Laser-solid interaction and energy accumulation in melt ejection formation during T2 copper oscillation laser welding

The phenomenon of melt ejection during pure copper laser welding exhibits the characteristics of randomness, accidentality, and abruptness. However, the current understanding of this mechanism cannot be used to conduct a detailed analysis of its random and sudden occurrence. Laser oscillations create a constantly changing environment for the laser, keyhole, and molten pool. In this study, a detailed analysis of the formation and development mechanisms of melt ejection was conducted under different laser-oscillation profiles and frequencies. Based on process observations and a self-designed energy and keyhole distortion calculation model, it was found that the random interaction between the laser and solid base metal contributed significantly to the sudden formation of the molten pool by transferring the heat accumulation zone to the rear bottom of the keyhole, which created an abnormal keyhole bulge. This was caused by the very low absorption rate of the solid surface of the T2 copper. The oscillation trajectory of the laser spot changed the frequency and time span when the laser moved near the liquid–solid boundary, as well as the heat input rate at different positions in the molten pool, resulting in a difference in stability with different oscillation strategies. Based on these results, we hypothesized that the formation of melt ejections is closely related to the random contact of a fluctuating keyhole with a highly reflective solid interface. Severe melt ejection was caused by the continuous accumulation of laser energy. This hypothesis led to a better understanding of the instabilities during the laser welding of highly reflective materials and can be applied to analyze most cases where melt ejection occurs.