On the elimination of solidification cracks in fusion welding of Al7075 by TiC-nanoparticle enhanced filler metal

Abstract

Solidification cracks in fusion welding of high strength and precipitation hardened aluminum alloys have been a major challenge for many years. In this research, the weldability of 3-mm thick Al7075 sheets in fusion arc welding was investigated. Welding was conducted with (heterogeneously) and without (autogenously) TiC-nanoparticle enhanced Al7075 filler metal. Results show that the nanoparticles were effective in eliminating welding solidification cracks completely. Two mechanisms are proposed: (1) TiC nanoparticles act as nucleants in the early stage of solidification to alter the microstructure from dendritic to equiaxed and to refine the grains; (2) TiC-nanoparticles act as nucleants in the late stage of solidification to change the shape of eutectic precipitates from continuous to discontinuous and to promote the formation of the Al-Cu-Mg phase at the expense of the deleterious Mg(Cu,Zn)2 (η) phase. Tensile mechanical properties of autogenous weld could not be obtained due to large solidification cracks. In contrast, the heterogenous joint showed relatively good tensile strength and ductility that, however, were slightly lower compared to unwelded Al7075. This was directly linked to the high amount of porosity in the fusion zone due to the use of non-optimal welding parameters adopted in this study to promote solidification cracking for better comparative study and investigation of effects of nanoparticles.