Sub-microsecond vapor plume dynamics under different keyhole penetration regimes in deep penetration laser welding

Abstract

It is well-known that distinct vapor plume dynamics occur during deep penetration laser welding under different keyhole penetration states. However, there is little knowledge about the physical characteristics of vapor plumes (velocity, pressure, flow patterns, etc) located inside transient keyholes of varying penetration regimes in laser welding. This lack of knowledge is primarily because mesoscale vapor plumes are highly dynamic and generally invisible. Based on a well-tested three-dimensional multiphase laser welding model, we conducted a computational study on vapor plume dynamics inside transient keyholes during the fiber laser welding of 304 austenite stainless steel as a function of keyhole penetration regimes. We observed three keyhole regimes of penetration: full penetration, partial penetration and no penetration. We then physically analyzed the vapor plumes in these regimes. We determined that the vapor plume velocities and pressures in all three regimes were uneven and oscillated following the dynamic keyhole with a characteristic timescale in sub-microseconds. Only when the keyhole approached the full penetration regime did vapor plumes begin to violently eject from the bottom of the keyhole opening, whereas in the partial penetration regime, even when the bottom part of the keyhole was open, most of the vapor plume ejected from the upper keyhole opening. This latter observation was similar to that in the no penetration mode. We studied the physical mechanism of this behavior by analyzing the keyhole temperature and vapor plume velocity distributions. We determined that the upward ejection of the vapor plume from the upper keyhole opening was the result of an uneven micro-meter scale boiling phenomenon of the transient keyhole governed by Fresnel absorptions dependent on the local inclination angle of the keyhole wall. Similarly, we determined that the ejection of the vapor plume from the bottom of the keyhole opening resulted from pressure differences between the inside and outside of the keyhole (as long as there was a relatively stable open state at the bottom of the keyhole opening). Additionally, we conducted quantitative studies on the velocity and pressure of vapor plumes in transient keyholes for all three regimes. We observed a decrease in the average velocity of vapor plumes at the upper keyhole opening, and an increase in average velocity at the bottom opening when the penetration regime moved from no penetration to full penetration. Moreover, the pressure distributions of vapor plumes decreased and became more uniform as the penetration regime varied from no penetration to full penetration. For the investigated process parameters used for the fiber laser welding of 1 mm thick 304 stainless steel, the vapor plume pressure decreased approximately 500–1200 Pa inside the millimeter scale keyhole. The findings in this study give the first physical insights into vapor plume dynamics inside transient keyholes as a function of keyhole penetration states during deep penetration laser welding. Moreover, our findings can be used as theoretical references for welding process parameter optimization in industrial applications.