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

利用超短激光脉冲的微钻孔是一个高度复杂的过程，涉及在非常小的时间和空间尺度上在极端条件下相互作用的多种机制。在金属钻孔过程中会出现几个问题，例如孔内和入口处的碎片，孔壁上的条纹，多孔和磨损出口的形成。由于微孔的边缘质量和形状精度是大多数应用的相关标准，因此上述现象限制了工艺适用性。这变得更加相关的稳定增加的商业可用的激光脉冲能量，允许非常深的孔。在本项目中，将通过全面诊断和广泛的数值模拟，对上述质量降低、相互依赖的现象的形成进行研究。因此，钻通孔的过程分为三个阶段：（1）在钻凿开始时出现表面结构，（2）在钻凿期间孔形状和孔壁上的结构的演变，以及（3）形成通孔出口的几何形状。在激光微加工中，由于超短脉冲激光与材料之间的相互作用，自组织纳米结构和微结构的形成是固有的。在激光冲击钻孔中，结构贯穿上述钻孔阶段发展，并且假定结构的形成受到先前结构的显著影响。本文所解决的问题是，这种微结构的生长是如何导致冲击钻削中所产生的并且通常是有限的孔质量的。因此，该项目的目标是确定基本机制和相应的工艺参数，这些参数在三个钻孔阶段中控制微观结构的形成和发展，特别是要澄清先前结构的影响。这将允许确定壁的质量和钻孔的孔出口对加工参数的依赖性。为此，IFSW和IFT的技能应结合起来，即全面的诊断和复杂的数值建模。现有的数值模型已经包含了激光加工过程中的物理效应。模型调整，以重现测量的增长的结构将允许因此，确定决定性的物理效应。即IFSW和IFT完美地相互补充，以更好地理解潜在的物理机制。该项目的愿景是，了解上述现象的物理原因将有可能显着减少其影响，并允许使用非常高效和快速的冲击钻孔过程，用于高精度应用。

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.