Effect of beam oscillation frequency on spattering in remote laser stitch welding of thin-gage zinc-coated steel with keyhole penetration

In the paper, the effect of beam oscillation frequency on zinc-induced spatter behavior was investigated for remote laser welding of thin-gage zinc-coated steel. The experimental study showed that welding with zigzag beam oscillation of 5 cycles/mm exhibited significantly improved weld aesthetics and less spatter-induced mass loss compared to the baseline condition by conventional straight-line welding. In addition, the weld quality varied with the oscillation frequency. To understand the above-mentioned phenomena, numerical simulations of welding processes by straight line and zigzag oscillations were carried out to study process physics including keyhole opening, zinc evaporation rate, molten pool flow and keyhole wall temperature. It was found that with keyhole penetration mode, variation of keyhole opening exhibited consistent frequency of 135 Hz across welding with the straight line and welding with various beam oscillation frequencies. Welding with beam oscillation frequency larger than the keyhole oscillation of 135 Hz was observed to have a relative stable keyhole and reduced process spatter as long as the frequency is not too high to cause reduced laser energy absorption and inadequate recoil pressure to hold keyhole stable. Further analysis showed that beam oscillation led to a wide and less dense laser energy distribution with low energy gradient, and an adequate oscillation frequency resulted in consistent laser energy input along the feed direction, such as 5 cycles/mm. Together they contributed to large molten pool mass, few hot spots in the laser keyhole rear wall, stable zinc evaporation rate and reduced peak magnitudes of recoil pressure. All these are beneficial to reduction of keyhole opening variation amplitude and prevention of spatter. The application of spatter occurrence measures Zout and Zp proposed in our previous work further confirmed the observation that the welding with oscillation frequency higher than the keyhole oscillation frequency has lower possibility of spattering.