Formidable: Microstructure formation during laser drilling

Laser drilling enables the production of the smallest holes in almost any material. Today, the technology is mainly used in micro material processing, for example to produce injection nozzles, filters or for the generation of cooling holes in turbine blades. The use of ultrashort laser pulses (pulse duration less then 1*10-12s) is intended to minimize the heat input into the material to be processed to avoid, for example, quality-reducing melt buildup. In so-called laser-beam percussion drilling, the hole is produced with many, successive laser pulses without moving the laser beam relative to the workpiece. Percussion drilling with ultrashort pulse lasers is a highly complex process in which a large number of interaction mechanisms between the laser beam and the material interact on very small temporal and spatial scales. In this process, structural formation phenomena occur in particular during the drilling of metals, the causes of which are still unclear. They lead, for example, to undesirable deposits inside the borehole and at the borehole entrance. In addition, the formation of multiple holes and frayed borehole exits can occur. Since the edge quality and shape accuracy of microholes are critical for most applications, the above phenomena limit the applicability of the process. It is assumed that the quality-reducing nano- and microstructures are formed by self- organization during the interaction between the ultrashort laser pulse and the material. They are formed successively during the entire drilling process. The formation of new structures is significantly influenced by previously formed structures. The aim of this collaborative project between the Institute of Manufacturing and Photonic Technologies at the Vienna University of Technology and the Institute of Beam Tools at the University of Stuttgart is to identify the fundamental mechanisms and the corresponding processing parameters that control structure formation during the drilling process and to clarify their influence on the final borehole quality. In Stuttgart, structure formation during laser-beam percussion drilling is investigated experimentally in a diagnostic manner, while in Vienna, extensive multiphysics simulations of the process are performed. The experimental data serve as input for the numerical modeling, while the analysis of the simulation results helps to improve the diagnostic setup in such a way that the decisive effects can be specifically observed. Only the combination of simulative and experimental work makes it possible to understand the physical mechanisms of structure formation and development in percussion drilling with ultrashort laser pulses, ultimately enabling the development of measures to optimize processing quality and open up new applications.