Microstructure evolution and grain refinement mechanism of ultrasonic-assisted low-frequency pulse VPPA welded joint of AA2195-T8 Al-Cu-Li alloy

Variable polarity plasma arc welding is a widely utilized welding technique for the AA2195-T8 alloy. However, coarse grains adversely affect the weld's mechanical properties. This study integrates ultrasonic-assisted and low-frequency pulse welding processes to achieve grain refinement and enhance mechanical properties. The results indicated that high-amplitude ultrasound produced a superior grain refinement effect. However, this method also introduced numerous microscopic defects in the weld, compromising the mechanical properties. In contrast, low-frequency pulse welding exhibited less effective grain refinement than ultrasonic assistance without microscopic defects. The optimal grain refinement was achieved when low-amplitude ultrasound was combined with the high-difference pulse welding process, resulting in the best grain refinement effect. The weld grain morphology primarily consisted of refined equiaxed grains, with an average size reduction of 82 % (18.8 μm). The coarse second phase near the grain boundary was the Al-Cu-Mg-Ag eutectic phase. T1 and σ phases, along with dislocation lines, were prevalent in the grains. Compared to conventional welds, the tensile and yield strengths improved. However, the presence of some microcracks led to a reduction in elongation. The cavitation and acoustic streaming effects induced by ultrasonic assistance facilitated liquid convection and oscillation, resulting in more uniform second phases and disrupting coarse dendritic structures. Furthermore, the chilling effect of the pulse current promoted grain nucleation and accelerated molten pool flow, compensating for the limited grain refinement effect associated with low-amplitude ultrasound. At the late stage of solidification, the plasticity of the intercrystalline liquid film decreased, while the ultrasonic oscillation increased the strain, contributing to the formation of microscopic defects.