Analysis Of Heat Transfer And Fluid Flow In The Micro-Waterjet Guided Laser Precision Drilling Process

Micro-waterjet guided laser processing is an internationally advanced technique based on guiding a laser beam inside a thin, high-speed waterjet. The process combines the advantages of laser processing with those of waterjet cutting. It provides a new method for thin and heat-sensitive materials processing with a high degree of precision. In the meantime, comparing to the conventional laser process this kind of technology has more influencing factors. The interactions between the laser, waterjet and material are much complicated. The study gives insight into dynamic process of micro-waterjet guided laser drilling process, analyzes the heat transfer and flow structure based on the thermal physics and flow dynamics. The physical and mathematical models based on Finite Volume Method (FVM) are developed to present the physics process. The research works and main achievements:1. A heat transfer model for micro-waterjet guided laser drilling is presented. In the simulation boundary conditions updating in real time are used to deal with the solid-liquid interface in the drilling process. The continuous process is treated as three stages to analyze the heat transfer and fluid flow process with heat transfer, state changing, material removal and moving boundary given in detail. The rules of temperature distribution and cavity propagation are obtained. The penetration speed can be predicted by the simulation results, and the cooling effect of waterjet is analyzed quantitatively.2. Dual-pulse laser generally can enhance material removal rates while minimizing redeposition and heat-affected zones. The process of waterjet guided dual-pulse laser drilling is simulated. The drilling times vary with the delay time of dual-pulse and an optimal delay time for drilling of silicon is predicted (0.6ms for present dual-pulse laser mode). The highest efficiency can be achieved with the optimal delay time, while the reasons and mechanism are analyzed and given.3. This study also presents the flow model in the melting pool for micro-waterjet guided laser drilling on the silicon substrate. The flow field and the temperature distribution in the melting pool and drilling cavity at different moments are calculated. The penetration speed of cavity is higher than moving speed of stagnation zone interface at the beginning for cavity forming, and then the stagnation zone suspends to move with cavity deepening. The influences of three waterjet speeds on the heat transfer and flow structure are compared. It is shown that the waterjet speed almost does not affect the flow structure in the cavity (0.1mm depth), but higher velocity has better cooling effect.Research significances: a new method dealing with the moving boundary of micro-waterjet guided laser drilling process is proposed. The analysis of simulation results deepens the understanding of the drilling mechanism. From the coupling between influencing factors the optimal parameter groups have been obtained. The results are very useful in improving the micro-waterjet guided laser machining process, and it provided the basis for the processing craft improvement and machining quality enhancement.