Defects and defect avoidance in cold forging.

1 EindhovenUniversityofTechnologyFacultyof MechanicalEngineeringDepartmentofProduct ionEngineeringandAutomation(WPA) :D3 WPAreportnr. , ,it widerangeofdefects,buttheycanbedividedin :dimensionalinaccuracies, , ,expertsystemsandneuralnetworkshowever, (massief) , , , , ,diegeschrevenisindeIaatstetienjaar, :Onderwerp:Toelichting:Omschrijving:1)2) (Defectsincoldforging). :breukofscheulVorming,ongewensteop-pelVl akteruwheid,onvlakheid,maatafwijkingen, , :gereedschapsslijtage,slechtesmering, , ) ' ,a ,onalocalaswellasa ,whichitselfisthekeyissuefortheeconomicd evelopmentofa ,ifrequired, reasonfortheincreasingpopularityofthecol dforgingprocesses,notonlyintheautomobile industrywhere80%ofthecoldforgedproductsa reused12) (3) strongpressuretowardsminimizationofcosts ; ; (orderquantity)percomponent,duetotheincr easingdiversityofshapes; (shorterspanoflife); ; ,insteadofusingseveralcomponents,compone ntsareintegratedintooneproduct; goodsurfacequality.

2 Thefirst, , , ,costswhichdecreasethecompetitivenessofa [ , ), [ :Ductility:theabilitytodeformplasticaHyw ithoutfractureina standardtest, :theabilityofa materialtodeformwithouttheoccurrenceofan ydefectina oftenreferredtoasthelimitofformabilityof thesheetmaterial, ,inthispapertheattentionisconfinedtoforg ingandwhenweareawareofthevariationsinusa geoftheabovetermsfromcountrytocountry,in dustrytoindustryaswellasbetweenindividua ls, ,givenabove, [ :thepropertiesofa productthatdonotconformtothedesignspecif ications, defectcanhavesuchoccurrencesascracksinth ecomponent, (7)hasmadea classificationoftheworkabilitylimitsfora llformingprocesses(figure1). , ,otherthancracking,suchasroughening( ),scratching, :shapeanddimensionalinaccuraciesandposit ionalerrorssuchasfolding,underfilling,no n-concentricproducts,fins, , , )canbeusedaswell,butonlyasa , )andJohnsonandMamalislllJ.]]]

3 ,Q)Q)~VlQ)Q)..Q).-'+ ~UE~_E!tiou~CQ)Q)>,ltIoltlltl0 ,+-'c.~..,CYcQ)Q)~~~.!!?>.. ;~>,"0ltI" ;g~~~..,Q)~~ ,Q):5' ~ ,"'Q)..,OJOaJQ).., ' )' :2-=Eoc'-Q)UcaJ<r~Db0=-"00' ,Q,QOJ0 VlVlOJ0'cN.!l1.~L 0'rn0cEE:::JQ) ,>,>, ~Q) ,..,.. "001000=.2S32OJ:::J'-UVl"0 VlQ)Q)ll)'+->,.cQ)! ""10:O-'~ >.<(])ll) ~Q)~rn>UlQ) \-'iiigE'+-UEE0>. ,> :!C(])~CY'+-'+-10-5ro0>EOJ'"alQ~al"OU"OL -=ll)..5ll).~0>..,:::J:::JOJg-.~(]).cOJC Q)UlQ) ,0 >lflbc::;;;EUEll)EU.~v.~~ro>.~ro>~~~~'li }Ern-'+-~Q).-~6Q)-'".~-'"._ll)._ltI:::JI O5-Q)S:.~5-;;=. " ;;::xQ)x'+-0" " ~-g.~:::J~Q)Q)"'.. )o<Ll.~g-5Q)g"5 (]) !'!10 EECDZ0:2v;-<;;:::JCI:cCI:"DltIltiCI:.u1O ::Jll)~.Dro :::JE~ uniquecauseofadefectortheymaybeinterlink ed, classificationofthepossiblecausesofdefec tsl81 ClThemetallurgicalnatureofthestartmetal, includingchemicalcomposition,mechanicala ndphysicalproperties,distribution,geomet ry andvolumefractionofsecondphasesorinclusi ons, , ,includinggeometry,heattreatment,wear,to olstiffnessandmechanicalpropertiesaswell astoolsurfacecoating(ifany).

4 C4 Tribologicalvariables, , ,includingsequence, , ,itcertainlyisinmostcasestheonlyonethati srecognizedasa Thisisa , (seefigure2) , ,forexample,areincludedinthe- forindustryimportant- processofupsettingl121 Upsettingisdefinedastheaxialcompressiono faworkpieceinordertoenlargethecross-sect ionalareaovereitherthewholeorpartofitsle ngthl13].6(I})(~~)(\~)ExternalExternalsh earExternalmixedlongitudinalcrackscracks cracksFigure2a-Crack,a , "upsettingoperations, , haslongbeenrecognizedthattheinducedtensi lestressesatthesurfacecanleadtopremature fracture[ , ,inliteraturealsocalledtheupsetability, (interfacial). (=height-diameterratio). showninfigure3[ ~I-tt~h~.err;..I-ttQI], )~crackseClraliaExternalmixedcracksExter nallongitudinalcracksCDWc(\jL+-'(f) :zFigure3 Influenceoffrictionandaspectratioonex- ,thefractureconditioncanbewrittensimplya sl13,241: e=a--!

5 L6j2(1)where efand ,forwhich e= - and3 , UsingtheLevy-VonMisesstress-strainincrem entrelations, ,LeeJVllllandothersll2,28-321haveconfirm edthatthisfractureconditionisvalidfora ] alsoshowsthatfracturepointsforbending,up settingandedgecrackingin8rolling,allfall onthesamestraightlinefora well-knownthattheductilityofa materialisstrain-historydependent(33).Th ishasbeenconfirmedinpapersbyBilligman[34 1,byGillandBaldwinn[35]andbyLuntz[361, [37).Effectofsurfacedefectsonupsetabilit yInindustrialupsettingandheadingoperatio nsonwireandrod,theworkabilitylimitisofte ndeterminedbythepresenceoflongitudinalsu rfacedefects,suchasgrooves, ,moreoften,frompriorformingprocessessuch aswiredrawing[ , , ,andthatvariationsinthedefectrootradiuso nlyhada guide totheworkabilitylimitofa newpart,careshouldbetakentoremainwithint herangeofpermissiblesurfacestrainsofa ,thesurfacestrainpathsshouldalwaysfallbe lowthefractureline(seefigure3).]]]

6 , ,eithera materialwitha higher"a-value"shouldbechosen, ,if fractureis foundtooccur, [38-40)showthatthisapproachisa [4lJalsofindsthatthisconditionis , , (j-Cracks(seefigure4)aredefinedaslongitu dinalcrackswhichoccuratthebottomoftheinn erpartofthespecimeninupsettingwitha 'nallongitudinalcracksFigure4{j-Crack,a crackwhichoccursatthebottomoftheinnerpar tofthespecimeninupsettingwitha (diameter,height,taper).Figures5 and6 showtheconditionsunderwhich{ (9( [9],[42]and[43]. ')'-crack(seefigure7)isdefinedasa .shearingcrackwhichappearsatthecornerofe xtrudedmaterialorasa surfacecrackinvariousprocesses[9J Theshearcrackoccursattheboundarybetweena de~ "?ltool%%~testpiece1013crack~~.. ~~.E~ ;..ETaper(< , >'(i) :::> :S45 CDimensionofspecimen:15d*30hLubricant:Jo hnsonwax0<!))]]]}}

7 L=1051015 Punchdiameterd (mm)Figure5 Distributionchartofcrackingtypes[91,doMa terial:S45 CDimensionofspecimen:15d*30hLubricant:Jo hnsonwax32---- '1craCKOJCl.<0f-1oo51013craCKTaper(q,)do -dh15---tooltestpiecePunchdiameterd (mm)Figure6 Distributionchartofcrackingtypes[91,Ashe arcrackisa wronglychosenpreviousheadingstep[261,The surfacecrackhasa widerangeofcauses:11 ShealclacksSUlfaceclacksFigure7-y Crack, ,shearcracks,thatoccuratthecorner[9]stre ssesinducedbyfinishingprocesses[26]skini nclusionsornon-metallicparts[26]previous processsteps[261lubricationexplosion[26] expansionoftheslugsurface[42]feedmarksth atareproducedwhentheslugsweremachined[42 ].Thesecrackscanbepreventedbyselectingsu itabledieandworkpiecedimensionsandbya showsresultsfromtestsdonebyOkamotoetal.[ ,Do,tothatofthediecavity,d, '1+ :2nocrackI'"crack1 Material:S45 CDimensionofspecimen:15d*30hLubricant:Jo hnsonwax00123Oo/dFigure8 Distributionchanof"1-ando-cracksintwodir ectionfreeextrusionwithoutsideconstraint [ (seefigure9)isdefinedasacrackwhichoccurs atmidheightofthematerialintheflangeintwo directionfreeextrusionwithoutsideconstra intl9J Thematerialisextrudedforwardandbackward; therefore, ,a !]]]]]]

8 9)concludefromthechartinfigure8thatthera tiooftheinitialdiameterofthespecimen,Do, tothatofthediecavity,d, [ [9]and[45]. (seefigure10)is definedasa crackthatoccursatmidheightorjustabovethe bottomradiusoftheinsidesurfaceandadvance sinthecircumferentialdirectioninupsettin g[ , [91, andtherapidchangeofthevelocityfieldinthe cornersofa backwardextrudedproduct[441 Thechangeofthevelocityinducesa :anannealingoperation[44)forwardinsteado fbackwardextrusion[44)taperedslugsinthec aseofforwardextrusion(44)anashighaspossi blehydrostaticpressure[441changingthepun chshape[91 FromthechartlDfigure6canbeconcludedthati ncaseofS45 Csteele-cracksoccurwhenthetaper,4>,ofthe punchissmallerthan1andthepunchdiameteris between5 [9]alsoconcludethatingenerale-crackingis observedbetween1'/and{3rangesincaseofa smallpunchtaper 1).]]]]]]]]}

9 (seefigure11) [ ].Figure11r-Crack, :choosinga materialwitha [47J raisingthehydrostaticpressurewithcounter -actingpressuresattheleadingendsofthebra nches[47].modifyingthedesignofthecross-s ectionofthebranches[ [47]and[48].Parketal.[46) ] "'crackAnl1-crack(seefigure12)isdefineda sa 'f'J-Crack,a crackontheupperandsidesurfacesoftheconca veportionoftheuppersurfaceofa (26),thepunchshapeorasurfacedefectcaused bya suitablepunchshape(9)orbya 'f' (9) [9],[26]and[42]. ,,-crackAK-crack(seefigure13)isdefinedas a ,,-Crack,a crackwhichisdefinedasa [49] , :atoolowductilityoftheworkpiecemateriald uetocoresegregationofinclusions; :insertionofanintermediateannealingstepi ntheextrusionsequence;selectionofanother dieanglefora givenreductionperpass;orselectionofa [ [Sljtrytopredicttheformationofa ,, , highsemiconeangleanda smallareareductionscaninduceasubstantial negativepressurealongtheaxisofthebillet, ,L-crack(seefigure14)isdefinedasa peripheralcrackthatoccursatthecentreof17 a ,a crackthatoccursatthecentreofa [9] (seefigure15)isdefinedasa ,a crackthatiscausedbya ] (seefigure16)isdefinedasa microscopiccrackwhichoccursattheboundary betweentheupperandlowerdeadmetalzonesand ata ,a :atoolowductilityofthematerial[ previousprocessstep[ [ beingdisplayedinliteratureaboutsurfaceim perfectionssuchasroughening, ,sothatthechoiceofalubricantandthekindof surfacepreparationstillis ( Defects )maybeexplainedthroughseveralrel atedmechanisms.]]]]]]

10 Tearingofweldments,plowing,a , ,fatigue,erosion,cavitation,andthermalef fects, ,a deeplyplowedgrooveiscreated(figure17).Th esegroovesexposefresh,uncontaminatedsurf aces,whichproducea , , ,becomeimbedded, , [S21 Whenthereis a strongchemicalaffinitybetweenthetoolmate rialandtheworkpiece, , ]Thedescriptionofplowingandtearingofweld mentsdemonstratestIlata ; verythinlayer,oneorderofmagnitudehighert hanthesurfaceroughness, ,asDautzenbergandKals[54)state, ,thewrinklesoftenfoldintocausea , metallurgicalpointofview, coarsegrainsizethiscanleadtoextremesurfa cerougheningduringthedeformationofthewor kpiece. ].Thelubricantpreventsthetoolsurfacefrom makingtheworkpiecesurfacesmoothand,conse quently,inducesdeteriorationoftheproduct ' workpiecematerialwitha smallgrainsizeora carefulheattreatmentofthebillet, , machiningoperationsaretobekeptata ,surfacetreatmentandsomegeneralguideline sconcerninglubricationcanbefoundinsevera lhandbooks, ( reportaboutlubricationaspectsincoldforgi ng[ widerangeofdefects,includingsuchdefectsa sbuckling,non-concentricity, , [56] [53} (materialstock).]]