Development of electric resistance sintering process for the fabrication of hard metals: Processing, microstructure and mechanical properties

Accep ed Manusc ip De elopmen o elec ic esis ance sin e ing p ocess o he ab ica ion o ha d me als: P ocessing, mic os uc u e and mechanical p ope ies M.A. Lagos, I. Ago e, T. Schube , T. Weissgae be , J.M. Galla do, J.M. Mon es, L. P akash, C. And eouli, V. Oikonomou, D. Lopez, J.A. Cale o PII: S0263-4368(17)30055-0 DOI: doi: 10.1016/j.ij mhm.2017.03.005 Re e ence: RMHM 4426 To appea in: In e na ional Jou nal o Re ac o y Me als and Ha d Ma e ials Recei ed da e: 31 Janua y 2017 Re ised da e: 7 Ma ch 2017 Accep ed da e: 12 Ma ch 2017 Please ci e his a icle as: M.A. Lagos, I. Ago e, T. Schube , T. Weissgae be , J.M. Galla do, J.M. Mon es, L. P akash, C. And eouli, V. Oikonomou, D. Lopez, J.A. Cale o , De elopmen o elec ic esis ance sin e ing p ocess o he ab ica ion o ha d me als: P ocessing, mic os uc u e and mechanical p ope ies. The add ess o he co esponding au ho was cap u ed as a ilia ion o all au ho s. Please check i app op ia e. Rmhm(2017), doi: 10.1016/j.ij mhm.2017.03.005 This is a PDF ile o an unedi ed manusc ip ha has been accep ed o publica ion. As a se ice o ou cus ome s we a e p o iding his ea ly e sion o he manusc ip . The manusc ip will unde go copyedi ing, ypese ing, and e iew o he esul ing p oo be o e i is published in i s inal o m. Please no e ha du ing he p oduc ion p ocess e o s may be disco e ed which could a ec he con en , and all legal disclaime s ha apply o he jou nal pe ain. b ough o you by COREView me ada a, ci a ion and simila pape s a co e.ac.uk p o ided by TECNALIA Publica ions ACCEPTED MANUSCRIPT DEVELOPMENT OF ELECTRIC RESISTANCE SINTERING PROCESS FOR THE FABRICATION OF HARD METALS: PROCESSING, MICROSTRUCTURE AND MECHANICAL PROPERTIES M.A. Lagos1, I. Ago e1, T. Schube 2, T. Weissgae be 2, J.M. Galla do3, J.M. Mon es3, L. P akash4, C. And eouli5, V. Oikonomou5, D. Lopez6, J.A. Cale o6 1 TECNALIA RESEARCH & INNOVATION 2 F aunho e Ins i u e o Manu ac u ing Technology and Ad anced Ma e ials IFAM, B anch Lab D esden 3 Uni e si y o Se ille 4 KYOCERA UNIMERCO TOOLING AS 5 MIRTEC 6 ALEACIONES DE METALES SINTERIZADOS (AMES) ABSTRACT This wo k p esen s he de elopmen o he Elec ical Resis ance Sin e ing (ERS) p ocess o he ab ica ion o Ha d Me als. The composi ions o he ma e ials p oduced we e WC wi h 6 and 10 w % o Co. In addi ion o he speci ic cha ac e is ics o he echnology, he cha ac e iza ion o he p oduced pa s is p esen ed and compa ed o ma e ials ob ained by con en ional p ocesses. The pa s p oduced by ERS p esen densi ies compa able o he ones ob ained by con en ional me hods. The mic os uc u al compa ison shows a conside able g ain size educ ion in he ERS ma e ials which consequen ly b ings a ha dness inc ease. ERS ma e ials show simila ac u e oughness o con en ional ones. The e y as sin e ing allows pe o ming he p ocess wi hou any p o ec i e a mosphe e, he e o e making his p ocess e y a ac i e o he p oduc ion o ma e ials ha need o be sin e ed unde non- oxidising en i onmen s. The o al du a ion o he cycle, including hea ing, holding ime and cooling is ew seconds. Finally, some conside a ions abou he scale up and possible indus ializa ion o he echnology a e explained. Keywo ds: Elec ical sin e ing, ERS, FAST, cu ing ools, ungs en ca bide 1. INTRODUCTION ECAS (elec ic cu en assis ed sin e ing) ga he s a amily o consolida ion me hods in which mechanical p essu e is combined wi h elec ic and he mal ields o enhance pa icle bonding and densi ica ion. The p ima y pu pose o imposed elec ic cu en is o p o ide he equi ed amoun o esis i e hea . The exp ession ield-assis ed sin e ing echniques (FAST) ha e become also popula in li e a u e o denomina e hese echnologies. Indeed, he speed o he p ocesses is he mos ema kable cha ac e is ic and he common ac o among hese echniques. ECAS echniques can be classi ied wi h espec o he p ocessing ime [1]. In con en ional ECAS p ocesses, like SPS (Spa k Plasma Sin e ing) [2], low cu en densi ies and ol age a e applied ( ypically < 10 V and 1 kA/cm2). Wi h hese condi ions, p ocessing ime is in he ange o minu es and o ha eason a con olled a mosphe e is needed. Howe e , in e y as p ocesses like EDS (Elec o Discha ge Sin e ing) [3] and ERS (Elec ic Resis ance Sin e ing) [4-6] highe cu en densi ies a e applied ( ypically > 5 kA/cm2) and he p ocessing ime is ew seconds. The sho du a ion o he cycle pe mi s he p ocessing in ai wi hou any p o ec i e a mosphe e. This is a e y impo an ad an age om he economic poin o iew. In EDS, he elec ic discha ge is p oduced wi h a high- ol age ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT capaci o bank, whe eas in ERS he cu en is p oduced by a low- ol age ans o me (a ound 10 V). The e o e, he cu en applied o he sample is easie o con ol in ERS and his imp o es he homogenei y o he p oduc s. Elec ical esis ance sin e ing (ERS), was al eady desc ibed in 1933 by Taylo [7] and la e modi ied by C eme [8] in 1944. Howe e , up o now, he ERS p ocess has been used o he consolida ion o high conduc i e ma e ials (pu e me als) a lab scale, ob aining e y small pieces [4-6]. The de elopmen o medium ol age machines (40 V) opens he possibili y o ob aining la ge pa s and using ma e ials wi h lowe elec ical conduc i i y like composi es and ce me s. WC–Co cemen ed ca bides ha e been widely used o cu ing ools o a ious ma e ials and o he machine pa s which a e equi ed o show high esis ance o ic ional wea . Thei mechanical p ope ies can be modi ied o e a b oad ange by changing he con en o he binding Co phase and he WC g ain size [9-13]. Cemen ed ca bides a e usually p oduced by sin e ing wi h he pa icipa ion o a liquid cobal phase. Howe e , he p esence o his phase du ing he WC–Co sin e ing, s imula es he g ow h o he WC g ains. In gene al, wi h dec easing WC pa icle size p ope ies such as ha dness, wea esis ance, and ans e se up u e s eng h o he composi e a e imp o ed [14-16]. The e ec o he nanos uc u e in he WC-Co ca bides p ope ies has been ex ensi ely s udied [17- 19]. I is known ha ac u e oughness dec eases wi h inc easing ha dness in con en ional composi es, whe eas he inc ease o ha dness in nano-s uc u ed composi es does no u he educe hei bulk ac u e oughness. This implies ha di e en dominan oughening mechanisms exis in he con en ional and nanos uc u ed composi es. Ve y as sin e ing p ocesses o e a unique oppo uni y o a oid he liquid phase sin e ing and hus limi he g ain g ow h. This pape p esen s he de elopmen o he ERS echnology o he p ocessing o ha d me als. The new pilo plan scale equipmen is shown. An elec ic p ess o 15 was combined wi h medium ol age ans o me s in a ully au oma ed ERS equipmen . Mic os uc u al and mechanical cha ac e iza ion o he ma e ials a e p esen ed and compa ed o comme cial sin e -hipped ca bides. In addi ion, some impo an aspec s o he echnology a e desc ibed. 2. MATERIALS AND METHODS WC-Co powde s (submic on size) we e used wi h he ollowing composi ion: 6 w % o Co and 10 w % o Co. G anula ed powde s om a comme cial sou ce we e p ocu ed wi h o ganic wax and a p e- ea men was pe o med in o de o elimina e o ganic componen s. The sphe ical mo phology o he g anules wi h a size be ween 100-200 µm can be obse ed in Figu e 1A. Each g anule is composed o e y ine powde (size less han 1 mic on). The su ace o one g anule is obse ed in Figu e 1 B. Appa en densi y o he g anula ed powde was 3,7 g/cm3. A B Figu e 1. WC-6Co powde , A) SEM image o he g anula ed powde , magni ica ion X200, B) SEM image o he su ace o he g anules, magni ica ion X20000. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Fo he ERS p ocessing, he powde was illed in an alumina based ce amic die be ween wo coppe elec odes (see he diag am o he ERS machine in Figu e 2A). The dimensions o he p oduced pa s we e 22 mm in diame e wi h a hickness o 10-14 mm. The maximum applied cu en densi y was be ween 4-5 kA/cm2 wi h a holding ime o 500 ms. Maximum load was 100 MPa. An image o he p o o ype machine used in his wo k is p esen ed in Figu e 2B. This ully au oma ed ERS machine was de eloped wi hin he EU unded EFFIPRO p ojec . Maximum ol age o he machine is 40 V wi h a maximum cu en o 35 kA. The p essu e is applied using a 15 ones elec ical p ess. A B Figu e 2. A) Diag am o he echnology, B) P o o ype machine, de eloped wi hin he EFFIPRO p ojec Densi y was measu ed by A chimedes me hod using a Me le AE 240 weigh balance and po osi y was analysed acco ding o he s anda d UNE-EN ISO 4605:1978. Mic os uc u e and semi- quan i a i e chemical composi ion we e analysed by op ical and SEM mic oscopy (Zeiss N ision 40 and Zeiss EVO50). Ha dness Vicke s (HV30) was measu ed acco ding o he s anda d UNE-EN ISO 6507-1:2006. KIC was calcula ed om he leng h o he adial c acks o igina ed in he co ne s o he Vicke s inden a ions acco ding o he o mula p oposed by She y e al. [20]. The analysis o he magne ic coe ci i y HC and sa u a ion magne isa ion MS was pe o med acco ding o DIN ISO 3326 using a Koe zima CS 1.096, om Fö s e GmbH. The W con en o he Co binde was calcula ed om measu emen s o magne ic momen [21]. Measu emen o la ice pa ame e o he Co binde phase was made wi h an X- ay di ac ome e using CuKα adia ion on he su ace o es -pieces which we e elec oly ically e ched o emo e WC. The elec oly e consis ed o 4-mol solu ion o NaOH in eagen g ade. 3. RESULTS AND DISCUSION 3.1. Cha ac e is ics o he ERS p ocessing o he ab ica ion o WC-Co Figu e 3 shows he clea ela ionship be ween he cu en applied and he global densi y ob ained in he ERS samples, measu ed by he A chimedes me hod. Inc easing he densi y o cu en , he global densi y o he samples is inc eased. The ma e ial wi h 10 % o Co p esen ed be e esul s o densi ica ion compa ed o he ma e ial wi h 6 % o Co. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Figu e 3: Rela ionship be ween he densi y and he elec ic cu en applied (densi y includes he ex e nal po ous laye ; co e o he samples was ully dense) The ERS samples ypically consis o a dense co e su ounded by a po ous su ace laye as i is shown in Figu e 4A. The densi y alues p esen ed in igu e 3 show alues be ween 90 and 95% o he heo e ical densi y due o he ac ha he ou e po ous laye is included. I has been obse ed ha inc easing he cu en densi y, he a ea o he ex e nal po ous laye dec eases. The hickness o he ou e po ous laye in con ac wi h he me allic punches ( op and bo om o he sample) is hinne han ha in con ac wi h he ce amic die walls (see igu e 4B and 4C). A e emo ing his laye , samples ob ained using ERS show e y high densi y, wi hin ypical equi ed alues o ha d me al ools. Po osi y a he co e is e y low (A02 acco ding o he s anda d UNE-EN ISO 4605:1978). A ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT B C Figu e 4. C oss sec ion o a pa p oduced by ERS, dis ibu ion o he po osi y: A) Whole sample, B) Ex e nal po osi y, con ac wi h he punches (scale ba 100 mic ons), C) Ex e nal po osi y, con ac wi h he ce amic die (scale ba 100 mic ons) In ERS p ocess, he elec ical cu en passes ac oss he ma e ial and he hea ing is p oduced by joule’s e ec . Tempe a u e in he ma e ial depends on di e en ac o s. One o he mos impo an ones is he g een densi y dis ibu ion o he pa s. I is known ha in a double sided p essing, g een densi y o he pa s is lowe a he co e. Whe e po osi y is highe , elec ical esis ance is also highe . This ac p oduces highe empe a u e a he co e o he ma e ial du ing esis ance hea ing. Ano he impo an ac o is he hea losses p oduced by conduc ion om he ma e ial o he dies and elec odes; his also inc eases he empe a u e g adien om he co e o he pa s o he bo de s. Fo ha eason, ex e nal po osi y is p oduced by he as cooling o he ma e ial in con ac o he coppe punches and ce amic die. Based on p elimina y expe imen al da a, i seems ha i is possible o imp o e his phenomenon educing he ex e nal po osi y by using ce amic dies wi h lowe he mal conduc i i y. 3.2. Mic os uc u al cha ac e iza ion o he p oduc s Mic os uc u es o he ma e ials ob ained by ERS a e shown in Figu e 5 (6 and 10 % Co). In bo h cases mic os uc u e is composed o polygonal WC g ains su ounded by Co me allic ma ix (da ke in he images). Figu e 5. HRSEM images o polished c oss sec ions o ERS/WC6Co (le ) and ERS/WC10Co. In addi ion, Figu e 6 p esen s o compa ison wi h con en ionally sin e ed ma e ial ion-e ched c oss sec ions o ma e ials wi h 6 % o Co ob ained by ERS and Sin e HIP, using in bo h cases he same s a ing powde s. A B Figu e 6. HRSEM images o polished and ion-e ched c oss sec ions o ERS/WC6Co (A) and Sin e HIP ma e ial (B) ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT C ys al g ain size was es ima ed by he linea in e cep s me hod (based on DIN EN 623-3). This me hod is based on he numbe o a e age leng hs o in e cep s h ough each c ys al/g ain along a line d awn ac oss he ma e ial su ace. The measu ed leng hs o in e cep s o WC and he Co binde o WC6Co a e gi en in Table 1. The eal phase sizes a e abou 1.4x la ge han he measu ed in e cep s owing o he measu ing p ocedu e. The measu emen s o phase sizes we e done a e an ion e ching o he polished sample o imp o ing he image quali y. The measu ed leng hs o in e cep s o WC gi e a WC-g ain size o abou 274nm o ERS and 326nm o he con en ional p ocess. The eason o his e y low g ain g ow h a e he e y sho p ocessing ime and e y as cooling a e du ing he ERS p ocess. The size o he Co phase is simila in he con en ional and ERS p ocesses. Table 1. Compa ison o phase sizes o di e en ly p ocessed ha dme al WC6Co P ocess Phase Leng h o in e cep s (nm) Numbe o measu ed egions d10 d50 d90 ERS WC 73 196 441 934 Co 28 73 218 310 Sin e -HIP WC 89 233 528 932 Co 28 77 195 417 Compa ing hese esul s wi h p e ious li e a u e, g ain size ob ained in ERS samples is simila o he one epo ed by Spa k Plasma Sin e ing o nanoc ys alline powde s [22]. I is impo an o ake in o accoun ha in he p esen wo k submic on powde s we e used ins ead o nanome ic ones. The e y low g ain g ow h obse ed du ing p ocessing opens he possibili y o nano-c ys alline ha d me als. The con igui y o ERS WC6Co was measu ed wi h abou 67% WC/WC combined wi h 33% WC/Co bounda ies. The co esponding alues o con igui y o he Sin e -HIP samples we e 56% WC/WC and 44% WC/Co. Con igui y dec eases wi h inc easing binde con en and is also dependen on he p ocessing his o y o he ca bide [23-24]. I is di icul o de e mine he e ec o con igui y on he mechanical p ope ies due o he in luence o o he mic os uc u al cha ac e is ics; howe e , i is belie ed ha ha dness inc eases wi h inc easing con igui y [24]. Rega ding he c ys allog aphic phases, Figu e 7 p esen s he XRD pa e ns o samples wi h 10 % o Co. ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Figu e 7. XRD pa e n, samples ob ained by ERS, WC -10Co The e ealed main phases a e WC-hexagonal and Co-cubic; aces o Co-hexagonal we e also de ec ed in bo h samples. Measu emen s o la ice pa ame e o he Co binde phase we e made on he su ace o es -pieces which ha e been elec oly ically e ched o emo e WC. F om he XRD measu emen s la ice pa ame e s o Co we e calcula ed and he concen a ion o W and C in he binde we e es ima ed. The la ice pa ame e o he cc phase can be exp essed as [21]: a = ao+ Q·mw + R·mc whe e ao, Q and R a e cons an s wi h alues o 0.3548 nm, 0.00036 and 0.0012, espec i ely. The a omic pe cen ages o W and C a e gi en by mw and mc espec i ely. Ob ained alues a e p esen ed in Table 2. 3.3. Mechanical p ope ies Ma e ials ob ained by ERS (6 and 10 % Co) p esen ed highe ha dness compa ed o con en ional ma e ials ob ained by Sin e -HIP (see Table 2). A ha dness inc ease o he 5 % was obse ed o he 6 % Co and and inc ease o he 8 % o he 10 % Co. Table 2. Summa y o p ope ies o di e en ly p ocessed ha d me als Compo- si ion P ocess Densi y (g/cm3) Ha dness HV30 F ac u e oughness K1C (MPa√m) HC (Oe) MS (10-7 T*m3*kg-1) Co binde composi ion (w %) W Co binde composi ion (w %) C WC6Co ERS 14.7 1960±15 9.6±0.5 470 101 9.8 0.04 Sin e - HIP 14.8 1860±15 9.5±0.5 385 103 8.8 0.03 WC10Co ERS 14.3 1750±20 10.3±0.5 420 150 9.0 0.14 Sin e - HIP 14.4 1620±15 10.2±0.5 320 154 13.9 ~0 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT In ERS ma e ials, a small a ia ion o he ha dness was obse ed be ween he co e and he su ace o he samples, p esen ing a small inc ease o he ha dness a he su ace. This phenomenon is no comple ely clea a he momen . This can be linked o a sligh ly highe g ain size obse ed a he co e o he samples. Samples a e hea ed by Joule’s e ec du ing ERS and he empe a u e a he co e o he samples is sligh ly highe han a he su ace. This longe ime a high empe a u e can p oduce a e y small inc ease in g ain size. Ano he possible explana ion is he mig a ion o Co om he co e o he samples; his mig a ion can p oduce small mic o-po osi y and he local dec ease o he ha dness. Howe e , no signi ican a ia ions in he Co con en we e obse ed. The applicabili y o he Hall-Pe ch equa ion o ha dness p edic ions: H = E1 + F1·dWC-1/2 has been examined o ma e ials wi h nominally 6 w % Co (app oxima ely 10 ol%). H is he ha dness and dwc he g ain size (WC a i hme ic mean linea in e cep ). E1 and F1 a e cons an alues. Leas squa es eg ession analysis was used o i a linea plo o he da a wi h alues o he cons an s E1 and F1 o 913 and 601, espec i ely. The ha dness measu ed on he ERS samples does no mee he men ioned co ela ion (see Figu e 8); he p edic ion should p oduce a highe ha dness as measu ed based on an in e cep o 200nm. The e o e, i is no clea , i his ela ionship be ween ha dness and WC g ain size can u he be ex ended o he ul a ine g ades. Figu e 8. Rela ionship be ween ha dness and g ains size. ERS ma e ials ma ked as ed ci cles [21] F ac u e oughness (K1c) e eals simila beha iou o he di e en ly p ocessed samples (con en ional and ERS) wi h alues in he ange o 9-10 MPa m0.5 (see able 2). I is known ha ac u e oughness dec eases wi h inc easing ha dness in con en ional composi es, whe eas he inc ease o ha dness in nano-s uc u ed composi es does no u he educe hei bulk ac u e oughness [17-19]. The e y sho p ocessing ime used in ERS (in he o de s o ew seconds) helps con olling he g ain g ow h and hus ob aining ine mic os uc u es. I is belie ed ha his educ ion in g ain size inc eases he ha dness bu main aining he ac u e oughness cons an . The mic os uc u al cha ac e isa ion o he WC–Co ha dme als is o en pe o med using magne ic measu emen s. By measu ing he magne ic sa u a ion MS o he coe ci i y HC, i is possible o ob ain an app oxima ion o he WC g ain size, cobal con en and e en de e mine he p esence o addi ional phases. Gene ally, an inc ease o coe ci i y is obse ed wi h a dec ease in WC g ain size. The compa ison o he measu ed HC alues o ERS wi h Sin e -HIP samples con i ms he educ ion in g ain size obse ed in ERS ma e ials. ACCEPTED MANUSCRIPT