This document discusses friction and lubrication in metal forming processes. It describes two models of friction - the Coulomb friction model which applies at normal stresses below the workpiece's flow stress, and the Tresca friction model at higher normal stresses. The key lubrication mechanisms are also outlined, including dry lubrication, hydrodynamic lubrication using high-speed fluid films, and boundary film lubrication preferred for sheet metal forming. Methods for measuring friction coefficients like the ring test and double cup extrusion test are also presented.

1. Friction and Lubrication

2. 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 .

3. 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 .

4. 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 .

5. 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

6. 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.

7. 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.

8. 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 .

9. Lubrication Mechanisms

10. 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 .

11. 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

12. 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.

13. Hydrodynamic Lubrication
Parameters affects on hydrodynamic
lubrication :
• Process type .
• Process velocity and lubrication (fluid)
velocity .
• Viscosity of the fluid used at lubrication .

14. 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 .

15. 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.

16. 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 .

17. Measurement of Friction
(Ring Method)

18. 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.

19. Thank You