MEL232 Forging

8/2/2019 MEL232 Forging

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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.

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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 , ,

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Substituting in equation (7),

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Mean forging pressure,

Forging load,

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MEL232 Forging

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