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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 – PowerPoint PPT presentation

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Title: Metal Forming


1
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

2
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
3
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
4
Bulk deformation processes
Forging
Rolling
Traditionally Hot
Extrusion
Drawing
5
Sheet deformation processes (Press working/
Stamping)
Drawing
Bending
Shearing
Actually Cutting
6
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

7
1. General mechanics principle
  • The underlying mechanics principle for metal
    forming is the stress-strain relationship see
    Figure 1.

Figure 1
8
  • True Stress Applied load divided by
    instantaneous value of cross-section area
  • True strain Instantaneous elongation per unit
    length of the material

9
  • In the forming process we are more interested in
    the plastic deformation region (Figure 1)

Plastic deformation region
10
  • 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)
11
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)

12
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
13
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
14
  • 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
15
  • 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
16
where C ? strength constant m ? strain-rate
sensitivity exponent
C and m are determined by the following figure
which is generated from the experiment
17
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