Module 8

Overview of processes

1

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Metal forming

Principle of the process

Structure and configurtion

Process modeling

Defects

Design For Manufacturing (DFM)

Process variation

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Principle of Metal Forming

3

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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 the shape of the die

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

• One difference between plastic forming and metal forming is

Plastic: solids are heated up to be polymer melt

Metal: solid state remains solid status in the process

- (temperature can be either cold, warm or hot)

Metal Forming

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Metal forming is divided into: (1) bulk and (2) sheet

Metal Forming

Bulk: significant deformation

massive shape change

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

RollingForging

Extrusion Drawing

Traditionally Hot

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Sheet deformation processes (Press working/ Stamping)

BendingDrawing

Shearing

Actually Cutting

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We discuss:

1. General mechanics principle

2. 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 strain: Instantaneous elongation per unit length of the material

0ln

0 LL

LdLL

L

L0: the initial length of a specimen

L: the length of the specimen at time t

the true strain at time t

True Stress: Applied load divided by instantaneous value of cross-section area

AF /

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More interested in the plastic deformation region

Plastic deformation region

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The stress-strain relation in the plastic deformation region

nK

where

K= the strength coefficient, (MPa),

= the true strain, σ=the true stress,

n= the strain hardening exponent.

Remark: Flow stress (Yf) is used for the above stress (which is the stress beyond yield). The equation is called flow curve.

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As deformation occurs, increasing STRESS is required to continue deformation

Flow Stress: Instantaneous value of stress required to continue deforming the material (to keep metal “flowing”)

FLOW STRESS

nKfY

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Average stress: total stress in one complete operation (e.g., exclusion)

Integrating the flow stress along the trajectory of straining, from zero to the final strain value defining the range of interest

nkY

n

f

1

AVERAGE FLOW STRESS

Average flow stress Max. strain during deformation

Strength Coefficient

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.

According to the statement of the problem, we have

4/3fY of fY

333.075.0)1/(1

75.0)1/(

75.0

nn

KnK

YYnn

ff

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The above analysis 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 of the straining

- Strain affecting flow stress

hv /h

Speed of deformation (could be equal to velocity of ram)

Instantaneous height of work-piece being deformedh

mf CY

Flow stress

Strain Rate

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mf CY

whereC strength constantm strain-rate sensitivity exponent

C and m are determined by the following figure which is generated from the experiment

nKfY

Strength coefficient but not the same as K

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C and m are affected by temperature

Temperature Up

C Down

m Up

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mnf AY

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 stress=45,000 lb/in2. Determine C and m

23000=C(10)^m45000=C(300)^m

From these two equations, one can find m=0.1973

Solution: