Dynamic change process of a recast layer in Nd:YAG millisecond laser trepan drilling

Abstract

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.