Diffusion Bonding Between TZM Alloy and Wre Alloy by Spark Plasma Sintering

Journal of Alloys and Compounds 764 (2018) 582e590

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Journal of Alloys and Compounds

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Diffusion bonding between TZM alloy and WRe alloy by spark plasma sintering

* Zhi Yang a, 1,KeHua, c, 1, , Dawei Hu b, d, Cuiliu Han a, Yinggang Tong b, d, Xinyu Yang a, ** Fuzhi Wei b, d, Jiuxing Zhang a, , Yan Shen b, d, Jian Chen b, d, Xiaogang Wu a a School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China b Beijing Institute of Control Engineering, Beijing 100190, China c National Engineering Research Center of Powder Metallurgy of Titanium & Rare Metals, Guangdong Institute of Materials and Processing, Guangzhou 510650, China d Beijing Engineering Research Center of Propulsion Technology, Beijing 100190, China article info abstract

Article history: Successful solid-state diffusion bonding was achieved between TZM alloy and WRe alloy without in- Received 11 February 2018 terlayers using spark plasma sintering (SPS) in the temperature range of 1300e1600 C for 30 min. Both Received in revised form the TZM and WRe alloys recrystallized during the bonding process. There were no intermetallics, dis- 14 May 2018 continuities, microcracks, or pores, and sufﬁcient diffusion occurred in the TZM/WRe joint bonded at Accepted 11 June 2018 1500 C for 30 min. The TZM/WRe joint bonded at 1500 C for 30 min exhibited excellent mechanical Available online 12 June 2018 properties (shear strength of 498 ± 32.5 MPa and tensile strength of 475 ± 19.8 MPa) and thermal shock performance. The fracture morphology after shear and tensile test was both characterized as mixed Keywords: Spark plasma sintering transgranular cleavage and intergranular rupture. The TZM/WRe joint (1500 C/30 min) was free from Diffusion cracks and maintained high strength after experiencing repeated thermal shock 1500 times, and this was Interfacial microstructure due to negligible residual thermal stresses and further diffusion in the TZM/WRe interface during the Mechanical property thermal shock process. Thermal shock © 2018 Elsevier B.V. All rights reserved.

1. Introduction [5e9]. However, effective utilization of WRe alloy (especially when the alloy contains a high Re content) is limited by its high cost and Titaniumezirconiumemolybdenum (TZM) alloy has been high density. Developing reliable techniques for joining WRe alloys widely used in aerospace fields in applications such as the exhaust to other metals (e.g., TZM alloy) is therefore indispensable for valve of torpedo engines, rocket nozzles, gas pipelines, and nozzle expanding the application of these alloys in the field of aerospace. inserts, because of its high melting point, large elastic modulus, Conventional diffusion bonding, fusion welding, and brazing are small thermal expansion coefficient, good thermal conductivity, commonly used to join refractory alloys [10e13]. However, these strong corrosion resistance, and excellent high-temperature me- methods have different limitations. Conventional diffusion bonding chanical properties [1e4]. Tungstenerhenium (WRe) alloy is requires high temperature and long dwell time, and it is thus both currently considered to be an important engineering material for time- and energy-consuming. In fusion welding, new alloys or ultra-high temperature structural applications because it has many coarse grains may form in the molten pool, where microcracks are favorable properties, such as high melting temperature, high prone to generate under a tensile stress, and this leads to lower recrystallization temperature, high strength, good plasticity, low tensile strength. During brazing, the melting point of the brazing vapor pressure, and low brittle-ductile transition temperature filler metal is lower than that of the parent materials. Hence, at elevated temperature, the mechanical properties of the brazing joint may not meet application requirements. Spark plasma sintering (SPS) is a new method for the rapid consolidation of powders * Corresponding author. School of Materials Science and Engineering, Hefei using strong, pulsed DC current flowing through powders or dies to University of Technology, Hefei 230009, China. ** Corresponding author. generate joule heat, and it is characterized by a rapid heating rate, E-mail addresses: [email protected] (K. Hu), [email protected] (J. Zhang). short sintering time, and low energy consumption [14e16]. In 1 Contributed equally to this work. https://doi.org/10.1016/j.jallcom.2018.06.111 0925-8388/© 2018 Elsevier B.V. All rights reserved. Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590 583

À recent years, this method has been used to join metallic or ceramic 100 C$min 1. The TZM/WRe joints were cooled at a cooling rate of À materials/parts via atom diffusion on the bonding interface approximately 15 C$min 1 from the bonding temperature to [17e19]. Herein, we call it “SPS-assisted diffusion bonding” to 600 C; this was then followed by the furnace cooling to room distinguish it from conventional diffusion bonding. Compared with temperature. conventional diffusion bonding, an additional electric field is introduced during SPS-assisted diffusion bonding. Under the elec- 2.2. Performance evaluation and microstructure analysis tric field, the electrical migration effect can accelerate the atom diffusion rate [20,21], and this contributes to improving the per- Thermal shock performance of the TZM/WRe joint was evalu- À formance of joints. Although there are an increasing number of ated in vacuum (residual cell pressure 1.0 Â 10 2 Pa) using a high papers related to SPS-assisted diffusion bonding of the same or frequency inductive localized heating system. The TZM/WRe joint dissimilar materials, there are few reports in the literature about was cyclically heat-treated at 1200 C for a duration of 5 min for À systematic studies on diffusion bonding of TZM and WRe alloys via 1500 cycles. The heating rate was approximately 500 C$min 1, and À the SPS method. In the present paper, TZM/WRe joints are suc- the cooling rate was approximately 50 C$min 1. Room tempera- cessfully diffusion-bonded using the SPS method. The effects of ture shear tests were performed with a specially designed jig using diffusion bonding temperature on the microstructure and perfor- a CMT5115 testing system. Test pieces with dimensions of 4L mance of TZM/WRe joints are studied in detail. mm Â 2W mm Â 3T mm (L, W, and T represent length, width, and thickness, respectively) were machined from the bonded alloys, À and a constant loading speed of 0.5 mm min 1 was used in the 2. Experimental procedure shear tests. Fig. 3 shows a schematic diagram of the shear test. Room temperature tensile tests were performed using a CMT5115 2.1. SPS-assisted diffusion bonding process À testing system under a constant loading speed of 0.5 mm min 1. Tensile specimens with a gauge of hexagonal cross-section were Extruded TZM and WRe alloys, provided by Advanced Tech- prepared using EDM. Fig. 4 shows photos of the tensile testing bars. nology & Materials Co., Ltd. (AT&M), were selected for this study. In both the tensile and shear tests, three samples were tested for The main chemical compositions of these two materials are shown each process. Tensile and shear fracture morphologies were in Table 1. Fig. 1 shows the microstructures of the as-received TZM examined using field-emission scanning electron microscopy (FE- and WRe alloys. The as-received WRe alloy has a fibrous micro- SEM, Sigma, Zeiss, Germany). The hardness (HV ) was evaluated on structure, and the TZM alloy has a microstructure composed of a 3 a Digital Vickers hardness tester (XHVT-50Z, Shanghai Shangcai large number of carbide particles that are dispersed homoge- Tester Machine Co. Ltd., China) with a load of 3.0 kgf. neously in deformed grains. From both the TZM and WRe alloys, Cross-sections of the TZM/WRe joints before and after thermal work pieces were machined (via electrical discharge machining shock were cut perpendicularly and prepared via mechanical pol- (EDM)) into a cylindrical shape with dimensions of ishing and chemical etching for metallographic examination. The Ø11 mm Â 33 mm along the extrusion direction. Prior to diffusion etching agent was prepared by mixing a concentration of 25% bonding, the bonding surfaces of the work pieces were ground and ammonia water with a concentration of 30% hydrogen peroxide polished to produce a surface roughness (R ) that was lower than a solution in a ratio of 1:1. The microstructure was observed using FE- 0.05 mm. The work pieces were then cleaned with acetone in an SEM. Chemical composition and elemental intensity profiles of the ultrasonic bath for 10 min and then air-dried. chemical species (W, Mo, and Re) across the bonding interface were The TZM alloy and WRe alloy work pieces were assembled analyzed using energy dispersive X-ray spectrometry (EDS) following a top-to-bottom stacking sequence in a graphite die with attached to FE-SEM. The diffusion distances of W, Mo, and Re across an inner-diameter of 11.4 mm, and the work pieces were diffusion- the bonding interface were then determined on the basis of the EDS bonded via SPS (LaboxTM-300, Sinter Land Inc., Japan). Graphite profiles, and the final measurement results were each the average foils with a thickness of 0.2 mm were placed between the punches of ten tested values taken from different areas. and work pieces and between the die and work pieces for easy removal and for a significant reduction in temperature in- 3. Result and discussion homogeneity. In addition, the exterior of the die was covered with a porous graphite felt (thickness of ~5 mm), which was used as a 3.1. Microstructural and morphological analysis thermal insulator to reduce radiation loss and a possible temperature gradient [22]. A schematic diagram of the graphite die-work Successful solid-state diffusion bonding was achieved between pieces-punches assembly is shown in Fig. 2. Diffusion bonding was TZM and WRe alloys using SPS under all of the employed experi- performed in vacuum (residual cell pressure < 8 Pa), and a constant mental conditions. The TZM/WRe joint bonded at 1600 Cwas pressure of 20 MPa was applied from the beginning of the heating seriously deformed. Fig. 5 shows back-scattered electron (BSE) step to the end of the cooling step. The pulse sequence was 40:7. images of the TZM/WRe joints bonded in the temperature range of The diffusion bonding temperature was measured and adjusted 1300e1600 C before chemical etching and secondary electron (SE) using an optical pyrometer aimed at a through-hole (with a images after chemical etching. Local unbonded regions were diameter of 2.0 mm) in the external die wall. The diffusion bonding observed at the TZM/WRe interface when the bonding temperature temperature range was between 1300 C and 1600 C, and the hold was 1300 C, as shown in Fig. 5(a2). With an increase in the bonding time was 30 min. The heating rate was approximately temperature to the range of 1400e1500 C, the TZM/WRe bonding interface was flat and free from discontinuities, microcracks, pores, Table 1 and other bonding defects. When the bonding temperature was Chemical compositions (in weight percent) of the as-received TZM and WRe alloys. further increased to 1600 C, the specimen underwent severe Material Elements plastic deformation and a few micropores were localized at the TZM/WRe interface (as indicated by the arrow in Fig. 5d2). Ti Zr Mo C W Re Furthermore, from comparing the microstructures shown in Figs. 1 TZM 0.4e0.6 0.07e0.12 balance 0.01e0.04 ee and 5, it is observed that both of the parent materials (TZM and WRe ee ee balance 24.0e26.0 WRe alloys) recrystallized in the bonding temperature range of 584 Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590

Fig. 1. Microstructures of the as-received TZM and WRe alloys.

Fig. 2. Schematic diagram of the graphite die-work pieces-punches assembly during SPS-assisted diffusion bonding.

Fig. 4. Photos of the TZM/WRe joints prepared by EDM for tensile test.

compounds, solid solution was formed at the TZM/WRe interface. This result is consistent with the WeMo and MoeRe binary phase diagrams [23,24]. The diffusion distances of Mo, W, and Re across the bonding interface increased with an increase in the bonding temperature from 1300 C to 1500 C. The element diffusion distances, however, decreased when the bonding temperature further increased to 1600 C. Fig. 7 shows the diffusion distances of W and Mo across the TZM/WRe interface at different bonding temperatures, and these were calculated according to the EDS profiles. The distance that Mo diffused into the WRe substrate was greater than the diffusion distance of W in the TZM substrate regardless of the bonding temperature. For self-diffusion, the activation energy is associated with the melting point of the materials, that is, the Fig. 3. Schematic diagram of the shear test for the TZM/WRe joints. material that has the higher melting point requires more energy to activate atom self-diffusion [25]. The diffusion rate of W was, 1300e1600 C, that is, the fibrous grains transformed to equiaxed therefore, slower than that of Mo because of its higher melting and coarsened with an increase in the bonding temperature. point, and this leads to the diffusion distance of W in the TZM Fig. 6 shows the elemental diffusion distribution curves (Mo, W, substrate being less than that of Mo in the WRe substrate. By the and Re) obtained from line-scan analysis of EDS across the TZM/ same token, the frequency of atom jumping or migration increases WRe interface bonded at 1300, 1400, 1500, and 1600 C for 30 min. with an increase in bonding temperature, and in turn, this causes The profile curves varied continuously and smoothly across the the rate of atomic diffusion to increase. Hence, the diffusion dis- TZM/WRe interface, and this suggests that instead of intermetallic tances of Mo and W increased to 4.28 ± 0.25 mm and 2.37 ± 0.25 mm, Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590 585

Fig. 5. Microstructures of the TZM/WRe joints bonded at temperatures of (a1) and (a2) 1300 C, (b1) and (b2) 1400 C, (c1) and (c2)1500 C, (d1) and (d2) 1600 C. The insets in each image are higher magniﬁed. On the left are the BSE images of the TZM/WRe joints before chemical etching with the SE images after chemical etching, the right. respectively, when the bonding temperature was increased from alloys is likely to be promoted by the unique “SPS effects” such as 1300 C to 1500 C (holding time was still 30 min). Frei et al. [26] electromigration, reduction of the oxide surface, and instantaneous and Toyofuku et al. [27] studied the effects of pulsed current on local overheating on the contact surface. The diffusion distances of neck growth in SPS of copper spheres to copper plates and tungsten Mo and W, however, decreased when the bonding temperature was wires to tungsten plates, respectively. Their results showed that the 1600 C. The main cause may be severe plastic deformation of the mechanisms of electromigration and/or reduction of the oxide sample at high temperature. A large number of lattice defects and surface from evaporation leads to enhanced atomic diffusion dur- residual stress were produced at the bonding interface when the ing SPS. Furthermore, Chikui et al. [28] reported that instantaneous TZM/WRe joints suffered severe plastic deformation. The energy high temperature (up to 500 C) was generated on the contact barrier for atomic diffusion increased accordingly, and then atomic surface in the initial stage of the power supply (about 10 s) during diffusion was blocked. SPS-assisted diffusion bonding of SUS304 stainless steel. Therefore, we believe that the atomic diffusion between the TZM and WRe 586 Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590

Fig. 6. Elemental diffusion distribution curves (Mo, W, and Re) obtained from line-scan analysis of EDS across the TZM/WRe interface bonded at (a) 1300 C, (b) 1400 C, (c) 1500 C and (d) 1600 C for 30 min.

Fig. 7. Diffusion distances of W and Mo across the TZM/WRe interface at different Fig. 8. Shear strength of the TZM/WRe joints obtained at different bonding bonding temperatures (calculated according to the EDS proﬁles and ten tested values temperatures. taken from different areas).

WRe joints was the same as that of the element diffusion distance, 3.2. Mechanical properties evaluation that is, the shear strength ﬁrst increased and achieved a maximum value of 498 ± 32.5 MPa at 1500 C and then decreased when the Fig. 8 shows the shear strength of the TZM/WRe joints bonded at bonding temperature ranged from 1300 C to 1600 C. Fig. 9 shows different temperatures. Variation in the shear strength of the TZM/ Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590 587

Fig. 9. Shear fracture surfaces of the TZM/WRe joints bonded at (a) and (b) 1300 C, (c) and (d) 1400 C, (e) and (f) 1500 C, (g) and (h) 1600 C. On the left are the fracture surfaces on the TZM side and on the right are those on the WRe side. the fracture morphology of the TZM/WRe joints after shear testing. the WRe side (region B). The fracture surface was full of trans- Insufﬁcient local plastic deformation at the contact zone resulted in granular cleavage when the bonding temperature was further poor physical contact (as indicated by the arrows in Fig. 9 (b)) and increased to 1600 C. EDS results (Fig. 9g and h) show that the Mo then in limited diffusion between the TZM and WRe alloys when content was more than 95 at.% on the fracture surface, and this the bonding temperature was 1300 C. As a result, the fracture indicates that the failure occurred on the TZM substrate. It also surface was basically featureless. The shear failure of the TZM/WRe suggests that the TZM alloy recrystallized and coarsened enough at joint bonded at 1400 C was characterized by intergranular fracture high bonding temperature and that the performance of the because of good physical contact but inadequate elemental diffu- bonding seam was superior to that of the recrystallized TZM alloy. sion. When the bonding temperature increased to 1500 C, the On the other hand, tensile testing was carried out for the TZM/ fracture mode was mixed with transgranular cleavage and inter- WRe joint bonded at 1500 C. Table 2 lists the tensile strengths of granular rupture. Fig. 10 shows the element mapping at the shear three tensile test pieces under the same bonding and testing con- fracture of the TZM/WRe joint bonded at 1500 C. The Mo and W ditions. The tensile strength of the TZM/WRe joint bonded at contents were comparable with the observations from element 1500 C was higher than 450 MPa. Elongation or plastic strain is mapping. Transgranular cleavage mainly occurred on the TZM side scarce because of recrystallization and grain growth of both the (region A), whereas the intergranular fracture mainly occurred on TZM and WRe alloys during SPS diffusion bonding. Fig. 11 shows the 588 Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590

Fig. 10. Element mapping at the shear fracture surface of the TZM/WRe joint bonded at 1500 C. Red and green is for Mo and W, respectively. Images above are the fracture surfaces on the TZM side and the fracture surfaces on the WRe side are below. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the Web version of this article.)

Table 2 Table 3 Tensile strengths of the TZM/WRe joints bonded at 1500 C. EDS results of different regions on the tensile fracture surface shown in Fig. 11.

Sample Tensile strength (MPa) Region Element (at.%)

test piece 1 453.4 Mo W Re test piece 2 479.6 A 92.08 7.92 e test piece 3 492.1 B 7.67 68.34 23.99 tensile fracture morphology of the TZM/WRe joint, and the corre- 3.3. Thermal shock performance sponding EDS results are listed in Table 3. The tensile fracture mode was the same mixture of transgranular cleavage and intergranular Three TZM/WRe joints bonded at 1500 C for 30 min were rupture as that after shear testing, that is, the transgranular selected to evaluate the thermal shock performance. All of the TZM/ cleavage mainly occurred on the TZM side (region A in Fig. 11b) and WRe joints withstood thermal shock 1500 times, and no cracks the intergranular fracture mainly happened on the WRe side (re- were found within the bonding seam. Fig. 13 presents the micro- gion B in Fig. 11b). structures of the TZM/WRe joints after thermal shock. The thermal Fig. 12 shows the hardness of the TZM and WRe alloys before expansion coefﬁcients (CETs) of Mo, W, and Re are approximately and after SPS-assisted diffusion bonding. After diffusion bonding, À À À À À À the same (4.8 Â 10 6 K 1, 4.5 Â 10 6 K 1, and 6.2 Â 10 6 K 1, the hardness of the recrystallized TZM and WRe alloys was lower respectively [29]). Only a small amount of alternating thermal than that of the as-received parent alloys before bonding, and it stresses were generated and accumulated in the bonding interface decreased with an increase in bonding temperature. For instance, when the TZM/WRe joints suffered cyclic rapid heating and cooling the hardness of the parent TZM and WRe alloys before diffusion during thermal shock. Therefore, no microcracks were found at the bonding was 557 ± 19 HV and 197 ± 10 HV, respectively; and these TZM/WRe interface when the TZM/WRe joints experienced thermal values decreased to 441 ± 11 HV and 173 ± 3 HV, respectively, when shock 1500 times. From observing the microstructure at higher the bonding temperature was 1500 C. magniﬁcation (Fig. 13b), “white stripes” and “gray stripes” (indicated by the white and black arrows) penetrated into the TZM and WRe substrates, respectively. The compositions of these “stripes”

Fig. 11. Tensile fracture surface of the TZM/WRe joints bonded at 1500 C. Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590 589

Table 4 EDS results of the spots shown in Fig. 13b (at. % in unit).

Element Spot

sp1 sp2 sp3 sp4 sp5 sp6 sp7 sp8

Mo 100 98.62 92.44 87.35 0 15.34 8.63 2.57 W 0 1.38 7.56 12.65 74.37 63.75 68.25 73.13

4. Conclusions

SPS was an effective method for diffusion bonding of the TZM alloy to the WRe alloy without any interlayers. The interfacial microstructure, mechanical properties, and thermal shock performance of the TZM/WRe joints were investigated in detail. The following conclusions were drawn:

(1) TZM/WRe interfaces were well bonded in the temperature range from 1300 C to 1600 C. In contrast to W, there was Fig. 12. Hardness of the TZM and WRe alloys before and after SPS-assisted diffusion more sufficient diffusion of Mo in the WRe substrate because bonding. of the lower self-diffusion activation energy. Both the TZM and WRe alloys recrystallized during SPS-assisted diffusion bonding. There were no discontinuities, microcracks, or were examined using EDS (spots marked in Fig. 13b), and the re- pores, and there was sufficient diffusion in the TZM/WRe sults are shown in Table 4. The “white stripes” in the TZM substrate joint bonded at 1500 C for 30 min. contained W, and the W content decreased when the distance was (2) The TZM/WRe joint that was bonded at 1500 C for 30 min far from the bonding interface. Obviously the “gray stripes” in the exhibited excellent mechanical properties. The room tem- WRe substrate contained Mo, and the content of Mo also decreased perature shear strength was 498 ± 32.5 MPa, and the tensile with long distances from the bonding interface. This means that strength was 475 ± 19.8 MPa. The corresponding fracture more sufficient diffusion occurred along the grain boundaries mode was a mixture of transgranular cleavage and inter- (because the activation energy required for atom diffusion along granular rupture. the grain boundary is much lower than that for atom diffusion (3) The TZM/WRe joints bonded at 1500 C for 30 min withstood within the grains) during thermal shock. Moreover, after chemical repeated thermal shock 1500 times because of the negligible etching, some recrystallized Mo grains grew across the bonding residual thermal stresses and further diffusion that occurred interface into the WRe substrate (marked by an ellipse in Fig. 13e). in the TZM/WRe interface during the whole thermal shock This also suggests that further diffusion occurred in the TZM/WRe process. joints during the process of thermal shock. On the other hand, the shear strength of the TZM/WRe joints after experiencing thermal shock 1500 times decreased to Acknowledgments 313.6 ± 26 MPa (Fig. 8). The main causes probably are: first, residual thermal stress accumulated at the bonding interface, and second, This work was financially supported by the National Natural the grains coarsened because of the long exposure time at high Science Foundation of China (No. 51504100) and the National De- temperature (1200 C) during the whole thermal shock process. fense Basic Scientific Research Program of China (No. JSZL2016203C006).

Fig. 13. Microstructure of the TZM/WRe joints after thermal shock 1500 times. The TZM/WRe joints were bonded at 1500 C for 30 min. On the above are the BSE images of the thermal shocked TZM/WRe joints before chemical etching with the SE images after chemical etching, the below. 590 Z. Yang et al. / Journal of Alloys and Compounds 764 (2018) 582e590

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