Ultra grain refinement and mechanical properties improvement of all-weld-metal for medium-thick Al-Li alloy via laser beam oscillation and in-situ alloying

The formation of weld cracking in high-strength Al-Li alloys has been a longstanding challenge attributed to undesirable coarse columnar grains and inhomogeneous microstructure distribution in the fusion zone of welds. At present, grain refining has been recognized as the most effective approach to inhibit cracking and enhance the mechanical properties of welded joint. Herein, a novel welding strategy exploiting laser beam oscillation and in situ alloying with Zr was proposed to obtain ultra-fine equiaxed grains in all-weld metal for medium-thick Al-Li alloy. Macro-scale simulation for heat-mass transfer and micro-scale simulation for microstructure evolution were conducted to investigate the formation of ultra-fine equiaxed grains. Firstly, the oscillating laser beam causes the periodical fluctuation of molten metal in the molten pool, which promotes adequate diffusion of Zr elements during welding. Secondly, the homogeneous Zr element scattering in the molten pool has sufficient cooling conditions to precipitate the L12-Al3Zr phase in all-weld metal. The cooling rate during solidification is less than the critical temperature calculated by the time-dependent nucleation theory, which creates favorable conditions for forming primary Al3Zr. Finally, dense efficient nucleation sites prompt the formation of ultra-fine grains in all-weld metal. Compared with the single laser welding joint, the grain size decreases from 106.7 to 7.7 & mu;m, and the tensile strength increases from 228 to 379 MPa. The microhardness of the fusion zone is equivalent to that of heat affected zone. These findings suggest that further improvements in mechanical properties cannot achieved by improving the process of fusion welding.