Numerical simulation of melt hydrodynamics induced hole blockage in Quasi-CW fiber laser micro-drilling of TiAl6V4

Abstract

Laser drilling in the melt expulsion regime often inherits defects such as melt debris around the hole vicinity, recast layer deposition, melt shadow, taper, barreling and melt flow induced hole blockage. Thus, it is imperative to study and scrutinize the mechanism responsible for defects in laser drilling. In this paper, a two-dimensional free surface numerical model of quasi-CW drilling is presented which describes complex time-varying melt flow patterns considering the effects of recoil pressure, surface tension, Marangoni shear stress which are all dependent on surface temperature. It has been found out that in case of laser heating during initial pulses, recoil pressure is the dominating factor resulting in the expulsion of melt near hole entrance. However, during solidification surface tension induced backflow of melt creates a shadow at hole entrance. In the later stages, the drag force induced by surface tension constraints the recoil pressure, limiting the penetration depth due to which significant amount of melt remains inside the drilled hole, leading to melt re-closure induced hole blockage. The predicted hole depth dimensions and defects such as recast layer, melt shadowing, and hole blockage agrees reasonably well with experimental micrographs and literature data.