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Based on flow of material
Open Die Forging
Closed Die Forging/ Impression Die Forging
Based on equipment used
- Smith forging
- Drop forging
- Press forging
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Let us assume that length (l) is much larger than
width and thickness and it remains constant.
So, it is a case of plane-strain deformation.
Lateral flow of material causes increase in width.
Let us assume the width has increased to 2a.
Let us try to estimate load required (P) in forging to
reduce the thickness of the plate to a thickness h.
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1
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The Von-Mises yield criterion for plane strain
condition is
Substituting in equation (1)
2
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In cold working under well lubricated
conditions, the frictional conditions at the tool-
work piece interface are such that there is a
sliding motion between the two. The workpiece slides over the tool surface.
It is called sliding friction or slipping friction.
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If sliding friction conditions exist, friction is
described by Coulombs law of friction:
= p
where is frictional shear stress at the interfacep is normal stress and
is coefficient of friction (0.1-0.3)
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Case (1): Sliding friction:From Coulombs law of friction,
So, equation (2) becomes3
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Integrating equation (3) both sides
Constant of integration can be found out by the
boundary condition
At , , so
4
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So,
Substituting in equation (4)
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The pressure increases exponentially as we
move from edge to the centre of the plate and
builds up to a maximum at the centre.
This is called friction hill.
Centreline is neutral surface.
5
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At centre,
So, peak pressure
At the edge, , So,
At any point
At centre,
At the edges,
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6
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Where
Forging load,
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An alternative approach to treat friction in metal
working operations is to consider the friction to be
sticking.
Sticking friction conditions exist particularly in hotworking processes and in those situations where
lubrication is inadequate.
This approach considers that the work piece in
contact with the tools can be represented as a materialof constant shear strength.
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This interface shear strength can be expressed as a fraction
of yield strength of the material in pure shear (k):
= m k
where m is the interface friction factor.
Values of m vary from 0 to 1.
m = 0 indicates perfect sliding.
m = 1 indicates perfect sticking.
Since accurate determination of m value in hot working is
difficult, m is taken to be 1 in the analysis.
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Case (2): sticking fr iction
Where m is the interface friction factor and
kis YSin pure shear.
according to Von-Mises yield criterion
In this case equation (2) becomes
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Integrating on both sides
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Applying the boundary condition,
At , ,
7
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Substituting in equation (7),
8
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Mean forging pressure,
Forging load,
9
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