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Short communication

Experimental study of friction in sheet metal forming

L. Figueiredo, A. Ramalho∗, M.C. Oliveira, L.F. Menezes

CEMUC– Mechanical Engineering Department, University of Coimbra, Portugal

a r t i c l e i n f o

Article history:

Received 3 September 2010

Received in revised form 4 February 2011

Accepted 4 February 2011

Keywords:

Friction

Tribology

Sheet metal forming

a b s t r a c t

During deformation of the sheet metal over a tool, contact occurs only at the peak asperities of both

surfaces. In the contact areas the processed material flows over the tool’s surface, therefore all the mod-

els used to study forming processes must include a way to take into account the contact with friction

phenomena. More widely used friction models are based in the Amontons-Coulomb theories.Unfortunately experience shows that for most applications the available models cannot accurately

describe the friction phenomena. The determination of the friction coefficient in a sheet metal forming

process is a complex procedure, because many variables influence the friction mechanisms. The aim of

this research work is to apply an experimental approach in order to bridge simple benchmark friction

experiments with real sheet forming applications.

Two different techniques were used to assess friction, namely unidirectional crossed cylinders slid-

ing with linear increase of the load and an equipment which allows measuring the friction coefficient

under stretch-forming conditions in a sheet metal forming process. The tested materials are a cold-rolled

advanced high-strength steel, DP600, and an aluminium 1100 alloy against heat-treated AISI D3 steel. The

test protocols were established to allow the study of several effects: sliding speed, the surface roughness,

the lubricant effect, the load and the running-in effect. The differences between the two techniques are

widely discussed and laser profilometry and scanning electron microscopy are used to help understand

the prevalent friction mechanisms.

The present study allows concluding that: the friction results obtained by a load-scanning test are

always higher than values assessed by a draw-bead test; roughness of the die material plays an important

roleon the friction coefficient; a significant reduction of friction was attained in multi-pass load-scanningtests due to running-in effect.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, numerical simulation has been widely accepted in

the optimisation of forming processes owing to the advantage of

the notable progress of computer capabilities. Significant benefits

can be obtained especially on time-to-market andstart-upcosts by

utilizing simulations. However, the advantage of applying numer-

ical simulation of sheet metal forming operations results depends

on the correct modelling of several topics [1,2]. Among these, con-

tact conditions definition in conjunction with friction modellingassumes a decisive role. In fact, tribological properties, and fric-

tional processes, are important factors determining the result of

forming [3]. However, tribology itself comprises the interaction of

different factors connected to the sheet metal surface. Thus, exper-

imental research in sheet metal forming follows two directions:

∗ Corresponding author. Tel.: +351 239790735.

E-mail address: [email protected] (A. Ramalho).

- to understand contact conditions during sheet metal forming;

- to assessthe influence of specific variablesin sheet metal forming

operations.

Therefore, experimental research should include classic tests,

where eachinfluencingparameter could be isolated and controlled,

and technological modelling tests to allow transferability of the

results fromthe laboratoryto the differentindustrial metal forming

operations.

Concerning classic tests, Podgornik et al. [4] performed a study

to evaluate galling properties of tool materials for metal forming

operations comparing several tribological test methods. Among

several possibilities, it was proved that load scanning is a very

simple and suitable method to evaluate galling properties of tool

materials. This method was successfully applied to study the effect

of the surface treatment and the roughness of tool material as well

the behaviour of different lubricants [4–8].

The assessment of friction during sheet metal forming oper-

ations is a very complex task, and the laboratory test selected

presents a fundamental importance on the friction results. Tisza

and Fülöp [3] classified friction tests for sheet metal forming as a

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1652 L. Figueiredo et al. / Wear 271 (2011) 1651–1657

function of the main performed operations: stretch forming; deep

drawing; stretch drawing.

Stretch forming was used by several authors [9–11] to study

the influence of several parameters on friction during sheet metal

forming. New die materials and surface treatments have been

investigated [10,11] as well as the effect of blank material, surface

roughness and lubricant viscosity. Instrumented deep-drawing

tests were used especially to evaluate the effect of lubricants

[12–14]. The stretch-drawing type tests are widely used to investi-

gatethe influence of several material and technological parameters

[15–23].

Among stretch-drawing tests, the draw-bead type test intro-

duced by Nine [24] assumes an outstanding role. Sanchez [25]

published results from an interlaboratory exercise with this tech-

nique which reveals very accurate experimental results.

In spite of the abundant number of laboratory tests that have

been reported, effective numerical simulation is still hindered by

accuracyof the contact and friction modelling. This difficulty can be

understood due to the nature of the friction phenomena. The main

objectives of this study are:

- Understand friction during sheet metal forming using comple-

mentary experimental methods: load-scanning and draw-beadfriction tests.

- Compare theresults obtainedby twodifferent experimental tech-

niques: one, the load scanning, with good control and which

allows the study of the effect of each variable; the second, draw

bead, replicates the sheet metal forming conditions, so the trans-

ferability of the results is assured.

- Testing the abilities of the recently developed draw-bead tester

in different contact conditions.

2. Experimental details

2.1. Load-scanning tests

The load-scanning test was done with two opposite cylindrical

surfaces, with cross relative position, i.e. with the axes in perpen-

dicular directions. Relative sliding motion during testing forms 45◦

in relation to each specimen axis, therefore the contact spot moves

along a contact path on each specimen. This test procedure derives

from the research work of Hogmark [4–7,26].

This type of test, with point-contact geometry,can be done with

a constant normal load or varying the load, using different loading

waves, during the test. The sliding velocity is another test param-

eter that can be adjusted. The equipment also allows changing the

diameter of specimens, their roughness and the lubrication. More-

over tests can be done applying single or multi-pass conditions.

The equipment developed at the University of Coimbra has a

high precision of motion and positioning and is numerically con-trolled [8]. The sliding motion corresponds to the movement of

the horizontal base, where the specimen is fixed on a three-axis

piezoelectric load cell. Normal load is applied by a spring, with

well-defined constant rigidity, controlling the vertical motion of

the upper specimen. Therefore, both the specimen path and the

loading wave are numerically controlled. Both normal and tangen-

tial forces, measured by the load cell, were acquired in real-time

during the test.

Default testconditionsin thisresearchwere:load linear increase

from 0 to 75 N with a sliding speed of 1 mm/s. Contact occurs

between a cylinder with 5mm diameter of the die steel, against

a sheet specimen with cylindrical shape. In this study sheets with

a thickness of 1mm were conformed to obtain a cylindrical shell

with a contact radius of 5mm Fig. 1.

Fig. 1. Detail of load-scanning test assembly.

The friction coefficient, as established by the Amontons-

Coulomb model, corresponds to the linear proportionality ratio

between the friction force and the normal load [27], Eq. (1). There-

fore, each pair (friction force; normal force) allows the estimation

of the friction coefficient value, as plotted in Fig. 2a.

F = N (1)

Fig. 2. Effectof thenormal load in thefriction coefficient.



L. Figueiredo et al. / Wear 271 (2011) 1651–1657 1653

Fig. 3. Draw-bead friction test. (a) Configuration of the deformation geometry. (b)

Outline of thetest assembly.

For these tests performedwith increasing load, the friction coef-

ficient is better determined as a slope, because in that case the

frictioncoefficient is the relationship between the friction force and

normal load increase and thus the offset errors are nullified [27].

This approach, Fig. 2b, allows the verification of the applicability of

the linear Amontons-Coulomb model and permits the calculation

of a safe friction coefficient value that is applicable to the entire

loading range.

2.2. Draw-bead friction test

The draw-bead test allows the simulation of the bending and

unbending in a sheet metal forming process and to measure the

friction coefficient during the sliding of the sheet against a die dur-

ingthe forming process [25]. To do this type of test, test equipment

was especially designed in order to be used in conjunction with

a classical electromechanical tensile test machine. With this test

equipment it is possible to measure the forces associated with the

forming process. The required configuration is shown in Fig. 3.

These tests are performed with a constant velocity and the

pulling and normal force data is acquired by two load cells with

a rate of 100Hz.

Fig. 4. Evolution of friction coefficient with increasing roughness (DP600 against

D3).

The pulling force of the sheet is the sum of the force of bending

plus the friction force. Thus, in order to find the friction coefficient

in the process it is very important know the contribution of these

two effects to the pulling force. To resolve this problem two types

of test are made. The first one, the “A type test” is made with the

five rotary rolls built with bearings. In this assembly the pulling

force measurement just takes into account the deformation force,

because with these rollers the tests are done with negligible fric-

tion. In the second one, the “B type test”, the rollers 1, 2 and 3,

Fig. 4, are changed to fixed rollers. In this type of test the pulling

measurement takes into account the deformation and the friction

force.

The friction coefficient is evaluated in these tests according to

Eq. (2) [24], where F fs is the total pulling force, acquired in “B type

test”, F fr is the pulling force without friction, acquired in “A type

test” and F n is the normal force, measured during “B type test” by

the corresponding load cell, Fig. 3b.

=F fs − F fr

F n(2)

Default test conditions in this research were:fixed relative posi-

tions of the rolls, constant speed of 1 mm/s and a sliding distanceof 100mm. The values of the force components used in Eq. (2) to

calculate the friction coefficient are the average of the measure-

ments in the steady-state part of the test. For each test condition

a minimum of three repetitions were considered, further the aver-

age value of friction coefficient and the standard deviation were

also considered.

2.3. Materials

The tool steel AISI D3, tempered and quenched, was used as die

material.Two cold-rolled sheetmaterials, witha thicknessof 1 mm,

were used in the friction test against AISI D3, namely: DP600 dual-

phase steel and aluminium alloy AA 1100. Table 1 summarises the

material properties.Table 2 summarises the different material combinations used

in the different series of tests. All tests were done with lubrica-

tion. Before testing, both sliding surfaces were cleaned with ethylic

alcohol and lubricated with stamping oil, Fuchs Renoform MZAN

54. The average quantity of lubricant applied was around 25g/m2.

Table 1

Mechanical properties.

Property Material

AA 1100 DP600 AISI D3

Hardness (GPa) 0.8 5.87 7.75

Yield stress (MPa) 105 362 –

Ult. tensile stress (MPa) 110 651 –




Table 2

Summary of test conditionsand material combination.

Study Materials Experimental technique

Load-scanning Draw-bead

Comparison load-

scanning/draw-bead

DP600 againstD3

AA1100 againstD3

Lubricant Fuchs MZAN54

Normal load: linear0–75 N

Contact radius: 5 mm

Sliding speed:1 mm/s

Rz of D3:1.66and 2.48m

Roll diameter:21 mm


Rz of D3: 1.66 and 2.48m

Influence of roughness DP600 against D3


Roll diameter:21 mm


Rz of D3: 1.66, 2.48, 3.66 and 4.47m

Influence of running-in DP600 against D3

AA1100 againstD3


Normal load: linear0–75 N

Contact radius: 5 mm

Multi-pass test: 5 passes

Rz ofD3: 1.66m

Lubrication regime DP600 against D3

Lubricant viscosity:

28.03–60.18mPa s

Roll diameter:21 mm

Sliding speed: 1–8.47mm/s

Rz ofD3: 1.66m

In the lubrication tests, in addition to the stamping oil, three

paraffinicindustrial mineral oils, with viscosities ISO32, 46 and68,

were used.

Mahr-Rodenstock RM600 3D laser topography equipment was

usedto assessthe roughness. Philips XL30 scanning electron micro-scope was used to investigate the wear surfaces.

3. Results and discussion

3.1. Comparison between draw-bead and load-scanning friction

tests

In order to compare the results obtained by load-scanning and

draw-bead testing systems, a set of tests was performed for both

pairs of materials under study. Table 3 summarises the obtained

results.

Experimental results obtained for the aluminium alloy demon-

strated the effect of the die material roughness on the friction. In

this case, the effect of abrasion by steel asperities is determinant inthe result. Therefore, an increase of the roughness induced a rise in

the friction coefficient. Comparingfrictioncoefficients obtained for

the two tested pairs, in the same roughness conditions, the highest

value corresponds to the steel sheet material, which should be due

to the higher yield stress value.

Comparing the results obtained by both techniques one can see

that the same evolution has been obtained in both cases. How-

ever, load scanning always produced lower friction values. This

difference should be due to the highest contact pressure on the

load-scanning test. In fact, the increase in contact pressure could

induce a reduction in the friction by changingthe transition from a

mixed to a boundary lubrication regime [28,29]. The contact pres-

sure in the draw-bead test can be calculated applying the formula

developedfor flatbeltdrives andbandtypebrakes [30]. The contact

pressure, p, is a functionof the maximum pulling force in the metal

sheet band, F fs, the band width, w, and the roll radius, r , Eq. (3). The

values obtained for the maximum pressure are around 8 MPa. In

the load-scanning friction test the contact occurs between crossed

cylinders with equal radius; therefore, is a Hertzian type contact.

Table 3

Die roughness and friction coefficients (average values and standard deviation)

measured by draw-bead and by load-scanning tests.

Materials D3 Roughness

Rz /Ra (m)

Draw b ead C OF Load scanning C OF

DP600/D3 1.66/0.22 0.14 ± 0.019 0.083 ± 0.005

AA1100/D3 1.66/0.22 0.11 ± 0.0014 0.075 ± 0.018

AA1100/D3 2.48/0.27 0.13 ± 0.0007 0.11 ± 0.015

Considering the contact geometry and the maximum normal load,

the maximum pressure value was 3130 MPa.

p =F fs

wr (3)

In spite of the differences obtained by the two techniques, the

values agree with the majority of the published results. Using

a hybrid numerical–experimental approach, Subramonian [31]

achievedvalues in the range of 0.08–0.09, thereforesimilar to those

obtained from load scanning. However, other authors [25,32] mea-

sured values around 0.15, which agree with the values obtained in

the current work applying the draw-bead technique.

3.2. Influence of roughness

In order to further evaluate the effect of surface roughness,

draw-bead tests were done with the DP600 sheet against AISI D3

sliding cylinders, under different conditions. The D3 specimens

were polished with different polishing routines, from emery paper

grit P320 up to P2500, which corresponds to the test conditions of

the results presented in the previous section. The different emery

papers were applied with the rolls in rotation in a lathe, using a

similar procedure to that used to prepare metallographic speci-

mens. Before the tested rolls were cleaned in an ultrasonic bath of

ethylic alcohol. The roughness of the cylinder sliding surfaces was

measured by laser roughmeter equipment; Table 4 summarises the

obtained results.

Fig. 4 shows the evolution of the friction coefficient with the

die surface roughness. One can see a clear tendency of an increase

of the friction coefficient with the increase of the Rz peak-to-peak

roughness parameter. However this growth tends to stabilise for

the highest roughness values.

Considering the difference of hardness between the tested

materials, and the lowsliding speed,abrasion wasthe main contactmechanism. In fact, as displayed in Fig. 5, the wear occurs mainly

by grooving. Therefore single asperity models [33] can be used to

explain the increase of the friction coefficient with the raise of sur-

face roughness. Furthermore, bearing in mindthe low sliding speed

used in these tests (1mm/s)and thelubricant viscosity (40 mm2/s),

Table 4

Roughnessparameters of different tested die surfaces (m).

Ra Rz Rk Rpk Rvk

P2500 0.22 1.66 0.57 0.48 0.14

P1000 0.27 2.48 0.84 0.55 0.44

P600 0.48 3.66 1.22 0.67 0.38

P320 0.52 4.47 1.97 1.45 0.38




Fig. 5. Micrographof the contact surface. The scratchesparallel to the sliding direc-

tion allow identification of abrasion by grooving as themain contact mechanism.

Fig. 6. Running-in effect in (a) dual-phase steel DP600 and (b) aluminium alloy

AA1100.

Fig. 7. Evolution of thefriction coefficient forthe pairs D3/DP600 D3/AA1100 with

the number of passes.

Fig. 8. Effect of the number of sliding passes in the aluminium wear track surface

morphology for the D3/AA1100 pair: (a) 1 pass; (b) 3 passesand (c) 5 passes.

boundary lubrication was the expected regime. In this lubrication

regime, friction is significantly dependent on the roughness.

3.3. Effect of running-in

Load scanning was used to investigate the evolution of the fric-

tion in multi-pass tests. Two pairs of material were investigated

in this study: AISI D3 cylinders against DP600 sheet steel and AISI

D3 cylinders against aluminium alloy 1100. To verify the running-

in effect multi-pass tests are made from one to five passes in the

same track. The roughness of the AISI D3 cylinder was always

Ra = 0.27m and Rz =2.48m.

Both materials tested reveal a reduction of the friction coeffi-

cient with the increasing of the number of passes, Fig. 6. Fig. 6a and




Fig. 9. Effectof thenumber ofsliding passesin thespectraof thealuminium rough-

ness profiles forthe D3/AA1100 pair (AR - as-received AA1100 sheet).

b displays the evolution of the tests corresponding to the 1st and

5th passes respectively for the pair D3/DP600 and D3/Al1100.

A significant decrease of the friction coefficient has been ver-

ified in both cases; however the pair D3/AA1100 demonstrated a

stronger reduction. In fact the friction coefficient tends to be very

small after the first four passes, Figs. 6a and b and 7. Blau [34]

identified several parameters contributing to the running-in phe-

nomenon. Among them, roughness is one of the most important,

especially on boundary-lubricated contacts, as is the present case.

In fact observing the wear tracks obtained in the AA1100 surfaceafter the 1st, 3rd and 5th passes, Fig. 8, it is clear that abrasion is

the main wear mechanism involved and that the surface becomes

smoother with theincreasein the numberof passes. To understand

this effect, the spectra of the roughness profiles were analysed for

the as-received AA1100 sheet and on the wear track after the 1st

and 3rd passes, Fig. 9. By these results one can conclude that the

running-in effectis strongly effectivein thereduction of the rough-

ness, especially the higher wavelength components. Furthermore,

the running-in effect occurs predominantly in the first pass. This

strong and quick effect of running-in could be influenced by the

higher value of pressure resulting from the point-contact geome-

try. The present study was restricted to the running-in effect of the

metal sheet;therefore, the results canonlybe used forunderstand-

ing the cases where the sheet contacts several times with the tool,as is the case of progressive metal forming.

3.4. Effect of lubrication regime

Sheet metal forming operations involve a wide range of contact

pressures and relative sliding speeds. Therefore, considering lubri-

cated contacts, which is the current practice, the friction depends

stronglyon both surface and oilproperties. TheStribeckcurve is the

usualway to analyse theevolutionof the friction coefficient accord-

ing to both operation parameters and lubricant characteristics. The

Stribeck curve plots the coefficient of friction as a function of the

Hersey parameter H , defined as H = v/ p, where is the dynamic

viscosity of the lubricant, v the speed and p the apparent contact

pressure.

Table 5

Draw-bead test conditions to study lubrication effect.

Sliding speed [mm/s] Dynamic viscosity [mPa s] H [m]

1.00 28.03 1.44E-11

1.00 39.20 1.96E-11

2.00 39.20 4.00E-11

3.00 28.03 4.39E-11

3.00 39.20 6.02E-11

3.00 40.71 6.51E-11

4.00 39.20 8.01E-115.00 39.20 1.01E-10

6.00 39.20 1.24E-10

6.00 60.18 1.93E-10

7.50 39.20 1.53E-10

7.50 60.18 2.41E-10

8.47 39.20 1.75E-10

8.47 40.71 1.82E-10

Fig. 10. Variation of the friction coefficient as a function of the contact conditions

using a Stribeck type curve.

Especially importantfor metal forming is to know the transition

conditionsboundary/mixed/hydrodynamic lubrication regimes. To

obtain a wide range of the Hersey parameter, as well as the stamp-

ing oil, three paraffinic industrial mineral oils were used and the

test speed was ranged from 1 to 8.3 mm/s. Table 5 summarises the

test conditions and the obtained Stribeck curve is plotted in Fig. 10.

4. Conclusions

Tribological characteristics involved in sheet metal forming

havebeen investigatedusing twoexperimentalapproaches: a load-

scanning type tester and a recently developed draw-bead type

device. The following conclusions can be drawn from this study:

1. Comparing the results obtained by both experimental tech-

niques loadscanning always producedlower friction values. This

difference could be due to the highest contact pressure on theload-scanning test.

2. Roughness of the die material has a significant effect on the fric-

tion coefficient.

3. A significant effect of reduction of friction by the running-in

effect has been achieved by multi-pass load-scanning tests. The

reduction of the friction occurs especially by the attenuation of

the roughness components with high wavelength.

Acknowledgement

The authors are grateful to the Portuguese Foundation for Sci-

ence and Technology (FCT) for the financial support for this work

(project PTDC/EMETME/74152/2006).




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experimental study of friction in sheet metal forming