Heat transfer and melt flow of keyhole, transition and conduction modes in laser beam oscillating welding

Melting modes have significant effects on the heat transfer and fluid flow in laser beam oscillating welding (LBOW), which in turn affects the welding quality and solidification microstructure. In this paper, a computational fluid dynamics (CFD) model for LBOW of 2219 aluminum alloy is developed to study the melting/solidification, fluid flow pattern, keyhole dynamics and energy absorption under different welding modes: keyhole, transition and conduction mode. A hybrid heat source model is developed to describe the influence of the vapor heat and multiple reflections of the laser rays on the keyhole surface. The predicted molten pool dimensions and weld profiles are in good agreement with the experimental results. By increasing the oscillating frequency, the line energy density significantly decreases due to a higher scanning speed that results in a shallower keyhole with wider opening. The laser rays are more likely to leak out after 2-3 reflections which reduces the energy absorption. Thus, the melting mode will transfer from keyhole to conduction mode. With further increase of the oscillating frequency, the heat transfer will be dominated by heat conduction when the energy density is below the threshold for keyhole formation. Our work lay the foundations for optimizing the LBOW process to obtain sound joints.