Experimental Investigation and Theoretical Modeling of Ultrashort Pulse Laser Ablation

摘要

Ultrashort pulse laser ablation has opened doors to many applications that require very high accuracy and precision. To fully harness the potential of these systems an optimized process and an adapted process strategy is required. For surface structuring, it can be shown that for metals and many other materials, the ablation process shows maximum efficiency at optimum fluence. The corresponding material removal rate depends on the threshold fluence and the energy penetration depth of that material. For achieving high efficiency and high machining quality it is necessary to maintain optimum working conditions consistently. Laser ablation depends on several parameters such as pulse width, frequency, scanning speed, overlap ratios, etc., precise control of these parameters is essential to obtain a superior quality cut.;The author has done a thorough review on theoretical modeling on the fundamental mechanisms of laser ablation, including Two Temperature Model (TTM), Molecular Dynamic (MD) Simulation and MD coupled with TTM, as well as Hydrodynamic Model. These models significantly advance the basic understanding of the laser ablation process. One of the most important findings of the fundamental research is the discovery of the logarithmic ablation law which relates the ablation depth with the applied fluence &phis;, the threshold fluence of the material &phis;th and the energy penetration depth delta. A simple mechanistic surface generation model was developed based on the logarithmic ablation law. The model predicts the ablation depth within 20% error using the material constants calibrated from the literature. If properly calibrated, the modeling accuracy can be further improved. The surface roughness cannot be accurately predicted due to significant stochastic components on the surface generation that were not accounted for in the mechanistic model. In general, with a 3W Coherent Helios laser with 532nm wavelength, the ablation depth per pulse of steel is between 7nm to 14nm when the pulse energy increases from 5 to 50 microjoules if the Gaussian beam has a spot size of 30 microns. Both the pulse energy and spot size can be modulated by adjusting the optical components. An accurate process model is critical in creating 3D features using ultra-short pulse laser ablation. These simulation models are robust and can be used for any material just by using the appropriate ablation threshold and energy penetration depth values of that material. Finally, the author has carried out a series controlled experiments to investigate some process parameters that could influence the surface roughness, including the laser pulse frequency, laser pulse width, scanning speed of the scan head, and size of step over between adjacent scan paths. For the laser system developed in house, the optimal combination to achieve the best surface quality is as follows: pulse frequency 20 KHz, pulse width 15 microseconds, overlap ratio in both scanning direction and cross direction to be around 40% and scanning speed of 300 mm/s. With the optimal process conditions, the author successfully created a series of miniature trapezoidal features on the sidewall of the piston rings. These surface textures are reported to significantly improve the tribological performance of the piston ring and significantly reduce the friction loss during the combustion processes.