Nucleation mechanism in oscillating laser welds of 2024 aluminium alloy: A combined experimental and numerical study

The grain structure of the weld metal significantly influences the mechanical properties of welds. However, the microstructure formation during oscillating laser welding (OLW) has not been investigated. This study aimed to clarify the nucleation mechanism in oscillating laser welds of 2024 aluminium alloy via combined numerical and experimental investigation. The numerical results revealed that the flow direction of the molten metal always aligned with that of the laser beam movement, which led to periodic fluctuations of the flow field in the molten pool. The trajectory of all particles near the liquidus temperature was helical owing to the periodic fluid flow. Further, the cooling curves obtained during welding indicated that periodic temperature fluctuations existed at different positions in the molten pool and contributed to the reheating of the molten metal during solidification. The dendrite side arms were pinched-off because of reheating-induced remelting and the capillary-driven pinching caused by surface energy. The nucleation mechanism was identified as dendrite fragmentation using an overlap welding procedure. The grain structure and size of the microstructure were unchanged for the different overlap welding cases. Furthermore, the dendrite arms successively underwent pinch-off, involvement, preservation, and nucleation during OLW and thereby refined the grain in OLW welds. This study provides a theoretical basis for grain refinement in the welds of aluminium alloys.