Mechanical engineering department

Manufacturing Technology

Dr. Ahmed Al-Zahrani

Assistant Professor

Lecture 06 MEE-332
Processing of Metallic Materials Metal
Chapter 14
Metal –Forging process and equipment
Introduction
 The most common type of forging equipment is the
hammer and anvil. Principles behind the hammer and anvil
are still used today in drop-hammer equipment.

10th century anvil

Introduction
Introduction
 Forging: a Process in which the work-piece is shaped by
compressive forces applied through various dies and tools.

 Typical parts made by forging and related processes: Shafts, gears, bolts, turbine
blades, hand tools, dies, and components for machinery, transportation, and farm
equipment.

Turbine Shafts Gear

 Alternative processes: Casting, powder metallurgy, machining, and

fabrication.
Introduction
 Cold forged parts have good surface finish and
dimensional accuracy.
 Hot forging requires smaller forces, but it produces
dimensional accuracy and surface finish that are not as
good as in cold forging.

 Each process has its own advantages and limitations regarding external and
internal characteristics :

Material properties Dimensional accuracy Surface finish Economics

of production
14.2 Open die forging
 Most open-die forgings generally weigh 15-500 Kg,
forgings as heavy as 275 tons have been made.

 Sizes may range from very small parts (nails, pins, and
bolts) up to 23 m for ships shafts.

 Upsetting or flat-die forging (Fig. 14.3): The die surfaces in

open-die forging may have simple cavities, to produce
relatively simple forgings.
14.2 Open die forging

Figure 14.3: (a) Ideal deformation of a solid cylindrical specimen compressed between
flat frictionless dies, an operation known as upsetting. (b) Deformation in upsetting with
friction at the die-workpiece interfaces. Note barrelling of the billet caused by friction.
14.2 Open die forging
 Barreling is caused primarily by frictional forces at the die-work-
piece interfaces that oppose the outward flow of the materials at
these interfaces.
 Barreling can be minimized by using an effective
Lubricant.
 Barreling can also occur in upsetting hot work-pieces between
cold dies. The material at and near the interfaces cools rapidly, while
the rest of the work-piece remains relatively hot. Thus, the material
at the ends of the work-piece has higher resistance to deformation
than the material at its center. Consequently, the central portion of
the work-piece expands laterally to a greater extent than do its
ends.
 Barreling from thermal effects can be reduced or
eliminated by using heated dies.
14.2 Open die forging -cogging

 Cogging (drawing out):

An open-die forging operation in which thickness of

a bar is reduced by successive forging steps at specific
intervals. Because contact area per stroke is small, a long
section of a bar can be reduced in thickness without
requiring large forces or machinery
 14.2 Open die forging - Cogging

Figure 14.4: (a) Schematic illustration of a cogging operation on a rectangular bar.

Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by
heating the workpiece and hammering it numerous times along the length of the part. (b)
Reducing the diameter of a bar by open-die forging; note the movements of the die and the
workpiece. (c) The thickness of a ring being reduced by open-die forging.
14.2 Open die forging
 The forging force, F, in an open-die forging operation
on a solid cylindrical piece:

 Yf : flow stress of the material, stress required to

continue plastic deformation of the work-piece at a
particular true strain.
 µ : coefficient of friction,
 r and h: radius and height of the work-piece
 14.2 Open die forging

 Example14.1: A solid cylindrical slug made of 304 stainless steel is 150 mm in

diameter and 100 mm high. It is reduced in height by 50% at room temperature
by open-die forging with flat dies. Assuming that the coefficient of friction is 0.2,
calculate the forging force at the end of the stroke.
14.3 impression-die and closed-die
forging
 In impression-die forging, the work-piece acquires the
shape of the die cavities while being forged between two
shaped dies (Fig. 14.5).

 The thin flash cools rapidly, and because of its frictional

resistance, it subjects the material in the die cavity to high
pressures, thereby encouraging the filling of the die cavity.
14.3 impression-die and closed-die
forging

 FIGURE I4.5 (a) through (c) Stages in impression-die forging of a solid round billet.
Note the formation of flash, which is excess metal that is subsequently trimmed off.
(d) Standard terminology for various features of a forging die
 14.3 impression-die and closed-die
forging
 Pre-forming processes, such as
fullering and edging (Figs.14.7b
and c), are used to distribute the
material into various regions of the
blank.
 In fullering material is distributed
away from an area.
 In edging, it is gathered into a
localized area.
 The part is then formed into the
rough shape of a connecting rod by
a process called blocking, using
blocker dies.
 The final operation is the finishing of
the forging in impression dies that
give the forging its final shape. The
flash is removed later by a
trimming operation.
 14.3 impression-die and closed-die
forging
 The blank is placed
on the lower die and,
as the upper die
begins to descend, the
blank’s shape
gradually changes, as
is shown for the
forging of a
connecting rod in fig.
14.7a.
 14.3 impression-die and closed-die
forging

 The forging force, F, required to

carry out an impression-die forging
operation can be estimated from the
formula
F = KYfA
 K: a multiplying factor (Table 14.2)
 Yf : flow stress of material at the
forging temperature,
 A: is projected area of forging,
including the flash.
14.3 impression-die and closed-die
forging
 In true closed-die or flash-less forging ,flash does not
form and the work-piece completely fills the die cavity.
Consequently, the forging pressure is very high,

 Undersize blanks prevent complete filling of die cavity.

 Oversize blanks generate excessive pressures and may

cause dies to fail.
14.3.1 Precision forging
 In order to reduce the number of additional finishing operations
required-hence the cost-the trend has been toward greater precision
in forged products (net-shape forming).
 Special dies produce parts having greater accuracies than those
from impression die forging and requiring much less machining.
 Process requires higher capacity equipment because of greater
forces required to obtain fine details on part.
 Precision forging requires special and more complex dies, precise
control of billet’s volume and shape, accurate positioning of the billet
in die cavity, and hence higher investment. However, less material is
wasted, less subsequent machining required.
Forgeability of Metals; Forging Defects

 Forgeability is defined as the capability of a material to

undergo deformation without cracking
 2 simple tests:
1. Upsetting test
greater the deformation prior to cracking, the greater
the forgeability of the metal
2. Hot-twist test
maximum number of turns
occurs then becomes the
forging temperature for
maximum forgeability
Forgeability of Metals; Forging Defects

Forging Defects
 When there is an insufficient volume of material, the
web will buckle and develop laps
 If the web is too thick, excess material flows will
develops internal cracks
Forgeability of Metals; Forging Defects

Forging Defects
 Internal defects may develop from
1. Nonuniform deformation of the material in the die
cavity
2. Temperature gradients throughout the workpiece
during forging
3. Microstructural changes caused by phase
transformations

 Forging defects can cause fatigue failures

Die Designing for forging
 ……………Die Designing for forging

Forging Dies required during development:

Preforming die ( open dies are required )

Blocking Die

Forging Die

Trimming Die

Coining Die
Die Design, Die Materials, and Lubrication

 Design of forging dies includes shape and complexity

of the workpiece, ductility, strength and sensitivity to
deformation rate and temperature, and frictional
characteristics
 Workpiece intermediate shapes should be planned so
that they properly fill the die cavities
 Software is available to help predict material flow in
forging-die cavities
Die Design, Die Materials, and Lubrication

Preshaping
 In a properly pre-shaped workpiece:
1. Material should not flow easily into the flash
2. Grain flow pattern should be favorable for the products’
strength and reliability
3. Sliding at the workpiece–die interfaces should be
minimized in order to reduce die wear
Die Design, Die Materials, and Lubrication

Die Design Features

 The parting line should locate at the largest cross
section of the part
 For simple symmetrical shapes, the parting line is a
straight line at the center of the forging
 For complex shapes, the line may not lie in a single
plane
 Draft angles are needed to facilitate removal of the
part from the die
 Selection of the proper radii for corners and fillets is to
ensure smooth flow of the metal into the die cavity and
improving die life
Die Design, Die Materials, and Lubrication

Die Materials
 Requirements for die materials are:
1. Strength and toughness at elevated temperatures
2. Hardenability and ability to harden uniformly
3. Resistance to mechanical and thermal shock
4. Wear resistance

Lubrication
 Greatly influences friction and wear
 Affects the forces required, die life, and material flows
into the die cavities
Die-manufacturing Methods and Die Failures

 Die quality and life are significant aspects of the total

manufacturing operation and quality of the parts
produced
 Manufacturing methods that used to make dies are
casting, forging, machining, grinding, electrical and
electrochemical methods and lasers for small dies
 Process of producing a die cavity in a die block is
called die sinking
 Hubbing, cold or hot, also used to make small dies with
shallow cavities
 Surface profile and finish are improved by finish
grinding and polishing
Die-manufacturing Methods and Die Failures

 Dies are machined from forged die blocks by high-

speed milling, turning, grinding, and electrical
discharge and electrochemical machining
 Dies are subjected to finishing operations such as
grinding, polishing, and chemical and electrical
processes for surface finish and dimensional accuracy

Die Costs
 Cost of a die depends on its size, shape complexity,
application and surface finish required
 Large number of parts are made from one set of dies
 Die cost per piece made is a small portion of a part’s
manufacturing cost
Die-manufacturing Methods and Die Failures

Die Failures
 Failure of dies results in
1. Improper die design
2. Defective die material
3. Improper finishing operations
4. Overheating and heat checking
5. Excessive wear
6. Overloading
7. Improper alignment
8. Misuse
9. Improper handling of the die
Die-manufacturing Methods and Die Failures

Die Failures
 The proper design of dies and selection of die
materials is important
 Large cross sections and clearances of a die is needed
to withstand the forces
 Overloading of tools and dies can cause premature
failure
Economics of Forging
 Depending on the complexity of the forging, tool and
die, costs range from moderate to high
 Costs are spread out over the number of parts forged
with that particular die set
 The more expensive the material, the higher the cost of
the material relative to the total cost
 Size of forgings also has some effect on cost
Economics of Forging
CASE STUDY 14.2
Suspension Components for the Lotus Elise Automobile
 Lotus group investigated the use of steel forgings to
reduce cost and improve reliability and performance
Economics of Forging
CASE STUDY 14.2
Suspension Components for the Lotus Elise Automobile
 Used advanced software tools to reduce the number of
components and to determine the optimum geometry
HW assignment

 Reading assignment: Chapters 14

 Review Questions:
 Problems:
Forging Machines
Hydraulic Presses
 Operate at constant speeds and are load limited
 Hydraulic presses are slower and involve higher initial
costs but require less maintenance
Forging Machines
Mechanical Presses
 They are either the crank or the eccentric type
 Mechanical presses are stroke limited since speed
varies from a maximum to zero
 Due to linkage design, very high forces can be applied
in this type of press
 Mechanical presses are preferred for forging parts with
high precision
Various Forging Operations
Coining
 A closed-die forging process used in the
minting of coins, medallions and jewellery
 Marking parts with letters and numbers can
be done rapidly through coining
 Lubricants cannot be applied in coining,
because they can become entrapped in the
die cavities, and being incompressible,
prevent the full reproduction of die surface
details.
Various Forging Operations
Heading
 Also called upset forging
 An upsetting operation performed on the end of a
round rod or wire in order to increase the cross section
 Products are nails, bolt heads, screws, rivets, and
fasteners
 Production rates: 100’s pieces per min for small parts

https://www.youtube.com/watch?v=HJz38gue9RE
 14.4 Various Forging Operations-
Heading

 Rules for upset forging:

1. Max Length of unsupported metal
that can be upset in one blow
without buckling: 3 times the
diameter of bar.
2. lengths of stock greater than 3
times diameter may be upset
successfully such that the diameter
of the cavity is no more than 1.5
the diameter of the bar.
3. For stock length more than 3
times bar diameter and where the
diameter of cavity is not more than
1.5 bar diameter, length of
unsupported metal beyond the
face of the die must not exceed
the diameter of the bar.
See Video Bolt Forging
https://www.youtube.com/watch?v=uI12RTlM1Sw
Various Forging Operations
Piercing
 A process of indenting the surface of a workpiece with
a punch in order to produce a cavity or an impression
 Piercing force depends on:
1. Cross-sectional area and the tip geometry of the punch
2. Strength of the material
3. Magnitude of friction at
the sliding interfaces
Various Forging Operations
CASE STUDY 14.1
Manufacture of a Stepped Pin by Heading and Piercing Operations
 A stepped pin is made from SAE 1008 steel
 Cold-forging steps is used to produce this part
 Various Forging Operations
Hubbing
 Process consists of pressing a
hardened punch with a tip geometry
into the surface of a block of metal
 Hubbing force can be estimated from
3(UTS)(A)
 UTS is obtained from Table 2.2 and A
is the projected area of the impression
Orbital Forging
 Upper die moves along an orbital path
and forms the part incrementally
 Operation is quiet, and parts is formed
within 10 to 20 cycles of the orbiting die
https://www.youtube.com/watch?v=BqBuh5q9exg
Various Forging Operations
Incremental Forging
 In this process, a tool forges a blank into a shape in
several small steps
 Similar to cogging where the die penetrates the blank
to different depths along the surface
Various Forging Operations
Isothermal Forging
 Known as hot-die forging process where it heats the
dies to the same temperature as workpiece
 Complex parts with good dimensional accuracy can be
produce

Rotary Swaging
 A solid rod or tube is
subjected to radial impact
forces by a set of reciprocating
dies of the machine
Various Forging Operations
Tube Swaging
 The internal diameter and/or the thickness of the tube
is reduced with or without the use of internal mandrels

https://www.youtube.com/watch?v=_JbiWPW5jM0

https://www.youtube.com/watch?v=BoyQGrYMQUE