Wear in sheet metal forming

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Faculty of Technology and Science Materials Engineering

Karlstad University Studies

2008:10

Anders Gåård

Wear in sheet

metal forming

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Karlstad University Studies 2008:10

Anders Gåård

Wear in sheet

metal forming

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Anders Gåård. Wear in sheet metal forming Licentiate thesis

Karlstad University Studies 2008:10 ISSN 1403-8099

ISBN 978-91-7063-168-9

© The author

Distribution:

Karlstad University

Faculty of Technology and Science Materials Engineering

SE-651 88 Karlstad SWEDEN

Phone +46 54 700 10 00 www.kau.se

Printed at: Universitetstryckeriet, Karlstad 2008

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The general trend in the ar body manufa turing industry is towards

low-seriesprodu tionandredu tionofpress lubri antsand ar weight. The

limiteduseofpresslubri ants,in ombinationwiththeintrodu tionofhigh

andultra-highstrengthsheetmaterials, ontinuouslyin reasesthedemands

oftheformingtools. Toprovidethemeansofformingnewgenerationsofsheet

material,developmentofnewtoolmaterialswithimprovedgallingresistan e

is required, whi hmayin lude tailored mi rostru turesintrodu ing spe i

arbides and nitrides, oatings and improved surfa e nish. In the present

work,thewearme hanismsinrealformingoperationshavebeenstudiedand

emulated ona laboratory s ale by developing atest equipment. The wear

me hanisms identied inthe real forming pro ess, were distinguished into

a sequen e of events onsisting of initial lo al adhesive wear of the sheets

resultingintransferofsheetmaterialtothetoolsurfa es. Su essiveforming

operationsledtogrowthofthetransferlayerandinitiationofs rat hingofthe

sheets. Finally,s rat hing hangedintosevereadhesivewear,asso iatedwith

grossma ros opi damage. Thewearpro esswasrepeatedinthelaboratory

test equipment insliding betweenseveral toolmaterials, ranging from ast

ironto onventionalingot asttoolsteelstoadvan edpowdermetallurgytool

steel,against dual-phase arbonsteelsheets. By useof thetest-equipment,

sele tedtoolmaterialswererankedregardingwearresistan einslidingagainst

ferriti -martensiti steelsheetsatdierent onta tpressures.

Wear insheetmetalforming is mainlydeterminedbyadhesion; initially

betweenthetoolandsheetsurfa eand subsequently,afterinitiation ofma-

terial transfer, between a sheet to sheet onta t. Atomi for e mi ros opy

for e urvesshowedthatadhesion issensitivetoboth hemi al omposition

andtemperature.Byalloyingofironwith18wt.%Crand8wt.%Ni,alloying

initself,or hangesin rystal stru ture,ledto anin reaseofthreetimes in

adhesion at room temperature. Hen e, alloyingmay be assumeda promis-

ing way for ontrol of adhesive properties. Additionally, fri tional heating

shouldbe ontrolledtoavoidhighadhesionas,generally,adhesionwasfound

toin reasewithin reasingtemperatureforallinvestigatedmaterials.

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The work presentedinthis li entiatethesis hasbeen arriedoutat theDe-

partmentof Me hani al-and MaterialsEngineering,KarlstadUniversity.

First ofall, Iwould like to thankmy supervisorsPavelKrakhmalevand

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.

ThankyouMagnus andStureatUppsalaUniversity. Itwasagreatpleasure

doing workwith you.

Se ondly,Iwouldliketo thankallparti ipating ompaniesfor supplying

of materials andfruitfuldis ussions:

UddeholmTooling AB

Volvo Car BodyComponents,Olofström

Swedish Steel AB

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

tinuoussupportand love.

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Thisli entiatethesis omprises thefollowing papers,whi h will be referred

tobytheir roman numerals

Paper I

A.Gåård,P.Krakhmalevand J.Bergström

"Wearme hanismsindeepdrawingof arbonsteel- orrelationtolaboratory

testing"

Tribotest 14 (2008)1

Paper II

M.Hanson, A.Gåård, S.Hogmark,P.Krakhmalev andJ, Bergström

"Comparisonof twotest methods for evaluationofforming tool materials"

A epted for publi ationinTribotest 14(2008) 2

Paper III

A.Gåård,P.Krakhmalevand J.Bergström

"Gallingresistan eof oldworktoolmaterialsinslidingagainst arbonsteel"

Tribologyletters 26 (2006)67

Paper IV

A.Gåård,J.HirvonenGrytzelius,P.Krakhmalev,H.M.Zhang,J.Bergström

"Experimental study of therelationship betweentemperatureand adhesive

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

mi ros opyinultra-highva uum"

Submittedto Journalof AppliedPhysi s

Paper V

A.Gåård

"Wearinsheet metalforming- aliterature review"

KarlstadUniversitystudies(2008)

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The author hasalso ontributed to the following papers, although they are

notin ludedinthis thesis

A

A. Gåård,P.KrakhmalevandJ. Bergström

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

prepared byDMLS"

Journalof alloysand ompounds421 (2006) 166

B

P.V. Krakhmalev, J.Sukumaranand A.Gåård

"Ee tof mi rostru tureon edgewearme hanisms inWC-Co"

InternationalJournalofRefra toryMetalsandHardMaterials25(2007)171

C

P.V. Krakhmalev, J.Sukumaranand A.Gåård

"How hardmetalsrea tto wear: Nanoisnotalwaysthebest"

Metal powder report62 (2007) 30(PaperB,republished bythepublisher)

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Contents 5

1 Introdu tion . . . 6

2 Sheet metalforming(SMF). . . 9

2.1 SMFpro esses . . . 9

2.2 TribologyinSMF . . . 10

2.3 Fri tion inSMF . . . 10

2.4 Wear . . . 11

2.4.1 Adhesivewear . . . 11

2.4.2 Abrasive wear. . . 12

2.5 Materials . . . 12

2.6 Toolmaterials investigated inthis study. . . 13

3 WearinSMF,paperI . . . 14

4 Tribologi al testing,paperI-II . . . 17

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

4.2 Comparison of dierent test methods, SOFS and the Uppsala Load-s anner . . . 19

5 Material ranking, paperIII . . . 22

6 Adhesion . . . 25

7 Final remarks . . . 28

7.1 Pra ti alimpli ation. . . 28

8 Con lusions . . . 29

Bibliography 31

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In manyof theappli ations usedindailylife,surfa es arefor edin onta t

and moved relative to ea hother. Hen e, they aresubje t forfri tion,wear

and/or surfa e damage to some extent. Toensure long-term reliabilityof a

system,thesetribologi alphenomenahavetobe ontrolled,whi hisrealised

bypropermaterials sele tion,surfa e modi ation andlubri ation.

Dependingonappli ation,dierenttribologi al onditionsprevail,whi h

are distinguished inso- alled losedand open systems. Inthe losedtribo-

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

initialwearandfa ilitateslubri ation. Therefore,thetribologi al onditions

are relatively well establishedand life length predi tion withreasonable a -

ura yispossible. Intheopensystems,oneofthesurfa esisalwaysrenewed

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.

Figure1: S hemati representationofroughness hangesduring running-in

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

sheetmetalforming(SMF)industry. Intheintera tionbetweenthetooland

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 thereis virtuallyno running-in,wear

insheetmetalformingissto hasti innatureandtoollifelengthpredi tions

are di ultto make.

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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-

natedparan, whi h is relatively toxi and have a negative environmental

impa t, and today,theuse of them islimited. A large fra tion of theSMF

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 tionofhighandultra-highstrengthsheetmaterialsandlight-weight

materials,su hasaluminiumandtitanium, hasintensiedthedevelopment

ofnew tool materialsand deeperinvestigations ofwear me hanisms.

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

optimisethetribologi al onditions. Generally,toolwearo ursduetotrans-

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[13℄.

Figure2: S hemati representationofthegallingpro ess

To prote tthetoolsurfa es, developmentof newtoolmaterialswithim-

provedgallingresistan eisrequired,whi hmayin ludetailoredmi rostru -

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

andimproved surfa e nish[1,315℄.

Several testmethodsareinusefortribologi alstudiesof thewearme 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

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torytestsin ompletelyrepresentthetribologi al onditionsinarealforming

pro ess [7,12,1621℄.

The obje tive of this thesis is to gain deeperunderstanding of thewear

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 esfrom the automotive indus-

try,toensurethatthetribologi al onditionsareemulated,andbyevaluation

of existingtribometers. The materials underinvestigationare arbon steel

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

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

Only drytest onditions weretested.

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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 dierent

typesofSMFpro essesexist,butthemostwidelyusedarebending,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

bodypanels and tins and upsfor the food industry. To optimise theSMF

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

me hani al onditions areof greatimportan e.

2.1 SMF pro esses

In both stret hing and deep drawing pro esses, the sheetsare 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 deepdrawing, thesheets areallowed to slide fromthe blank-

holderarea into thedie avity,Fig. 4.

Figure3: S hemati representationofthestret hingpro ess[22℄

Thematerialtransportduringtheformingpro esshastobe ontrolledto

avoidgeometri aldeviationsandwrinklingoftheworkpie eandproperma-

terialowisrealisedbyarestrainingfor e,obtained bythefri tionbetween

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

strainingfor esarerequired,drawbeadsareused,whi haresemi- ylindri al

protrusionslo ated onthe die. Astheblank passesthedrawbead itis sub-

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

riseto an additional restrainingfor e [1,2328℄.

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2.2 Tribology inSMF

Tribologyis denedasthes ien e andte hnologyof intera ting surfa esin

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 esoftheintera tionsthattakepla eattheinterfa e

ontrol thefri tion,wearandlubri ationbehavior. Allthephenomenahave

tobein ludedasasystemapproa h,tounderstandthepro essesresponsible

for fri tion andwearinaspe i appli ation.

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

je ted tomaterialmovementand arethereforeoftribologi alinterest. 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

dieradius with hara teristi lo almaximum. Sheetmetalthi kness, blank-

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

and magnitude ofthestress maximum.

2.3 Fri tion in SMF

Fri tionis theresistan eto motionwhentwosurfa es in onta tmovetan-

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 esleadtogeometri al deviationsoftheformedsheetandto tool

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

ensure an a eptableprodu tand to minimise toolwear.

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asthe ratio betweenthe fri tion for e and thenormal load and is assumed

independent ofnormalload, sliding speedand apparentareaof onta t[30,

31℄.

µ = F ^F

F ^N

⁽¹⁾

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

nents, onsistingof a ontribution due to:

Fri tiondueto deformationofthesofter surfa ebyplowing ors rat h-

ing,

µ P

Fri tiondue to hemi alintera tionswithformationof adhesivebonds

betweenmatingsurfa e asperities,

µ A

µ = µ P + µ A

⁽²⁾

Changes infri tion during SMFoperations ofteno urdue to wearand

dependingonwearme hanism,eitherofthetwo omponentsmaydominate.

Bothdeformationandadhesiveme hanismsaredis ussedindetailinpapers

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 theformingindustry,wearandsurfa edamage su has

ploughingand adhesionofsheetmaterial, isdetrimentalfor thetoolperfor-

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

thegalling pro ess. As for fri tion, wear is a systemresponse and altering

ofany parametersmay hangetheoperative wearme hanism. Similarlyto

fri tion,wear ouldo urdue to bothadhesiveand abrasiveme hanisms.

2.4.1 Adhesivewear

Duringadhesivewear,parti lesaretransferredfromonesurfa etotheother

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

adhesive bondsare reatedat theasperity onta ts inthe interfa e Fig. 5,

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whi h onstitutestherealareaof onta t. Whenthesurfa esareslidrelative

to ea hother,theadhesivebondmaybreakeitherat theinterfa e,or inone

of thematingbodies. Ifbreakageo urs inone ofthematerials,materialis

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

of adhesive wear is dis ussed and distinguished into dierent ategories de-

pendingontheseverity. Duringloading,theasperitiesdeform,elasti ally,or

plasti ally,ee ting therealareaof onta t. For du tilematerials,jun tion

growthduringanimposedslidingmotion,resultsinanin reasedreal onta t

area, whi h mayleadto omplete seizureof thesurfa es[30,31℄.

2.4.2 Abrasive wear

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

fromasofter ounter-surfa e. Theme hanismisdistinguishedintotwo-body

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

abradantisathirdlooseparti le. Generally,two-bodyabrasionisresponsible

forhigherwearratesthanthree-bodyabrasion. Inmany ases,abrasivewear

isaresultofadhesivewear,whi hmaygenerateabrasiveparti lespossessing

high hardnessbyoxidation anddeformation hardeningphenomena.

2.5 Materials

Dependingon sheet quality, dierent types of tool materials arein useand

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

Cast tools possess a great e onomi al benet by oering near net shape

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

heat treatment,arene essary. A disadvantagefor the ast tools isthat the

materials aresubje tedto porosityand hemi al segregation. As theother

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

arbon ontentis onsiderably higher,2 wt.% or greater. The solidi ation

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

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stronglypromote graphiteformation[33℄.

Forformingofmedium-andhigh-strengthsheetmaterials, oldworktool

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℄ [3436℄. All lasses have high arbon ontent for

hardness, but dierin alloying element ontent whi h ee tshardenability,

arbidetype anddistribution. Type[O℄steels areoil quen hed dueto arel-

ativelylowamount ofalloying elements. Highwearresistan eand hardness

isprovided byhigh- arbon martensite. Thehigh- arbon martensiteis tem-

peredatlowtemperatures,resultinginnedispersionsof arbides. Type[A℄

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

ontent,hardenabilityissu ienttopermitmartensiteformationonair 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

abrasionresistan eprovidedbylargefra tionsofalloy arbides. Someofthe

alloy arbidesareprodu edbysolidi ationand oexistwithaustenite dur-

ingaustenitizing and some areprodu edduring tempering. Type [D℄ steels

arehardenableinair,[33,37℄.

Alongwiththe onventionallyingot astandforged oldworktoolsteels,

powder metallurgy (PM)toolsteels areused to some extentin theforming

industry. ThePMmaterials ompriseamu hmorehomogeneousmi rostru -

turewitha moreuniformdispersionofsmall re-enfor ementsparti les.

2.6 Tool materials investigated in this study

The dierent types of tool materials investigated in this study, where ast

nodular iron,D2typetoolsteeland powder metallurgy toolsteel,Fig. 6.

Figure6: Typi almi rostru turesofthematerialsusedin thepresentwork(mag-

ni ation is not the same in order to illustrate the dierent features). D2 old

work toolsteel with relatively largeand elongated arbides a), ast nodular iron

GGG70Lwithgraphitenodulesb) andpowdermetallurgytoolsteel )

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

term isusedtodes ribesevereadhesivewear, hara terisedbyimmenseand

lo alisedma ros opi transferofsheetmaterialontothetoolsurfa es. How-

ever,investigationsofrealdeepdrawingdiesandsheetsfromtheautomotive

industry, paper I, showed that wearo ured asa sequen e of events, where

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

drawing dies weremade of ast nodular ironand toolsteel, usedinforming

of medium-strength arbonsteel sheetmaterial.

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

of mi ro-s rat hes intheslidingdire tion, Fig. 7. Most possibly,s rat hing

wasaresultoflo altransferofsheetmaterialtothetoolsurfa esandimplies

that intheinitialstage ofthewearpro ess,wearis limitedtolo al transfer

of sheet materialto thetoolsurfa es.

Figure7: Ma ros opi allyunwornrealsheetsurfa efromtheautomotiveindustry

ontainingmi ros rat hesandsurfa eplasti deformation. Thearrowindi atesthe

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 hingwasfollowedbytransformationintoseveradhesive

wearwith hara teristi gross ma ros opi damage ofthesurfa es, Fig. 8.

Hen e,wearwasdenedasana umulative pro ess,whereadhesiveand

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

lating an open tribo-system inlaboratory simulationof SMF. The dierent

stages ofthegalling pro ess were distinguishedas;

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sheetmaterial. Abrasives rat hing, followedbyanin reasingamountof adhesive

wearwithin reasingslidingdistan eandnaltransformationintogross,severe,ad-

hesiveweara). Typi alappearan eoftheabrasivewear-regionb)andtheadhesive

nalwear-region ). Thearrowindi atesthetoolsliding dire tion.

1. Therst stage, hara terised byinitiationof transferofsheet material

to thetoolsurfa e bylo al adhesive wear;

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

lumpsoftransferred sheet materialo ur;

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

adhesive wear;

Examinationoftherealformingdiesshowedthatbuild-upofsheetmate-

rialhado urred,andthatthetoolsweresubje tforabrasives rat hingand

pit-like removalof material, Fig. 9. The damagesobserved are detrimental

astheya eleratethegalling pro ess.

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

heredsheetmaterial,asindi atedinthegurea)andanabrasives rat hwiththe

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

indi ate theslidingdire tion.

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

Toperformlaboratorysimulationsoftribologi alphenomenas,the onditions

inthea tualappli ationhavetobewellunderstood. Basedontheresultsof

damagedrealdiesandsheets,paperI,atribometerforsimulationofwearin

SMF,hastoemulateanopentribo-systemandtorea hrelativelylongsliding

distan esallowingfora umulationofsurfa edamage. Thetribometerbuilt

andusedthroughoutthetribologi altestinginthisresear hwasaSlider-On-

Flat-Surfa e(SOFS)tribometer,Fig. 10,des ribedindetailinpaperI. 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 reasingnormalload against a atsheet material. At the end of a tra k,

thetoolwasliftedandreturnedtothepositionofthenexttra k. Inthisway

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

material.

Figure10: S hemati representationoftheSOFStribometer

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

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

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

deformationofthesheetsurfa eandlo altransferofsheetmaterial. Further

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

severeadhesive 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 normalload of300N. Initial attening of the

tra k entreduring a),transformationintob) andnaltransformationintosevere

adhesivewear ). Thearrowsindi ate theslidingdire tion.

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

orrelatedto theinitiation ofs rat hing,Fig. 12.

Figure 12: Typi almorphology of a SOFS toolsurfa e showing transfer of sheet

materialto thetoolsurfa e,SEMa) andopti alprolometryb).

Bymonitoringofthe oe ientoffri tiondiagrams,thetransformations

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

values orrespondingto severeadhesivewear,Fig. 13,

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hangestosevereadhesivewear

4.2 Comparison of dierent test methods, SOFSand 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 dierent tri-

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

To exploit dieren es and similarities between dierent tribometers is im-

portant to establish the ee t of the dierent onditions. The LS is based

onthe on ept oftwo rossed ylindri alrods, one usually representing the

toolandtheothertheworkmaterial,Fig. 14. Therodsareslidagainstea h

otherunder a ontinuously in reasingload. The sliding is arranged insu h

awaythatea hpoint alongthesliding tra konea hspe imenrepresentsa

unique load. The sliding distan e is very short, typi ally between0.5 - 1.5

mm,and o urs ina dire tion 45 degrees from the extensionof thesliding

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

slidingspeed0.001 -0.1 m/s.

A omparisonofresultsobtainedusingtheSOFSandtheLStribometers

ontoolsteelslidingagainsttoolsteelin ontinuouslyin reasingloading,pa-

perII,showedthattherearedieren esbetweenthetwotestequipments. In

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

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tions,withtypeofwearme hanismdependingontoolmaterial ombination,

Fig. 15. UsingtheLS,wearo urredonlyforone material ombinationand

fortheother,fri tionwasstableandnosignofwear,orsurfa edamage,was

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

bilityin terms of fri tion diagrams and wearme hanisms for ea h material

ouple and testblo k.

Figure 15: Wearat dierent sliding distan es,beginningof thetra ka), interme-

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

toolsteelplateusingSOFSat ontinuouslyin reasingloading. Images orrespond

to fri tiondiagrame)in Fig. 16

Thedieren esobservedshowthatbothequipmentshaveadvantagesde-

pending onparameter of interest. In theSOFS, 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 umulationof surfa edamage inSOFS ledto wear,whi h

ouldnotbedete tedusingtheLS.Therefore,theSOFSismoresuitablefor

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forinvestigationsof fri tionat dierentloads withdierentlubri ants.

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

testingatsimilar onditionsandmaterials

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

gallingresistan eisbyobservationofSOFSfri tiondiagrams,where hanges

of the oe ient of fri tionindi ate a hangeinwear me hanisms, Fig. 17.

However, fri tiondiagrams have to be omplementedbymi ros opyto or-

relate the hanges to thea tual wearme hanisms.

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

bometer a). Thein reasein fri tion for the astiron tool materialGGG70L or-

respondedto theonset of s rat hing due to transferred tool material onthe tool

surfa e b)

Dierent riteria ouldbeusedtodenegalling,i.e. initiationofs rat h-

ingor theon-setofsevereadhesivewear. Dis-regardlessofwhat isused, the

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

distan etogallinginitiation. BySOFS-testingatdierent onta tpressures,

itispossibleto onstru tdiagramswith onta tpressureversus riti alslid-

ing distan eto galling,to illustratethebehaviourofthe materials.

InpaperI,theSOFSwasusedfor toolmaterialrankingagainstmedium-

strength ferriti -martensiti sheet material at dierent 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-setofs rat hingofthesheets. Thenodulariron auseds rat hingofthe

sheetsalreadyat slidingdistan eslessthan100 mfor thelowestloadof100

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.

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linesrepresentameanvalueofthe riti alslidingdistan euntiltransitionintothe

unstablefri tionstage orrespondingtos rat hing

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

feredfromsubstantialtransferofsheetmaterialalreadyafterafewmetersof

sliding,Fig. 19. However, asseen inFig. 18, despitethe materialtransfer,

thePMtoolmaterialdidnot ause s rat hing, even after1000msliding.

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

sliding(a- )andthenodularironsurfa e after5,15and 80m ofsliding(d-f)ata

normalload of600N.Thearrowsindi atethedire tionofsliding.

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

terial,Fig. 20.

Figure20: Graphitenodulebeforeslidinga)andgraphitenodulepull-outandlling

ofthevoidwithsheetmaterialafterSOFStestingb).

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Based on thendings inpapers Iand III, the main me hanismresponsible

for the galling pro ess is related to adhesionat theasperity onta ts when

thetwosurfa esarebroughtinto onta t. Adhesion ausesinitiationofsheet

materialtransfer 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

onditionsif other me hanisms,su hasdiusion arenegle ted.

AsseeninpaperIII,thetoolsurfa eswerealmost oatedbysheetmate-

rial. If layergrowth o urs due to adhesion, thematerials wereexpe ted to

auses rat hingatarelatively similarway. However,thiswasnotobserved.

The materials behaved very dierently and thePM material did not ause

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

layerwasde-atta hed,whi h ouldbeduetolowadhesionbetweenthesheet

materialandthetoolsurfa 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 measurementsusing AFM where thetip is

broughtinto onta twiththespe imen[38℄a)anda typi alfor e urveb)

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

proximatelythree timeshigher adhesion omparedto low- arbonsteel, Fig.

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wherestainless steeland titanium,generally,are veryproneto galling.

Figure 22: Adhesiveforeversustemperature urvesfortitanium,f steelandb

steel againstsili on

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

fa es [31℄. Asseenin Fig. 22, asigni ant dependen eof adhesionon tem-

peratureexistsand ageneraltrendofin reasingadhesionwithtemperature

wasobservedfortheinvestigatedmaterials. Similartrendhasbeenobserved

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

urveswereshifted. But,to usethediagramforma ros opi impli ations, it

should benotedthatthematerialshaveverydierentthermal ondu tivity.

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

are on dierent points on the temperature axis, with higher temperatures

favouring titanium andstainless steel.

Theadhesiontenden ywasrelatedto theele tron workfun tion(EWF)

of thematerials. The EWF istheminimumenergyrequired for anele 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 lowestadhesivefor e.

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andb steel

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7.1 Pra ti al impli ation

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

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

thepossibilityofa hievingrelativelylong slidingdistan es. Wear was har-

a terised asamulti-stagepro ess,whereadhesive andabrasive me hanisms

intera teddue toa umulationoftoolsurfa edamage. TheSlider-On-Flat-

Surfa e(SOFS)tribometerusedinthepresentwork,didrepeat ertain on-

ditions informingoperationsand isableof rea hingslidingdistan es inthe

kilometre range.

Byusing theSOFS, several toolmaterials were ranked regardinggalling

resistan eand theresults wereingood onjun tionto industrialexperien e.

However,maybethemostinterestingobservationwasthatalltools,irrespe -

tivelyofperforman e,weresubje ttotransferofsheetmaterial,eventhough

dieren esintheperforman ewereten-fold. De-atta hmentand removalof

thetransferred layerwasassumedasa possible explanation.

By investigation of several metalli materials regarding adhesive for es

using AFM, itwasdemonstratedthat thematerialspossessed substantially

dierentadhesionproperties. Alloyingofironwith18%Crand8%Ni,ledto

substitutionofironatomsandtransitionofb tof rystalstru 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. Supportingthisapproa hindire tly,dierent hemi al omposition

of theinvestigated toolmaterialsinpaperIIImaybean explanationto de-

atta hmentof thetransferlayerof thePMmaterial dueto low adhesion.

Finally,AFM adhesionmeasurement showed thatadhesion wasstrongly

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

mized,whi h ouldberealizedbyusingtoolswithhighthermal ondu tivity

to dissipatetheheat.

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Thefollowing major on lusions an bedrawn fromthepresent resear h:

The mainme hanism ontrolling wearinsheet metal formingis adhe-

sion

Gallingisa multi-stage pro ess hara terisedas:

1. Initiation oflo al transferofsheet materialto thetoolsurfa e

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

sheets

3. Transition into severeadhesivewear

TheSOFStribometer anbeusedasalaboratorytestmethodsimulat-

ingwearinsheetmetalformingandtoevaluatetoolmaterialsregarding

gallingresistan 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, ledto an in rease of 3 times inadhesion at room

temperature

Adhesion for several metalli materials was found dependant on tem-

peratureand,therefore,fri tionalheatingshouldbe ontrolledtoavoid

highand abruptadhesion

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(34)

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