Energy efficiency and melt ejection analyses of a dual-laser micromachining method

This thesis is concerned with the understanding of a melt removal method which is based on combining a short laser pulse with the primary continuous laser beam called in this thesis dual laser micromachining (DLM). In this method, the continuous laser beam melts the surface and subsequently the short laser pulse ejects the molten material. In previous studies, this method has demonstrated improvements in material removal rates, however, there is a considerable range of results both in efficiency and quality. The energy consumed to produce the melt pool is the major part of the required energy budget in DLM (the sum of the energy consumed in melting and ejection processes). This important melting part has been ignored in the previous DLM studies. Moreover, there is still uncertainty in the melt ejection mechanism. This thesis considers the energy efficiency and quality characteristics also provides insight into the mechanism of melt ejection to facilitate its incorporation into the existing studies.

For the energy efficiency analysis, a contribution to the knowledge has been made through a combined analysis of theoretical and experimental result of the energy used in the DLM method. Theoretical calculations derived from a one-dimensional heating model has been performed for the melt pool size against the melting process parameters. The minimum energy required to remove the molten material was calculated from the surface energy at the liquid-solid interface that separates the liquid and creates new surfaces. It was found that the ejection energy can be considered to be negligible in comparison to the energy required to form the same mass of melt pool. Therefore, the energy model has focused on the melting process to optimise the DLM energy. The key finding was that the most efficient melting occurs at the maximum melt depth when the surface starts vaporises. The decrease in the energy required for the combined lasers is primarily due to the optimisation of the irradiation time in the melting process. At this most efficient melting process, the theoretical calculation has shown that there would be a reduction in total energy consumption of three times comparing DLM to practical conventional vaporisation found in the literature.

The relative energy efficiency of the DLM method has been demonstrated experimentally and compared to findings published in the literature. Two lasers were used, a continuous wave fibre laser to create a molten pool while a nanosecond pulse Nd:YAG laser was used to eject the molten material by vaporising the molten pool surface to generate recoil pressure. The experimental melt depths of the melting laser only were compared with the theoretical calculations. It was found that the experimental melt data align with the theoretical calculation at low melting time values. However, after that, the experimental results depart significantly from the linear theoretical trend. The most efficient was found at experimental melting time 9 ms, however, it is less than the theoretical melting time of 15 ms. The DLM method created holes with 18-28 µm maximum depth from 20-31 µm maximum melt depths at melting times in the range of 9-60 ms at the same order. At optimised DLM method of 9 ms melting time, of the total energy, 95% of the energy was delivered in the melting process and 5% in the ejection. This key finding shows a good agreement between the experimental results with the theoretical calculation that predicted negligible energy required from ejection laser. The DLM result was compared to findings published in the literature. The results have shown that DLM method can increase material removal efficiency compared with the conventional processes by approximately 2 to 6 times. This comparison result confirms the theoretical reduction in total energy consumption of 3 times comparing DLM to vaporisation machining (ablation).

For the quality analysis, this thesis presents an analysis of the geometry and metallurgical features via sectioning and imaging of the DLM holes. Analysis of the material quality shows that were found free from microcracks and with a small amount of redeposited material at the workpiece surface along the periphery of the created hole. Moreover, the micrographs show low porosity in the solidified molten material.

Melt ejection mechanism results constitute a novel contribution in the field. It has been discovered that the material is ejected by the effect of the pressure pulse generated at the surface and travelled through the target material. This pulse is converted into a tension pulse at a certain position inside the melt pool as a result of mismatching from high to low impedance during travelling inside the material. Upon this tensile pulse, spallation can occur and eject the molten material that takes place when the tensile stress exceeds the tensile strength of the liquid material. The spallation of a laser-melted material by nanosecond laser pulse was studied experimentally and theoretically to find the magnitude and position of the tensile stress. DLM method was experimentally and theoretically demonstrated in different setup regimes. The key finding is that both the simulations and experiments showed the molten material is spalled by the tensile pulse close to the liquid-solid interface leaving behind a residual molten material along the bottom of the hole.