When galvanized steel sheets are closely overlapped and welded by laser lap-welding, a large amount of molten metal spatters, resulting in a poor surface appearance of the weld and weakened strength of the welded joint, as compared with that of cold-rolled steel sheets. Whereas, in the case of Al-coated steel sheets, even when closely overlapped and welded by laser lap-welding, no spattering occurs. Thus, a good surface appearance of the weld is obtained, but the welded joint has lower strength. In both cases above, it is known that if a clearance of about 0.1mm is provided between steel sheets, laser lap-welding produces a good surface appearance of the weld and the welded-joint strength equal to that of cold-rolled steel sheets.
This report discusses specifically how, in laser lap-welding of overlapped Al-coated steel sheets, Al of the coated layer comes to enter the weld metal, also specifically how to reduce the joint strength, as well as what behaviors of Al when a clearance is provided between steel sheets. When steel sheets are closely overlapped and welded, Al becoming molten on the base-metal side of the bond of the overlapped face becomes swallowed up by the bath streams of the molten pool, flowing into the molten pool, then forming the Fe-Al intermetallic compound, while not being sufficiently stirred. It is considered that when subjected to the tensile shear test, the Fe-Al intermetallic compound starts to fracture, thereby causing a partial loss of the weld metal and a reduction in the joint strength. On the other hand, when a clearance is provided between steel sheets, it may be inferred that the fusion Al on the base-metal side of the bond stays in place without flowing into the molten pool, consequently not forming the Fe-Al intermetallic compounds within the weld metal.