Incapability of Drilling With a High-Power-Density Beam

This paper theoretically identifies the factors affecting the keyhole collapse during drilling with a high-power density laser or electron beam from fundamental principles of thermal physics. Laser drilling is widely used in components, packaging, and manufacturing technologies. The approach adapted in this paper is to probe the quasi-steady 1-D supersonic or subsonic flow behavior of the two-phase vapor-liquid dispersion in a vertical keyhole of varying cross section, paying particular attention to the transition between the annular and slug flows. Drilling with a pulsed laser beam can evidently change Mach number, ejected mass flux at the keyhole base, energy absorption and evolution in the keyhole, drilling speed, and surrounding pressure. The results find that thermal drilling is susceptible to becoming incapable for higher absorbed energy, drilling velocity, and Mach number, ejected mass flux at the keyhole base, and lower surrounding pressure resulting in a shock wave for a supersonic flow in the keyhole. A subsonic flow usually gives rise to keyhole collapse. The predicted results agree with physical intuition and exact closed-form solutions derived in the absence of friction, entrainment and energy absorption. Controlling the factors to enhance efficiency and quality of drilling is therefore provided in this paper.