Electrical discharge consolidation of Al and Ti powders

Elec ical discha ge consolida ion o Al and Ti powde s F. Te ne o1 * , R.M. A anda2, P. U ban1, R. As acio1 and J.M. Mon es1 1Ad anced Ma e ials Enginee ing G oup, Escuela Técnica Supe io de Ingenie ía, Uni e sidad de Se illa, 41092 Se illa, Spain 2Ad anced Ma e ials Enginee ing G oup, Escuela Técnica Supe io de Ingenie ía, Uni e sidad de Huel a, 21071 Huel a, Spain Abs ac In his pape , elec ical-discha ge-consolida ion (EDC echnique) expe imen s we e ca ied ou wi h an equipmen based on he echnology de eloped o he s ud welding echnology. The main ad an age o he EDC echnique is i s high speed, on he o de o milliseconds o less, which makes i pa icula ly in e es ing when a high inal po osi y is aimed, o when he inhe en nanos uc u e o he powde s needs o be p ese ed. Compac s o Ti and Al we e consolida ed wi h his echnique, bo h di ec ly om loose powde s and om cold p essed g een compac s. Two di e en con igu a ions (200 V – 66 mF and 800 V - 1.1 mF) we e es ed. Rela i e densi y, mic oha dness, elec ical esis i i y and me allog aphic and SEM s udies we e ca ied ou o unde s and he changes in powde pa icles caused by he elec ical discha ge. The use o a high capaci y in he capaci o s bank, despi e he use o a lowe ol age, esul s in a be e consolida ion p ocess. Keywo ds: Elec ical Discha ge Consolida ion; Capaci o s bank; Po osi y; Al powde ; Ti powde . 1. In oduc ion The con en ional powde me allu gical p ocessing ou e essen ially consis s o cold compac ion o a mass o me al powde o ob ain a g een compac , ollowed by u nace sin e ing o p o ide he inal o nea - inal pa . Al hough his is he ou e mos equen ly used in he indus y, new al e na i es a e cons an ly being in es iga ed o imp o e, a leas pa ially, he p oblems and sho comings o he cu en echnology. In his sense, he di ec use o elec ici y as a means o consolida ing powde s (me allic and ce amic) has been sugges ed on nume ous occasions. The g ea di e si y o p oposed me hods can be g ouped unde he gene ic name o Elec ic Cu en Assis ed Sin e ing Techniques (ECAST) o by he e m FAST (Elec ic Field Assis ed Sin e ing Techniques). In ac , he speed o he p ocesses is he mos ema kable ea u e and he common ac o o all hese echniques. In addi ion o he ime sa ings, he high speed o elec ical consolida ion echniques ep esen s an impo an ad an age, as i o en makes he use o acuum o ine a mosphe es unnecessa y. These echniques, like he con en ional ou e, p o ide a pa whose shape is i ually he inal o m. Howe e , one o he p oblems equen ly men ioned by esea che s who ha e expe imen ed wi h FAST echniques ela es o he du abili y o he dies used, especially in cases whe e he die mus be elec ically insula ing. The p oblem o choosing a ma e ial wi h accep able du abili y and cos -e ec i eness is o g ea in e es and equi es ho ough s udy. Only wi h he sa is ac o y esolu ion o his p oblem will elec ical consolida ion echniques be able o ind sui able niches o he indus ial scale. Howe e , on he labo a o y scale, i is easy o ind su icien ly accep able solu ions o wo k wi h. In he so-called Elec ical Discha ge Consolida ion (EDC), he elec ic cu en passing h ough he powde is also o high cu en in ensi y [11-18], howe e , compa ed o he ERS echnique, he ol age can each much highe alues. This combina ion o high in ensi y and mode a e (o high) ol age can be achie ed by discha ging he ene gy s o ed in a capaci o bank, a echnique also known in he elec ical ield o welding. Because he elec ic cu en p oduces e y as mic o-welds a he in e pa icle con ac s, he equi ed consolida ion ime can be ex ao dina ily sho , as li le as a ew milliseconds o e en mic oseconds. As Okazaki [11] concluded, he high speed o he EDC p ocesses allows he nanos uc u e o amo phous cha ac e o he powde s o be p ese ed, which is one o he mos impo an ad an ages o he echnique. * Co esponding au ho . Tel.: +34954487305 E-mail e ne [email p o ec ed] Else ie . Licencia C ea i e Commons CC BY-NC-ND In gene al, in he con en ional ou e, he cold compac ion s ep is su icien o ac u e and o pa ially emo e he oxide (and/o hyd oxide) laye s su ounding he me al powde pa icles, al hough in cases whe e his p ocess is no e icien , i may be use ul o apply an ex usion p ocess o he powde s. The emo al o oxide laye s in he EDC p ocess is p obably one o i s g ea es s eng hs and a consequence o he applica ion o ela i ely high s esses. These ol ages induce he dielec ic b eakdown o he laye s, making hem conduc i e and allowing he passage o a high cu en in ensi y [11]. The hea gene a ed by his high elec ic cu en so ens (o e en mel s) he me al unde he oxide laye , so ha i e en ually ac u es. Subsequen pa icle ea angemen p ocesses emo e he emaining oxides om he con ac zones, a phenomenon ha is e ec i e e en when no p essu e is applied. The e o e, he EDC echnique makes i possible o ob ain compac s wi h high po osi y. This p esen s an impo an di e ence be ween he EDC p ocesses wi h espec o he ERS echnique, whe e he ini ial esis i i y o he powde s is e y high and hinde s sin e ing, as he cu en in ensi y h ough he powde agg ega e is negligible due o he lowe ol age applied o he powde column, and no su icien o b eak he oxide laye s. The main ad an age o he EDC echnique is an ex ao dina y sho consolida ion ime (a ound milliseconds o mic oseconds), which makes unnecessa y he use o acuum o p o ec i e a mosphe es. This as p ocess allows o p ese e he inhe en s uc u e o he ma e ial and o ob ain high-po osi y compac s. This wo k desc ibes he EDC expe imen s ca ied ou wi h di e en powde s (Al and Ti), and he ob ained compac s p ope ies a e discussed. 2. Expe imen al P ocedu e and equipmen s 2.1. Desc ip ion o he EDC Equipmen Figu e 1 shows a diag am o he basic componen s o he expe imen al equipmen . Figu e 1. Diag am o he basic componen s o he EDC equipmen : (1) he mally and elec ically insula o die, (2) powde , (3) elec odes (uppe and lowe ), (4) elec ic insula o (uppe and lowe ), (5) connec ion wi es o he capaci o s bank, (6) ex e nal powe supply (elec ical ne wo k, 220 V), (7) pneuma ic p ess. Two di e en con igu a ions we e used: con igu a ion I uses a cha ging ol age o 200 V and a capaci o s bank wi h a capaci ance o 66 mF, whe eas con igu a ion II uses a cha ging ol age o 800 V and a capaci ance o only 1.1 mF. Bo h capaci o s’ banks we e adap ed om s ud welding equipmen s. 2.2. Ma e ials and Expe imen al P ocedu e Two di e en comme cial powde s we e s udied: Al AS61 (wi h 0.15% Fe as main impu i y) and Ti Sejong-325 (0.0038% Fe, 0.08% N, 0.035% Si, 0.003% Mn and 0.45% O). These powde s we e analysed ega ding hei pa icle size by lase di ac ion (Mal e n Mas e size 2000), and hei mo phology was s udied by scanning elec on mic oscopy (SEM, Philips XL-30 coupled wi h BSE and EDS de ec o s). Thei comp essibili y cu es we e ob ained by a comp ession es pe o med wi h a uni e sal es ing machine (Ins on 5505). The powde s absolu e densi y was de e mined using a helium pycnome e (Accupy II1340). Figu e 2 shows he pa icle size dis ibu ion and comp essibili y cu es o he wo powde s. Size dis ibu ion cu es show a na ow and symme ic shape o he Al AS61 and Ti Sejong-325 powde s. These mo phological cha ac e is ics we e co obo a ed by SEM obse a ion o he powde s, as shown in Figu e 3. a) b) Figu e 2. (a) G anulome ic, and (b) comp essibili y cu es o Al and Ti powde s. Figu e 3. SEM mic og aphs o s udied powde s: (a) Al AS61, (b) Ti Sejong-325. EDC expe imen s we e ca ied ou bo h s a ing wi h loose powde s and s a ing wi h cold p essed g een compac s. In his las case, a cylinde die o 8 mm diame e was used o p ess he compac s. Comp essibili y cu es show ha di e en p essu es a e needed in he di e en powde s in o de o each a simila ini ial no malised po osi y (de ined as he a io be ween he po osi y and he ap po osi y) in g een compac s. Ini ial ela i e po osi ies o 0.1 and 0.25 we e chosen o Al powde , and 0.25 and 0.40 o Ti powde . Thus, wo di e en p essu es we e applied o each powde : 990 and 490 MPa o Al and 1076 and 553 MPa o Ti powde . The p essu e used o each ype o powde depends on he p ope ies o each powde o allow he compac o ha e su icien consis ency o be handled. The powde s (loose o p e-compac ed) we e subjec ed o elec ic discha ge ollowing h ee di e en ias: (1) discha ging om con igu a ion I (200 V-66 mF), (2) discha ging wi h con igu a ion II (800 V-1.1 mF) and (3) discha ging wi h con igu a ion II + I (one s age a e he o he ). The ob ained compac s we e cha ac e ised ega ding hei ela i e densi y (by weigh and geome ic measu es), he Vicke s mic oha dness HV (by S ue s Du amin-A300) and he elec ical esis i i y (by he ou p obes me hod, Mic ohmme e CA 10 Chau in A noux). Mo eo e , diame ical sec ions o he compac s we e obse ed by op ical mic oscopy and ac u e su aces o mechanically b oken compac s we e s udied by SEM. 3. Resul s and Discussion 3.1. Rela i e Densi y, Elec ical Resis i i y and Vicke s Mic oha dness Figu e 4a, 4b and 4c espec i ely show ela i e densi y, mic oha dness and elec ical esis i i y o loose powde s and highe -p essu e p e-compac ed compac s consolida ed wi h con igu a ion I (200 V-66 mF). Fo he p e- 0 2 4 6 8 10 12 0.01 0.1 1 10 100 1000 Volume, % Pa icle size, mm Al Ti 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0200 400 600 800 1000 1200 1400 Rela i e Densi y P essu e, MPa Al Ti (a) (b) compac ed compac s, esul s a e also shown in Figu e 4d, 4e and 4 , bo h be o e he EDC (g een compac s) and a e he di e en discha ge con igu a ions. As expec ed, EDC compac s ob ained om loose powde s show a lowe ela i e densi y han he p e iously p essed ones (Figu e 4a); his is due o he compac ion p essu e. The low densi y o he compac s p epa ed om loose powde s is also accompanied by a low mic oha dness, as shown in Figu e 4b, wi h a educ ion o 20-40%. This is due o he highe po osi y o hese compac s, which also inc eases hei esis i i y, as shown in Figu e 4c. On he o he hand, ela i e densi y and mic oha dness o p e-p essed compac s consolida ed wi h con igu a ion II (800 V-1.1 mF) esul simila o hese p ope ies ob ained in g een compac s (Figu e 4d and 4e). Bo h p ope ies a e imp o ed when con igu a ion I (200 V-66 mF) is used. Finally, compac s ha we e subjec ed o he double discha ge cycle (II + I) show a sligh ly highe ela i e densi y and mic oha dness han any o he s. I migh be caused by he b eakdown o he oxide laye s o he powde pa icles aking place du ing he con igu a ion II s age, making easie he la e elec ical cu en passing when con igu a ion I is applied. Figu e 4. Compa ison o (a) ela i e densi y, (b) Vicke s mic oha dness and (c) elec ical esis i i y o high-p essu e p e-p essed compac s and compac s consolida ed om loose powde s by EDC, bo h wi h con igu a ion I; and (d) ela i e densi y, (e) Vicke s mic oha dness and ( ) elec ical esis i i y o p e-p essed compac s wi h he wo di e en p essu es, consolida ed wi h bo h con igu a ions (II + I). Finally, he elec ical esis i i y o he compac s consolida ed wi h con igu a ion I esul s e y simila o he one ob ained a e consolida ing wi h con igu a ion II + I, whe eas ha o he compac s consolida ed wi h he con igu a ion 0 10 20 30 40 50 60 70 80 90 100 Al Ni Fe Ti Rela i e Densi y, % Powde s + Con igu a ion I Compac s + Con igu a ion I 0 10 20 30 40 50 60 70 80 90 100 Al Ni Fe Ti Rela i e Densi y, % Powde s + Con igu a ion I Compac s + Con igu a ion I 0 10 20 30 40 50 60 70 80 90 100 Al Ni Fe Ti Rela i e Densi y, % Powde s + Con igu a ion I Compac s + Con igu a ion I (a) (b) (c) (d) (e) ( ) II is almos he same ha he one o he g een compac s (Figu e 4 ). The e o e, he EDC wi h con igu a ion II does no ha e an impo an e ec on powde s consolida ion; howe e , i may a o he elec ical cu en passing in a subsequen discha ge. 3.2. Mic os uc u al S udy Fo he Al compac s, op ical mic og aphs we e ca ied ou on a diame ical sec ion o he compac , di e en ia ing wo zones: ou e (pe iphe y) and cen e (Figu e 5). Since he elec ic cu en akes p e e en ial pa hs in i s passage h ough he powde column, a li le homogeneous po osi y dis ibu ion is o be expec ed. Howe e , he highe po osi y obse ed a he pe iphe y o he diame ical sec ion is inc eased by he loss o ma e ial du ing he mechanical cu ing and me allog aphic p epa a ion p ocess. This di e ence is mo e p onounced in he samples ob ained om loose powde , whe e consolida ion is poo e . Howe e , in he samples ha we e p e iously p essed, he di e ences a e smalle , due o he g ea e consis ency o hese compac s. Figu e 5. Op ical mic og aphs o he Al compac s: (a) and (b) pe iphe y and cen e, espec i ely, o he compac consolida ed om loose powde s and discha ging wi h con igu a ion I; (c) and (d) pe iphe y and cen e, espec i ely, o he compac consolida ed om a p e-p essed g een compac (990 MPa) and subsequen discha ge wi h con igu a ion II; (e) and ( ) pe iphe y and cen e, espec i ely, o he compac consolida ed om a p e-p essed g een compac (990 MPa) and subsequen discha ge wi h con igu a ion II + I. On he o he hand, Figu e 7 shows di e en mic og aphs aken on ac u e su aces o he di e en compac s ob ained a di e en p essu es: (a) none (loose powde ), (b) 490 MPa and (c) 990 MPa. Con igu a ion I was used o (a) (b) (c) (d) (e) ( ) pe o m he EDC o he compac ob ained om loose powde , and a combined con igu a ion (II + I) o he cold- p essed compac s. Figu e 7. SEM mic og aphs o Al AS61 compac s: (a) loose powde s and con igu a ion I, (b) g een compac p essed a 490 MPa and subsequen discha ge wi h con igu a ion (II + I) and (c) g een compac p essed a 990 MPa and subsequen discha ge wi h con igu a ion (II + I) and de ails o he possible join s (mic o-welds) be ween pa icles. Figu e 7a shows he SEM mic og aphs o he ac u e su ace o he compac ob ained om loose powde and discha ge o he I equipmen . I can be seen ha , as is ypical o compac s ob ained om loose powde s, he e ec o he p e e en ial elec ic cu en pa hs is mo e clea ly mani es ed, wi h mo e po ous a eas (ci cled in igu e 7a). In e - pa icle bonds can be seen, o example, in he squa es shown in Figu e 7a, al hough in gene al, he e a e no many such bonds ac oss he su ace s udied. I can be seen ha he compac s p essed a 990 MPa show sligh ly be e consolida ion (no signi ican po e size in Figu e 5.11c) han he compac s p essed a 490 MPa (po es ci cled in Figu e 7b) and he compac s ha we e no p e iously p essed (clea ly highe po osi y, ci cled in Figu e 7a). This co obo a es he esul s ob ained o he p e iously measu ed p ope ies ( ela i e densi y, elec ical esis i i y and Vicke s mic oha dness) whe e i was s a ed ha he compac s p essed a 990 MPa had he lowes ela i e densi y and he lowes elec ical esis i i y, which a ou ed consolida ion. On he o he hand, he p e iously p essed compac s ha e de o med pa icles so ha no possible join s be ween hem can be seen. Howe e , he compac s ha ha e no been p e iously p essed do allow some join s be ween pa icles o be seen (join s be ween pa icles su ounded by a squa e in Figu e 7a, co esponding o he compac ob ained om loose powde ). In addi ion, he di e en mo phology o he ac u e is clea ly seen, being mo e homogeneous and less po ous a highe applied p essu e. This is due, as men ioned abo e, o he ac ha he compac s p essed a highe p essu e inc ease he pa hs o he passage o he cu en , which imp o es hei consolida ion. As wi h Al and Ti compac s clea ly shows ha he cen al zone o he compac has ewe po es han he pe iphe y. Figu e 8 shows, as expec ed, ha he compac s ob ained om loose powde s and discha ging wi h con igu a ion I p esen highe po osi y han he compac s ha we e p e iously p essed. Figu e 8: (a) cen e and (b) pe iphe y o Ti compac p e iously p essed a (553 MPa) and wi h a discha ge o combined con igu a ion (II+I) . Figu e 9 shows he mic og aphs o Ti compac s p e-p essed a 1073 and 553 MPa and subsequen discha ge wi h combined con igu a ion (II + I), and compac ob ained s a ing om loose powde s and con igu a ion I discha ge. I can be seen ha , as be o e, he mos po ous compac s a e he non-p ep essed compac s (ci cled in Figu e 9a), while he p essed compac s show lowe po osi y. The homogenei y o he p essed compac s is e iden in Figu e 9b and Figu e 9c. The di e ences in he compac s subjec ed o di e en p essu es and he same discha ge cycle (con igu a ion II + I) a e no no iceable in hese mic og aphs. No join s be ween pa icles caused by he passage o he elec ic cu en can be seen in any o he cases, bu i can be obse ed ha he pa icles o he p essed compac s a e de o med due o he p essu e o which hey ha e been subjec ed. Figu e 9 SEM mic og aphs o Ti compac s: (a) unp essed powde s and discha ge wi h con igu a ion I, (b) p e-p essed a 553 MPa compac s and subsequen discha ge wi h a combined modali y (II + I) and (c) p e-p essed a 1076 MPa compac s and subsequen combined discha ge (II + I). 4. Conclusions Comme cial powde s o Al and Ti ha e been consolida ed by passing h ough hem an elec ical cu en om a capaci o s bank, achie ing a low ela i e densi y (high po osi y). P e-compac ed powde s each a e he EDC p ocess a highe ela i e densi y (lowe po osi y) and highe ha dness (a) (c) (b) han he ob ained in compac s p epa ed s a ing om loose powde . The di e en discha ge con igu a ions applied show ha con igu a ion II (800 V - 1.1 mF) a o s he b eaking o he oxide laye co e ing powde pa icles, howe e i has no signi ican e ec on he mic oha dness and elec ical esis i i y. On he o he hand, con igu a ion I (200 V – 66 mF) has a c i ical con ibu ion on hese p ope ies. The highe mic oha dness is ob ained applying a discha ge wi h a combined con igu a ion (II+I) . Acknowledgemen s This esea ch was unded by Minis e io de Economía y Compe i i idad (Spain) and Fede (EU) h ough he esea ch p ojec s DPI2015-69550-C2-1-P and DPI2015-69550-C2-2-P. Re e ences [1] G.F. Taylo . 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