Metal forming 2

Amr Shehata Fayed, Ph.D.
Assistant Professor of Mechanical Engineering
Materials Engineering Department
Faculty of Engineering
Zagazig University
amrfayed@yahoo.com

Dr. Amr Shehata Fayed

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.

Dr. Amr Shehata Fayed 2

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

Dependent Variables in Metal Forming
Dependent variables 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


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

Necking starts at
maximum engineering
stress or, equivalently,
at maximum tensile
load.


Material Behavior in Metal Forming

• In plastic region,
metal's behavior is
expressed by the flow
curve:

K = strength coefficient (MPa); and n = strain hardening
exponent. Stress and strain in flow curve are true stress
and true strain.


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”.

Where: Yf = flow stress, that is, the yield strength as a
function of strain


Relevance of the Flow Curve
The flow curve is used to determine the new yield strength
after a plastic deformation process.
The flow curve is used to judge the formability of metals.
The flow curve describes the hardening behavior of
metals during plastic deformation in terms of equivalent
strain, equivalent strain rate and temperature.
The flow curve is a property of each individual metal.
various experiments with different stress and strain-rate
states should yield the same flow curve for same strain-
rate value and same temperature.


Ranges of Equivalent Strain & Strain
Rates in Metal Forming Processes

Therefore, the flow curves should be up to these strains and
strain rates..


Some Ugly Facts about the
Determination of Flow Curves

It is not possible to obtain flow curves up to the required
plastic strains and strain rates practically.
It is extremely difficult to have tests with homogeneous
deformation.
The flow curves obtained by different tests do not
coincide for the same material.


Reasons for Deviations in the Flow
Curves Obtained by Different Tests

Effect of stress state,
Effect of equivalent stress equation,
Effect of anisotropy (Bauschinger effect),
Effect of experimental inaccuracies (e. g. friction),
Effect of temperature (heating of the specimens),


Flow Curve: Mathematical Representation (1)
(Cold Flow Curves)

For cold flow curves the flow stress increases only 3 to
10 % for an increase of one order in the strain-rates.

Average Flow Stress
The average flow stress (or
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.

Where: Yf = average flow stress; and
= maximum strain during deformation

Typical Values of K and n


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


Temperature Ranges Metal Forming

There are three temperature ranges in metal forming
processes:

where Tm is the melting point of the metal


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


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


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. While, in other cases, metal is
simply not ductile enough to be cold worked.

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


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.
Finishing machining is reduced.
Less scaling and steel decarburization compared to
hot working.


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.
Capability for substantial plastic deformation of the
metal - far more than possible with cold working or
warm working.


Advantages of Hot Working

Workpart shape can be significantly altered.
Lower forces and power than in cold working.
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.


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


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:
Metal flow in the work is retarded.
The forces and power to perform the operation
are increased.
Rapid wear of the tool occurs.
Friction and tool wear are more severe in hot working

Friction in Metal Forming

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.


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:
Reduced sticking, forces, power, tool wear
Better surface finish
Removes heat from the tooling


Homework (2)
1. If K = 600 MPa and n = 0.2 for certain metal. During a
forming operation, the final true strain that the metal
experienced = 0.73. Determine the flow stress at this
strain and average flow stress that metal experienced
during the operation.
2. A particular metal has a flow curve with parameters;
strength coefficient = 35000 lb/in2 and strain hardening
exponent = 0.26. A tensile specimen of the metal with
gage length = 2 in is stretched to a length = 3.3 in.
Determine the flow stress at this new length and the
average flow stress that the metal was subjected to during
deformation.


Homework (2), cont.

3. Why is the term press working often used for sheet-
metalworking processes?
4. Mention some of the advantages of cold working relative
to warm and hot working.
5. Why is friction generally undesirable in metal forming
operations?
6. What are the main differences between bulk deformation
and sheet metalworking processes?


Metal forming 2

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