Faculty of Technology and ScienceMaterials Engineering

Karlstad University Studies2008:10

Anders Gåård

Wear in sheet metal forming

Karlstad University Studies

2008:10

Anders Gåård

Wear in sheet metal forming

Anders Gåård. Wear in sheet metal forming

Licentiate thesis

Karlstad University Studies 2008:10ISSN 1403-8099ISBN 978-91-7063-168-9

© The author

Distribution:Karlstad UniversityFaculty of Technology and ScienceMaterials EngineeringSE-651 88 KarlstadSWEDENPhone +46 54 700 10 00

www.kau.se

Printed at: Universitetstryckeriet, Karlstad 2008

Abstra t

The general trend in the ar body manufa turing industry is towardslow-series produ tion and redu tion of press lubri ants and ar weight. Thelimited use of press lubri ants, in ombination with the introdu tion of highand ultra-high strength sheet materials, ontinuously in reases the demandsof the forming tools. To provide the means of forming new generations of sheetmaterial, development of new tool materials with improved galling resistan eis required, whi h may in lude tailored mi rostru tures introdu ing spe i� arbides and nitrides, oatings and improved surfa e �nish. In the presentwork, the wear me hanisms in real forming operations have been studied andemulated on a laboratory s ale by developing a test equipment. The wearme hanisms identi�ed in the real forming pro ess, were distinguished intoa sequen e of events onsisting of initial lo al adhesive wear of the sheetsresulting in transfer of sheet material to the tool surfa es. Su essive formingoperations led to growth of the transfer layer and initiation of s rat hing of thesheets. Finally, s rat hing hanged into severe adhesive wear, asso iated withgross ma ros opi damage. The wear pro ess was repeated in the laboratorytest equipment in sliding between several tool materials, ranging from astiron to onventional ingot ast tool steels to advan ed powder metallurgy toolsteel, against dual-phase arbon steel sheets. By use of the test-equipment,sele ted tool materials were ranked regarding wear resistan e in sliding againstferriti -martensiti steel sheets at di�erent onta t pressures.

Wear in sheet metal forming is mainly determined by adhesion; initiallybetween the tool and sheet surfa e and subsequently, after initiation of ma-terial transfer, between a sheet to sheet onta t. Atomi for e mi ros opyfor e urves showed that adhesion is sensitive to both hemi al ompositionand temperature. By alloying of iron with 18wt.% Cr and 8wt.% Ni, alloyingin itself, or hanges in rystal stru ture, led to an in rease of three times inadhesion at room temperature. Hen e, alloying may be assumed a promis-ing way for ontrol of adhesive properties. Additionally, fri tional heatingshould be ontrolled to avoid high adhesion as, generally, adhesion was foundto in rease with in reasing temperature for all investigated materials.

1

PREFACE

The work presented in this li entiate thesis has been arried out at the De-

partment of Me hani al- and Materials Engineering, Karlstad University.

First of all, I would like to thank my supervisors Pavel Krakhmalev and

Jens Bergström for their experien ed guidan e and support. Also, I thank

my olleagues at Karlstad University for support and pra ti al guidan e.

Thank you Magnus and Sture at Uppsala University. It was a great pleasure

doing work with you.

Se ondly, I would like to thank all parti ipating ompanies for supplying

of materials and fruitful dis ussions:

� Uddeholm Tooling AB

� Volvo Car Body Components, Olofström

� Swedish Steel AB

Finally, I would like to thank my family, Teresa and Alva for their on-

tinuous support and love.

2

List of en losed papers

This li entiate thesis omprises the following papers, whi h will be referred

to by their roman numerals

Paper I

A. Gåård, P. Krakhmalev and J. Bergström

"Wear me hanisms in deep drawing of arbon steel- orrelation to laboratory

testing"

Tribotest 14 (2008) 1

Paper II

M. Hanson, A. Gåård, S. Hogmark, P. Krakhmalev and J, Bergström

"Comparison of two test methods for evaluation of forming tool materials"

A epted for publi ation in Tribotest 14 (2008) 2

Paper III

A. Gåård, P. Krakhmalev and J. Bergström

"Galling resistan e of old work tool materials in sliding against arbon steel"

Tribology letters 26 (2006) 67

Paper IV

A. Gåård, J. Hirvonen Grytzelius, P. Krakhmalev, H.M. Zhang, J. Bergström

"Experimental study of the relationship between temperature and adhesive

for es for low-alloyed steel, stainless steel and titanium using atomi for e

mi ros opy in ultra-high va uum"

Submitted to Journal of Applied Physi s

Paper V

A. Gåård

"Wear in sheet metal forming- a literature review"

Karlstad University studies (2008)

3

Other publi ations

The author has also ontributed to the following papers, although they are

not in luded in this thesis

A

A. Gåård, P. Krakhmalev and J. Bergström

"Mi rostru tural hara terization and wear behavior of (Fe,Ni)-TiC MMC

prepared by DMLS"

Journal of alloys and ompounds 421 (2006) 166

B

P.V. Krakhmalev, J. Sukumaran and A. Gåård

"E�e t of mi rostru ture on edge wear me hanisms in WC-Co"

International Journal of Refra tory Metals and Hard Materials 25 (2007) 171

C

P.V. Krakhmalev, J. Sukumaran and A. Gåård

"How hardmetals rea t to wear: Nano is not always the best"

Metal powder report 62 (2007) 30 (Paper B, republished by the publisher)

4

Contents

Contents 5

1 Introdu tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Sheet metal forming (SMF) . . . . . . . . . . . . . . . . . . . . 9

2.1 SMF pro esses . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Tribology in SMF . . . . . . . . . . . . . . . . . . . . . 10

2.3 Fri tion in SMF . . . . . . . . . . . . . . . . . . . . . . 10

2.4 Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4.1 Adhesive wear . . . . . . . . . . . . . . . . . . 11

2.4.2 Abrasive wear . . . . . . . . . . . . . . . . . . . 12

2.5 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.6 Tool materials investigated in this study . . . . . . . . . 13

3 Wear in SMF, paper I . . . . . . . . . . . . . . . . . . . . . . . 14

4 Tribologi al testing, paper I-II . . . . . . . . . . . . . . . . . . 17

4.1 Slider-On-Flat-Surfa e (SOFS) tribometer . . . . . . . . 17

4.2 Comparison of di�erent test methods, SOFS and the

Uppsala Load-s anner . . . . . . . . . . . . . . . . . . . 19

5 Material ranking, paper III . . . . . . . . . . . . . . . . . . . . 22

6 Adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7 Final remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7.1 Pra ti al impli ation . . . . . . . . . . . . . . . . . . . . 28

8 Con lusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Bibliography 31

5

1 Introdu tion

In many of the appli ations used in daily life, surfa es are for ed in onta t

and moved relative to ea h other. Hen e, they are subje t for fri tion, wear

and/or surfa e damage to some extent. To ensure long-term reliability of a

system, these tribologi al phenomena have to be ontrolled, whi h is realised

by proper materials sele tion, surfa e modi� ation and lubri ation.

Depending on appli ation, di�erent tribologi al onditions prevail, whi h

are distinguished in so- alled losed and open systems. In the losed tribo-

systems, the same surfa es are involved in the pro ess over time and the

surfa es have the possibility to run-in under light loading, Fig. 1. During

running-in, oarse surfa e protrusions are smoothed, whi h prevents gross

initial wear and fa ilitates lubri ation. Therefore, the tribologi al onditions

are relatively well established and life length predi tion with reasonable a -

ura y is possible. In the open systems, one of the surfa es is always renewed

and the system is not able to run-in as the losed system. Generally, this

leads to severe tribologi al onditions and predi tion of wear is more om-

plex.

Figure 1: S hemati representation of roughness hanges during running-in

One of the major appli ations in whi h open tribo-systems exist is the

sheet metal forming (SMF) industry. In the intera tion between the tool and

the sheet the tool surfa e is stationary, while the sheet surfa e is renewed

at every new forming operation. Generally, the sheets possess a relatively

rough surfa e and onsequently, sin e there is virtually no running-in, wear

in sheet metal forming is sto hasti in nature and tool life length predi tions

are di� ult to make.

6

In the past, wear in press shops was limited by using an ex ess of lu-

bri ation, in addition to the existing lubri ant on the as-re eived sheet for

orrosion prote tion. However, e� ient lubri ant oils often ontain hlori-

nated para�n, whi h is relatively toxi and have a negative environmental

impa t, and today, the use of them is limited. A large fra tion of the SMF

industry is the automotive industry and the general trend in the ar body

manufa turing is towards low-series produ tion and redu tion of press lu-

bri ants and ar weight. The limited use of oils, in ombination with the

introdu tion of high and ultra-high strength sheet materials and light-weight

materials, su h as aluminium and titanium, has intensi�ed the development

of new tool materials and deeper investigations of wear me hanisms.

Today, extensive resear h is ondu ted on the tool/sheet intera tion to

optimise the tribologi al onditions. Generally, tool wear o urs due to trans-

fer and a umulation of sheet material onto the tool surfa es, referred to

as galling, Fig. 2. The adhered sheet material reates unstable fri tional

onditions, loss of dimensional toleran es and s rat hing of the sheet/tool

surfa es [1�3℄.

Figure 2: S hemati representation of the galling pro ess

To prote t the tool surfa es, development of new tool materials with im-

proved galling resistan e is required, whi h may in lude tailored mi rostru -

tures, introdu ing of spe i� (MC, M(C,N)) arbides and nitrides, oatings

and improved surfa e �nish [1, 3�15℄.

Several test methods are in use for tribologi al studies of the wear me h-

anisms involved in the pro ess, ranging from very simple laboratory meth-

ods, su h as the pin-on-dis set-up, to semi-industrial methods like the U-

bending- and bending under-tension (BUT) test equipments. However, the

semi-industrial methods require large quantities of spe ially prepared strip

7

oils, whi h make them rather expensive and time- onsuming, while labora-

tory tests in ompletely represent the tribologi al onditions in a real forming

pro ess [7, 12,16�21℄.

The obje tive of this thesis is to gain deeper understanding of the wear

me hanisms in sheet metal forming of arbon steel, by developing a test

equipment for simulation of the pro ess. The design of the equipment is

based on studies of real die- and sheet surfa es from the automotive indus-

try, to ensure that the tribologi al onditions are emulated, and by evaluation

of existing tribometers. The materials under investigation are arbon steel

sheets and several tool materials, ranging from ast nodular iron and on-

ventionally ingot ast tool steel to advan ed powder metallurgy tool steels.

Only dry test onditions were tested.

8

2 Sheet metal forming (SMF)

In the sheet metal forming pro ess, an initially �at sheet is plasti ally de-

formed into a desired shape by me hani al deformation. Several di�erent

types of SMF pro esses exist, but the most widely used are bending, stret h-

ing and deep drawing. Bending is found in most assembly industries due

to its �exibility and the two latter are often found in the forming of ar

body panels and tins and ups for the food industry. To optimise the SMF

pro ess and to ensure a su essful forming operation, the tribologi al- and

me hani al onditions are of great importan e.

2.1 SMF pro esses

In both stret hing and deep drawing pro esses, the sheets are lamped be-

tween a blank-holder and a die. The pro esses are distinguished by that

in stret hing, there is no material transport in the blank-holder area, Fig.

3, whereas in deep drawing, the sheets are allowed to slide from the blank-

holder area into the die avity, Fig. 4.

Figure 3: S hemati representation of the stret hing pro ess [22℄

The material transport during the forming pro ess has to be ontrolled to

avoid geometri al deviations and wrinkling of the work pie e and proper ma-

terial �ow is realised by a restraining for e, obtained by the fri tion between

the sheet and the blank-holder/die. In omplex tools, or where high re-

straining for es are required, drawbeads are used, whi h are semi- ylindri al

protrusions lo ated on the die. As the blank passes the drawbead it is sub-

je ted to a sequen e of bending, unbending and reverse bending that gives

rise to an additional restraining for e [1, 23�28℄.

9

Figure 4: Prin iple of the deep drawing pro ess [22℄

2.2 Tribology in SMF

Tribology is de�ned as the s ien e and te hnology of intera ting surfa es in

relative motion. By the intera tions, for es are transmitted, the surfa es

hemi al and physi al nature and topography is altered and energy is on-

verted. The onsequen es of the intera tions that take pla e at the interfa e

ontrol the fri tion, wear and lubri ation behavior. All the phenomena have

to be in luded as a system approa h, to understand the pro esses responsible

for fri tion and wear in a spe i� appli ation.

In deep drawing, the blank-holder and the die radius are regions sub-

je ted to material movement and are therefore of tribologi al interest. Gen-

erally, highest onta t pressures are developed at the die radius [29℄ and

onsequently wear is often more severe in that region. Finite element sim-

ulations show that the onta t pressure is errati ally distributed over the

die radius with hara teristi lo al maximum. Sheet metal thi kness, blank-

holder for e, fri tion oe� ient and die radius all in�uen e on the lo ation

and magnitude of the stress maximum.

2.3 Fri tion in SMF

Fri tion is the resistan e to motion when two surfa es in onta t move tan-

gentially relative to ea h other. As opposed to several other me hani al

appli ations, fri tion in deep drawing should not be minimised. The re-

straining for e is ne essary throughout the forming pro ess to ontrol the

movement and the plasti �ow of the sheet material. However, too high

fri tion for es lead to geometri al deviations of the formed sheet and to tool

damage. Therefore, it is of highest importan e to ontrol the pro ess to

ensure an a eptable produ t and to minimise tool wear.

10

The oe� ient of fri tion, a ording to Amontons-Coulomb law, is de�ned

as the ratio between the fri tion for e and the normal load and is assumed

independent of normal load, sliding speed and apparent area of onta t [30,

31℄.

µ =FF

FN

(1)

Additionally, the oe� ient of fri tion may be divided into two ompo-

nents, onsisting of a ontribution due to:

� Fri tion due to deformation of the softer surfa e by plowing or s rat h-

ing, µP

� Fri tion due to hemi al intera tions with formation of adhesive bonds

between mating surfa e asperities, µA

µ = µP + µA (2)

Changes in fri tion during SMF operations often o ur due to wear and

depending on wear me hanism, either of the two omponents may dominate.

Both deformation and adhesive me hanisms are dis ussed in detail in papers

I-III.

2.4 Wear

Wear is the removal of material from one, or both, of two solid surfa es in

moving onta t. For the forming industry, wear and surfa e damage su h as

ploughing and adhesion of sheet material, is detrimental for the tool perfor-

man e. Surfa e defe ts often a t as initiation points for wear or a elerate

the galling pro ess. As for fri tion, wear is a system response and altering

of any parameters may hange the operative wear me hanism. Similarly to

fri tion, wear ould o ur due to both adhesive and abrasive me hanisms.

2.4.1 Adhesive wear

During adhesive wear, parti les are transferred from one surfa e to the other

and are either permanently, or temporarily, atta hed to the surfa e. The

adhesive bonds are reated at the asperity onta ts in the interfa e Fig. 5,

11

Figure 5: Real area of onta t

whi h onstitutes the real area of onta t. When the surfa es are slid relative

to ea h other, the adhesive bond may break either at the interfa e, or in one

of the mating bodies. If breakage o urs in one of the materials, material is

transferred from one surfa e to the other. In [32℄, several di�erent regimes

of adhesive wear is dis ussed and distinguished into di�erent ategories de-

pending on the severity. During loading, the asperities deform, elasti ally, or

plasti ally, e�e ting the real area of onta t. For du tile materials, jun tion

growth during an imposed sliding motion, results in an in reased real onta t

area, whi h may lead to omplete seizure of the surfa es [30,31℄.

2.4.2 Abrasive wear

Abrasive wear o urs when a hard surfa e, or parti le, uts material away

from a softer ounter-surfa e. The me hanism is distinguished into two-body

abrasion, for example as in utting and in three-body abrasion, where the

abradant is a third loose parti le. Generally, two-body abrasion is responsible

for higher wear rates than three-body abrasion. In many ases, abrasive wear

is a result of adhesive wear, whi h may generate abrasive parti les possessing

high hardness by oxidation and deformation hardening phenomena.

2.5 Materials

Depending on sheet quality, di�erent types of tool materials are in use and

for low strength arbon steel sheets, ast nodular iron is often su� ient.

Cast tools possess a great e onomi al bene�t by o�ering near net shape

produ ts. Often, no pro essing steps ex ept for �nishing ma hining and

heat treatment, are ne essary. A disadvantage for the ast tools is that the

materials are subje ted to porosity and hemi al segregation. As the other

steels, ast iron is based on the Fe-C system, with the ex eption that the

arbon ontent is onsiderably higher, 2 wt.% or greater. The solidi� ation

of the arbon ri h melt provides formation of either ementite or graphite.

12

Alloying elements, with sili on as the major, are Al, B, Cu, Ni and Ti, whi h

strongly promote graphite formation [33℄.

For forming of medium- and high-strength sheet materials, old work tool

steels are used. The materials are distinguished into three types depend-

ing on alloying element ontent and a ording to AISI lassi� ation, these

are type [O℄, [A℄and [D℄ [34�36℄. All lasses have high arbon ontent for

hardness, but di�er in alloying element ontent whi h e�e ts hardenability,

arbide type and distribution. Type [O℄ steels are oil quen hed due to a rel-

atively low amount of alloying elements. High wear resistan e and hardness

is provided by high- arbon martensite. The high- arbon martensite is tem-

pered at low temperatures, resulting in �ne dispersions of arbides. Type [A℄

has omparable properties to types [O℄. But due to higher alloying element

ontent, hardenability is su� ient to permit martensite formation on air ool-

ing. The slow ooling rate minimises distortion and promotes dimensional

stability during heat treatment. AISI type [D℄ tool steels are high- arbon

and high hromium materials. Type [D℄ steels possess very high wear and

abrasion resistan e provided by large fra tions of alloy arbides. Some of the

alloy arbides are produ ed by solidi� ation and oexist with austenite dur-

ing austenitizing and some are produ ed during tempering. Type [D℄ steels

are hardenable in air, [33, 37℄.

Along with the onventionally ingot ast and forged old work tool steels,

powder metallurgy (PM) tool steels are used to some extent in the forming

industry. The PM materials omprise a mu h more homogeneous mi rostru -

ture with a more uniform dispersion of small re-enfor ements parti les.

2.6 Tool materials investigated in this study

The di�erent types of tool materials investigated in this study, where ast

nodular iron, D2 type tool steel and powder metallurgy tool steel, Fig. 6.

Figure 6: Typi al mi rostru tures of the materials used in the present work (mag-

ni� ation is not the same in order to illustrate the di�erent features). D2 old

work tool steel with relatively large and elongated arbides a), ast nodular iron

GGG70L with graphite nodules b) and powder metallurgy tool steel )

13

3 Wear in SMF, paper I

In sheet metal forming, wear is generally referred to as galling. Often, the

term is used to des ribe severe adhesive wear, hara terised by immense and

lo alised ma ros opi transfer of sheet material onto the tool surfa es. How-

ever, investigations of real deep drawing dies and sheets from the automotive

industry, paper I, showed that wear o ured as a sequen e of events, where

severe adhesive wear took pla e at the �nal stage. The investigated deep

drawing dies were made of ast nodular iron and tool steel, used in forming

of medium-strength arbon steel sheet material.

Examination of ma ros opi ally unworn sheet surfa es showed presen e

of mi ro-s rat hes in the sliding dire tion, Fig. 7. Most possibly, s rat hing

was a result of lo al transfer of sheet material to the tool surfa es and implies

that in the initial stage of the wear pro ess, wear is limited to lo al transfer

of sheet material to the tool surfa es.

Figure 7: Ma ros opi ally unworn real sheet surfa e from the automotive industry

ontaining mi ros rat hes and surfa e plasti deformation. The arrow indi ates the

sliding dire tion.

Additional forming operations led to growth of the transferred layer of

sheet material, with formation of lumps ausing ma ros opi s rat hing of

sheets. Finally, s rat hing was followed by transformation into sever adhesive

wear with hara teristi gross ma ros opi damage of the surfa es, Fig. 8.

Hen e, wear was de�ned as an a umulative pro ess, where adhesive and

abrasive me hanisms intera ted, whi h emphasises the importan e of emu-

lating an open tribo-system in laboratory simulation of SMF. The di�erent

stages of the galling pro ess were distinguished as;

14

Figure 8: Typi al appearan e of the wear pro ess observed on the formed DP600

sheet material. Abrasive s rat hing, followed by an in reasing amount of adhesive

wear with in reasing sliding distan e and �nal transformation into gross, severe, ad-

hesive wear a). Typi al appearan e of the abrasive wear-region b) and the adhesive

�nal wear-region ). The arrow indi ates the tool sliding dire tion.

1. The �rst stage, hara terised by initiation of transfer of sheet material

to the tool surfa e by lo al adhesive wear;

2. The se ond stage, where ma ros opi s rat hing of the sheets by the

lumps of transferred sheet material o ur;

3. The third stage, where the intensity of s rat hing hanges to severe

adhesive wear;

Examination of the real forming dies showed that build-up of sheet mate-

rial had o urred, and that the tools were subje t for abrasive s rat hing and

pit-like removal of material, Fig. 9. The damages observed are detrimental

as they a elerate the galling pro ess.

15

Figure 9: Opti al pro�lometry images of worn deep drawing die surfa es. Tool

steel with multiple abrasive s rat hes in the sliding dire tion and build-up of ad-

hered sheet material, as indi ated in the �gure a) and an abrasive s rat h with the

abradant left at the end of the s rat h on the ast iron material b) The arrows

indi ate the sliding dire tion.

16

4 Tribologi al testing, paper I-II

4.1 Slider-On-Flat-Surfa e (SOFS) tribometer

To perform laboratory simulations of tribologi al phenomenas, the onditions

in the a tual appli ation have to be well understood. Based on the results of

damaged real dies and sheets, paper I, a tribometer for simulation of wear in

SMF, has to emulate an open tribo-system and to rea h relatively long sliding

distan es allowing for a umulation of surfa e damage. The tribometer built

and used throughout the tribologi al testing in this resear h was a Slider-On-

Flat-Surfa e (SOFS) tribometer, Fig. 10, des ribed in detail in paper I. The

tool geometry used was a double- urved dis shaped spe imen, diameter

50 mm and edge radius 5 mm, slid under either onstant or ontinuously

in reasing normal load against a �at sheet material. At the end of a tra k,

the tool was lifted and returned to the position of the next tra k. In this way

unidire tional sliding was simulated up to 1000 m on a 1 x 1 m real sheet

material.

Figure 10: S hemati representation of the SOFS tribometer

Comparison of sheet and tool morphologies after SOFS-testing to the

morphology of the real worn sheet and dies, showed that the laboratory

17

test onditions were in good agreement to the onditions found in real SMF

operations. Wear of the sheets showed a similar appearan e, with initial

deformation of the sheet surfa e and lo al transfer of sheet material. Further

sliding lead to lump growth and s rat hing with �nal transformation into

severe adhesive wear, Fig. 11.

Figure 11: Evolution of the tra k morphology in SOFS testing of D2 tool mate-

rial and sheet material DP600 at a normal load of 300N. Initial �attening of the

tra k entre during a), transformation into b) and �nal transformation into severe

adhesive wear ). The arrows indi ate the sliding dire tion.

On the tools, transfer and build-up of sheet material was observed and

orrelated to the initiation of s rat hing, Fig. 12.

Figure 12: Typi al morphology of a SOFS tool surfa e showing transfer of sheet

material to the tool surfa e, SEM a) and opti al pro�lometry b).

By monitoring of the oe� ient of fri tion diagrams, the transformations

in wear me hanisms were distinguished as hanges in fri tion, with highest

values orresponding to severe adhesive wear, Fig. 13,

18

Figure 13: S rat h morphology at a hange in the fri tion diagram. S rat hing

hanges to severe adhesive wear

4.2 Comparison of di�erent test methods, SOFS and the

Uppsala Load-s anner

Apart from wear, other parameters su h as the oe� ient of fri tion and

lubri ation are of importan e for the SMF pro ess. Several di�erent tri-

bometers are urrently in use and one is the Uppsala Load-S anner (LS).

To exploit di�eren es and similarities between di�erent tribometers is im-

portant to establish the e�e t of the di�erent onditions. The LS is based

on the on ept of two rossed ylindri al rods, one usually representing the

tool and the other the work material, Fig. 14. The rods are slid against ea h

other under a ontinuously in reasing load. The sliding is arranged in su h

a way that ea h point along the sliding tra k on ea h spe imen represents a

unique load. The sliding distan e is very short, typi ally between 0.5 - 1.5

mm, and o urs in a dire tion 45 degrees from the extension of the sliding

tra k on the test rods. The appli able load range is 50 - 2500 N and the

sliding speed 0.001 - 0.1 m/s.

A omparison of results obtained using the SOFS and the LS tribometers

on tool steel sliding against tool steel in ontinuously in reasing loading, pa-

per II, showed that there are di�eren es between the two test equipments. In

the SOFS tribometer, wear o urred on all investigated material ombina-

19

Figure 14: S hemati representation of the Uppsala Load-S anner [17℄

tions, with type of wear me hanism depending on tool material ombination,

Fig. 15. Using the LS, wear o urred only for one material ombination and

for the other, fri tion was stable and no sign of wear, or surfa e damage, was

observed, Fig. 16. Both test methods displayed individually good repeata-

bility in terms of fri tion diagrams and wear me hanisms for ea h material

ouple and test blo k.

Figure 15: Wear at di�erent sliding distan es, beginning of the tra k a), interme-

diate b) and �nal distan e of 10 m ), of a D2 tool steel wheel slid against PM

tool steel plate using SOFS at ontinuously in reasing loading. Images orrespond

to fri tion diagram e) in Fig. 16

The di�eren es observed show that both equipments have advantages de-

pending on parameter of interest. In the SOFS, one surfa e is ontinuously

in onta t with the ounter-surfa e, while in the LS, both surfa es are re-

newed. Hen e, a umulation of surfa e damage in SOFS led to wear, whi h

ould not be dete ted using the LS. Therefore, the SOFS is more suitable for

20

investigation of wear in sheet metal forming. However, the LS is preferable

for investigations of fri tion at di�erent loads with di�erent lubri ants.

Figure 16: Coe� ient of fri tion for the LS (a- ) and SOFS (d-f) equipments in

testing at similar onditions and materials

21

5 Material ranking, paper III

As seen in paper I, a possible way of distinguishing materials regarding

galling resistan e is by observation of SOFS fri tion diagrams, where hanges

of the oe� ient of fri tion indi ate a hange in wear me hanisms, Fig. 17.

However, fri tion diagrams have to be omplemented by mi ros opy to or-

relate the hanges to the a tual wear me hanisms.

Figure 17: Fri tion diagram for two tool materials in sliding using the SOFS tri-

bometer a). The in rease in fri tion for the ast iron tool material GGG70L or-

responded to the onset of s rat hing due to transferred tool material on the tool

surfa e b)

Di�erent riteria ould be used to de�ne galling, i.e. initiation of s rat h-

ing or the on-set of severe adhesive wear. Dis-regardless of what is used, the

oe� ient of fri tion diagrams ould be used to extra t the riti al sliding

distan e to galling initiation. By SOFS-testing at di�erent onta t pressures,

it is possible to onstru t diagrams with onta t pressure versus riti al slid-

ing distan e to galling, to illustrate the behaviour of the materials.

In paper I, the SOFS was used for tool material ranking against medium-

strength ferriti -martensiti sheet material at di�erent onta t pressures.

The results are summarised in a onta t pressure versus riti al sliding

distan e-diagram in Fig. 18. The riterion used for tool failure was the

on-set of s rat hing of the sheets. The nodular iron aused s rat hing of the

sheets already at sliding distan es less than 100 m for the lowest load of 100

N. The powder metallurgy tool steel showed no s rat hing even after 1000

m of sliding at the highest load of 500 N while ast and forged tool steels

possessed intermediate performan e. The results were in good onjun tion

to industrial experien e.

22

Figure 18: Conta t pressure versus riti al sliding distan e to galling diagram. The

lines represent a mean value of the riti al sliding distan e until transition into the

unstable fri tion stage orresponding to s rat hing

Mi ros opy of the worn tool surfa es showed that all tool materials suf-

fered from substantial transfer of sheet material already after a few meters of

sliding, Fig. 19. However, as seen in Fig. 18, despite the material transfer,

the PM tool material did not ause s rat hing, even after 1000 m sliding.

Figure 19: SEM mi rographs of the Van ron 40 surfa e after 5, 15 and 130 m of

sliding (a- ) and the nodular iron surfa e after 5, 15 and 80 m of sliding (d-f) at a

normal load of 600 N. The arrows indi ate the dire tion of sliding.

23

For the nodular iron, initiation of materials transfer was related to de-

stroying of graphite nodules whi h were subsequently �lled with sheet ma-

terial, Fig. 20.

Figure 20: Graphite nodule before sliding a) and graphite nodule pull-out and �lling

of the void with sheet material after SOFS testing b).

24

6 Adhesion

Based on the �ndings in papers I and III, the main me hanism responsible

for the galling pro ess is related to adhesion at the asperity onta ts when

the two surfa es are brought into onta t. Adhesion auses initiation of sheet

material transfer and growth of the transfer layer. However, initiation and

growth have to be separated into two pro esses. The �rst is determined

by adhesion between the tool and the sheet, while growth depends on a

sheet/sheet onta t. Therefore, growth is expe ted to o ur at a similar

onditions if other me hanisms, su h as di�usion are negle ted.

As seen in paper III, the tool surfa es were almost oated by sheet mate-

rial. If layer growth o urs due to adhesion, the materials were expe ted to

ause s rat hing at a relatively similar way. However, this was not observed.

The materials behaved very di�erently and the PM material did not ause

s rat hing even at sliding distan es of 1000 m. Most possibly the transfer

layer was de-atta hed, whi h ould be due to low adhesion between the sheet

material and the tool surfa e.

Therefore, adhesion measurements on the asperity level were attra tive

to distinguish between materials regarding tenden y for sti king. In paper

IV, a STM/AFM apparatus was used to measure adhesive for es at room

temperature and at elevated temperatures in ultra-high va uum o urring

between stainless steel, low-alloyed arbon steel and pure titanium and a

Si-tip, Fig. 21.

Figure 21: Prin iple of adhesive for e measurements using AFM where the tip is

brought into onta t with the spe imen [38℄ a) and a typi al for e urve b)

At room temperature (RT), stainless steel and titanium possessed ap-

proximately three times higher adhesion ompared to low- arbon steel, Fig.

25

22. The RT observations are in good agreement to ma ros opi experien e,

where stainless steel and titanium, generally, are very prone to galling.

Figure 22: Adhesive fore versus temperature urves for titanium, f steel and b

steel against sili on

During sliding, fri tional heating auses a temperature rise of the sur-

fa es [31℄. As seen in Fig. 22, a signi� ant dependen e of adhesion on tem-

perature exists and a general trend of in reasing adhesion with temperature

was observed for the investigated materials. Similar trend has been observed

for SiC-SiC material ouples [39℄. However, at elevated temperatures, the

urves were shifted. But, to use the diagram for ma ros opi impli ations, it

should be noted that the materials have very di�erent thermal ondu tivity.

Hen e, at a given loading and similar oe� ient of fri tion, the materials

are on di�erent points on the temperature axis, with higher temperatures

favouring titanium and stainless steel.

The adhesion tenden y was related to the ele tron work fun tion (EWF)

of the materials. The EWF is the minimum energy required for an ele tron

to es ape from the Fermi level to a point outside the bulk metal. Higher

EWF implies a more inert surfa e and, hen e, adhesion was expe ted to

de rease. The EWF values for the investigated materials are illustrated in

Fig. 23. It is seen that higher adhesive for es were measured for materials

demonstrating lowest adhesive for e.

26

Figure 23: Ele tron work fun tion versus adhesive for e for pure titanium, f steel

and b steel

27

7 Final remarks

7.1 Pra ti al impli ation

In this thesis, it has been shown, that to fully simulate wear in SMF on a

laboratory s ale, the tribometer has to emulate an open tribosystem, with

the possibility of a hieving relatively long sliding distan es. Wear was har-

a terised as a multi-stage pro ess, where adhesive and abrasive me hanisms

intera ted due to a umulation of tool surfa e damage. The Slider-On-Flat-

Surfa e (SOFS) tribometer used in the present work, did repeat ertain on-

ditions in forming operations and is able of rea hing sliding distan es in the

kilometre range.

By using the SOFS, several tool materials were ranked regarding galling

resistan e and the results were in good onjun tion to industrial experien e.

However, maybe the most interesting observation was that all tools, irrespe -

tively of performan e, were subje t to transfer of sheet material, even though

di�eren es in the performan e were ten-fold. De-atta hment and removal of

the transferred layer was assumed as a possible explanation.

By investigation of several metalli materials regarding adhesive for es

using AFM, it was demonstrated that the materials possessed substantially

di�erent adhesion properties. Alloying of iron with 18% Cr and 8 % Ni, led to

substitution of iron atoms and transition of b to f rystal stru ture, that

led to three times rise in adhesive for e. Hen e, alloying may be assumed

a promising way for ontrol of adhesive properties, but requires additional

resear h. Supporting this approa h indire tly, di�erent hemi al omposition

of the investigated tool materials in paper III may be an explanation to de-

atta hment of the transfer layer of the PM material due to low adhesion.

Finally, AFM adhesion measurement showed that adhesion was strongly

related to temperature and, therefore, fri tional heating should be mini-

mized, whi h ould be realized by using tools with high thermal ondu tivity

to dissipate the heat.

28

8 Con lusions

The following major on lusions an be drawn from the present resear h:

� The main me hanism ontrolling wear in sheet metal forming is adhe-

sion

� Galling is a multi-stage pro ess hara terised as:

1. Initiation of lo al transfer of sheet material to the tool surfa e

2. Growth of the transfer layer and subsequent s rat hing of the

sheets

3. Transition into severe adhesive wear

� The SOFS tribometer an be used as a laboratory test method simulat-

ing wear in sheet metal forming and to evaluate tool materials regarding

galling resistan e

� Adhesion was found sensitive for hemi al omposition. By alloying

of iron with 18wt.% Cr and 8wt.% Ni, alloying in itself, or hanges

in rystal stru ture, led to an in rease of 3 times in adhesion at room

temperature

� Adhesion for several metalli materials was found dependant on tem-

perature and, therefore, fri tional heating should be ontrolled to avoid

high and abrupt adhesion

29

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34

Wear in sheet metal forming

The general trend in the car body manufacturing industry is towards low-series produc-tion and reduction of press lubricants and car weight. The limited use of press lubricants, in combination with the introduction of high and ultra-high strength sheet materials, continuously increases the demands of the forming tools. To provide the means of forming new generations of sheet material, development of new tool materials with improved galling resistance is required, which may include tailored microstructures, introducing of specific(MC, M(C,N))carbides and nitrides, coatings and improved surface finish. In the present work, the wear mechanisms in real forming operations have been studied and emulated on a laboratory scale by developing a test equipment. The wear mechanisms identified in the real forming process, were distinguished into a sequence of events consisting of initial local adhesive wear of the sheets resulting in transfer of sheet material to the tool surfaces. Successive forming operations led to growth of the transfer layer and initiation of scratching of the sheets. Finally, scratching changed into severe adhesive wear, associated with gross macroscopic damage. The wear process was repeated in the laboratory test-equipment in sliding between several tool materials, ranging from cast iron to conventional ingot cast tool steels to advanced powder metallurgy tool steel, against dual-phase carbon steel sheets. By use of the test-equipment, selected tool materials were ranked regarding wear resistance in sliding against ferritic-martensitic steel sheets at different contact pressures.

Wear in sheet metal forming is mainly determined by adhesion; initially between the tool and sheet surface interaction and subsequently, after initiation of material transfer, between a sheet to sheet contact. Atomic force microscopy force curves showed that adhesion is sensitive to both chemical composition and temperature. By alloying of iron with 18wt.% Cr and 8wt.% Ni, alloying in itself, or changes in crystal structure, led to an increase of 3 times in adhesion at room temperature. Hence, alloying may be assumed a promising way for control of adhesive properties. Additionally, frictional heating should be controlled to avoid high adhesion as, generally, adhesion was found to increase with increasing temperature for all investigated materials.

Karlstad University StudiesISSN 1403-8099

ISBN 978-91-7063-168-9