Laser drilling has extensive applications in modern industry. To improve the quality of microholes drilled by a high-power millisecond pulsed laser, the laser trepan drilling technique was studied in depth in this paper. Processing parameters, such as trepanning speed (TS) and rotation number (RN), were experimentally studied for their influence on a recast layer (RL), and the detailed change process of the RL in laser repairing stage was also analyzed. It was discovered that the RL thickness would decrease with lower TS and larger RN, but micro edge cracks would appear at the hole entrance if the two parameters exceeded some appropriate ranges. The reduction of the RL during laser trepanning was neither linear nor uniform; it was a dynamic change process. The minimal RL obtained in this study was about 1 µm at the hole entrance. The difference in laser trepanning diameter has no influence on the RL. The mechanism of RL reduction in laser trepan drilling is that the recast material may intermittently be remelted by the rotating laser beam, and once it is remelted, it can be pushed downward by the vaporization-induced recoil pressure and assist gas. This process can continue only if the absorbed energy can support and keep the molten material in liquid state; therefore, the thickness of the RL is inversely proportional to the duration of the remelted material remaining in liquid state per unit length. The theoretical analysis is consistent with the experimental results.

中文翻译：

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Nd：YAG毫秒激光trepan钻孔中重铸层的动态变化过程

激光钻孔在现代工业中具有广泛的应用。为了提高大功率毫秒脉冲激光打孔的质量，本文深入研究了激光开孔钻技术。通过实验研究了加工速度（TS）和转数（RN）对重铸层（RL）的影响，并分析了激光修复阶段RL的详细变化过程。人们发现，随着TS的减小和RN的增大，RL的厚度将减小，但是如果两个参数超出某个适当的范围，则在孔的入口处会出现微边缘裂纹。激光打孔过程中RL的降低既不是线性的也不是均匀的。这是一个动态的变化过程。在这项研究中获得的最小RL在孔入口处约为1 µm。激光穿透直径的差异对RL没有影响。激光节流钻削中降低RL的机理是，重铸材料可以通过旋转的激光束间歇地重熔，一旦重熔，它就可以通过汽化引起的反冲压力向下推动并辅助气体。仅当吸收的能量可以支撑并保持熔融物料处于液态时，此过程才能继续；因此，RL的厚度与每单位长度保持液态的重熔材料的持续时间成反比。理论分析与实验结果吻合。一旦重熔，它就可以通过气化引起的后坐压力和辅助气体向下推。仅当吸收的能量可以支撑并保持熔融物料处于液态时，此过程才能继续；因此，RL的厚度与每单位长度保持液态的重熔材料的持续时间成反比。理论分析与实验结果吻合。一旦重熔，它就可以被气化引起的后坐压力和辅助气体向下推。仅当吸收的能量可以支撑并保持熔融物料处于液态时，此过程才能继续；因此，RL的厚度与每单位长度保持液态的重熔材料的持续时间成反比。理论分析与实验结果吻合。