Gas-Assisted Laser-Metal Drilling: Theoretical Model

A theoretical model for gas-assisted low-power laser-metal interaction is developed. The steady-state, one-dimensional heat transfer and two-dimensional axisymmetric flow equations for the gas and molten metal regions are solved to obtain the melting front velocity. The model is based on the mechanism of gas-assisted molten metal expulsion and does not apply to situations where incident laser fluxes are high enough to produce significant vaporization at the metal surface. The times required to drill a hole in sheets of aluminum, copper, 304 stainless steel, and low-carbon steel for both argon and oxygen assist gases are obtained. In the case of oxygen-assisted drilling, the effects of change in absorptivity of the surface due to oxide formation and the difference in the melting point of the oxide and metal are considered. The competing effect of these two factors determines whether use of oxygen as an assist gas improves the process efficiency. The model is compared with experimental values of the drilling time obtained using a Nd-YAG laser, and reasonably good qualitative and quantitative agreement is found, although better quantitative agreement could be obtained by adjusting the absorptivity.