A study of keyhole behavior and weldability in zero-gap laser welding of zinc-coated steel sheets at subatmospheric pressures

Experiments were conducted to investigate how the keyhole responds to a decrease in the ambient pressure using a coaxial observation method in the laser welding of zinc-coated steel sheets. To understand the effect of zinc coating on keyhole dynamics, both zinc-coated and uncoated DP 590 steel sheets were studied using a 2kW multi-mode fiber laser in a zero-gap lap-weld configuration. Two energy density conditions (1830W, 12.5mm/s and 1230W, 21.2mm/s) and four ambient pressures (101.3, 10, 1 and 0.1kPa) were considered systematically. For these conditions, time-averaged 3-D keyhole shapes were reconstructed by analyzing top and bottom surface video images. This study showed that zinc evaporation was much more intensified than the evaporation of steel at subatmospheric pressures, which made the keyhole highly fluctuating and elongated in the welding direction for the zinc-coated steel. Also, the bottom aperture opening time increased as the ambient pressure was decreased due to the enhanced effect of evaporation, which led to less energy absorption and smaller melt pools. The weld quality became poorer as the spacing of weld chevrons were wider and more erratic, and more fumes with less light emission were observed at reduced ambient pressures.