| Compared with traditional drilling methods,laser drilling,which is widely used in many industries especially in manufacturing industry,has the advantages of high efficiency,good flexibility and high quality.But there are still many defects in millisecond pulsed laser drilling,such as recast layer,micro cracks and oxide layer,and so on.In this thesis,a method of ultrasonicassisted laser trepanning is proposed.The principle exploration and experimental study for laser trepanning of circular and square holes in nickel super-alloy GH4037 and stainless steel 304 sheets were carried out.The principle of ultrasonic-assisted laser trepanning was explained through describing laser beam characteristics,the physical process of laser drilling,and the formation principle of blind holes and through holes during ultrasonic-assisted laser trepanning.Besides,the construction of the ultrasonic-assisted laser trepanning platform,experimental equipment and material processing method were introduced.3-D physical and finite element models for laser trepanning of circular and square holes were established by assuming and simplifying the model.The boundary conditions and latent heat were considered in the simulation model.The Gaussian moving heat source model and the application of heat source model were also introduced.The influence of pulse energy and pulse width on hole geometry,hole diameter,hole taper,hole circularity and temperature field of laser trepanning was simulated via using the life-death element method.The simulated results were compared to the corresponding experimental results,providing the guidance for the formulation of subsequent experimental methods and the selection of parameters.Ultrasonic field was applied in the process of Nd:YAG laser trepanning of circular holes.The effects of ultrasonic power and pulse energy on geometrical and metallurgical quality of circular holes trepanned in nickel super-alloy GH4037 were studied.In addition,the effects of ultrasonic vibrations on chemical compositions surrounding the entrance of laser-trepanned circular holes were analyzed based on energy dispersive spectrum(EDS)analysis.It was shown that the axial ultrasonic vibrations coupled into the process of laser trepanning of circular holes could effectively improve the hole entrance morphology and reduce metallurgical defects such as micro cracks.In addition,a relatively high ultrasonic power is helpful for improving the quality of laser trepanning of circular holes,especially reducing the hole taper angle,recast layer thickness and micro cracks.However,there was no obvious improvement for oxide layer on inner hole sidewall of lasertrepanned circular holes with ultrasonic assistance.During laser trepanning of square holes,for the problem of irregular formation at the initial location of the laser trepanning path,the whole path of laser trepanning of square holes was optimized.For laser trepanning of stainless steel 304 sheets,the effects of ultrasonic power,pulse width and pulse energy on hole quality were investigated by adding ultrasonic vibrations into the process of Nd:YAG laser trepanning of square holes.Moreover,the EDS analysis was also carried out for showing the influence of ultrasonic vibrations on the chemical compositions of the entrance corner of laser-trepanned square holes.It was shown that the increase of pulse width decreased the hole side length and hole taper,while increasing recast layer thickness.The square hole side length,taper and inner wall recast layer thickness increased with pulse energy.The increase of ultrasonic power slightly influenced the side length of the trepanned square hole,but it could reduce hole taper angle and recast layer thickness.In addition,the morphology of hole entrance could be effectively improved while reducing micro cracks through using ultrasonic vibration assistance.It was also found that the ultrasonic vibrations had small influence on the oxidation of the inner wall for the square hole trepanned in the stainless steel 304 sheet. |
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