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In this work, the asymmetric weld fusion geometry in oscillating laser beam welding (OLBW) of aluminium alloy was reported and investigated using a numerical approach. A multi-physics heat transfer and fluid flow model of OLBW was developed and validated with the corresponding experimental results. The weld fusion geometry, temperature fields, and fluid flow behaviours for four commonly used oscillation modes, line, circle, eight, and infinity, were calculated to analyse the origin of the asymmetric weld fusion geometry. The results show the asymmetry of local heat input along beam travelling path and the fluid flow pattern in the molten pool are the main factors that result in the asymmetric weld fusion geometry in OLBW. [ABSTRACT FROM AUTHOR]

• A new welding method of combining oscillating laser with vacuum environment were carried out for a deeper sound weld. • With the increase of the oscillating amplitude, the penetration depth first increases and then decreases. • The mechanism by which beam oscillation increases grain size in a vacuum environment has been elucidated. • The tensile strength of the joints welded by OLBW under vacuum all achieved 90% of the base metal. Beam oscillation in the laser welding process has demonstrated numerous advantages in suppressing spatters, eliminating pores, and reducing gap sensitivity, particularly for joining thin plates. This study presents the welding of thick section aluminum alloy using laser beam oscillation in a vacuum environment. During vacuum laser welding, the effect of beam oscillation variables, which include oscillating path, amplitude, and frequency, on weld seam morphology, microstructures, and mechanical properties was symmetrically evaluated. The findings indicate that in a vacuum environment, beam oscillation with a small amplitude can optimize the contour of the weld sidewall and increase the penetration depth, which is different from that observed in an atmospheric environment, where the melting penetration depth decreases with increasing oscillating amplitude. The deep penetration weld seam in a vacuum environment can achieve an extremely small size of 19 ∼ 30 µm. The addition of small oscillations causes a slight increase of the average grain size. The stirring effect of beam oscillation was weakened because of the minimized weld pool size under vacuum. This increased grain size is mainly attributed to the changes in the size of the molten pool caused by the oscillation cause changes in solidification parameters which accelerate grain growth. However, increasing the oscillation frequency can greatly increase the strength of the stirring effect and the refinement ability of small-sized grains. On further increasing the oscillation frequency to 200 Hz, the grown grains are refined. The results showed the tensile strength of the vacuum laser weld joints with and without beam oscillation can achieved a value of about 90 % of the base metal, which is superior to the strength of the joints welded by atmospheric pressure laser welding and can meet the vast majority of welding needs. [ABSTRACT FROM AUTHOR]