Thermal and Thermomechanical Phenomena in Picosecond Laser Copper Interaction

Abstract

Thermal and thermomechanical phenomena in laser metal interaction are of great importance in terms of understanding the underlying mechanisms in laser materials processing, optimizing the efficiency of laser micro-machining, and minimizing laser induced damage. In this work, Molecular Dynamics (MD) simulation is carried out to investigate picosecond laser copper interaction. A method has been developed to account for the laser beam absorption in, and the thermal transport sustained by, free electrons. Superheating is observed, and an evident temperature drop is revealed at the solid-liquid interface, which moves at a speed of 4400 m/s. However, the later phase change from solid to liquid happens in the target simultaneously and no visible movement of solid-liquid interface is observed. The results show that the laser induced stress wave consists of a strong compressive stress and a weak tensile stress. After reflection at the back side of the MD domain, the strong compressive stress becomes a strong tensile stress, which results in a visible drop of the number density of atoms. In the presence of this strong tensile stress, voids have formed in the region near the back side of the MD domain, indicating that the strong tensile stress in laser materials interaction plays an important role in terms of inducing structural damage.

MD simulation of a copper rod under thermal shock

“…In Ref. [12], Wang adopted the same boundary conditions and estimated that the stress wave was traveling at a speed of about 5,000 m/s. So we may conclude that our MD simulation method is feasible and our simulation results are fairly accurate.…”

“…Xue et al [9] considered heat transfer balance phenomenon of films. Wang et al [10][11][12] investigated heat transfer and phase change problem during laser material interaction, and thermal and thermomechanical phenomena in picosecond laser material interaction. Alper et al [13] studied temperature-dependent behavior of solid copper.…”

Melting of Bulk Gold During Continuous Heating: A Molecular Dynamics Study

“…Similarly, nonequilibrium melting has been observed in intense laser irradiation. [19][20] Considerable superheating achieved in ultrafast dynamic loading (heating rate ~10 9 -10 12 K/s) and its rarity in conventional melting experiments (heating rate ~1K/s) indicates the important role of heating rates in achieving superheating.…”

“…Superheating has been proposed to explain the sharp drop of temperature at successive shock states as pressure is increased. [19][20][21][22][23] In shock wave loading experiments, the rise time of the shock in solids is of nanosecond order. For shocks strong enough to induce melting, the temperature increase is of 10 3 K, so the heating rate is about 10 12 K/s.…”

“…Some approaches are started to study melting processes and mechanisms experimentally [12][13][14][15][16][17][18][19][20][21][22][23]. Shock-state sound speed and temperature measurements on metals demonstrate non-equilibrium melting.…”