Because the fluid flow behavior in a molten pool is closely associated with the mechanical properties of the joint, the analyses and regulation of welding temperatures and flow fields are of great practical importance. The main objectives of this study are to reveal the interaction of the oscillating laser beam with the molten pool and to elucidate the underlying relationship between the welding temperature and flow fields as well as the microstructure of the joint.

The welding experiments were conducted using an oscillating laser wire-filling welding system with a butt joint configuration. A three-dimensional transient multiphysical numerical model was constructed to simulate the oscillating laser welding process of 5083 aluminum alloy. The microstructures of the joints were observed using an AOSVI M320P-HK830 polarizing microscope (PM) and a Zeiss Ultra 55 LE scanning electron microscope (SEM). The chemical compositions of the different phases of the joints were analyzed using an XFlash^?4010 energy dispersive spectroscope (EDS) detector. X-ray diffraction (XRD) analyses of the joints were performed using a D8 Advance X-ray diffractometer for phase identification. Nondestructive X-ray testing was performed to detect weld pores.

The results show that molten pool stirring produced by an oscillating laser beam facilitates heat transfer and thus reduces the temperature gradient of the molten pool. In addition, the oscillating laser beam stirs the molten pool to drive the fluid flow in the same direction and form vortices, thereby increasing the stability of the molten pool. When the oscillaton frequency is 100 Hz, a vortex with a flow rate of up to 0.7 m/s is formed, which significantly improves the stability of the molten pool. Furthermore, degassing the molten pool, refining the grain size of the weld, and promoting the precipitation of the β phase in the weld can be achieved by virtue of the oscillating laser beam stirring the molten pool. As the oscillation frequency increases from 0 to 100 Hz, the area fraction of β phase in the weld increases from 3.37% to 12.3%, whereas the weld porosity decreases from 5.02% to 0.2%.

Oscillating laser beam stirs the molten pool to facilitate heat transfer and reduce the temperature gradient of the molten pool. The stirring of the molten pool caused by the oscillating laser beam drives the fluid to flow in the same direction and forms vortices, which thereby increase the stability of the molten pool and result in a sound weld. Oscillating laser welding refines the grain size of the weld and hence increases the amount of grain boundary by stirring the molten pool, thereby promoting the precipitation of β phase in the weld. The molten pool created by oscillating laser welding is wide and shallow compared with that created via conventional laser welding; thus, the gas bubbles in the former have sufficient time to leave the molten pool, which reduces the gas porosity. Meanwhile, the stirring effect of the oscillating laser beam on the molten pool helps gas bubbles escape from the molten pool surface, which contributes to the reduction in weld porosity.