From conduction to keyhole transition on copper using blue laser: Bead-on-plate process modeling and analysis of physical phenomena

The growth of electric mobility has stimulated a large and expanding interest in achieving new manufacturing techniques that would enable more efficient production with less waste. Due to their high levels of flexibility, efficiency, and monitoring capabilities, laser-based processes are essential technologies in this context. Due to high laser absorption of copper for wavelengths under 450 nm, blue lasers are among the most promising laser sources currently on the market and are generating significant interest from battery manufacturers. From a manufacturing point of view, high absorption leads to the possibility of having conduction welding, with calmer melt pool, instead of keyhole welding, characterized by more turbulent molten pool dynamics and higher porosity. On the other hand, consolidated near-IR lasers only lead the possibility of keyhole welding due to poor copper absorptivity with 1064 nm wavelength. In this paper, the transition from conductive to keyhole regime was investigated and predicted through experimental and numerical analyses. From an experimental point of view, the transition is characterized by rapid change of laser absorption and the consequent variation of the seam geometry. Following experimental results, the surface vapor pressure threshold responsible for the transition to the keyhole regime is calculated in accordance with the process parameters employed. Finally, the comparison between experimental and predicted data confirmed the assumption that the recoil pressure is the main physical parameter determining the transition from conduction to keyhole mode, under certain conditions in terms of threshold fluence and irradiance.