Effects of pulse parameters on porosity rate and flow characteristics of molten pool in pulsed laser welding

Flow behavior determines the formation and evolution of porosity and other defects in laser welding, but the existing investigations of liquidity of molten pool are restricted by qualitative analysis methods, so it is of great significance to quantitatively characterize the flow patterns and flow characteristics of the pulsed laser welding for understanding the porosity suppression mechanism. A series of welding experiments was carried out by using 1.5 kW slab CO2 laser under a variety of pulse parameters, and the images of molten pool and keyhole were captured by using a high-speed camera during welding. Porosity rate was also obtained by image processing of x-ray inspection of weld joints. The flow characteristics of molten pools including speed and vortices from these images were extracted and analyzed quantitatively. Effects of different pulse frequency and duty ratios on penetration depth, weld width, and flow characteristics of the molten pool were investigated and compared with each other. A three-stage flow pattern of molten pool for pulse laser welding and the respective characteristics of each stage were revealed. The analysis result shows that the velocity at the starting time in a pulse impact phase is about three times of that in the steady-state phase and the primary mechanism for suppression of pores lies in the impact and stirring on molten pool in pulse laser welding. The other suppression mechanisms of pores for welding joints are also discussed.