Estimation of current path area during small scale resistance spot welding of bulk metallic glass to stainless steel S. Fukumoto 1 , A. Soeda 2 , Y. Yokoyama 3 , M. Minami 2 , M. Matsushima 1 and K. Fujimoto 1 The current path area is a significant factor in estimating the temperature distribution via numerical modelling for resistance spot welding. This paper presents a method for the estimation of the current path area at the faying surface during small scale resistance spot welding between bulk metallic glass and stainless steel. Observation of cross-sections and fracture surfaces reveals the welding process at the faying surface for both dissimilar and similar welding. The equipotential surface that depends on the difference between the contact area of the electrode- to-sheet and sheet-to-sheet interfaces is estimated by numerical modelling. The current path area at the faying surface is estimated by measuring the electric potential between the sheets, taking into account the current distribution. Keywords: Current path area, Dynamic resistance, Resistance spot welding, Equipotential surface, Fringing field, Microjoining Introduction Bulk metallic glasses (BMGs) are candidates for use as widely functional materials because of their unique properties, such as a low Young’s modulus and high strength. In order to take advantage of their unique properties, they need to be welded to other conventional structural alloys, such as stainless steels. However, it is well known that most amorphous alloys are embrittled by the application of heat, which leads to structural relaxation, phase separation, and crystallisation. Fast heating and cooling rates are necessary for the successful welding of BMGs. BMGs were recently reported to be successfully welded without crystallisation via fusion processes, such as electron beam welding, 1,2 laser welding, 3,4 and re- sistance spot welding (RSW). 5 Among them, RSW is the only process that does not require a controlled atmo- sphere, because the molten metal is never exposed to the atmosphere. This is a significant advantage, particularly for the fusion welding of Zr based BMGs, because they are sensitive to oxidisation. Although RSW is a suitable fusion process for similar welding of BMGs, there are issues that need to be resolved before the application of RSW to dissimilar welding of BMGs. The formation of brittle intermetallic compounds (IMCs) when two different metals are welded is unfavourable. In dissimilar welding of a Zr based BMG to stainless steel via small scale resistance welding, the stainless steel can melt, forming a brittle IMC weld nugget at the weld interface, which degrades the joint quality. 6 Therefore, controlling the distribution and hysteresis of the temperature is vital in order to avoid either the crystallisation of BMGs or the formation of brittle IMCs at the RSW interface. However, it is impossible to measure the temperature in the weld directly using a thermocouple because a high current, on the order of several hundreds of amperes in small scale RSW, flows between sheets. Moreover, when at a small scale, the weld zone is too small to place a thermocouple in. Harlin et al. 7 reported that the appro- ximate distribution of temperature in RSW steels could be estimated by microstructural observation. However, the weld nugget of BMGs is invisible because no signi- ficant microstructural change is observed in both the weld and the heat affected zone (HAZ) if no crystal- lisation occurs. 5 Therefore, it is difficult to approximate the temperature distribution via microstructural obser- vation in the case of RSW of BMGs. Cho et al. 8 observ- ed nugget formation in a 1?4 mm thick steel directly by using half-section-truncated dome-type electrodes and a high speed camera. However, the phenomenon observed by using half-section electrodes is different from what is observed by using regular electrodes. Moreover, as described above, it is difficult to directly observe the molten nugget in small scale welding owing to its small size. Therefore, numerical simulations, such as finite element method (FEM) analysis, are often applied to calculate the temperature distribution and hysteresis in RSW. Wei et al. 9 precisely calculated the temperature hysteresis and distribution, accounting for the electro- magnetic force, heat generation at the electrode-to-sheet and sheet-to-sheet interfaces, and dynamic electric resistance. The model carefully considered the contact 1 Graduate School of Engineering, Osaka University 2 Graduate Student of Osaka University 3 Institute for Materials Research, Tohoku University *Corresponding author, email fukumoto@mapse.eng.osaka-u.ac.jp ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 3 September 2012; accepted 31 October 2012 DOI 10.1179/1362171812Y.0000000085 Science and Technology of Welding and Joining 2013 VOL 18 NO 2 135