Mechanical behavior of dissimilar friction stir welded T-joints between aluminum alloy and steel: role of kissing bonds, intermetallic compounds, and microstructural evolution

This study investigates the mechanical behavior of dissimilar friction stir welded (FSWed) T-joints of AA6061/steel, focusing on the roles of kissing bonds (KBs), intermetallic compounds (IMCs), and microstructural evolution. The T-joints are fabricated at the welding speeds of 100–250 mm/min and characterized by SEM, EDS, XRD, and tensile testing for both skin and stringer loading. The results show that both KBs and IMCs are unavoidable and strongly dependent on welding conditions. At low welding speeds, thick and brittle IMC layers consisting of Fe₂Al₅ and FeAl₃ phases together with interfacial micro-cracks promote crack initiation and reduce stringer strength. Increasing welding speed led to a reduction in IMC thickness from ~ 1.0 μm to nearly undetectable levels. The high welding speed suppresses IMC formation but produces unfavorable KB geometry that promotes crack propagation, leading to low stringer strength. The maximum stringer strength of approximately 71 MPa is achieved at 150 mm/min, whereas the skin strength reaches about 207 MPa. The evolution of KB geometry generates the favorable crack tip angles of approximately 60–75°, which promotes crack propagation along the interface. The KB geometry plays a dominant role in fracture behavior, while IMCs mainly affect crack initiation through interfacial brittleness. Microstructural evolution has a secondary effect, mainly influencing skin strength. The optimal welding speed is obtained at 150 mm/min with a balance between reduced IMC brittleness and favorable KB geometry, resulting in the highest joint strength.