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Friction

Friction During Metal Forming

 

Friction always plays an important role in the forming processes. In the case of the Coulomb friction law:

 

FR = μ FN

 

that derived from the rigid body condition, the friction coefficient μ depends on the ratio of the tangential stress to the normal stress. Since the friction coefficient is not constant over the contact area, the average value is used.

 

In the plastic forming process, other friction laws are also assumed to be valid, such as Newtons friction law and Navier-Stocke Law.

 

Major influence factors on the friction are:

  • Materials of tool and workpiece
  • Surface roughness
  • Surface state, e.g., oxidize and lubrication material
  • Forming parameters, such as temperature, speed, deformation zone geometry, specific pressure on the deformation zone

In the table 1, friction coefficients for several materials in various forming processes are presented.

 

Table 1: Friction coefficient for hot and cold forming [101]

1. Upsetting & free forging

 

Werkstoff

 

C-alloyed Steels and high-alloy

Cr-Steels

Austenical

Cr-Ni-Steels

Cupper

Aluminium

Hot forming

0,30 - 0,40

0,20 - 0,30

0,25 - 0,35

0,18 - 0,32

 

Cold forming

0,10 - 0,25

0,10 -0,25

0,15 - 0,30

0,12 - 0,25

 

2. Rolling

 

Material

Cold rolling

Hot rolling

 

Without lubrication

With lubrication

T, C

μm

 

Aluminium

0,13 - 0,30

0,08 - 0,15

400

0,42 - 0,48

 

Cupper

0,10 - 0,25

0,04 - 0,10

900

0,42 - 0,45

 

Nickel

-

-

1100

0,40

 

Zink

0,25 - 0,30

0,12 - 0,15

-

-

 

Brass

0,12 - 0,15

0,04 - 0,10

800

0,34 - 0,42

 

Low carbon steel

0,20 - 0,30

0,05 - 0,10

900 - 1100

0,20 - 0,40

 

Spring steel

0,20 - 0,30

0,06 - 0,12

900 - 1100

0,30 - 0,42

 

Austenite steel

0,20 - 0,30

0,05 - 0,12

900 - 1100

0,21 - 0,42

 

Tool steel

0,20 - 0,30

0,06 - 0,12

900 - 1100

0,22 - 0,44

 

Steel (0,2%C)

 

 

 

 

 

- Polished roll

0,08 - 0,15

0,05 - 0,07

 

0,21 - 0,27

 

- Smooth roll

-

0,10 - 0,12

900 - 1100

0,27 - 0,30

 

- Bar rolling

-

-

 

0,30 - 0,50

 

- Section rolling

-

-

 

0,50 - 0,60

 

3. Extrusion

Process variation

 

Material

Without lubrication

With lubrication

Hydrostatic extrusion

 

 

Steel

-

0,02 - 0,05

 

 

 

Aluminium

0,15 - 0,25

 

 

 

 

Copper

0,15 - 0,30

 

0,06 - 0,085

 

Brass

0,20 - 0,30

 

 

 

 

Lead

0,10 - 0,15

0,03 - 0,05

 

 

 

4. Drawing

Process variation

 

 

 

Material

Normal drawing with static die

Rotated die

hydrodyna-

mic

drawing

Drawing by risen temperature

Steel

0,03 - 0,08

0,02 - 0,05

0,006 - 0,02

0,04 - 0,20

 

 

Normal drawing with lubrication

 

 

 

1 % lubrication soap in water

Lubrication oil

Water

Sintered soda in water

 

Aluminium

0,04

0,07

0,14

0,21

 

Cupper

0,09

0,12

0,19

0,27

 

 

 

The friction coefficient decreases when the speed increases; and the coefficient goes up when the strain increases. Fig 1 shows the influence of average pressure, speed and temperature on the friction coefficients. The friction coefficient gets lower when the pressure increases. The temperature has different effect on the friction in different value range. In the low temperature and very high temperature, increased temperature reduces the friction. However, in certain temperature range, scale formation and property of the scale play an important role. So in this range (for steel rolling, about 500-800˚C depending on steel grade), when temperature rises, amount of scale also increase, as a result, the friction gets higher. When the temperature becomes still higher (over 800 for many steels), scale becomes softer, so this effect actually brings down the friction.

 


Fig. 1: Influences of average pressure P (left), rolling speed (middle) and rolling temperature (right) on the friction coefficient [101]

 

 

 

References:


[100] A. Hensel and T. Spittel: Kraft- und Arbeitsbedarf bildsamer Formgebungsverfahren. VEB Deutscher Verlag fur Grundstoffindustrie Leipzig, 1978.

[101] Arno Hensel: Technologie der Metallformung -Eisen- und Nichteisenwerkstoffe. Dt. Verl. Fur Grundstoffind., 1990. ISBN 3-342-00311-1.

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