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
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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
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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.
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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.
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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
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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)
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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.
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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
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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.
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