By: Xinwei Wang

Department of Mechanical Engineering, Iowa State University

This work reports on the pioneering molecular dynamics (MD) modeling of shock waves in laser-material processing. For pulsed laser-assisted material processing with an ambient gas, the fast melting, vaporization, and phase explosion of the target is a very complicated process and will form a strong shock wave in the ambient gas. Formation of the shock wave and the interaction between the shock wave and the plume play critical roles in processing control. In this work, the dynamics and internal structure of shock waves in picosecond laser-material interaction are explored at the atomistic level by tracking the movement of individual atoms. The pressure of the shock wave, its propagation, the interaction zone thickness between the plume and ambience, the inside velocity profile at nanoscales are evaluated to study the effect of the laser absorption depth, ambient pressure, and laser fluence. Due to the strong constraint from the compressed ambient gas, the ablated plume could stop moving forward and mix with the ambient gas, or move backward to the target surface, leading to surface redeposition. Under smaller laser absorption depth, lower ambient pressure, or higher laser fluence, the shock wave will propagate faster and have a thicker interaction zone between the target and ambient gas. Plume splitting and secondary shockwave due to strong constraint of the ambient gas are observed and explored to reveal their underlying physics.