High Performance 7075‐T6/6061‐T6 Dissimilar Joint Fabricated via Laser‐MIG Hybrid Welding Assisted by Nanotreating and Postweld Heat Treatment

Crack‐free dissimilar joining of 7075‐T6/6061‐T6 aluminum alloys has been realized by laser‐MIG hybrid welding technique assisted with nanotreating. The weld metal is composed of fine equiaxed dendrites. Indirectly promoted nucleation and physical impedance on grain growth brought by nanoparticles are the primary reasons for the refined grain structure. Increased grain boundary due to grain refining leads to enhanced grains coalescence during solidification and improved hot cracking resistance. Heat‐affected zone softening takes place and the microhardness minima of the as‐welded joint locates at heat‐affected zone of 6061 side with value of 65.8 Hv. The ultimate tensile strength of the as‐welded joint reaches 233.5 MPa, comparable to that of the 7075‐T6/6061‐T6 dissimilar joints fabricated using friction stir welding. Postweld solution and artificial aging heat treatment further enhances the mechanical properties of the joint. This combined heat treatment increases the tensile strength and ductility by 15% and 36%, respectively. The key strengthening mechanisms after heat treatment are region‐specific: the η′ phases in the 7075 side, the coexistence of η′ and T′ phases in the weld metal, and the β″ phases in the 6061 side. Elimination of the soft zone and uniform deformation contribute to the improved ductility of the heat‐treated joint.