| Laser drilling is different from traditional mechanical processing,and has the characteristics of high precision,easy operation and wide applicability.With the increasing requirements of machinery manufacturing industry for micro-hole processing quality,laser drilling has a broader application prospect.Laser drilling essentially is a thermophysical process,and there are process defects such as micro-cracks,recast layers and small depths during processing.In order to reduce these defects and improve the processing quality and performance of micro-hole,a new method of additional current nanosecond laser drilling is proposed in this paper.Two different types of materials,metal copper and semiconductor silicon wafer,are selected for experimental research.The external current was applied to improve the defects generated in laser processing,effectively increasing the energy absorption of the laser and thereby improving processing efficiency.Firstly,finite element simulation is used to construct a mathematical model of nanosecond laser perforation phase transition to analyze the mechanism and physical drilling of nanosecond pulsed laser.The ablation threshold experiments perforation of laser processing are constructed to obtain the single pulse ablation thresholds of copper and silicon by the area extrapolation method.In addition,the ablation threshold experiments with multiple pulses are designed to obtain the ablation thresholds of the materials at different pulse.The experimental results show that the ablation threshold of the material decreases with the increase of the number of pulses;when the number of pulses is less,the ablation threshold decreases faster,and when pulses exceed a certain fixed value,the ablation threshold changes slowly.Secondly,the orthogonal text method is used to optimize the laser processing parameters of copper,and to analyze the influence of parameters such as repetition frequency,pulse energy,processing speed,scan distance and off-focus distance influence on processing quality of laser annular ring cutting,that is entrance hole diameter and roundness.The comprehensive analysis of the experimental results are used by intuitive analysis and extreme deviation analysis.Considering effects of each factor on the two kind experiments,the optimized process parameters are determined by the comprehensive balance method as follows: repetition frequency 10 k Hz,pulse energy 150 μJ,processing speed 150 mm/s,scan distance 0.005 mm,and off-focus distance 0 mm.Finally,some comparative experiments of nanosecond laser drilling on copper and silicon at assisted current or no current and processing times are conducted to analyze the effects on hole shape,micro hole depth,hole wall quality and microstructure,and the distribution of induced magnetic field strength around the energized metal are simulated.The results show that the assisted current can improve the laser drilling efficiency and the quality of the inner wall of the micro-hole;the entrance diameter of copper decreases with the increment of current,and the entrance diameter of silicon increases with the increment of current.The main reason is that additional current affects laser drilling of copper is due to the fact that additional current generates an induced magnetic field around the metal material,which restrains the expansion of the plasma,reduces the shielding effect of the plasma,and enhances the absorption of laser energy by the copper wafer.The main reason of the additional current affects laser drilling of copper is that assisted current generates an induced magnetic around the metal material,which restrains the expansion of the plasma,reduces the plasma shielding effect,and enhances the laser absorption by the copper.The main reason of assisted current influencing laser punching silicon is due to increment of free electrons inside the semiconductor material by the assisted current,and more free electrons colliding with laser photons to enhance the absorption of laser by silicon. |
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