Research On Ultrasonic-assisted Pulsed Laser Underwater Processing Technology

Ultrasound-assisted underwater pulsed laser processing technology is an emerging composite processing technology.This technology can effectively reduce the slag on the of material processing surface and improve the processing quality and the processing efficiency of metal materials and brittle materials.However,when ultrasonic waves are introduced into underwater laser processing,it produces complex physical phenomena such as ultrasonic driving fluid flow,ultrasonic effect on cavitation bubbles,ultrasonic effect on material molten pool,evaporation impact,plasma impact and other complex physical phenomena during processing.Based on an overview of the research status of ultrasonic-assisted underwater laser processing,this thesis conducts an in-depth analysis of the mechanism of ultrasonic-assisted underwater pulsed laser processing and studies the optimization and selection of processing parameters.1.Analyze the interaction of multi-physical fields in ultrasonic-assisted underwater pulsed laser processingA systematic analysis of the combined effects of ultrasonic-assisted underwater pulsed laser processing is carried out for the first time.Mainly includes: The driving influence of ultrasonic waves on the water is briefly analyzed.The acoustic streaming velocity increases with the increase of ultrasonic power and decreases with the water layer thickness.Theoretical calculation of the interference effect of the quartz glass,the water layer and the laser beam,and the total interference effect of the quartz glass and the water layer on the laser beam is very small.The shielding and impact effects of plasma are analyzed.In this thesis,the plasma shielding effect can be ignored,but the impact pressure generated by plasma can promote the etching of the material.According to the acoustic streaming model considering the cavitation bubbles,the amplitude of the sound pressure acting on the processing area is deduced,so as to establish the relationship between the ultrasonic parameters and the cavitation bubbles.The effect of ultrasonic vibration speed on the molten pool during the heating process of the material is studied.The results show that the lower the ultrasonic vibration speed(that is,the smaller the ultrasonic vibration energy),has less effect on the molten pool.In addition,the effect of ultrasonic vibration on laser processing with different pulse widths is studied.The results show that the ultrasonic vibration in the nanosecond laser has little effect on the molten pool.2.The temperature field and material removal of ultrasonic-assisted underwater pulsed laser processing are studiedIn this thesis,a numerical model of ultrasonic-assisted underwater high-frequency pulsed laser processing is established,and the relationship between the processing depth and width in the numerical model and the groove depth and width in the experiment is derived.A single crystal silicon material as the sample,the internal temperature field distribution and material removal changes of the workpiece during the machining process are analyzed,and the numerical model proposed is verified through experiments.The results show that the error between the numerical model calculation results and the experimental results is within10%.Therefore,the accuracy,feasibility and effectiveness of the proposed model are verified.The effect of ultrasonic power,water layer thickness and cavitation bubble interference coefficient on the temperature field and material removal during material processing is systematically studied.The results show that as the thickness of the water layer increases,the processing surface of the material stays at a high temperature for a shorter period,thus material removal rate reduction.Ultrasonic power has little effect on temperature field and material removal.The cavitation bubble interference coefficient has a significant influence on the temperature field and removal of material.As the interference coefficient of cavitation bubbles increases,the maximum temperature on the workpiece decreases and even drops below the temperature at which the material can be removed.3.The formation of micro-cracks in the material during ultrasonic-assisted underwater pulsed laser processing is studied.A thermal stress model was established.Single crystal silicon as an example,the influence of different ultrasonic power,different water layer thickness and cavitation bubble interference coefficient on the formation and propagation of micro-cracks at the groove bottom was analyzed.The results show that the thermal stress obtained by different ultrasonic power is the same.The maximum thermal stress on the workpiece decreases with the increase the water layer thickness.the cavitation bubbles interference coefficient increases,which leads to the decrease of the thermal stress on the workpiece.The relationship between the groove aspect ratio and the microcracks length at the groove bottom is discussed.The microcrack length is consistent with the changing trend of the groove aspect ratio.This indicates that the formation of microcracks at the groove bottom is in elastic mechanics.Therefore,the model assumes that thermoelastic forces are feasible.The impact of plasma impact on the formation of micro-cracks at the groove bottom under different ultrasonic power and different thickness of the water layer is analyzed.The result analysis shows that the plasma has little effect on the formation of cracks at the groove bottom.In addition,The influence of cavitation bubble pulsation and collapse impact of different ultrasonic power and different water layer thickness on the formation of microcrack at the groove bottom is analyzed.The results show that the microcrack length at the groove bottom first decreases and then increases with the increase of ultrasonic power and first increases and then decreases with the increase of the water layer thickness.It shows that cavitation bubble pulsation and collapse impact are the main reasons for the formation of micro-cracks at the bottom of the slot.4.The optimization of the processing parameters of ultrasonic-assisted underwater pulsed laser processing is studied.Based on the analysis in the previous chapters,select the process parameters that have a greater impact on the processing effect and have better controllability for experimentation.The orthogonal test method is used to optimize the process parameters in processing,and the best process parameter combination is verified through experiments.Besides,the microelectron microscope image of the groove obtained according to the best process parameters verifies that the plasma has little effect on the formation of micro-cracks at the groove bottom.According to the time series diagram of cavitation bubbles in different experimental conditions,it is verified that the interference of cavitation bubbles has a more significant impact on the processing effect.Finally,the process mechanism of different process parameters is summarized.