Ablation And Micromachining Of Metals With Short And Ultra-short Laser Pulses

Firstly, an atomic-level computer modeling program is developed to study the ultra-short pulse laser ablation of metals with molecular dynamics method. The physical phenomenons during the ablation process, such as heating, melting, overheating, overcooling, formation and propagation of the stress wave, elastic oscillation, vaporization, spallation, decomposition and so on, are analyzed from the simulation results. The snapshots of atomic distributions within the ablated area at different laser fluences and times are presented, from which three different types of phase change - void, heterogeneous and homogeneous nucleation can be identified. The dependences of melt depth, gas-phase atom number and ablation yield on the laser fluences are also determined. The simulation results show that the relaxation of stress wave under the condition for the stress confinement can play an important role in ultra-short pulse laser ablation of metals, which determines the behavior of phase change and induces fracture/spallation of material. According to the laser fluences, the ultra-short pulse laser ablation of metals can be distinguished into four types - change of surface morphology, ablation of multi-pulse accumulation, mechanical spallation and thermodynamic decomposition.Secondly, a novel approach to restrain the formation of the burr during nanosecond laser ablation is reported. An assistant laser pulse, separated from the primary processing laser pulse with the pulse duration of 21 ns by temporal pulse shaping method, is used to control the formation of the melt deposit. The effect of the assistant pulse on the morphologies of the melt pools is investigated with the aid of microscope. The results of machining a groove on steel sample with the shaped pulse show a reduction of the burr at the borders of the ablation zone. The contribution indicates a potential method for obtaining an efficient ablation as well as good processing quality in short pulse laser microfabrication. On the other hand, nanosecond double-pulse laser drilling method is developed. The double-pulse herein represents two closely conjoint pulses with about 52 ns interpulse separation, which is acquired by temporal pulse shaping. Percussion drilling with such double-pulse is performed in stainless steel samples with different laser fluences, sample's thickness, repetition rates and ambient pressures. The experimental results show that the drilling rates of double-pulse drilling are more than one order of magnitude higher than that of conventional single-pulse drilling in air. Differences in the processing results between single-pulse and double-pulse with various processing parameters are investigated. In addition the ablation mechanisms of the double-pulse drilling are discussed.Thirdly, the effect of quasi-vacuum environment during pulse laser ablation with high repetition rate is investigated. The comparison between the ablation rates of pulse laser drilling in steel with high repetition rate and the ablation rates of pulse laser drilling in low ambient pressure shows that the quasi-vacuum environment can be the main reason for the increase of ablation rate during pulse laser ablation with high repetition rate. The density distributions of ambient air are investigated in two different time scales. Both experimental results and analysis show that the quasi-vacuum environment occurs during the pulse laser ablation and continues for several hundreds microseconds after laser pulse, which can significantly improve the ablation performance of the following pulses. In addition, the experiment of shock wave diagnosis during pulse laser microdrilling is introduced. The linear dependence of the propagation distance of shock wave on the hole depth suggests a potential method for monitoring the hole depth in the process.