WELDING RESEARCH between 270º and 325ºC for 30 min — Fig. 8. The shear strength of these diffusion soldered Pd/Ni joints using such a Sn/Ag/Sn interlayer decreased from the maximal value of 17.3 to 12.2 MPa when the bonding temperature was increased from 325º to 350ºC, which can be attributed to the high thermal stress during diffusion soldering at 350ºC. To further improve the bonding effect of the diffusion soldering of Pd/Ni joints, the Pd sheet was electroplated with a multilayer of 6-m Ni film, 4- m Sn, and 3-m Ag. The pretreated Pd sheet was diffusion soldered with a 4-m Sn-coated Ni plate at 300ºC for 30 min (Case III in Fig. 3). Figure 9 shows sound interfaces without any voids or cracks in the Pd/Ni joints sandwiched with a multilayer structure of Pd/Ni/Ni3Sn4/ Ag3Sn/Ni3Sn4/Ni. The perfect diffusion soldered interfaces resulted in a satisfactory bonding strength of 22.6 MPa. However, delamination of electroplated Ni film from the Pd surface occurred quite often during the subsequent electroplating processes of Sn and Ag layers on the Ni-coated Pd sheet. To solve this problem, the Ni-coated Pd sheet was preheated at 450ºC for 30 min and then electroplated with Sn and Ag layers before being diffusion soldered with the Sn-coated Ni plate (Case IV in Fig. 3). According to a Ni-Pd phase diagram reported by Nash et al., Ni and Pd form a continuous fcc solid solution with a minimum in both the liquidus and solidus boundaries (Ref. 19). The diffusion profile of the Ni element in the Pd matrix in Fig. 10 indicates that a Pd-Ni solid solution zone with a thickness of about 5 m appears between the Pd sheet and Ni layer. The micrographs of such Pd-Ni solid solution covered Pd/Ni joints using a Sn/Ag/Sn interlayer are shown in Fig. 11. In this case, a sandwich structure of intermetallic compounds Ni3Sn4/Ag3Sn/Ni3Sn4 formed between the Ni-coated Pd sheet and Ni plate. The apparent activation energy for the intermetallics growth of the upper Ni3Sn4 (Ni plate side) and lower Ni3Sn4 (Pd sheet side) in Fig. 11, as calculated from the measurements of intermetallic thicknesses in Fig. 12A and their Arrhenius plots in Fig. 12B, were 29.6 and 50.2 kJ/mol, respectively. The apparent activation energy for the growth of the upper Ni3Sn4 (Ni plate side) is near that for the Ni3Sn4 growth in Case I (26.8 kJ/mol). However, the growth of lower Ni3Sn4 (Pd sheet side) showed a much higher apparent activation energy than that in Case I. 448-s WELDING JOURNAL / NOVEMBER 2016, VOL. 95 As compared to the diffusion profile in Fig. 10, the Ni concentration in front of the Pd-Ni solid solution zone decreased gradually with the increase of bonding temperatures, as revealed in Fig. 13, which implies that the precoating Ni film on the Pd sheet was consumed during diffusion soldering. However, the thickness of the Pd-Ni solid solution zone changed slightly, which indicates most of the consuming Ni atoms reacted with the Sn atoms in the Ag3Sn intermetallic layer, leading to the growth of the Ni3Sn4 layer (Pd sheet side). Because the diffusion source of the Ni atoms from the Pd-Ni solid solution zone for the growth of lower Ni3Sn4 (Pd sheet side) is much less than that from the pure Ni plate for the growth of upper Ni3Sn4 (Ni plate side), the higher apparent activation energy for the Ni3Sn4 growth on the Pd sheet side (50.2 kJ/mol) in comparison to that on the Ni plate side (29.6 kJ/mol) could be resulted in. The bonding strengths of these Pd-Ni solid solution covered Pd/Ni joints (Case IV), as shown in Fig. 8, ranged from 18.9 to 24.1 MPa, which was higher than those bonded in Case II. For the evaluation of reliability, the Pd-Ni solid solution covered Pd/Ni joint diffusion soldered at 300ºC for 30 min was aged at 400ºC for 500 h. During this period of aging, the upper Ni3Sn4 intermetallic layer grew from about 2.6 to 7.8 m — Fig. 14. On the other hand, the Ag3Sn intermetallic layers grew from 4.3 to 12.4 m, accompanied by the consumption of the lower layer of Ni3Sn4, which was reduced from 1.7 to 0.2 m. It was also found that the bonding strengths did not obviously decay after this high-temperature storage test, which is optimistic for the longterm durability of the Pd/Ni joints during the operation of hydrogen purifiers at elevated temperatures above 350ºC. However, a leakage test with a hydrogen pressure of more than 1.7 MPa and further reliability tests are needed to ensure the real applicability of the diffusion soldered Pd cell for hydrogen purification. Conclusions Pd sheet and Ni plate can be bond- Fig. 14 — Microstructure of the diffusion soldered PdNicoated Pd/Ni couple (Case IV) bonded at 300°C for 30 min after hightemperature storage at 400°C for 500 h.
Welding Journal | November 2016
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