Multi-scale modelling on the dynamic microstructure evolution and solute segregation behavior for the adjustable-ring-mode (ARM) laser welding on Al-Mg-Si alloy

Abstract

To address the issue of how the special energy field affects solidification microstructure evolution and solute segregation for the novel ARM laser welding on Al-Mg-Si alloy, a macro-micro coupling multi-scale numerical model was built in this paper. As comparison, under the condition of pure core laser welding, the fundamental reason for formation of coarse columnar crystals with 586.37 μm width was that high and rapidly changing temperature gradient G and pulling velocity R inhibited the undercooling zone expansion, resulting in insufficient driving force for grain nucleation and then the promotion of rapid columnar crystal growth. A small average primary dendritic arm spacing (PDAS) of 4.1 μm was generated. However, under the condition of ARM laser welding, the significant reduction of nearly 50 % in G and R and their gently changing trends allowed the undercooling zone to expand, promoting the grain nucleation, inhibiting the columnar crystal growth by about 19.7 %, and increasing the average PDAS by 21.9 %. Further, adopting the ring beam weakened the solute micro-segregation and regional segregation. The slower cooling rate and thicker undercooling layer made the solute have more time and space for sufficient diffusion, reducing enrichment concentration from approximately 6 % to 4 %. And the uniform energy distribution in the molten pool ensured that solute diffusion behavior did not change significantly during the solidification process, with the average solute concentration variation shifting from an increase of 0.9 % to 1.4 % to a slight fluctuation around 1 %.