Effect of thermophysical properties on porosity and microstructure of laser welded cast and wrought aluminum alloy dissimilar lap joints

The influence of thermophysical properties of materials on the porosity and microstructure of dissimilar lap joints between laser-welded Al–Si–Mg cast and Al–Mg–Si–Cu wrought aluminum alloys was investigated. The low thermal conductivity and viscosity of the weld would promote bubble escape, while the high electrical conductivity of the base material would increase the melt depth and reduce the convection intensity in the melt pool, thus increasing the difficulty of bubble escape. The variation in thermophysical properties did not significantly impact the distribution of elements, the type of precipitated phases, and the dislocation density, however, it did affect grain morphology. Differences in thermal diffusion coefficients between base materials created conditions for constitutional supercooling, increasing the likelihood of a columnar-to-equiaxed transition. Moreover, the temperature difference between the liquid phase lines of the weld and base materials facilitated the liquid base material flushed into the weld to solidify before mixing, resulting in an “island” formation. This “island” acted as a heterogeneous nucleation site, leading to equiaxed grain layer formation in the lowest zone of the weld. The variations in thermophysical properties exerted an influence on the tensile strength by impacting both the melt depth and the grain morphology of the weld but did not yield a significant impact on the average microhardness.