Mechanism And Numerical Simulation Of Femtosecond Laser Ablating Metals And Alloy

Mechanism of femtosecond laser interaction with materials has been widely investigated in recent years. It is the foundation of femtosecond laser micromachining. In this paper, a hybrid simulation combined two-temperature model and molecular dynamics simulation is applied to investigate femtosecond laser interaction with metals and alloy. The physical process including temperatrure evolution, pressure distribution and atomic configuration is described in detail. The main work is summarized as follows:(1) The general physical process is analyzed and summarized in the domain of femtosecond laser interaction with metals, which includes the deposition of optic energy, the absorption and transfer energy in the material. The characteristic time is resolved in the special physical phenomenon. The basic calculation methods have been performed in detail.(2) On the foundation of one dimension two-temperature model, the influence of material properties and laser parameters are investigated on femtosecond laser interaction with typical metals. The simulation results show that the metal which has a bigger electron heat capacity needs more fluence to occur to ablation behavior. The different electron thermal conductivity model is the key to the depth of heat affect zone. The nonequilibrium time between electron and lattice under femtosecond laser is strongly dependent on the electron-phonon coupling factor. The stronger laser fluence is, the higher the temperature and the longer the coupling time. It is also observed that by the shorter pulse duration, the electron temperature is higher and the process is stronger. (3) We research on femtosecond laser ablation of transition metal (Ni), light metal (Al) and noble metal (Ag) using the hybrid modeling combined two-temperature model and molecular dynamics simulation, respectively. The results show that transition metal has the small and thin heat affect zone because of bigger electron heat capacity and less electron thermal conductivity. The mechanism of femtosecond laser interaction with metals can be described that at certain laser fluence, heterogeneous nucleation increases and surface material begins melting. The phase explosion may occur while increasing laser fluence and temperature reaching to critical point. If the laser fluence is high enough, it directly results in forming plasma.(4) The modeling of femtosecond laser ablation of B2 type NiTi alloy is resolved by combined two-temperature model and molecular dynamics simulation. The simulation results show that ablation threshold is near absorbed fluence of 29.3mJ/cm2 and the speed of laser-induced pressure wave is related to velocity of sound in material. Thermal ablation occurs in subsurface while the tensile wave passes through the subsurface of material. At low laser fluence, ablated material is formed by a big liquid cluster, however, at high laser fluence, the ablated component consists of small cluster, liquid droplets and single particles. A focused region of negative pressure is formed in the substrate. The fragmentation may occur by this negative pressure. The novel ablation mechanism results from laser induced thermomechanical wave.