A Model for Laser Hole Drilling in Metals

Abstract

A direct computer simulation technique is developed to analyze quantitatively the influence of the fluid flow and heat transfer in the transient development of a laser drilled hole in a turbine airfoil material, where the material removal is effected by vaporization and melt ejection. The coupled conduction heat transfer in the solid and the advection-diffusion heat transfer in the liquid metal, the fluid dynamics of melt expulsion and the tracking of solid–liquid and liquid–vapor interfaces have been mathematically modeled for the 2D axisymmetric case. The donor–acceptor cell method using the volume of fluid approach is used to solve the complex problem and a versatile numerical code has been developed. It takes into account all thermophysical properties including latent heat of vaporization, gravity, and surface tension driving forces. The novelty of this model is to treat the melted pool surface as a deformable free surface. The impressed pressure and temperature on the melt surface is provided by an 1D gas dynamics model whose vaporization kinetics are also discussed. The model is used to simulate drilling for a number of spatially and temporally varying laser intensity profiles. It is found that resolidification of melt (recast formation) occurred throughout the pulse interval and had significant effect on the developing hole geometry, while the effect of vaporization material removal on the hole geometry is found to be small. Comparison of the simulated results indicates the material removed per joule of energy absorbed appears to be inversely proportional to the square root of the peak beam intensity and the drilling rate appears to be proportional to the square root of the surface pressure.