Simulation of the removal mechanism of thermo-oxidized layer and surface morphological evolution on TA15 alloy under nanosecond pulsed laser ablation by multiphase flow model

Abstract

TA15 alloy is susceptible to form bluish-purple thermo-oxidized layer during thermal processing. It is accompanied by surface morphological defects. Laser ablation can remove the thermo-oxidized layer and modify the morphology. However, the pulse width is only a hundred nanoseconds and the spot is in the micron range. It makes the laser-matter interaction investigations extremely challenging. In this paper, a multiphase flow model is established to simulate the removal mechanism of thermo-oxidized layer and surface morphological evolution by nanosecond pulsed laser ablation with different ablation energy densities ( E ). The critical removal conditions of the thermo-oxidized layer were explored. The critical removal window of the thermo-oxidized layer was determined between 0.1 J/cm 2 and 0.35 J/cm 2 . In conjunction with thermodynamic calculations, the reasons for the minimum surface oxygen content between 3.5 J/cm 2 and 8.75 J/cm 2 were discussed. A multiphase flow model was used to analyze the formation of crater under time-dependent. Initially, the molten pool is mainly influenced by the recoil pressure and expands outward. Subsequently, the recoil pressure weakens. Instead, convection caused by surface tension and thermal buoyancy forms double vortex. Finally, depending on the low-temperature matrix, the molten pool solidifies rapidly and forms crater.