A study of resistance spot weldability of metallic coated steels and PVD coated electrodes

The resistance spot weldability characteristics of three commercially available electrode compositions, Cu/Cr, Cu/Cr/Zr, and Cu/Zr have been investigated when welding three metallic coated sheet steels: aluminium 45% zinc, galvanised and galvanneal steels. Electrodes were evaluated using weldability lobes and electrode life tests. The aggressive nature of the erosion process encountered for Al-45%Zn and galvanised steels masks the influence of small additions of alloying elements such as Zr on electrode life. However, for galvanneal steel where the erosion process is more gradual, the Cu/Cr/Zr electrode proved to be the most effective in obtaining maximum life. The development and composition of alloy layers which form on the tips of each electrode used to weld galvanneal steel have been identified from the outside as: a surface oxide sublayer (ZnO), a porous Fe-Zn sublayer (F phase: FesZnio), a hard and brittle gamma (y) brass (CusZng) sublayer, and a ductile beta (p) brass (CuZn) sublayer, and then the Cu/Cr substrate. The feasibility of depositing a surface coating onto Cu/Cr electrodes to mitigate the formation of alloy layers and thus extend electrode life was also investigated. Physical vapour deposition of Cr using unbalanced magnetron sputtering and filtered arc evaporation extended the life of Cu/Cr electrodes by 100% from approximately 420 to 840 welds when welding Al-45%Zn sheet steel. Although extension of electrode tip life was achieved it has been established that the life determining failure mechanism of coated electrodes is by gross plastic deformation of the electrode tip due to the extreme temperatures and repeated compressive forces generated during the welding operation. It is therefore proposed that future work should aim to improve the mechanical properties of the electrode material in order to provide a strong base capable of supporting a thin liquid metal resistant coating. This is essential to take full advantage of the benefits that thin film coatings have to offer.