Friction and Lubrication

Friction Effects in Metal Forming

• Forming loads and stresses transfers to die depends on friction , and reduced by lubrication .

• The surface quality of formed workpiece depends on friction and lubricant used .

• Wear of die can be reduced if lubricant films are applied which reduces friction .

Models of Friction

• If normal stress is less than flow stress of the workpiece

σn ≤σf Coulomb friction model .

• If normal stress is higher than flow stress

σn >>σf Tresca friction model .

The Coulomb Friction Model

The normal force that acts between the block and the underlying surface can be expressed by the equation

FN = mg

Actually the real area of contact will be very small at low contact pressures, because the contact between the block and the underlying body occurs only through the highest asperity tips of the two bodies .

The Coulomb Friction Model

The force required to move the block can be expressed in terms of the frictional shear stress, τi, as

The Coulomb coefficient of friction is then determined as the ratio between the friction force and the normal force on the block

The Tresca Friction Model

With higher contact pressure between die and workpiece, i.e., σf < σn < 3σf, where σ0 is the flow stress of the softer contact body, this body is subjected to local plastification deformation of the high asperity peaks of the softer contact body.

The Tresca Friction Model (Sticking)

• at sufficiently high contact pressures, namely, for σn > 3σf, the workpiece material will penetrate into the deepest depressions , Then the workpiece material will stick to the surface of the die, and sliding will not occure .

• This phenomenon is therefore named subsurface sliding.

The Tresca Friction Model

• Tresca’s friction model is commonly expressed by the following way:

ζ = ζi = mk• The parameter m is called the friction factor, and

can vary in the range 0 < m < 1 .• If m > 1 Sticking occurs .

Lubrication Mechanisms

Dry and Solid Lubrication

• This method is usually used at forming processes with very high pressure like forging or at hot forming as extrusion ,

• By depending on the surface oxide layer of the material as a lubricant .

• This method can not be used with all materials .

Dry and Solid Lubrication

In case of Al• Layer of Aluminum oxide will fail to reduce friction and sticking occurs

In Case of Cu :• Layer of Aluminum oxide will reduce friction and sliding occurs

Hydrodynamic Lubrication

• By a thick lubricant film that completely separates the workpiece from the die.

• This lubricant mechanism is favored in wiredrawing or rolling processes because the workpiece then moves with high speed into the dies.

Hydrodynamic Lubrication

Parameters affects on hydrodynamic lubrication :

• Process type .

• Process velocity and lubrication (fluid) velocity .

• Viscosity of the fluid used at lubrication .

Boundary Film Lubrication

• It is generally claimed that the most favorable lubrication condition in metal forming is boundary film lubrication.

• Suitable for sheet metal forming due to good finishing required .

Boundary Film Lubrication

• It is When this lubrication mechanism prevails, a thin – and very effective – lubricant film persists in the interface between workpiece and die .

• The value of the Coulomb coefficient of friction will commonly lie in the range 0.01 < μ < 0.1.

Measurement of Friction (Ring Method)

• A flat ring such as that shown is applied. It is lubricated and placed between two flat dies, where it is compressed down to less height.• From the ratio between height reduction and diameter reduction , The fiction coefficient can be determined .

Measurement of Friction (Ring Method)

Double Cup Extrusion Test (DCET)

• The test combines the single cup forward and single cup backward extrusion which reflects the real process conditions more accurately.

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