Laser Drilling of Transparent Media for the Study of Material Removal, Taper and Spatter Formation Mechanisms

摘要

This paper presents an investigation of the dynamics of drilling a visually transparent media using a CO_2 laser source. This enabled the use of a high-speed imaging system for observing, in real time, the behaviour of the drilling process below the workpiece surface. The work revealed that the transient drilling velocity within each laser pulse can vary considerably from the steady state drilling velocity as a result of the temporal pulse shape and the oscillation of the melt ejection rate. During beam breakthrough, both upward and downward melt ejections were observed to occur inside the hole cavity for a short period of time, after which the material was ejected downwards only. It has been shown in this work that the downward melt flow velocity increases with hole depth for a positive taper hole geometry (0.09-1.43 ms~(-1)) and decreases with hole depth for a negative hole geometry from 0.4 to 0.1 ms~(-1), as a result of the change in the assist gas velocity inside the hole cavity with respect to the hole taper geometry. The mechanisms of forming the positive and negative tapered holes in the transparent media have been correlated with the hole geometry and melt flow velocity.