrode and the work piece.
➢ The high intensity of the arc melts the work piece metal forming a small molten pool.
➢ At the same time, the tip of the electrode also melts and combines with the molten metal
of the work pieces there by filling the gap between the two work pieces.
➢ The deposited metal upon solidification bonds the joint to form a single piece of metal.
➢ Fig. 3.5 represents the MIG welding process and components.
Dr. Kiran M D, Asst. Professor, Dept. of Mechanical Engg, BMSIT&M, Bangalore Page 8
Introduction to mechanical Engineering
Advantages
➢ MIG welding is fast and economical
➢ The electrode and inert gas are automatically fed. This reduces the burden on the
operator, and also helps him to concentrate on the arc.
➢ Weld deposition rate is high due to the continuous wire feed.
➢ No flux is used. Hence, no slag formation. This results in clean welds
➢ Thin and thick metals can be welded.
➢ Process can be automated.
Disadvantages
➢ Equipment is costlier.
➢ Dross and porosity are the most prevalent quality problems in this process. However,
extensive edge preparation can eliminate this defect.
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