Microstructure and mechanical properties of nano-treated 7075 aluminum alloy joint fabricated by laser-MIG hybrid welding

Hot cracking and joint softening are the two main factors restricting the achievement of high-performance fusion welding joints of 7075 aluminum alloy. This paper utilized low heat input laser-melt inert gas welding (MIG) hybrid welding to butt weld 7075 aluminum alloy. The crack-free joint was obtained with the assistance of nano-treating. The weld microstructure was composed of fine equiaxed grains with an average size of 12.1 μm ± 5.9 μm, and there was a 20 μm columnar grain near the fusion line, indicating that the addition of nanoparticles promoted the transition from columnar grains to equiaxed grains and refined grain size. Additionally, the eutectic phase was also refined by nanoparticles. The fine equiaxed grains and the refined eutectic phase contributed simultaneously to the formation of the crack-free weld. The tensile strength, yield strength, and percentage elongation after fracture of the as-welded joint were 404 MPa ± 2.8 MPa, 360.5 MPa ± 0.7 MPa, and 1.5% ± 0.14% respectively. Post-weld heat treatment further enhanced the mechanical properties. The tensile strength, yield strength, and percentage elongation after fracture after solution treatment and aging treatment were improved to 529.5 MPa ± 20.5 MPa, 462.5 MPa ± 3.5 MPa, and 5.1% ± 2.5%.