Eliminating Cu-rich intermetallic compound layer in dissimilar friction stir welding of 304 stainless steel and 2219 Al alloy via ultralow rotation speed

The formation of the Cu-rich intermetallic compound (IMC) layer at the interface has always been the critical bottleneck of welding steel to 2xxx series Al alloys with high Cu content. In this study, we successfully eliminated the Cu-rich IMC layer by using a modified friction stir welding (FSW) method under an extremely low rotation speed of 200 rpm. At an appropriate thickness difference of the welded plates, defect-free FSW joint of 304 stainless steel to 2219Al alloy with high tensile strength of 210 MPa was obtained, and the sample failed in the 2219Al side rather than at the interface. It is found that a continuous Al-Fe-O amorphous layer of similar to 10 nm in thickness formed at the steel-Al interface, and only few Al2Cu IMC particles existed at the interface. The Al2Cu particles were blocked by the amorphous layer and cannot directly contact the steel, consequently realizing the excellent metallurgical bonding between the steel and 2219Al alloy. It is anticipated that this work will provide a feasible strategy for fabrication high-quality dissimilar joints between steel and 2xxx series Al alloys, which can broaden the application of steel-Al joints in the industries.