In-situ formed amorphous phase in aluminum/steel friction stir welds: Interface evolution and strength improvement

Aluminum/steel structures are widely proposed for weight reduction in aviation, aerospace, and automotive industries, whereas the applications of aluminum/steel structures are still limited due to the unreliable welding related to the intermetallic compounds. In this study, an amorphous layer was in-situ formed at the aluminum/steel interface, replacing the intermetallic compounds, and strengthening the welds. The effects of the plunge depth on the microstructure and mechanical properties of the Al/steel friction stir welds were further investigated. The amorphous phase was only formed when the welding tool was plunged to the interface precisely. Once the plunge depth was further increased, the amorphous layer would grow over the critical thickness of 18 nm and, subsequently, be replaced by the FeAl₃ and FeAl. Different interfacial microstructure led to the different strength and fracture characteristics. The ultimate load of 6237 N was achieved with the in-situ formed amorphous layer, and it was improved by 45 %, as compared to the previous results.