Formation of microstructures and their impact on the hole quality during percussion drill-ing with ultrashort laser pulses (Formidable)

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Micro drilling with ultrashort laser pulses is a highly complex process involving a multitude of mechanisms interacting at extreme conditions on very small temporal and spatial scales. Several issues occur during drilling in metal, such as debris inside the hole and at the entrance, striations on the hole wall, the formation of multi-holes and frayed exits. Since the edge quality and shape accuracy of microholes are relevant criteria for most applications, the mentioned phenomena are limiting the process applicability. This becomes even more relevant with the steadily increasing commercially available laser pulse energy which allows very deep holes. The formation of the mentioned quality-reducing, mutually interdependent phenomena will be in¬vestigated in this project for percussion drilling with comprehensive diagnostics and extensive numerical simulations. The process of drilling through-holes is therefore divided into three stages: (1) emergence of surface structures at the beginning of drilling, (2) evolution of the hole shape and structures on the hole walls during the drilling, and (3) formation of the geometry of the exit of the through-hole. The formation of self-organized nano- and microstructures in laser micro processing is inherent due to the interaction between the ultrashort laser pulse and the material. In laser percussion drilling, the structures develop throughout the above-mentioned drilling stages and it is assumed that the formation of structures is significantly influenced by the preceding structures. The herein addressed question is how the growth of such microstructures is responsible for the resulting and often limited hole quality in percussion drilling. The objective of the project is therefore to identify the fundamental mechanisms and the corresponding processing parameters that rule the formation and development of microstructures during the three drilling stages and in particular to clarify the influence of the preceding structures. This will allow determining the dependence of the quality of the walls and the hole exit of percussion-drilled holes on the processing parameters. For this, the skills of the IFSW and the IFT shall be combined, i.e. comprehensive diagnostics and sophisticated numerical modelling. The existing numerical model already includes the physical effects during laser processing. The model tuning to reproduce the measured growth of the structures will allow therefore to identify the decisive physical effects. I.e. IFSW and the IFT complement each other perfectly for gaining a better understanding of the underlying physical mechanisms. The project vision is that the understanding of the physical causes for the above-mentioned phenomena will make it possible to significantly reduce their influence and allows using the very efficient and fast percussion drilling process for high-precision applications.