Development of a deep drilling process in metals by means of ultrashort laser pulses

Laser drilling is one of the oldest applications in the field of laser material processing and is widely used in industry. The creation of large (~Ø500μm) and deep holes (~5mm) has only been possible by utilizing a melt-based ablation process so far, but material defects as melt layers or (delamination) cracks occur. For micro holes (~Ø100μm), ultrashort pulse (usp) lasers offer the possibility to create precise hole geometries without material defects. Despite of the superior hole quality compared to melt-based processes, no such large and deep holes have been created with usp-lasers so far because of the low average power, small ablation rates and other usp-drilling based phenomena. In this paper the development of a deep hole drilling process with a commercially available ultrafast laser beam source will be presented. The goal is to create large and deep holes by ultrashort pulse laser radiation which could only be created by a melt dominated process so far. In a first step, the principal approach to reach such high depths while maintaining a high material ablation rate is explained. Then, the boundary conditions that come along with this approach are discussed. With a prototype optical system, the feasibility of this concept is shown and some exemplary results are presented. The discussed deep drilling process allows to create precise and cylindrical holes with a diameter <200 μm up to an aspect ratio of 20 in metals without any metallurgical defects in a few minutes.