Friction, w ear and lubrication during sheet metal forming

4.2.3 Friction, w ear and lubrication during sheet metal forming

As p art icu larly im p o rt an t areas o f t ribo lo gy, frict io n , wear an d lu brica- t io n h ave a sign ifican t in flu en ce o n p ro cess en gin eerin g relat ed t o sh eet m et al fo rm in g. Th e p ract ice h as sh o wn t h at by a p u rp o sefu l ap p licat io n o f ap p ro p riat e d ie m at erials, t o get h er wit h p ro cess-o p t im ized lu bri- can t s, wear o f fo rm in g an d blan kin g d ies can be sign ifican t ly red u ced . Friction Frict io n p lays a m ajo r ro le in sh eet m et al fo rm in g. Frict io n is t h e resu lt o f in t ern al d efo rm at io n o f t h e d efo rm ed m at erial an d o f ext ern ally ap p lied lo ad s. In it s d ifferen t m an ifest at io n s, frict io n resu lt s in co m p lex wear m ech an ism s at t h e co n t act su rfaces bet ween t h e d ie an d t h e wo rk- p iece. Du rin g fo rm in g an d sh ear cu t t in g o p erat io n s, slid in g frict io n co n d it io n s ap p ly wh ere lu bricat io n is u sed . In d escribin g frict io n in sh eet m et al fo rm in g, as a ru le, t h e co efficien t o f frict io n µ [–] is u sed an d it is exp ressed as t h e frict io n fo rce d ivid ed by t h e n o rm al fo rce, in t h e fo rm : 179 Deep draw ing and stretch draw ing Titanium and titanium alloys M aterial R m R p0,2 or Fields of application, No. [Nmm 2 ] R eL [Nmm 2 ] A 80 [] examples chemical equipment construction, electro- deposition, aircraft and Ti 99,8 3.7025.10 290...410 min. 180 30 spacecraft construction, surgery and orthopaedics: protheses, bone screw s, implants and splints Ti 99,7 3.7035.10 390...540 min. 250 22 dito mechanical engineering, aircraft and spacecraft TiAl 6 V 4 F 89 3.7165.10 min. 890 min. 820 6 construction, electrical engineering, optics, precision mechanical engineering, medicine technology, fittings mechanical engineering, TiAl 5 Sn 2 F 79 3.7115.10 min. 790 min. 760 6 aircraft and spacecraft construction, fittings µ = [ ] F F or R N – Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 t h e frict io n al sh ear st ress d ivid ed by n o rm al co n t act st ress At lo wer slid in g velo cit ies, d ry o r m axim u m frict io n co n d it io n s ap p ly; t h at is t o say t h e d ies t h e wo rkp iece are p rim arily sep arat ed by t h e reac- t io n o r ad so rp t io n layers – so -called bo u n d ary layers – o f a m o lecu lar o rd er o f m agn it u d e, wh ereby so lid bo d y frict io n is also p o ssible in su b- m icro sco p ic ran ges. Here, t h e wo rkp iece slid es o n lu bricat in g film s o f m o lecu lar m agn it u d e, i. e. o n t h ese bo u n d ary layers. In t h e absen ce o f su fficien t lu bricat io n , t h e bo u n d ary layers can break u p lo cally an d weld jo in t s can d evelo p . Co efficien t s o f frict io n in t h e ran ge o f µ = 0.10 t o 0.15 are in d icat io n o f d ry frict io n co n d it io n s. Wit h an in crease in slid in g velo cit y, p o rt io n s o f t h e in t erface are su p - p o rt ed by h yd ro -d yn am ic flu id lu bricat io n . Th e t h ickn ess o f t h e lu bri- cat in g film in creases, bu t t h e lu bricat io n gap h [µ m ] st ill rem ain s sm all- er t h an t h e t o t al o f t h e su rface ro u gh n ess R t [µ m ] o f bo t h , t h e d ie an d t h e wo rkp iece, i. e.: Th e friction al forces F R [N] d rop sh arp ly an d com e close asym p totically to a lim itin g valu e, wh ich is th e tran sition p oin t to p u re h yd ro-d yn am ic frict io n . Th e in flu en ce o f slid in g velo cit y, n o rm al st ress an d lu brican t visco si- t y is at it s m axim u m in t h e area o f bo u n d ary o r m ixed frict io n . Here, t h e d ie wo rkp iece p air h as t h e t o p o grap h y o f t h e t ask o f h o ld in g t h e lu brican t fo r as lo n g as p o ssible d u rin g t h e slid in g act io n . As a ru le, all fo rm in g p ro cesses in vo lvin g bo u n d ary frict io n o p erat e wit h frict io n al co efficien t s o f µ = 0.05 t o 0.10. Wit h fu rt h er in creases in t h e slid in g velo cit y, co m p let e h yd ro -d yn am ic frict io n can be ach ieved in wh ich t h e lu brican t film h beco m es great er t h an t h e su m o f t h e su rface ro u gh - n ess R t o f bo t h wo rkp iece an d d ie. Th is co n d it io n , h o wever, can n o t be ach ieved in p ract ical m et al fo rm in g o p erat io n s: 180 Sheet metal forming and blanking µ τ σ = [ ] R N – h R R t die t workpiece + h R R t die t workpiece + Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 High lo cal p last ic d efo rm at io n s o ccu r d u rin g frict io n in sh eet fo rm in g, i. e. wit h t h e alt erat io n o f t h e su rface o f t h e wo rkp iece a t ran sfo rm at io n in t h e t o p o grap h y t akes p lace. Su rface t ran sfo rm at io n t akes p lace d ep en d in g u p o n t h e st at e o f st ress, st ren gt h o f t h e sh eet m et al m at erial an d o f t h e grain size an d align m en t . Th is su rface t ran sfo rm at io n wh ich can alt er co n sid erably t h e am o u n t o f lu brican t t h at is ret ain ed at t h e d ie m at erial in t erface. Th ese m ech an ism s receive m in im u m co n sid era- t io n in t h e laws o f frict io n , as d o es t h e in flu en ce o f frict io n al t em p era- t u re an d p ressu re o n lu brican t visco sit y d u rin g t h e slid in g o p erat io n . Th e co efficien t o f frict io n can be co n sid ered n eit h er as a m at erial variable n o r as a co n st an t fo r t h e d ie wo rkp iece p air. Ho wever, if t h e co efficien t o f frict io n is select ed fo r m akin g p red ict io n s, it s d ep en d en ce o n vario u s p ro cess p aram et ers m u st be co n sid ered . In sh eet m et al fo rm - in g, t h e fo llo win g su ch p aram et ers are o f m ajo r im p o rt an ce: – t h e m at erial p air an d t h e m et allic su rfaces wh ich are in co n t act wit h each o t h er, – t h e d ie geo m et ry, – t h e t em p erat u re-d ep en d en t p ro p ert ies o f t h e lu brican t as well as it s vo lu m e an d t h e lo cat io n o f lu brican t ap p licat io n , – t h e act u al n o rm al co n t act st ress, wh ich is a fu n ct io n o f t h e lo ad bear- in g su rface p o rt io n s o f bo t h su rfaces, – t h e m agn it u d e o f t h e slid in g velo cit y an d t h e frict io n al p at h o f t h e co n t act in g su rfaces an d – t h e su rface ch aract erist ics o f t h e wo rkp iece m at erial as well as t h e ch an ges o f t h ese ch aract erist ics t h at t ake p lace d u rin g t h e p ro cess. W ear In gen eral, t h e assessm en t o f t h e wear beh avio r o f d ies is d ifficu lt becau se o f t h e p resen ce o f several su p erim p o sed wear m ech an ism s. In o rd er t o d escribe t h e vario u s fo rm s o f d am age, wear m ech an ism s sh o u ld be d ivid ed in t o five basic fu n d am en t al p h en o m en a Fig. 4.2.8 : Defo rm at io n Un d er extern al load s, a m icro-geom etrical ad ap tation of th e su rface p air takes p lace, in itiated by th e local d eform ation at th e con tactin g p eaks of th e rou gh n ess of th e workp iece su rface. 181 Deep draw ing and stretch draw ing Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Wear at su rface layer Du rin g t h e ap p licat io n o f lo ad ch em ical react io n s bet ween t h e d ie an d t h e wo rkp iece m ay t ake p lace. Th ese are d u e t o t h e in flu en ces o f fric- t io n al h eat in g, lu brican t an d t h e su rro u n d in g m ed iu m wh ich lead t o t h e bu ilt -u p o f an in t erface layer at t h e d ie m at erial in t erface. Th e sh ear st ren gt h o f t h e in t erface layers is less t h an t h at o f t h e m et als, so t h at t h ey are rem o ved u n d er frict io n al sh ear st ress. Co n seq u en t ly, t h e co ef- ficien t s o f frict io n o ccu rrin g bet ween t h e d ie wo rkp iece su rfaces are lo w wh en an in t erface layer is p resen t . Th u s, it is p o ssible t o m ain t ain t h e fu n ct io n al cap abilit y o f t h e co n t act su rfaces even d u rin g sh o rt t erm o verlo ad in g t h ro u gh wear o f t h e in t erface layers. Ad h esio n Ad h esio n – also -called co ld weld in g – o ccu rs as a resu lt o f n u clear ad h e- sive fo rces bet ween t h e frict io n al p art n ers. Th e em ergen ce o f ad h esive fo rces p re-su p p o ses t h e fo llo win g: 182 Sheet metal forming and blanking Fig. 4.2.8 Depth effect of w ear mechanisms on metallic surfaces adsorption layer outer non- charac- teristic boundary layer inner charac- teristic boundary layer 3-5 Å 10-100 Å 50.000 Å 1 3 2 4 1 w ear at surface layer 2 adhesive w ear 3 abrasive w ear 4 fatigue w ear metal in chemically bonded form oxide or reaction layer layer damaged by forming, inhomogenous in terms of hardness, strength, degree of deformation, intrinsic tension and texture substrate material Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 – p last ic d efo rm at io n at t h e p eaks o f t h e ro u gh n ess o f co n t act in g su r- faces, – su rface exp an sio n bro u gh t abo u t by t h e fo rm in g p ro cess, wh ich rem o ves t h e ad h esio n p resen t in g bo u n d ary layers an d brin gs brigh t m et al su rfaces in co n t act wit h o n e an o t h er, – sim ilar m at erial st ru ct u res o f t h e frict io n al p art n ers. Th e great er is t h e d ifferen ce bet ween t h e m et als an d allo ys o f t h e fric- t io n al p art n ers, t h e lo wer is t h e t en d en cy t o d evelo p co ld weld in g. Th is t en d en cy is at it s lo west bet ween m et allic an d n o n -m et allic m at erials o f co rresp o n d in g h ard n ess. Abrasio n Abrasive wear in clu d es all p artin g or sep aration p rocesses th at take p lace at th e in terface of friction al p artn ers, wh ere m aterial is released in su b- m icroscop ic p articles th rou gh a m aterial rem oval p rocess. Th e d ifferen ce in th e h ard n ess between th e two su rfaces of friction al p artn ers is an im p ortan t factor for assessin g abrasive friction . Th e wear resistan ce of a d ie in creases with th e h ard n ess of its su rface. Du e to th e relative m ove- m en t of th e two su rfaces of th e friction al p artn ers, a p art of th e en ergy u sed is con verted in to h eat. With m ost m aterials, th e h ard n ess d rop s as th e tem p eratu re in creases an d th e resistan ce to wear is red u ced . Abrasive wear rep resen ts a gen erally in evitable lon g-term wear effect on th e d ie. Su rface fat igu e Fat igu e wear is co n sid ered t o be t h e sep arat io n o f m icro - an d m acro - sco p ic m at erial p art icles. Th is sep arat io n is cau sed t h ro u gh fat igu e crackin g, p ro gressive crackin g an d resid u al fract u re d u e t o m ech an ical, t h erm al o r ch em ical st ress-relat ed co n d it io n s o n t h e lo ad ed su rfaces. As a ru le, su rface fat igu e p lays a m in o r ro le in sh eet fo rm in g d ies. Ho wever, fat igu e is co m m o n in cu t t in g t o o ls, as an in creasin g n o rm al t en sio n st ress is ap p lied o n t o t h e fro n t su rface in ad d it io n t o a variable frict io n - al sh ear st ress t h at exist s o ver t h e wh o le ext ern al su rface area. Lubrication Lu brican t s are su bject t o d ifferen t t em p erat u res an d co m p ressive st ress- es d u rin g t h eir u se. Im p o rt an t variables in flu en cin g t h e co rrect ch o ice o f su it able lu brican t s are visco sit y, d en sit y an d co m p ressibilit y, wh ere- 183 Deep draw ing and stretch draw ing Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 by visco sit y d em o n st rat es t h e great est d ep en d en ce o n t h e t wo p aram e- t ers: p ressu re an d t em p erat u re. Part icu larly at co n t act p o in t s wit h h igh su rface p ressu re, at t en t io n m u st be p aid n o t o n ly t o t h e t em p erat u re, bu t also t o t h e in flu en ce o f p ressu re. Lu brican t s can be classified as fo l- lo ws: – wat er so lu ble, – n o n wat er so lu ble, – so lid lu brican t s, – fo ils an d varn ish es. Liq u id lu brican ts are p red om in an tly oil based . Th ese are gen erally blen d s with ou t a con sisten t com p osition , as th e p ortion s of p araffin , arom atic an d n ap h t h a co n t en t varies acco rd in g t o t h e o rigin o f t h e o ils. An im al- based , veget able an d syn t h et ic o ils also belo n g in t h is cat ego ry. Past e- t yp e lu brican t s are st abilized blen d s o f m in eral o r syn t h et ic o ils, greas- es an d waxes as well as so ap . Po wd ered o r n eed le sh ap ed h ard waxes an d h ard so ap s are u sed as so lid lu brican t s. Th e p ro p ert ies o f lu brican t s can be alt ered by ad d it ives an d ad ju st ed t o su it t h e resp ect ive ap p lica- t io n . Ad d it ives serve t o im p ro ve lo ad bearin g cap acit ies, bo n d in g st ren gt h , an d visco sit y-t em p erat u re-p ressu re ch aract erist ics, an d also assist in p reven t in g co rro sio n . Th e ad d it ives creat e eit h er p h ysical ad so rp t io n layers o r ch em ical react io n layers. By m ean s o f so called “frict io n m o d ifiers” fo r exam p le an im al an d veget able fat s, fat so ap et c. also belo n g in t h is cat ego ry, t h e lu brican t bo n d s it self t o t h e m et al su rface wit h o u t cau sin g an y ch em ical react io n . Th is t yp e o f p h ysically-act in g ad d it ives are, h o wever, in h eren t ly t em - p erat u re-d ep en d en t . Th e red u ct io n in co h esive an d ad h esive st ren gt h u n d er in creasin g t em p erat u res lead s t o an in crease in t h e co efficien t o f frict io n . Th e effect o f t h ese ad d it ives ran ges fro m a t o t al absen ce o f p h ysical ad so rp t io n u p t o a st able ch em ical bo n d fo rm at io n o f m et al- lic so ap . At h igh levels o f in t erface p ressu res, an t i-wear ad d it ives are u sed . Th ey fo rm a wear-red u cin g p ro t ect ive layer. A su b-d ivisio n o f t h is gro u p are t h e ext rem e p ressu re EP ad d it ives, wh ich fo rm react io n lay- ers u n d er h igh er t em p erat u res. Wit h a co m bin at io n o f select ive ad d i- t ives, in t h is ran ge lu brican t s can be o p t im ized t o su it ju st abo u t an y p art icu lar ap p licat io n . Based o n t h e d iscu ssio n given abo ve, t h e fo llo win g is a su m m ary o f t h e p erfo rm an ce req u irem en t s o f lu brican t s: 184 Sheet metal forming and blanking Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 – d evelo p m en t o f a co h esive p ressu re an d t em p erat u re-resist an t lu bri- cat io n film , wh ich sep arat es t h e su rfaces o f t h e wo rkp iece an d t h e d ie, – h igh ad h esive an d sh earin g st ren gt h as well as go o d wet t in g cap abil- it y, – n o in vo lu n t ary p h ysical o r ch em ical react io n s o n t h e su rface o f t h e frict io n al p art n ers, – easy an d co m p let e rem o val o f t h e lu brican t fro m t h e fin ish ed p art , – n o u n h ealt h y o r en viro n m en t ally h arm fu l su bst an ces.

4.2.4 Hydro-mechanical deep draw ing