Nanosecond time-resolved thermo-acoustics in refractory metals undergoing laser pulse-induced phase transition: Finite difference modeling

The presented work investigates the metal-to-liquid phase transition induced by a nanosecond pulsed-laser. The generation and propagation of the elastic waves by the surface thermal source is described by the coupled heat conduction and elastic wave equations and solved using a numerical finite difference time domain (FDTD) technique. During laser-induced melting the molten mass loses its rigidity and the generation of the shear waves is significantly influenced by a shallow melt pool, while the propagation of longitudinal waves remains less influenced. Numerical simulations are carried out to investigate the possibility to detect laser-induced melting in tungsten by utilizing this effect. In particular, the arrival of the shear waves in the epicentral wave form is monitored to detect the presence of molten material.