The impact of Adjustable-Ring-Mode (ARM) laser beam on the microstructure and mechanical performance in remote laser welding of high strength aluminium alloys

This paper introduced the use of an adjustable-ring-mode (ARM) laser beam to optimise the microstructure and mechanical performance of laser welded 6xxx high-strength aluminium alloy joints. A remote laser welding system equipped with transverse beam oscillation was employed and welding parameters, including the beam oscillation width and core/ring power ratio of the ARM laser beam, were investigated sequentially. Results showed that the core/ring power ratio at a constant total power has a limited control on the weld geometry when integrated with beam oscillation. Increasing the beam oscillation width, up to 2.5 mm, resulted in a wider weld interface and a threshold value of 3.1 mm was determined to activate the transition from interface failure to the fusion boundary failure subjected to tensile lap shear loading, leading to a significant improvement in the joint strength from 170 N/mm to 277 N/mm. Furthermore, results revealed that proper selection of core/ring power ratio at a constant total power can restrict the formation of columnar grains near the interface between two plates, resulting in grain refinement within the weld zone. Both extremely high power ratio, for example core alone mode (infinite power ratio) and extremely low power ratio, e.g. ring alone mode (zero power ratio), lead to a low thermal gradient and cooling rate at the solidification front. A compromised solution which balances the wider formation of equiaxed dendrites and better grain refinement was determined at the power ratio of 0.33, translating to the improvement in joint strength up to 400 N/mm.