Multi-Physics Modeling of Laser Beam Drilling with Temporally Shaped Pulses
Final Report Abstract
Laser beam drilling has always been driven by two main aspects: quality and efficiency. While high ablation rates and thus high efficiency can be obtained using long pulses and high pulse repetition rates, they come at the cost of quality aspects such as melt accretions and spattering. Using shorter pulses the influence of melt effects can be avoided enabling drilling with very high precision. However, this goes along with strongly reduced ablation rates and process efficiency. In this tension field this project focused on drilling with short pulses, where process efficiency is still very high, but melt defects already impair process quality. It was the aim of this project to use process simulations and process observations to increase process understanding and to develop a process model and process guidelines. For the process simulations, an existing thermo-fluiddynamic model for the simulation of laser beam processing, had to be extended to be able to reproduce the multi-phase multi-physics of laser beam drilling. Special focus within this project was put on drilling using temporarily shaped laser pulses, as they offer two main advantages. On the one hand, shaped laser pulses have the potential to increase process quality while maintaining high process efficiency by tuning evaporation and melt removal. On the other hand, they can be used as a tool to gain additional information about the process and improve process understanding overall. Therefore, different pulse shapes were compared during the project to increase process understanding and to identify pulse shapes beneficial to laser beam drilling regarding process efficiency and process quality. In the investigations, Gaussian-like and rectangular pulses and temporally pulsed shapes were used and compared. The shaped pulses were designed such that they consisted of a low-intensity plateau and a short high-intensity peak. Experiments and process simulations with single pulses showed that while pulse shapes with trailing peaks lead to stronger spattering, they also come with larger ablation depths. Regarding percussion drilling, the energy accumulated for the production of through holes could be reduced by 60%, increasing efficiency by a factor of 2.5. However, this comes at the cost of increased burr volume. Combining two pulse shapes, a percussion drilling process could be tailored such that burr volume remained minimal and the accumulated energy could be reduced by 33%. Time-resolved measurements showed that the position of the pulse peak significantly influences the velocity of the resulting shock wave resulting in faster shock waves for pulse shapes with peak positions located towards the end of the pulse duration. These higher velocities combined with the increased presence of molten material were identified to be connected to higher ablation rates and efficient drilling processes. Regarding plasma formation the investigations identified the pulse peak as the dominant cause for plasma formation, while the pulse plateaus rather maintain an already existing plasma or – depending on the intensity – the plasma even diminishes during the pulse. The setup and extension of the simulation model turned out to be the main challenge of this project. The numerical stability of the implemented compressible description of the process dynamics within the multiphase solver represented a very demanding task, but was successfully reached. In the simulations the process dynamics were reproduced and the experimental findings were confirmed. Comparing different pulse shapes, a significant persisting difference in plasma/vapor temperature and differences in the propagation of the vapor could be identified. Regarding the process model different characteristic process phases were identified. Using shaped pulses enables the tuning of the onset and duration of these phases enabling process optimization and tailoring of application-specific pulse shapes.
Publications
- (2017): Zeitliche Pulsformung in der Lasermikromaterialbearbeitung - Grundlegende Untersuchungen und Anwendungen. 1. Auflage. Bamberg: Meisenbach (Fertigungstechnik - Erlangen, 296)
Hertweck, Sasia Mareike
- (2019): Analysis of the Wavelength and Temperature Dependence and their Impact on Laser Material Processing. International Conference on Advanced Optical Technologies. Erlangen, Germany, 13.03.2019
Kohl, Stefanie
- (2019): Influence of Temporal Beam Shaping onto Ablation Efficiency and Process Dynamics of Laser Beam Drilling with Short Pulses. 15th Conference on Laser Ablation (COLA 2019). Kaanapali, 08.09.2019
Kohl, Stefanie
- (2019): Towards a better understanding of dynamics in metal processing with ultrashort laser pulses - numerical simulations. International Conference on Advanced Optical Technologies. Erlangen, Germany, 13.03.2019
Späth, Luisa
