Two-Pass Friction Stir Welding of AZ31/AA1050 Bimetallic Butt–Lap Joint

Abstract This study evaluated the feasibility of friction stir welding (FSW) of AZ31/AA1050 bimetallic plates produced by explosive welding. The investigated material had a total thickness of 10 mm (5 + 5 mm) and, after milling, 8 mm (4 + 4 mm). Butt–Lap joints were produced using both a single-pass and a two-pass FSW approach. The process parameters were 1600 rpm and 150 mm/min for the AZ31 alloy, and 600 rpm and 100 mm/min for the AA1050 alloy. In the single-pass variant, a tool with a 9.8 mm pin length was used, whereas in the two-pass process, tools with pin lengths of 4.8 and 3.8 mm were applied, with penetration depths ranging from 4.0 to 4.9 mm. The investigation of the joints properties involves microhardness analysis and tensile strength tests. The joints were subjected to microstructure analysis using scanning electron (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) techniques. The results showed that the single-pass process led to excessive plasticization of aluminum, intensive material mixing, and the formation of discontinuities. The most favorable results were obtained for the two-pass variant with a penetration depth of 4.0 mm, without crossing the interfacial boundary between the layers. It was found that when the total width of the mixed zone exceeded 0.4 mm, discontinuities appeared, while values above 0.6 mm resulted in voids and joint degradation. The microhardness of the AZ31 alloy was generally 80 to 85 HV0.1, locally increasing to about 115 HV0.1, whereas for AA1050 it increased from about 30 HV0.1 in the base material to 45 HV0.1 in the TMAZ and up to 60 HV0.1 in the stir zone. The tensile strength of the base material was 191.4 ± 14.6 MPa, while that of the FSW joint reached 142.0 ± 8.8 MPa, corresponding to a joint efficiency of approximately 74 pct. The results confirm that two-pass FSW is an effective method for joining Al/Mg bimetals, provided that the tool penetration depth is strictly controlled.