-
esal
iynol
Received 29 September 2014Received in revised form
Keywords:Vacuum diffusion bonding
Ma(PFM) for a fusion reactor as well as a heat sink material for
high power microelectronic devices. The
forming on a copper sheet was proposed as interlayer to join W
and Cu via vacuum diffusion bonding. It
cult to join them together.Several methods have been developed
to fabricate W/Cu Func-
tionally Graded Materials (FGM), such as hot isostatic
pressing(HIP) bonding [7], diffusion bonding [8], powdermetallurgy
[9e11],Vacuum plasma spraying [12], mechano-chemical progress
[13],inltration process [14], direct metal laser sintering [15,16],
eld-
ications [18e20].to fabricate W/Cuayed onto oxygen-ert gas
protectionthe contaminationn at 0.2% has been
lk material fabri-cation is powder metallurgy. However because
the WeCu systemexhibits mutual insolubility or negligible
solubility, WeCu pow-der compacts show very poor sinterability,
even by liquid phasesintering above the melting point of the Cu
phase. Althoughevery method has its own merits, the interface
compatibility,however, is still the core issue challenging the
existing processes.
Although the common welding defects such as cracking
anddistortion can be avoided through diffusion bonding
technology[22,23], the application of conventional fusion welding
to join the* Corresponding author. Tel.: 86 010 62772856.
Contents lists available at ScienceDirect
Vacu
els
Vacuum 114 (2015) 58e65E-mail address:
[email protected] (Y. Ling).clear Experimental Reactor
(ITER) because of many favorableproperties such as high melting
point, high sputtering threshold,high thermal conductivity and a
low coefcient of thermal expan-sion [1e5], and copper has been
proposed as the heat sink materialbehind the plasma facing
materials (PFM) due to its high thermalconductivity, high
electrical conductivity and high ductility [6,7].However, the large
difference in melting point and coefcient ofthermal expansion
between these two metals makes it very dif-
however is not compliant with ITER new specPlasma spraying is
another promising methodFGM. Tungsten has been successfully plasma
sprfree copper in thicknesses up to 1 mm under in[21]. The
intrinsic limitation of this technique isof oxygen and carbon due
to higher level of oxygefound in those coatings.
One of the most important methods for buThe refractory metal
tungsten is recommended as the leadingcandidate for the divertor
section of the International Thermonu-
demonstrated a good tensile strength of joint ~200MPawith
failurein the Cu alloy near the brazed joint, the high temperature
brazingAmorphous interlayerMicrostructureTensile strength
testFracture mechanism
1.
Introductionhttp://dx.doi.org/10.1016/j.vacuum.2015.01.0080042-207X/
2015 Elsevier Ltd. All rights reserved.was found that an
improvement in bonding strength and a decrease in bonding residual
stresses wasobtained by the bidirectional diffusion of Fe, in the
form of highly active amorphous state, in W and Cu atthe weld
temperature. The diffusion transition regions were formed near the
W/Cu interface which isconsisted of a solid solution zone and
various phases between the FeeW and FeeCu binary systems andtwo
different fracture phenomena was observed on the basis of the
microstructural characteristics. Withthe introduction of this new
kind amorphous coating as interlayer, the vacuum diffusion bonding
joint ofW and Cu heating at 950 C for an hour with a load of 30 MPa
showed a maximum tensile strength ofabout 146 MPa.
2015 Elsevier Ltd. All rights reserved.
assisted sintering [17], etc. Braze welding is also an
effectivemethod to fabricateW/Cu FGM. The use of CuMn base brazing
alloyAccepted 7 January 2015Available online 14 January 20155
January 2015immiscible properties in W and Cu, however, make it
difcult to join each other without introduction ofactive metals
like iron group elements. In this paper, pulse electro-deposited
FeeW amorphous alloyMicrostructure and mechanical propertibonding
joints using amorphous FeeW
Song Wang a, b, Yunhan Ling b, *, Jianjun Wang a, Gua Laboratory
of Special Ceramics and Powder Metallurgy, University of Science
and Techb Laboratory of Advanced Materials, Tsinghua University,
Beijing 100084, China
a r t i c l e i n f o
Article history:
a b s t r a c t
W/Cu Functionally Graded
journal homepage: www.of W/Cu vacuum diffusionloy as
interlayer
ing Xu a
ogy Beijing, Beijing 100083, China
terials (FGM) are promising materials to be used as plasma
facing materials
um
evier .com/locate/vacuum
-
dissimilar alloys is not feasible because of the large
difference inthe melting points between these alloys. In this
study, the pos-sibility of fabricating W/Cu FGM with a new kind of
amorphousFeeW coatings electrodeposited onto the Cu foils by
vacuumdiffusion bonding (VDB) is explored. A Cu foil with thickness
of30 mm was used as an interlayer and the effect of bonding
tem-peratures on the microstructural developments across the
jointand the resulting mechanical properties was investigated. In
the
previous works [24,25], the effect of the amorphous FeeWcoating
transformation from non-crystal to crystal on WeCucomposite
materials was studied in detail. During the bondingprocess, the
FeeW deposit undergoes a change from the amor-phous to nano
crystals of alloy compounds with grain sizes of58.6 nm, 26.3 nm for
W and Fe2W, respectively. In the currentresearch, the effect of
bonding temperatures on the microstruc-ture and mechanical
properties of the joint interfaces werestudied extensively.
2. Experimental details
2.1. Electrodeposition of amorphous FeeW coatings on Cu
FeeW amorphous alloys were electroplated using an
aqueoussolution containing 0.212e0.243 mol L1 ferrous sulfate
hepta-hydrate, 0.018e0.036 mol L1 sodium tungstate dehydrate
and0.26 M ammonium tartrate. Tartaric acid complex system
wasselected as the complex agent in the study. A pH value of 8.0
wasmaintained by adding either ammonia or dilute sulfuric acid.
Forevery electroplating, the current density was set to be0.05 A
cm2 and the plating temperature as 60 C, while the timewas xed to 8
min. Amorphous FeeW coatings were prepared byelectroplating onto
the surface of Cu foils. Cu foil is chosen as thesubstrate because
of its high thermal conductivity and excellentplasticity. Before
deposition, the Cu foil sample was electro-chemically polished with
phosphoric acid, washed by distilled
Fig. 1. Heating curve for tungsten and copper bonding.
S. Wang et al. / Vacuum 114 (2015) 58e65 59Fig. 2. (a) XRD
pattern for FeeW coating; (b) TEM se
Fig. 3. (a) SEM top view morphology of as-depositelected area
diffraction pattern for FeeW coating.d FeeW coating; (b)
Cross-section morphology.
-
uumS. Wang et al. / Vac60water and then immersed in the bath as
a cathode, in the mean-time the inert graphite was selected as the
counterpart anode.
2.2. Diffusion bonding using amorphous FeeW coating as
aninterlayer
The coated samples were transferred to the diffusion
bondingchamber and the amorphous FeeW interlayer was placed
betweenthe surfaces of W and Cu. Then they were bonded under
differentVDB conditions in a ZTY-50e23 vacuum hot pressing
furnace(104e103 Pa). The load used in this investigation was 30 MPa
forall joints. The isothermal bonding temperatures were designed
tobe 850 C, 900 C and 950 C, respectively. Fig. 1 showed
thetemperature schedule, the bonding duration was xed to 60 min
ateach welding temperature.
To evaluate the bonding performance of the joint
specimens,tensile strength tests were employed using a DWD-200D
universaltesting machine. The microstructures of the specimens
wereexamined using a eld emission scanning electron
microscope(FESEM, HITACH S4800) equipped with an Oxford energy
disper-sive spectrometer (EDS). X-ray diffraction (XRD, D/max 2500,
Cu-Ka) analysis was conducted on the samples to determine
thecrystallinity of the deposited and bonded surfaces, while
theamorphous as-deposited alloy sample was observed by
trans-mission electron microscopy (TEM, JEM-2010F).
Fig. 4. (a) The line scan map of VDB interface between W and Cu
bon114 (2015) 58e653. Results and discussion
3.1. Structure and surface morphology of the coatings
The X-ray diffraction pattern of the FeeW coating
electro-deposited at 0.05 A cm2 is presented in Fig. 2(a). Only one
broadenpeak is found at the angle of approximately 42, implying
that theas-deposited lm was amorphous. Fig. 2(b) showed the
TEMmorphology and the selected area electron diffraction pattern
ofthe FeeW coating. The concentric aureole of transmission
electronmicroscope further demonstrated that the FeeW coating is a
kindof amorphous material.
The SEM morphology of the FeeW coating was depicted inFig. 3(a).
Compact and smooth FeeW coating was successfully ob-tained by
electroplating, and the content of tungsten varies from 22to 30 at%
depending on the cathodic current density applied.Fig. 3(b)
displayed the cross-section prole of the amorphousFeeW coating, it
can be seen that the thickness of the coating is2.5 0.05 mm under
the given plating conditions.
3.2. Interfacial structure of the W/Cu composite
The bonding temperature is important in determination
theformation of reaction phases involving elemental diffusion.
Themelting point of copper is 1083 C, which is much lower than
ded at 900 C; (b) the curve of diffusion depth prole at 900
C.
-
uumS. Wang et al. / Vacthat of tungsten (3400 C), three
temperature scenarios belowcopper's melting point was planned to
investigate the diffusionbonding behavior. Needless to say that the
interlayer's crystalsize and elementary composition will change as
a function ofthe experimental bonding parameters such as holding
time,temperature and the pressure applied during the
bondingprocesses.
The amorphous FeeW coating has a high adhesive andjoining
activity on W and Cu, as seen from Figs. 4 and 5 a tightjunction
between the two contact surfaces is formed success-fully. Figs.
4(a) and 5(a) presented the line scan map of the VDBinterface
between W and Cu. The interdiffusion of iron elementwas found on
both sides of tungsten and copper foil. The resultssuggest that Fe
in the coating diffused into both W and Cusubstrates. From Fig.
4(b), the Fe diffusion depth is calculated tobe about 1.2 mm on the
side of tungsten and about 6.6 mm on thecopper side at 900 C. While
at 950 C, the diffusion depth of Featom is measured to be about 1.5
mm on the tungsten side, andabout 7.25 mm on the copper side, as
shown in Fig. 5(b), sug-gesting that Cu dissolves in the solid
solution of a-Fe and Fe issoluble in Cu solid solution likewise. It
should be mentionedthat Fe and W atoms are inclined to form
intermetallic com-pounds as Fe2W phase and Fe7W6 phase (Fig. 6)
once beyond its
Fig. 5. (a) The line scan map of VDB interface between W and Cu
bon114 (2015) 58e65 61solubility limit. One reason for that might
be the iron atomicdiffusive rate in W is much lower than that in
copper. Themelting point difference is also a probable factor
determiningthe diffusive behaviors. Figs. 4 and 5 revealed that the
diffusiondepth of Fe atom at 950 C is deeper than that at 900 C.
Higherdiffusion temperature promotes a good plastic deformation
ca-pacity for copper, resulting in better tightness of the
connectingsurface. The joint (under optimization bonding
conditions) be-tween W, FeeW interlayer and Cu is achieved when
inter-diffusion between the materials is present without the
forma-tion of voids and brittle phases such as Fe2W
intermetalliccompounds. The new phases were produced during
thebonding, which would control the ultimate
mechanicalproperties.
3.3. Tensile strength test
Tensile strength performance further conrms the
successfuljointing of W and Cu. The results of tensile strength
tests on W andCu alloys are listed in Table 1.
As seen, the diffusion joints heated at 900 C and 950 C for
aduration of 60 min is more successful as they are higher in
tensilestrength than those of joints obtained at 850 C. During
the
ded at 950 C; (b) the curve of diffusion depth prole at 950
C.
-
bonding diffusion the atoms on both contact surfaces diffuse
andsome intermetallic compounds (Fe2W and Fe7W6) will be formedand
could improve the bonding strength at a certain range and tocertain
extent. At 850 C the fracture location was the interface ofCu foil
and Cu, implying that the interlayer cannot joint well withCu for
the bonding temperature was too low. At 900 C and950 C the fracture
locations were both at the interface W sides.The surface of W might
be weakened during the hot-press pro-cess and the weakened area
might inuence the residual stresses[7]. From the fractographic
analysis of surfaces obtained aftertensile strength testing, the
fracture mechanisms can be dis-cerned by the microstructural
characteristics of each interfaceformed by diffusion (seen from
Figs. 7 and 8). Figs. 7 and 8demonstrated the fracture surface
morphology of FeeW inter-layer of a joint W/Cu obtained at 900 C
and 60 min and the EDSelement mapping of the fracture surface.
Figs. 7 and 8 demon-strated the fracture surface morphology of FeeW
interlayer of ajoint W/Cu obtained at 900 C and 60 min and the EDS
elementmapping of the fracture surface, while Fig. 9 described the
frac-
Fig. 6. (a) XRD pattern of the interlayer after heat treatment
at 950 C-60 min; (b) Thephase constitution.
Table 1Results of tensile strength test on W and Cu alloys.
Joint isothermal conditions s(MPa) Fracture location
850 Ce60 min 15 Cu foil/Cu900 Ce60 min 142 Interface W side950
Ce60 min 146 Interface W side
Fig. 7. Brittle fracture surface of FeeW interlayer o
S. Wang et al. / Vacuum 114 (2015) 58e6562ture surface
morphology of FeeW interlayer of a joint W/Cuobtained at 950 C and
60 min and the EDS element mapping ofthe fracture surface. The
fracture surfaces suggest the variousmechanisms as follows:
At different bonding conditions (900 Ce60 min and950 Ce60 min),
the cracks take place by two different mecha-nisms: brittle
fracture and ductile fracture. The brittle fracturealways occurs
through the hard metal W (seen from Figs. 7 andf a joint W/Cu,
bonded at 900 C for 60 min.
-
A: b
S. Wang et al. / Vacuum 114 (2015) 58e65 639(a)A), while the
brous brittle fracture appears through theFe2W and Fe7W6 phases
(Figs. 8(a)B and 9(a)A); and the ductilefracture emerges through
the soft metal copper (Figs. 8(a)B and9(a)C). From the above
morphology and element distributionanalysis, it might be concluded
that lower temperature (900 C)
Fig. 8. (a) Fracture morphology of the interlayer W/Cu, bonded
at 950 C for 60 min,is conducive to the diffusion of FeeW
(especially for Fe) to W
Fig. 9. (a) Fracture morphology of the interlayer W/Cu, bonded
at 950 C for 60 min, A andsubstrate without inducing much harmful
compounds or phaseseparation; higher temperature (950 C) enhanced
inter-diffusion of Fe and W but leading to voids formation and
phasesegregation. That how to ameliorate the constitution design
ofFeeW amorphous coating to prevent or reduce the formation of
rittle fracture and B: ductile fracture; EDS element mapping of
(b) Cu; (c) Fe; (d) W.in homogeneous phase await to be further
investigation.
B: brittle fracture; C: ductile fracture; EDS element mapping of
(b) Cu; (c) Fe; (d) W.
-
uumS. Wang et al. / Vac64W/Cu bulk PFM specimens were
successfully prepared by theVDB technique in this study, as seen
from Fig. 10. With the intro-duction of FeeW amorphous interlay,
the size of the specimen canbe enlarged to 100 100 mm, meeting the
requirements of thedivertor module's size.
4. Conclusions
Bonding of W and Cu with a novel amorphous FeeW coating asan
interlayer was successfully demonstrated. The amorphous
andnanostructured FeeW has good adhesive strength with
substrate
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diffusion transition regionnear the W/Cu interface was formed and
consisted of a solid so-lution zone and intermetallic compounds
with Fe2W phase andFe7W6 phase. The vacuum diffusion bonding joint
processed at950 C for 60 min with a load of 30 MPa showed the
maximumtensile strength of about 146MPa. In addition, the vacuum
diffusionbonding technique has been successfully employed to
preparerelatively large size W/Cu-PFM specimens.
From the fractographic analysis of surfaces obtained after
me-chanical testing, fracture mechanisms can be deduced by
depend-ing on the microstructural characteristics of the various
interfacesformed by the diffusion bonding. The fracture failures
originatedfrom two different mechanisms: brittle fracture and
ductile frac-ture. More efforts await to be invested in pursuit of
higher bondingbetween W and Cu via further amelioration of the
interfacecompatibility.
Acknowledgments
This research was funded by the National Magnetic
connementFusion Science Program under contract 2010GB106003 and
Na-tional Basic Research Program of China (973 Program) under
grantNo. 2011CB61050, and the NSAF Program (No. U1430118).
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S. Wang et al. / Vacuum 114 (2015) 58e65 65
Microstructure and mechanical properties of W/Cu vacuum
diffusion bonding joints using amorphous FeW alloy as interlayer1.
Introduction2. Experimental details2.1. Electrodeposition of
amorphous FeW coatings on Cu2.2. Diffusion bonding using amorphous
FeW coating as an interlayer
3. Results and discussion3.1. Structure and surface morphology
of the coatings3.2. Interfacial structure of the W/Cu composite3.3.
Tensile strength test
4. ConclusionsAcknowledgmentsReferences