Novel measures for spatter prediction in laser welding of thin-gage zinc-coated steel

This paper studies full-penetration remote laser stitch welding of thin-gage zinc-coated steel sheets in a lap joint condition. The spatter was classified as either Type A or Type B based on their characteristics, corresponding to low and high linear energies, respectively. The Type A spatter was mainly affected by the zinc vapor outgassing through the keyhole openings, while Type B was more influenced by the vulnerability of the keyhole rear wall to the impact of the high-pressure zinc vapor jet originating from the faying interface. Two measures, Z_out and Z_p, are proposed to predict the occurrence of Type A and Type B spatter. A thermo-fluid numerical model was developed to investigate the physics in the remote laser welding of zinc-coated steels, such as molten pool dimensions, keyhole stability, and zinc coating evaporation and to calculate Z_out and Z_p in the welding process. The effects of welding parameters, such as laser power and feed speed, on spatter occurrence were studied numerically and verified against experimental observations. It was found that the laser linear energy could directly affect values of Z_out and Z_p. A medium linear energy, striking a balance between Z_out and Z_p, could generate welds with much less spatter compared to a relatively low or high linear energy.