Influence of redesigned electrode system on interfacial microstructures and thermal behaviors in resistance micro-welding of stranded conductor to interconnector

Abstract

Mechanical and electrical properties of joints between stranded Ag-plated Cu conductors and Ag-plated Kovar interconnectors are crucial for the reliability and energy efficiency of space solar arrays. Parallel gap resistance welding (PGRW) is a conventional approach for fabricating the welds. However, the PGRW involves two critical issues, namely underheated wire-interconnector interfaces and overheated wires, which narrowed the weld lobe and resulted in unsatisfying joint properties. In this study, an indirect resistance spot welding (IRSW) with a redesigned electrode system was developed to address the issues. By the IRSW, 80% enlargement in the weld lobe and 18% reduction in joint resistance were achieved. The two processes regarding interfacial microstructure and thermal behaviors were comparatively investigated to understand the fundamental mechanism behind the advancements. Compared with the PGRW, the IRSW elevated the temperature history at the wire-interconnector interface, which reinforced the interfacial bonding by transforming the sold-state diffusion bonding to brazing. The IRSW decreased the temperature of the upper wires while uniformizing the temperature among the multi-layered wires (e.g. the peak temperature gap between the upper and low wires was narrowed from 118℃ to 15℃), which inhibited the formation of high-resistivity Ag-Cu solid solution. The process also reduced the temperature at the heat-affected zone of wires, which suppressed the softening. The key to achieving high-quality micro welds between Ag-plated Cu conductors and Ag-plated Kovar interconnectors was to obtain brazing-based interface bonding while suppressing alloy behavior through uniformized electric and thermal fields.