Numerical Simulation And Experimental Study Of Pulsed Laser Drilling On Nickel-Based Superalloy

Laser drilling, a non-contact machining method, has been widely used in the automotive industry, shipbuilding, aerospace, instrumentation, medical technology and many other industries. Compared with other micro-hole machining methods, laser drilling, which has become the most widely used micro-hole machining method, has many advantages such as low cost, high efficiency, capable of machining high density group holes and so on. This thesis carried out an investigation for laser drilling of nickel-based superalloy GH4037, including theoretical analysis, numerical simulation and experimental study.Based on laser-material interaction mechanism, the physical process of millisecond pulsed laser drilling was firstly analyzed, and the effects of light-induced plasma and other factors on energy absorption rate were discussed in detail. Several laser drilling methods together with their individual characteristics were also introduced.Afterwards, the physical model, mathematical model and finite element model for laser drilling were established. Simulation program for laser drilling was written via using the ANSYS parametric design language(APDL). Temperature fields for laser drilling at different pulse numbers were calculated, and hole geometries(including hole shape and hole size) drilled by using different pulse numbers were also predicted. The measured cross-sections, diameters and depths of the holes drilled under different pulse numbers were obtained through using control variable method. The simulated results were also compared with those from the experimental work to verify the accuracy of the calculated results and analyze the influence of pulse number on laser drilling, which might guide the experimental method development and experimental parameter selection for the future related work.In order to improve the drilling efficiency and ensure the drilling quality, the improved control variable method for laser drilling, which avoided the defects of traditional control variable method, was proposed according to the characteristics of laser drilling by using the penetration detection technique based on numerical simulation and experimental demonstration of the effects of pulse number on laser drilling.Lastly, based on the DMG LASERTEC 80 PowerDrill millisecond pulsed laser drilling facility, the effects of the defocusing amount, the assist gas, the beam expanding ratio, the pulse energy, the pulse width and the repetition rate on laser drilling were systematically studied by using the improved control variable method. It was shown that the effects of defocusing amount and beam expanding ratio on laser drilling were the most significant; when the defocusing amount was 0, the beam expanding ratio was 4, i.e., better quality of laser drilling could be obtained. The influence of pulse energy, pulse width and repetition rate on laser drilling was obvious. Besides, when the pulse energy was 1 J, the pulse width ranged from 0.8 ms to 1 ms, and the repetition rate was 60 Hz, i.e., better quality of laser drilling could be achieved. Moreover, when the assist gas was argon, the micro-cracks and roughness of the inner surface could be obviously reduced, but the drilling efficiency was relatively low, which was suitable for machining high-quality microholes in small-thickness materials. Furthermore, when the assist gas was oxygen, micro-cracks might occur in the inner surface, and the inner surface roughness was relatively large, however, the drilling efficiency was relatively high, which was suitable for machining relatively low-quality microholes in thick materials.