Thermal and Fluid Flow Characteristics and their Relationships with Porosity in Laser Welding of AA5083

A computational fluid dynamics (CFD) model was developed to numerically analyse the temperature and fluid flow during laser butt welding, and a heat source model was proposed, which, with adjustment, was suitable for both partial and full penetration welds over a wide range of conditions. The models were then used to study the laser welding of a 4 mm thickness aluminum alloy (AA5083), and the results predicted were analysed to better understand the formation of porosity in aluminium laser welds. The flow patterns predicted in the melt pool are essentially the same for various welding parameters: vortices can be found close to the surfaces of the weld pool (top surface only for partial penetration; both top and bottom surfaces for full penetration). The dimensions of the weld pool are predicted to increase with increasing laser power and decreasing welding speed, and the maximum velocity to increase with increasing laser power and welding speed. In partial penetration cases, there are no simple relations between porosity levels and laser power and welding speed. A sharp decrease in porosity content, when changing from partial to full penetration, was thought to be a result of an extra driving force from the outwards fluid flow at the weld pool bottom for the pores to escape out of the melt pool. In full penetration cases, the lower porosity content when using higher laser power and/or lower welding speed was attributed to the longer time for pores to escape before the weld pool solidifies.