ENM208
METAL FORMING
ANADOLU
UNIVERSITY
Industrial Engineering Department
2006
Saleh AMAITIK
Manufacturing Processes
Fundamentals of Metal Forming
Metal
Forming
includes a large group of
manufacturing processes in which plastic deformation
is used to change the shape of metal workpieces.
Deformation results from the use of a tool, usually a
die in metal forming, which applies stresses that
exceed the yield strength of the metal.
The metal deforms to take a shape determined by the
geometry of the die.
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Manufacturing Processes
Stresses in Metal Forming
Stresses applied to plastically deform the metal are
usually classified into five groups:
Compressive Stress. Deformation of a
solid body is achieved through Uni-axial
or Multi-axial compressive loading
Tensile Stress. A plastic deformed shape
is achieved through either Uni-axial or
Multi-axial deformation.
Combined tensile and compressive stresses.
A combination of tension and compression to
achieve plastic deformation.
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Stresses in Metal Forming
Bending Stress. Permanent deformation is
achieved by a bending load.
Shearing Stress A solid body in which
plastic deformed state has been achieved
by a shearing load.
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Manufacturing Processes
Material Properties in Metal Forming
To be successfully formed, a metal must possess certain properties.
Desirable material properties:
– Low yield strength and high ductility
These properties are affected by temperature:
– Ductility increases and yield strength decreases when work
temperature is raised
Strain rate and friction are additional factors that affect
performance in metal forming.
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Manufacturing Processes
Independent Variables in Metal Forming
Independent variables are those aspects of the process over which the
engineer has direct control , and they are generally selected or
specified when setting up a process.
Some of independent variables in a typical forming process
• Starting material.
• Starting geometry of the workpiece.
• Tool or die geometry.
• Lubrication.
• Starting temperature.
• Speed of operation.
• Amount of deformation
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Dependent Variables in Metal Forming
Dependent variables, these are the consequences of the independent
variable selection.
Example of dependent variables include:
• Force or power requirements.
• Material properties of the product.
• Exit (or final) temperature.
• Surface finish and dimensional precision.
• Nature of the material flow.
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Independent-Dependent Variables Relationships
The link between independent and dependent variables is truly the
most important area of knowledge for a person in Metal Forming
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Metal Forming Classification
Metal forming processes can be classified as:
1- Bulk Deformation Processes
- Characterized by significant deformations and massive shape changes
- "Bulk" refers to workparts with relatively low surface area-to-volume ratios
- Starting work shapes include cylindrical billets and rectangular bars
2- Sheet Metal Working Processes
- Forming and related operations performed on metal sheets, strips, and coils
- High surface area-to-volume ratio of starting metal, which distinguishes
these from bulk deformation
- Often called pressworking because presses perform these operations
- Parts produced in sheet-metal operations are called Stampings
- Tools used
1- The punch is the positive portion.
2- The die is the negative portion.
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Bulk Deformation Processes
The basic operations in bulk deformation are illustrated in the
following figures.
1- Rolling. Deformation process in which work thickness is reduced
by compressive forces exerted by two opposing rolls
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Bulk Deformation Processes
2- Forging. Deformation process in which work is
compressed between two dies
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Bulk Deformation Processes
3- Extrusion. Compression forming process in which the
work metal is forced to flow through a die opening to
produce a desired cross-sectional shape
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Bulk Deformation Processes
4- Drawing. Cross-section of a bar, rod, or wire is
reduced by pulling it through a die opening
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Sheet Metal Working Processes
1- Bending. Straining sheet metal around a straight axis
to take a permanent bend
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Sheet Metal Working Processes
2- Drawing. Forming a flat metal sheet into a hollow or
concave shape, such as cup, by stretching the metal.
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Sheet Metal Working Processes
3- Shearing. A shearing operation cuts the work using a
punch and die.
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Material Behavior in Metal Forming
The typical stress strain curve for most
metals is divided into an elastic region and
a plastic region
Plastic region of stress-strain curve is
primary interest because material is
plastically deformed
In plastic region, metal's behavior is
expressed by the flow curve:
where
  K
n
K = strength coefficient (MPa); and n = strain hardening exponent
Stress and strain in flow curve are true stress and true strain
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Flow Stress
For most metals at room temperature, strength increases when
deformed due to strain hardening. The stress required to
continue deformation must be increased to match this increase in
strength.
Flow stress. Is defined as the instantaneous value of stress
required to continue deforming the material – to keep the metal
“flowing”.
Yf  K
n
where
Yf = flow stress, that is, the yield strength as a function of strain
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Average Flow Stress
The average flow stress (also called the Mean flow stress) is the
average value of stress over the stress-strain curve from the
beginning of strain to the final (maximum) value that occurs
during deformation
Determined by integrating the flow
curve equation between zero and
the final strain value defining the
range of interest.
K
Yf 
1 n
_
Where
_
n
Yf = average flow stress; and
 = maximum strain during deformation process
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Typical Values of K and n
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Temperature in Metal Forming
For any metal, the values of K and n in the flow curve depend on
temperature
- Both strength and strain hardening are reduced at
higher temperatures
- In addition, ductility is increased at higher
temperatures
These property changes are important because;
Any deformation operation can be accomplished with lower forces
and power at elevated temperature
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Temperature Ranges Metal Forming
There are three temperature ranges in metal forming processes:
Category
Temperature Range
 0.3 Tm
Cold Working
Warm Working
0.3 Tm – 0.5 Tm
Hot Working
0.5 Tm – 0.75 Tm
Where Tm is the melting point of the metal
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Cold Working
Performed at room temperature or slightly above
Many cold forming processes are important mass
production operations
Minimum or no machining usually required
These operations are near net shape or
net shape processes
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Advantages of Cold Working
Significant advantages of cold forming compared to hot working
• Better accuracy, meaning closer tolerances
• Better surface finish
• Strain hardening increases strength and hardness
• Contamination problems are minimized
• No heating of work required
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Disadvantages of Cold Working
There are certain disadvantages or limitations associated with cold working
• Higher forces are required to initiate and complete
the deformation
• Heavier and more powerful equipment and stronger
tooling are required.
• Surfaces of starting workpiece must be free of scale
and dirt.
• Ductility and strain hardening limit the amount of
forming that can be done
- In some operations, metal must be annealed to allow
further deformation
- In other cases, metal is simply not ductile enough to
be cold worked
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Warm Working
Performed at temperatures above room temperature
but below recrystallization temperature.
Dividing line between cold working and warm
working often expressed in terms of melting point:
0.3Tm, where Tm = melting point for metal
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Advantages of Warm Working
The lower strength and strain hardening as well as higher
ductility of the metal at the intermediate temperatures provide
warm working the following advantages over cold working
• Lower forces and power than in cold working
• More intricate work geometries possible
• Need for annealing may be reduced or eliminated
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Hot Working
Deformation at temperatures above recrystallization
temperature
Recrystallization temperature = about one-half of melting point
In practice, hot working usually performed somewhat above 0.5Tm
Metal continues to soften as temperature increases above
0.5Tm, enhancing advantage of hot working above this level
Capability for substantial plastic deformation of the metal - far
more than possible with cold working or warm working
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Advantages of Hot Working
• Workpart shape can be significantly altered
• Lower forces and power required
• Metals that usually fracture in cold working can be
hot formed
• Strength properties of product are generally
isotropic
• No strengthening of part occurs from work
hardening
– Advantageous in cases when part is to be subsequently
processed by cold forming
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Disadvantages of Hot Working
• Lower dimensional accuracy
• Higher total energy required (due to the thermal
energy to heat the workpiece)
• Work surface oxidation (scale), poorer surface
finish
• Shorter tool life
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Strain Rate
The rate at which the metal is strained in a forming process is
directly related to the speed of deformation, v
In many forming operations, deformation speed is equal to the
velocity of the ram or other moving element of the equipment.
.
Strain rate is defined:
.where

v

h
= true strain rate (m/s/m) or simply (S-1); and
h = instantaneous height of workpiece being deformed
In most practical operations, valuation of strain rate is complicated by
- Work part geometry
- Variations in strain rate in different regions of the part
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Manufacturing Processes
Friction in Metal Forming
Friction in metal forming arises because of the close contact
between the tool and work surfaces and the high pressures that
drive the surfaces together in these operations.
In most metal forming processes, friction is undesirable for the
following reasons:
1. Metal flow in the work is retarded.
2. The forces and power to perform the operation are increased.
3. Rapid wear of the tool occurs
Friction and tool wear are more severe in hot working
If the coefficient of friction becomes large enough, a condition
known as sticking occurs.
Sticking in metal working is the tendency for the two surfaces in
relative motion to adhere to each other rather than slide
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Manufacturing Processes
Lubrication in Metal Forming
Metalworking lubricants are applied to tool-work
interface in many forming operations to reduce
harmful effects of friction.
Benefits obtained from the application of lubricants are:
1. Reduced sticking, forces, power, tool
wear
2. Better surface finish
3. Removes heat from the tooling
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Lubrication in Metal Forming
Consideration in
lubricant include:
choosing
an
appropriate
metalworking
• Type of forming process (rolling, forging, sheet
metal drawing, etc.)
• Hot working or cold working
• Work material
• Chemical reactivity with tool and work metals
• Ease of application
• Cost
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