Optical trepaning using axicon lenses

Percussion drilling and trepanning are two laser drilling methods. Circular laser spots are generally used in conventional laser drilling. An annular laser beam provides a new drilling mechanism. When an annular beam is focused on the workpiece surface, the material within the annulus is heated, melted (possibly vaporized), and removed, leading to the formation of a hole. This process, which we refer to as “optical trepanning”, involves no rotating optics or any motion of the workpiece. Based on the ray tracing technique, a refractive axicon lens system has been designed to transform a Gaussian circular laser beam into an annular laser beam. An analytical two-dimensional model was developed for optical trepanning. The analysis accounted for conduction in the solid, vaporization and convection due to the melt flow caused by an assist gas. Based on the model, the influences of pulse duration, pulse repetition rate, irradiance profiles and beam radius were investigated to examine their effects on the recast layer thickness, hole depth and taper. Experiments have been conducted on Inconel 718. Annular melt spots with different outer diameters are obtained with annular laser irradiation.