The comprehension of microhole formation during percussion drilling of metals using ultrashort laser pulses is still limited. The shape of a microhole can be impacted by factors like heat accumulation, the emergence of side channels, bending, and bulging. Understanding these defects is challenging due to constraints in conventional diagnostics. To address this issue, high-speed synchrotron x-ray imaging was employed to capture the spatial and temporal evolution of the microhole shape during laser percussion drilling of stainless steel. The recorded images reveal that heat accumulation leads to the creation of a melt film on the microhole walls, exhibiting dynamic fluctuations throughout the drilling process. Furthermore, a transversal widening or bulging of the microhole can be seen later in the process. Additionally, the emergence of side channels was observed in the region of maximum drilling depth, where the overall fluence on the microhole walls falls below the threshold fluence.
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.
High productive laser drilling processes usually employ an on-the-fly single-pulse drilling process. This process achieves a productivity of up to several hundred holes per second but is usually limited to hole diameters in the order of magnitude of ~100 μm and hole depths below 1 mm. Moreover, the geometrical and metallographic hole quality is limited. Larger or deeper holes can be drilled by means of percussion drilling processes which also results in an increased hole quality. However, the productivity for percussion drilling is only in the range of a few holes per second at best due to the positioning time of the optic for each hole. In this paper we present the development of a new drilling process which combines the advantages of an on-the-fly process with the achievable size and quality of a percussion drilling process. Instead of a single pulse, an elaborately designed short pulse burst is emitted to drill the hole during the relative movement between the drilling optic and workpiece. The impact of each pulse of the pulse burst on the final shape of the hole is evaluated by a systematic variation of the process parameters. A drilling process to achieve a hole diameter of Ø500 μm in 2 mm thick aluminum was designed and a drilling speed of 15 holes per second has been demonstrated with a relative standard deviation of less than 5% for the entry and exit diameter.
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