CAPACITOR DISCHARGE SPOT WELDING OF COATED MOLYBDENUM ALLOY SHEET

In this paper, the technique of applying insert-foils in capacitor discharge spot welding for a successful joining together of coated molybdenum alloy sheel is described. Methods have been developed, with which the main technical problems, such as sticking of coating to the electrode tip, cracking in the welded spot and deterioration of joint strength, were solved with satisfaction. Success in preventing sticking was achieved by inserting tin or lead foil (0.3~0.45 mm thick)between electrode tips and workpiece. Increase of strength at high temperature was obtained by inserting niobium or tantalum foil (0.03~0.05 mm thick) between the contact surfaces of workpieces. The effect of the shape of electrode tip on crack formation and sticking were investigated. It was shown that the electrode with domed tip was more effective in reducing cracking and sticking than that with flat tip. Metallographic examination showed that actually welding was not due to the formation of nugget, but due to the formation of solid phase bonding between niobium foil and molybdenum alloy sheet instead. Under the action of pulse current heating and electrode pressure, the coating in the joint centre melted and was squeezed out.