Reduction of cw laser melt-through times in solid materials by transverse gas flow

The effects of a subsonic transverse gas flow over a solid target surface irradiated by a high-energy cw laser are investigated. The coupling of gas-flow energy into the melt layer amplifies Kelvin-Helmholtz–type capillary instabilities. The subsequent breakoff and entrainment of the top of these unstable waves provides the mechanism which reduces the melt-through time tb for finite-thickness slabs. The analysis shows that for low to moderate laser power densities, the enhancement mechanism arises from melt removal thus increasing the heat conduction to the melt-solid interface, while for higher power densities the primary enhancement originates from suppression of front surface vaporization. The predicted behavior of tb with gas-flow Mach number agrees qualitatively with experimental results and shows that the observed increase in tb above Mach 0.5 is a natural consequence of the gas compressibility. The laser-induced melt is assumed to be incompressible and viscid, and the gas is assumed to be inviscid and to obey the equations of linearized compressible flow theory. The effect of surface tension at the gas-melt interface is also included.