esal

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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: yhling@mail.tsinghua.edu.cn (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

[7] Saito S, Fukaya K, Ishiyama S, Saito K. Mechanical propertied of HIP bonded Wand Cu-alloys joint for plasma facing components. J Nucl Mater

[11] Hashempour M, Razavizadeh H, Rezaie HR, Salehi MT. Thermochemicalpreparation of We25%Cu nanocomposite powder through a CVT mechanism.

Fig. 10. Pictures of the vacuum diffusion bonding W/Cu module samples.Mater Charact 2009;60:1232e40.[12] Pintsuk G, Smid I, Doring JE, Hohenauer W, Linle J. Fabrication and charac-

terization of vacuum plasma sprayed W/Cu-composites for extreme thermalconditions. J Mater Sci 2007;42:30e9.

[13] Cheng JG, Song P, Gong YF, Cai YB, Xia YH. Fabrication and characterization ofWe15Cu composite powders by a novel mechano-chemical process. Mater SciEng A 2008;488:453e7.

[14] Yang XH, Zou JT, Xiao P, Wang XH. Vacuum 2014;106:16e20.[15] Gu DD, Shen YF. Inuence of Cu-liquid content on densication and micro-

structure of direct laser sintered submicron WeCu/micron Cu powdermixture. Mater Sci Eng A 2008;489:169e77.

[16] Chen PG, Luo GQ, Shen Q, Li MJ, Zhang LM. Thermal and electrical propertiesof WeCu composite produced by activated sintering. Mater Des 2013;46:101e5.

[17] Rape A, Chanthapan S, Singh J, Kulkarni A. Engineered chemistry of CueWcomposites sintered by eld-assisted sintering technology for heat sink ap-plications. J Mater Sci 2011;46:94e100.

[18] Barabash V, Akiba M, Cardella A, Mazul I, Odegard Jr BC, PloEchl L. Armor andheat sink materials joining technologies development for ITER plasma facingcomponents. J Nucl Mater 2000;283e287:1248e52.

[19] Prokoev YG, Barabash VR, Khorunov VF, Maksimova SV, Gervash AA,2002;307e311:1542e6.[8] Wang CB, Shen Q, Zhou ZG, Zhang LM. Diffusion welding of 93W alloy to OFC

and structural control of 93W/OFC joint. J Mater Sci 2005;40:2105e7.[9] Gan KK, Chen N, Wang Y, Gu MY. SiC/Cu composites with tungsten coating

prepared by power metallurgy. Mater Sci Technol 2007;23:119e22.[10] Ryu SS, Kim YD, Moon IH. Dilatometric analysis on the sintering behavior of

nanocrystalline WeCu prepared by mechanical alloying. J Alloys Compd2002;335:233e40.due to its high diffusion activity. The 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).

References

[1] Zhou DH, Liu Y, Chen JL, Zhou ZJ, Yan R. Surface cracking behaviors of ultra-ne grained tungsten under edge plasma loading in HT-7 tokamak. PlasmaSci Technol 2013;15:605e8.

[2] Li YG, Zhou WH, Huang LF, Zeng Z, Ju X. Cluster dynamics modeling ofaccumulation and diffusion of helium in neutron irradiated tungsten. J NuclMater 2012;431:26e32.

[3] Loarte A. Power and particle control. Nucl Fusion 2007;47:S203e63.[4] Mayer M, Andrzejczuk M, Dux R, Fortuna-Zalesna E, Hakola A, Koivuranta S.

Tungsten erosion and redeposition in all-tungsten divertor of ASDEX upgrade.Phys Scr 2009;T138:014039e45.

[5] Mayer M, Likonen J, Coad JP, Maier H, Balden M, Lindig S. Tungsten erosion inthe outer divertor of JET. J Nucl Mater 2007;363e365:101e6.

[6] Mitteau R, Missiaen JM, Brustolin P, Ozer O, Durocher A, Ruset C. Recent de-velopments toward the use of tungsten as armour material in plasma facingcomponents. Fusion Eng Des 2007;82:1700e5.

114 (2015) 58e65Fabritsiev SA. Some problems of brazing technology for the divertor platemanufacturing. J Nucl Mater 1992;191e194(Part A):483e7.

[20] Singh KP, Pandya SP, Khirwadkar SS, Patel Alpesh, Patil Y, Buch JJU. Pre-qualication of brazed plasma facing components of divertor target elementsfor ITER like tokamak application. Fusion Eng Des 2011;86:1741e4.

[21] Zhou ZJ, Song SX, Yao WZ, Pintsuk G, Linke J, Guo SQ. Fabrication of thick Wcoatings by atmospheric plasma spraying and their transient high heatloading performance. Fusion Eng Des 2010;85:1720e3.

[22] Barrena MI, Gomez de Salazar JM, Merino N, Matesanz L. Characterization ofWCeCo/Ti6Al4V diffusion bonding joints using Ag as interlayer. Mater Charact2008;59:1407e11.

[23] Sabetghadam H, Zarei Hanzaki A, Araee A. Diffusion bonding of 410stainless steel to copper using a nickel interlayer. Mater Charact 2010;61:626e34.

[24] Wang S, Ling YH, Zhao P, Zang NZ, Wang JJ, Guo SB. Bonding tungsten, WeCu-alloy and copper with amorphous FeeW alloy transition. Fusion Eng Des2013;88:248e52.

[25] Zhao P, Wang S, Guo SB, Chen YX, Ling YH, Li JT. Bonding W and WeCucomposite with WeFe coated copper foil through hot pressing method. MaterDes 2012;42:21e4.

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