Xiaowu Ni

Mathematical modeling of laser induced heating and melting in solids

Abstract An analytical method for treating the problem of laser heating and melting is developed in this paper. The analytical method has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained. In addition, the depth profile and time evolution of the temperature of aluminum before melting and after melting are given, in which a discontinuity in the temperature gradient is obviously observed due to the latent heat of fusion and the increment in thermal conductivity in solid phase. Additionally, the calculated melt depth evolution of fused quartz induced by 10.6 μm laser irradiation is in good agreement with the experimental results.

Numerical simulation of laser-generated ultrasound by the finite element method

The results obtained from the finite element model of laser-generated ultrasound are presented in terms of temperature and displacement. According to thermoelastic theory, considering the temperature dependence of the thermophysical parameters of the material, the transient temperature field can be precisely calculated by using the finite element method; then, laser-generated surface acoustic wave forms are calculated in Al plates of various thicknesses. The elastic waves excited by a pulsed laser in a thin plate are typical Lamb waves, and the numerical results demonstrate that the surface vibration is mainly determined by the lower frequency components of the symmetric mode s0 and antisymmetric mode a0 of the lowest order in very thin plate materials. It is also indicated that, when the sample thickness increases, both the higher frequency components of the lower Lamb wave modes and the higher order Lamb wave modes should be considered. In a relatively thicker plate, the numerical model can still captur...

Laser-induced damage threshold of silicon in millisecond, nanosecond, and picosecond regimes

Millisecond, nanosecond, and picosecond laser pulse induced damage thresholds on single-crystal are investigated in this study. The thresholds of laser-induced damage on silicon are calculated theoretically for three pulse widths based on the thermal damage model. An axisymmetric mathematical model is established for the transient temperature field of the silicon. Experiments are performed to test the damage thresholds of silicon at various pulse widths. The results indicate that the damage thresholds obviously increase with the increasing of laser pulse width. Additionally, the experimental results agree well with theoretical calculations and numerical simulation results.

Mechanisms of laser drilling of metal plates underwater

Several metal plates with different thickness including copper, iron, aluminum, and stainless steel have been drilled in the surroundings of air and water, respectively, by a Q-switched pulsed Nd:yttrium–aluminum–garnet laser. It is observed that for the same metal plate less energy is needed to drill a hole in water than that in air, and the surface morphology of hole drilled in water is improved greatly than that in air by comparison of the scanning electron micrographs. The underlying mechanisms behind the efficiency and quality enhancement in water are further investigated by means of optical beam deflection technique. The experimental results show that due to the water confinement the peak amplitude and duration of the laser-ablation-generated impact underwater is much larger than that in air. During the underwater laser drilling, besides laser ablation effect, both the first and second liquid-jet-induced impulses by cavitation bubble collapse in the vicinity of a solid boundary are also observed and...

Shock-wave propagation and cavitation bubble oscillation by Nd:YAG laser ablation of a metal in water

A highly sensitive fiber-optic sensor based on optical beam deflection is applied for investigating the propagation of a laser-induced plasma shock wave, the oscillation of a cavitation bubble diameter, and the development of a bubble-collapse-induced shock wave when a Nd:YAG laser pulse is focused upon an aluminum surface in water. By the sequence of experimental waveforms detected at different distances, the attenuation properties of the plasma shock wave and of the bubble-collapse-induced shock wave are obtained. Besides, based on characteristic signals, both the maximum and the minimum bubble radii at each oscillation cycle are determined, as are the corresponding oscillating periods.

Time-resolved temperature measurement and numerical simulation of millisecond laser irradiated silicon

Thermal process of 1064 nm millisecond pulsed Nd:YAG laser irradiated silicon was time-resolved temperature measured by an infrared radiation pyrometer, temperature evolutions of the spot center for wide range of laser energy densities were presented. The waveforms of temperature evolution curves contained much information about phase change, melting, solidification and vaporization. An axisymmetric numerical model was established for millisecond laser heating silicon. The transient temperature fields were obtained by using the finite element method. The numerical results of temperature evolutions of the spot center are in good agreement with the experimental results. Furthermore, the axial temperature distributions of the numerical results give a better understanding of the waveforms in the experimental results. The melting threshold, vaporizing threshold, melting duration, and melting depth were better identified by analyzing two kinds of results.

Finite element model of laser-generated surface acoustic waves in coating-substrate system

The generation of ultrasound in coating-substrate systems subjected to laser beam illumination has been studied quantitatively by using the finite element method. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field can be obtained in different coating-substrate systems. According to the thermoelastic theory, these temperature gradient fields are taken as bulk sources to generate ultrasound in coating-substrate systems. The typical surface acoustic waves (SAWs) in systems, a slow coating on a fast substrate and a fast coating on a slow substrate, are obtained. The influence of the propagation distance and the coating thickness on the SAWs is analyzed.

Numerical simulation of laser-induced transient temperature field in film-substrate system by finite element method

The transient temperature fields generated by a pulsed laser in film-substrate system are obtained by using the finite element method. Time integrations of the semi-discrete finite element equations are achieved by using approximate one order derivative of temperature. The temperature dependences of material properties are taken into account, which has a great influence on the temperature fields indicated by the numerical results. The pulsed laser-induced transient temperature fields in aluminum/methyl-methacrylate system and aluminum/copper system are obtained, which will be useful in the research on thermoelastic excitation of laser ultrasonic waves in film-substrate system.

Effect of defects on long-pulse laser-induced damage of two kinds of optical thin films

In order to study the effect of defects on the laser-induced damage of different optical thin films, we carried out damage experiments on two kinds of thin films with a 1 ms long-pulse laser. Surface-defect and subsurface-defect damage models were used to explain the damage morphology. The two-dimensional finite element method was applied to calculate the temperature and thermal-stress fields of these two films. The results show that damages of the two films are due to surface and subsurface defects, respectively. Furthermore, the different dominant defects for thin films of different structures are discussed.

Thermoelastic finite element modeling of laser generation ultrasound

Thermoelastic finite element modeling of laser-generated ultrasound in aluminum plates is presented based on a numerical formulation for the transient response in terms of the characteristics of the source of the thermoelastic waves. The model accounts for the effects of thermal diffusion, as well as the finite width and duration of the laser source. The stress can be related to the laser energy and material properties. The numerical results indicate that the temperature dependence of the thermophysical parameters has a significant influence on the laser-generated surface-acoustic waves with high frequencies.