This document provides an overview of tribology and friction in cold forming of aluminum sheet metal. It discusses: 1) The importance of friction in sheet metal drawing and the different friction zones that exist. Both low and high friction is needed in different areas to control material flow. 2) How the microtopography of the sheet surface impacts friction and the mechanisms of friction. Surface treatments can make the surface isotropic and better retain lubricant. 3) Tests used to measure friction coefficients in strip drawing tests that simulate the forming process. Surface properties greatly influence friction behavior. 4) The tribological system involves the sheet surface, tool surface, and lubricant working together to prevent adhesion. Both surface roughness

1. TALAT Lecture 3702
Tribology in Cold Forming of Aluminium Sheet
13 pages, 12 figures
Basic Level
prepared by
K. Siegert and S. Wagner, Institut für Umformtechnik, Universität Stuttgart, and
F. Ostermann, Aluminium Technologie-Service, Meckenheim
Objectives:
− to appreciate the importance of friction in sheet metal drawing
− to describe the mechanism of friction and lubrication
− to show the importance of surface topography
− to learn about factors improving the problem of adhesion and about
− methods of determining the coefficients of friction in different tribological systems
Prerequisites:
− background in production engineering and sheet metal forming
− TALAT Lecture 3701
Date of Issue: 1996
© EAA – European Aluminium Association

2. 3702 Tribology in Cold Forming of Aluminium Sheet
Table of Contents:
3702 Tribology in Cold Forming of Aluminium Sheet .................................2
3702.01 Friction in Deep Drawing and Drawing of Car Body Parts ................... 3
3702.02 The Effects of Microtopography of the Sheet Metal Surface ................. 4
3702.03 Mechanism of Friction................................................................................ 7
3702.04 Effect of Surface Properties on Friction Behaviour ................................ 8
3702.05 The Tool Surface....................................................................................... 10
3702.06 Lubrication ................................................................................................ 11
3702.07 Literature/References ............................................................................... 12
3702.08 List of Figures............................................................................................ 13
TALAT 3702 2

3. 3702.01 Friction in Deep Drawing and Drawing of Car Body Parts
Apart from the formability of sheet materials the tribological conditions in the contact
zones between the sheet surface and the tool surface play an important part in
determining the procedural limits of the forming process. Friction in the various contact
zones affects the flow of the material in the tool and is used deliberately to control the
forming process.
The friction zones in deep drawing and in drawing of car body parts are illustrated
schematically in Figure 3702.01.01 and Figure 3702.01.02, respectively. The demands
made on the friction situation in these friction zones can vary greatly depending on the
type of part being drawn and on the forming procedure. In deep drawing, low friction is
required under the blankholder (zone 1) and at the drawing die curvature (zone 2), in
order to reduce drawing forces. At the punch edge (zone 3), friction needs to be as high
as possible, so that high forces are introduced into the cup wall at the transition zone
from punch to cup wall. If special areas have to be drawn out by stretch forming in the
bottom of the drawn part, low friction values are desirable at the punch face (zone 4). To
control material flow in the case of irregular drawn parts, such as, e.g. car body parts,
higher friction may be necessary in certain parts of the blankholder, which can be
achieved with locally higher surface pressure or with braking bulges (draw beads).
Friction Zones in Deep Drawing
FSt
FN FN
Blankholder 1 Punch
1
2 4
Drawing Die
3
Friction zones:
1 Drawing die and blankholder 3 Punch edges
2 Drawing die curvature 4 Punch-shank surface
alu
Training in Aluminium Application Technologies
Friction Zones in Deep Drawing 3702.01.01
TALAT 3702 3

4. Friction Zones in Drawing of Car Body Parts
FSt
FN FN
Blankholder 1 Punch
4
1 5
3
Drawing Die 2
Friction zones:
3 Punch edges
1 Drawing die and blankholder 4 Punch-shank surface
2 Drawing die curvature 5 Punch face
alu
Training in Aluminium Application Technologies
Friction Zones in Drawing of Car Body Parts 3702.01.02
3702.02 The Effects of Microtopography of the Sheet Metal Surface
The tribological system as a whole consists of the sheet surface, the tool surface and the
lubricant. The lubricant prevents abrasion and wear of the tool and workpiece surfaces,
and, particularly in the case of drawn aluminium parts, prevents adhesion at the tool
surface. Lubrication is, therefore, vital to the drawing of parts from bare aluminium. At
the same time, it is necessary to keep the use of lubricants as low as possible, since they
have to be removed after forming prior to any further operations such as joining or
surface treatment. The capacity of the sheet surface to absorb lubricant and thus the
precise surface microtopography of the sheet are correspondingly important.
The standard rolled surface of aluminium sheet is the so-called “mill-finish“ surface
with relatively low roughness coefficients. It is produced with tangentially ground rolls
and thus exhibits a directional roughness, which produces different tribological
behaviour parallel and transverse to the rolling direction. The topographic image of the
mill-finish surface is depicted in Figure 3702.02.01. Lubricant contained in the long
stretched roughness valleys will be squeezed or drained out under the force of the die
pressure before a significant hydrostatic pressure can build up. Such surfaces are,
therefore, rather prone to adhesion and abrasion during forming operations and exhibit
directionality in the coefficient of friction and with regard to the tendency of adhesion,
as will be seen later.
TALAT 3702 4

5. Surface Texture
anisotropic isotropic stochastic isotropic deterministic
Conventional Mill-Finish Spark Erosion, Laser Texturing,
Blasting with Hard Particles Electron Beam Texturing
Surface Preparation Technique
Source: P. Fuller, Alusuisse-Lonza
alu
Microtopographic Structures of Al Carbody Sheet 3702.02.01
Training in Aluminium Application Technologies
The anisotropy of the friction performance of the mill-finish surface can be overcome by
the use of roll surfaces prepared by blasting, electro-discharge or laser texturing. The
resultant isotropic structures of the surface microtopography is also shown in Figure
3702.02.01. These surface structures are characterised by closed pits which entrap the
lubricant. Flattening of the rims due to contact with the tool surface builds up
hydrostatic pressure in the entrapped lubricant, which helps to reduce the danger of
adhesion.
To model the friction behaviour and to study the various tribological effects and
parameters a number of special tests have been developed, see Figure 3702.02.02. Most
of these tests methods incorporate friction tests on sheet specimen strips, which are
drawn between a mock die and blankholder. Blankholder pressure and drawing force
can be monitored individually and the resulting friction coefficient can be measured.
Friction strip tests can be performed with and without simulating the drawing over a die
curvature.
TALAT 3702 5

6. Measuring Friction with Strip Drawing Tests
FN
FN
FG
FZ
FZ
FN
FZ
Strip drawing without bending Strip drawing with bending
FZ Drawing force
FG Reaction force
FN Blankholder force
Source: Wojtowicz / Littlewood and Wallace / Woska
alu
Training in Aluminium Application Technologies
Strip Drawing Tests 3702.02.02
Figure 3702.02.03 shows two friction strip testing methods without bending actions. In
the test set-up (a) the strip is drawn between two stationary dies, measuring an average
of the friction coefficients on both sides of the strip. As sheet qualities become
available, which have different surface treatments on either side of the sheet, it may be
necessary to determine the friction behaviour separately for both sides of the sheet. The
test set-up (b) employs a tool sled, which is moved on frictionless air bearings (cf.
Institut für Umformtechnik, Universität Stuttgart).
Forces and Friction Coefficients in
Strip Drawing Friction Tests
a) double-sided friction b) single-sided friction
2 FN 2
FN
1 1
FR1=FN µ1 FR=FN µ1
µ1 FZ=FR1+FR2 µ1
vZ vZ
µ2
FR2=FN µ2
3
2 FN 2
FR=FZ=FN (µ1+µ2) FR=FN µ1
1 Sheet strips
2 Gripping (drawing) jaws
3 Drawing sledge (air bearings)
Source: IfU - Stuttgart
alu Forces and Friction Coefficients in
Training in Aluminium Application Technologies Strip Drawing Tests 3702.02.03
TALAT 3702 6

7. 3702.03 Mechanism of Friction
The tribological conditions in sheet metal forming operations are characterised by rather
low relative velocities between work piece and die surfaces, by generally low pressures
in the macroscopic contact area between die and sheet metal and by relatively large
areas of macroscopic contact between die and work piece surfaces. Under these
conditions liquid or pasty lubricants can be employed to reduce the frictional forces
between dry metal surfaces. The low relative velocities do not provide conditions for
general hydrodynamic lubrication.
On a microscopic scale, however, there are zones between the die and work piece
surfaces separated by a thin layer of lubricant and zones of direct metallic contact. The
magnitude of metallic contact depends on a number of factors, among which the surface
roughness and its microtopological structure as well as the amount of lubricant are the
most important ones.
When the die surface meets the generally rougher and softer surface of the sheet metal
the area of direct metallic contact is at the first instant relatively small and restricted to a
few roughness peaks. Due to the high specific local pressure the peaks are flattened and
the die surface sinks deeper into the sheet surface. The lubricant enclosed in the
roughness valleys builds up a hydrostatic pressure and transmits the die pressure onto
the sheet metal surface. At the same time the excess lubricant is driven out of the
valleys, forced between the flattened roughness peaks and forms a thin boundary film
made up of tribo-chemical reaction products and reacting substances (e.g., metallic
soaps, E., P. additives among others). This situation is depicted in Figure 3702.03.01.
Mechanism of Friction
FN tool
τ
V
PHS kƒ τ
Vrel Adhesion
PHD (bridging due to
cold welding)
Hydrodynamic Hydrostatic Abrasion particle
pressure build-up pressure Levelling processes (elastic, plastic deformations)
(fluid friction) build-up
Lubricant film made up of tribo-chemical reaction products
and reacting substances (e.g. metallic soaps, E. P. additives
among others)
(µ liquid + µ interface + µ solid) = µ total = ƒ (t,s,v,....)
Source: IfU - Stuttgart
alu
Mechanism of Friction 3702.03.01
Training in Aluminium Application Technologies
TALAT 3702 7

8. The relative motion during the drawing operation builds up shear stresses in the
flattened peak zones. If the boundary layer ruptures due to motion or high die pressure
the metallic surfaces get into direct metallic contact. The result will be adhesion due to
local pressure welding. These metallic bridges rupture during further motion, partly
sticking to the die surface and partly breaking loose as abraded particles. At this stage
lubrication has broken down.
3702.04 Effect of Surface Properties on Friction Behaviour
The friction and adhesion behaviour of aluminium car body sheet materials have been
determined using the strip friction test method without bending. For standard mill-finish
surfaces the results are shown in Figure 3702.04.01. (The tests were carried out using a
drawing speed of 2 mm/sec, a drawing distance of 100 mm and lubricant Oest Platinol
V711/80). The effect of the directionality of the surface roughness becomes apparent
with respect to the point of seizure and to the friction coefficient.
2000
Drawing Force, FZ [N]
1500
AlMg0,4Si1,2 Strip Drawing Test
AlMg2,5 without Bending
AlMg5 v z = 2mm/s
1000
Lubricant:
Oest Platinol V711/80
500 Adhesion
0
0 2 4 6 8
Blankholder Pressure P [N/mm²]
N
Source: E. Mössle, 1983
Friction Behaviour of Al Carbody Sheet
alu
in Strip Drawing Tests 3702.04.01
Training in Aluminium Application Technologies
From Figure 3702.04.02 it is evident that the isotropic surfaces, such as „Lasertex“ or
„EDT“, perform much better with regard to seizing and galling. Good results are also
achieved with roll surfaces prepared by blasting. (The tool material was cast iron
GG25CrMo, drawing speed 100 mm/sec, drawing distance 370 mm and lubricant
mineral oil M100).
TALAT 3702 8

9. Figure 3702.04.03 exhibits the friction and adhesion behaviour of aluminium car body
sheet with surface treatments prepared by coil coating. While CrVI-conversion coatings
have little beneficial influence on adhesion when compared with the bare metal surface,
the organic coatings prove superior in this respect.
µ = 0,10
2000
AlMg0,4Si1,2-T4
Lasertex Lubricant: M100
Vz = 100 mm/s
Drawing Force, FZ [N]
1500
Tool: X155CrVMo121
1000 µ = 0,05
EDT
500
Mill-Finish
0
0 4 8 12 16
Blankholder Pressure P [N/mm²]
N
Source: S. Wagner, IfU, Universität Stuttgart, 1994
alu Effects of Microtopographic Structure 3702.04.02
Training in Aluminium Application Technologies on Friction Properties
3000
µ = 0,10
2500 O
n- nc
Strip Drawing Friction Test
M zi
g5 ona without Bending
M
Al ith B
Drawing Force, F [N]
w Lubricant: M100 Mill-Finish
2000
4
,2-T Vz = 100 mm/s
Z
Si1
g 0,4 azinc
AlM Bon Tool: 1.2379
1500 with
µ = 0,04
1000
Chip-Resist
500
0
0 8 16 24 32 40 48
Blankholder Pressure PN [N/mm²]
Source: S. Wagner, IfU, Universität Stuttgart, 1994
alu
Friction Properties of Coil Coated Aluminium 3702.04.03
Training in Aluminium Application Technologies
TALAT 3702 9

10. 3702.05 The Tool Surface
The second friction partner in the tribological system, the tool surface also makes an
important contribution to the tribological situation. Basically the same tool materials are
used for drawing aluminium body parts as for manufacturing steel bodies, for example
cast iron GG26, GG25CrMo and tool steel inserts for drawing edges, drawing beads and
cutting edges. To prevent the occurrence of adhesion, the roughness of the tool surface
in critical contact zones should meet the following requirements
Rz ≤ 1 µm
λp ≥ 0,46
where
Rz = average peak-to-valley depth
λp = degree of profile emptiness = Rp/Rt
Rp = peak to mean line height
Rt = peak to valley height
The adhesion tendency and the friction behaviour can be particularly effectively
influenced by surface treatment of the tool, as shown in Figure 3702.05.01. The type of
surface treatment which is particularly suitable for a specific case depends on the
technical and economical parameters, such as the type of tool material and the size of
the tool.
3000
Adhesion Paste Strip Drawing Test
boronised without Bending
2500 AlMg0.4Si1.2-T4
Ionitrided
vz = 2mm/s
Drawing Force, FZ [N]
(GG26)
2000 Lubricant:
TiC coated Oest Platinol V711/80
1500 Hardened
(X165CrMoW12)
Hard chrome
Grey cast iron
1000 plated
(GG26)
500
Bath nitrided
(Tenifer)
0
0 2 4 6 8
Blankholder Pressure P [N/mm²]
N
Source: E. Mössle, 1983
Effects of Tool Surface Treatments on Friction
alu
and Adhesion of Aluminium Carbody Sheet 3702.05.01
Training in Aluminium Application Technologies
TALAT 3702 10

11. 3702.06 Lubrication
The third tribological partner is the lubricant. Successful drawing of aluminium body
parts depends decisively on the choice of a lubricant and its application to the sheet
blank and the tool. Greasing of the sheet blank is generally performed by roll application
today. In particular cases it may be necessary to use pressure lubrication in specific areas
of the tool itself.
There is a large selection of lubricants available today. One important factor in the
choice of the lubricant is the dynamic viscosity. Figure 3702.06.01 shows the influence
of the dynamic viscosity on the friction coefficient as tested with unalloyed mineral oil
M10, M100 and M300, the numbers corresponding roughly with the dynamic viscosity
η (in 10-3 Ns/m2) at room temperature. The higher the viscosity the lower is the friction
coefficient.
Effect of Lubricant
1600 µ = 0,20 Viscosity
1400
Mill-Finish
on Friction in
µ = 0,16 Strip Drawing
Drawing Force, FZ [N]
1200
Tests
1000
µ = 0,12 Sheet alloy:
AlMg0,4Si1,2-T4
800
µ = 0,08 Tool material:
600 X 210 Cr 12
400 Drawing velocity:
µ = 0,04 vZ = 100 mm/s
200
Lubricant: Mineral Oil
0 M 10
0 1 2 3 4 5 6 7 8
M100
Blankholder Pressure PN [N/mm²]
M300
Source: IfU, Universität Stuttgart
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Friction Behaviour and Lubricant Viscosity 3702.06.01
Training in Aluminium Application Technologies
It is important also to realize that the viscosity of lubricants can vary significantly with
temperature. Figure 3702.06.02 shows the pressure and temperature dependence of an
alloyed lubricant (Oest Al2N). The lubricant exhibits a steep reduction in dynamic
viscosity with temperature increasing slightly above room temperature. Temperature
rises have been measured at the drawing die radius in the order of 10 to 15 °C. In critical
areas this temperature rise and the resulting decrease in dynamic viscosity must be taken
into account.
TALAT 3702 11

12. Dynamic viscosity behaviour of a compounded lubricant
dynamical viscosity η in mPa.s
60
40
20
0
C
RT
°
40
in
60
80
T
100
ure
150
rat
pe
200
m
1 50 100 150 200
Te
Pressure p in bar
Source: IfU - Stuttgart
alu Temperature and Pressure Dependence of the
Training in Aluminium Application Technologies Dynamic Viscosity of Lubricants (Oest AL2N) 3702.06.02
3702.07 Literature/References
Mössle, E., The effect of the sheet surface in drwaing of sheet parts made of aluminium
alloys (in German), Report No. 72, Institut für Umformtechnik, Universität Stuttgart,
1983, Springer-Verlag
Balbach, R.: Optimierung der Oberflächenmikrogeometrie von Aluminiumfeinblech für
den Karosseriebau, Report no. 97, Institut für Umformtechnik, University Stuttgart,
1988, Springer-Verlag
Woska, R.: Einfluß ausgewählter Oberflächenschichten auf das Reib- und
Verschleißverhalten beim Tiefziehen, Diss. TH Darmstadt, 1982
Siegert,K., Thoms,V.: Anforderungen an den Schmierstoff bei der Blechumformung in
Karosseriewerken, in: Blechbearbeitung 1986, VDI report no. 614, Dusseldorf, VDI-
Verlag, 1986
Siegert, K and Thoms, V. The use of lubricants to influence friction during forming of
body sheet (in German) ALUMINIUM, 1987, vol. 63, p 401-406
Lange, K.: (Editor) Umformtechnik - Handbuch für Industrie und Wissenschaft, vol. 3:
Blechbearbeitung, 2nd edition, Chapter 4 Tribologie der Blechumformung, Springer-
Verlag, Berlin 1990
Ostermann, F.: Principles of drawing aluminium body parts, in F. Ostermann (Editor)
Aluminium Materials Technology for Automobile Construction, english edition by Roy
Woodward, Mechanical Engineering Publications Ltd., London 1993
TALAT 3702 12

13. 3702.08 List of Figures
Figure No. Figure Title (Overhead)
3702.01.01 Friction Zones in Deep Drawing
3702.01.02 Friction Zones in Drawing of Car Body Parts
3702.02.01 Microtopographic Structuresof Al Carbody Sheet
3702.02.02 Strip Drawing Tests
3702.02.03 Forces and Friction Coefficients in Strip Drawing Tests
3702.03.01 Mechanism of Friction
3702.04.01 Friction Behaviour of Al Carbody Sheet in Strip Drawing Tests
3702.04.02 Effects of Microtopographic Structure on Friction Properties
3702.04.03 Friction Properties of Coil Coated Aluminium
3702.05.01 Effects of Tool Surface Treatments on Friction and Adhesion of Al
Carbody Sheet
3702.06.01 Friction Behaviour and Lubricant Viscosity
3702.06.02 Temperature and Pressure Dependence of the Dynamic Viscosity of
Lubricants (Oest Al2N)
TALAT 3702 13