Lap joints of an upper Al alloy (1.0-mm-thick A5052) and lower Zn-coated steel (1.2-mm-thick GA steel) were welded by a novel spot welding process for dissimilar metal lap joints using a new tool with the tip made of spherical ceramics, i.e., "Friction Anchor Welding." As a result, the Al atoms in the Al alloy diffused into the Zn-Fe layer on the GA steel, and the layer transformed into an altered layer which was mainly composed of Al-Fe intermetallic compounds. Because of this altered layer, a steel projection could not extend straight when the rotating tool was plunged through the Al alloy into the GA steel. Consequently, the height of the steel projection was small and exhibited a rugged shape while the steel projection was formed in the Al alloy. Furthermore, large amounts of Al-Fe intermetallic compounds existed near the steel projection. Additionally, Zn atoms in the Zn-Fe layer on the GA steel penetrated into the Al alloy and cracks occurred due to the Al-Zn eutectic melt. Thus, the tensile shear strength reached only about 2.7 kN/point, compared to that of the weld between A5052 and SPCC, which reached about 3.6 kN/point.
