Spot Welding of Zinc Coated Steel

Abstract

Generally, in spot welding of galvanized sheet, it is believed that better conditions than in welding of bare sheet are necessary, and that bare sheet sometime requires 300-400% of welding current.
We think that the difficulty of resistance welding of galvanized sheet is due to the reduction of the current density, by which the heat generation was decreased. As the cause, the following two can be considered:
i) Microscopically enlard area depending on softness of zinc coating. This means less contact resistance.
ii) Macroscopically enlarged area by spreading of fused zinc depending on the low melting point. On the basis of such consideration and some experimental demonstration, we could succeed in getting reliable weld even under lower class welding conditions, by using a low inertia holder. (See Table 3 and Fig. 11)
However, as the number of welding spots increases, nugget gradually becomes hard to form owing to the pick-up of zinc coating (even in such a case considerable shear strength can be obtained).
To prevent such pick-up, it is necessary to dress the electrode tips about every 1000 spots.