Metal Forming

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Metal Forming

• Metal forming includes a large group of
  manufacturing processes in which plastic
  deformation is used to change the shape of metal
  work pieces

• Plastic deformation a permanent change of
  shape, i.e., the stress in materials is larger
  than its yield strength

• Usually a die is needed to force deformed metal
  into a shape of the die so that the product has
  the same cross area as the die

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Metal Forming

• Metal with low yield strength and high ductility
  is in favor of metal forming

• One important difference between plastic forming
  and metal forming is

Plastic solids are heated up to be polymer melt
Metal solid remains a solid state in the whole
process
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Metal Forming
Metal forming is divided into (1) bulk and (2)
sheet
Bulk (1) there is a significant deformation
(2) there is a massive shape change
(3) surface area to volume of the work is small
Sheet Surface area to volume of the work is large
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Bulk deformation processes
Forging
Rolling
Traditionally Hot
Extrusion
Drawing
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Sheet deformation processes (Press working/
Stamping)
Drawing
Bending
Shearing
Actually Cutting
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In the following series of lecture, we discuss

• For general metal forming process, general
  mechanics principle / law / effect
• For individual processes
• - mechanics principles
• - design for manufacturing (DFM) rules
• - equipment

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1. General mechanics principle

• The underlying mechanics principle for metal
  forming is the stress-strain relationship see
  Figure 1.

Figure 1
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• True Stress Applied load divided by
  instantaneous value of cross-section area

• True strain Instantaneous elongation per unit
  length of the material

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• In the forming process we are more interested in
  the plastic deformation region (Figure 1)

Plastic deformation region
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• The stress-strain relationship in the plastic
  deformation region is described by

Where K the strength coefficient,
(MPa) ? the true strain, sthe true
stress n the strain hardening
exponent,
The flow stress (Yf) is used for the above stress
(which is the stress beyond yield)
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FLOW STRESS

• As deformation occurs, increasing STRESS is
  required to continue deformation

• Flow Stress Instantaneous value of stress
  required to continue deforming the material
  (i.e., to keep metal flowing)

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AVERAGE FLOW STRESS

• For many bulk deforming processes, rather than
  instantaneous stress, average stress is used
  (extrusion)

• The average flow stress can be obtained by
  integrating the flow stress along the trajectory
  of straining, from zero to the final strain value
  which defines the range of interest

Strength Coefficient
Max. strain during deformation
Average flow stress
Strain hardening exponent
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Example 1 Determine the value of the
strain-hardening exponent for a metal that will
cause the average flow stress to be
three-quarters of the final flow stress after
deformation.
Solution According to the statement of the
problem, we have
of
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• The above analysis is generally applicable to
  the cold working, where the temperature factor is
  not considered.

• The metal forming process has three kinds in
  terms of temperature (1) cold, (2) warm, (3) hot

• In the case of warm and hot forming, the
  temperature factor needs to be considered, in
  particular

Temperature up ? The (yield) strength down and
ductility up
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• Strain rate (related to elevated temperatures)

• Rate at which metal is strained in a forming
  process
• Such a strain rate can affect the flow stress

h
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where C ? strength constant m ? strain-rate
sensitivity exponent
C and m are determined by the following figure
which is generated from the experiment
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(No Transcript)
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C and m are affected by temperature
Temperature Up
C Down
m Up
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Even in the cold work, the strain rate could
affect the flow stress. A more general expression
of the flow stress with consideration of the
strain rate and strain is presented as follows
A is a strength coefficient, a combined effect of
K, C
All these coefficients, A, n, m, are functions of
temperature
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Example 2 A tensile test is carried out to
determine the strength constant C and strain-rate
sensitivity exponent m for a certain metal at
1000oF. At a strain rate 10/sec, the stress is
measured at 23,000 lb/in2 and at a strain rate
300/sec, the stress45,000 lb/in2. Determine C
and m
Solution
23000C(10)m 45000C(300)m From these two
equations, one can find m0.1973