Thermal and thermomechanical phenomena in laser material interaction

Xinwei Wang, Purdue University

Abstract

In recent years, laser technology has been widely used in materials processing, non-destructive detecting and characterizing. Knowledge of thermal and thermomechanical phenomena in laser material interaction is of great importance in terms of understanding and optimizing these processes. In this thesis, several aspects of these thermal and thermomechanical phenomena are studied. First, the photoacoustic (PA) wave induced by periodical laser heating is studied considering thermal and optical properties and geometry of the multilayer structure. An apparatus is developed to measure thermal conductivities up to a frequency of 20 kHz. Thermal conductivities of thin films and bulk materials with mirror-like or rough surfaces are successfully measured. Second, a generalized solution for the temperature and the thermoelastic wave induced by pulsed laser heating is formulated considering the non-Fourier effect and the coupling effect between temperature and strain rate. Calculation results reveal that with the same maximum surface temperature increase, a shorter pulsed laser induces a much stronger stress wave. The non-Fourier effect results in a higher surface temperature increase, but a weaker stress wave. The coupling effect attenuates the thermoelastic wave and extends its duration. Third, the temperature and the thermoelastic wave in a metal subjected to ultrashort pulsed laser heating are investigated implementing two-step heat transfer and coupling between lattice temperature and strain rate. Two-step heat transfer results in a lower peak surface temperature, a slower temperature variation, and a weaker stress. Moreover, the thermoelastic wave experiences a weaker attenuation and pulse width expansion. With the same surface temperature increase in lattice, the shorter the laser pulse, the stronger the stress wave and its attenuation. Finally, laser material interaction is studied using molecular dynamics simulations when phase change takes place. During the melting process, the solid-liquid interface moves much slower than the local sound, while the liquid-vapor interface moves as fast as the local equilibrium atoms. Superheating is observed at the melting interface. The laser-ablated material is found to burst out of the target as fast as a thousand meters per second. Displacement and stress waves, as well as formation of nanoparticles are clearly observed during laser material interaction.

Degree

Ph.D.

Advisors

Xu, Purdue University.

Subject Area

Mechanical engineering

COinS