Study of microstructure evolution of friction stir welded novel (Al-Zn-Mg)-Fe (HE700) cast alloys for automotive applications

Light weighting of automobiles would improve fuel efficiency and reduce emissions. Newly developed high-strength, high-elongation (Al-Zn-Mg)-Fe alloys for structural shape casting applications have the potential to fulfill the demand for significant lightweighting of components. Joining such shape casting alloys to form a high integrity component assembly is a critical process in structural lightweighting. Friction stir welding has recently emerged as an effective joining method for aluminium alloys. In this work, friction stir welding was used to join these new-generation (Al-Zn-Mg)-Fe cast alloys. The critical process parameters such as rotation and traverse speed were optimized through a detailed microstructural study and mechanical property evaluation of the welds. Three different tool rotation speeds (600, 800, and 1000 rpm) and traverse speeds (25, 50, and 75 mm/min) were the independent parameters. The welds made with 800 rpm and 25 mm/min traverse speed showed the best properties with no weld/microstructural defects. Friction stir welding also refined the microstructure and uniformly distributed the agglomerated Al-Fe-based intermetallic particles. The microhardness and tensile properties (longitudinal direction) of the stir zone increased significantly (Ultimate tensile strength = 380 MPa, Yield strength = 250 MPa) with appreciable ductility (31%) due to the microstructural refinement despite the dissolution of the strengthening precipitates such as Guinier-Preston zones and ηꞌ phases. The study was able to establish a viable window of processing parameters for joining the new generation Al-Zn-Mg-Fe alloys for structural lightweight components.