Study On Microstructure Evolution Of Copper Films Irradiated By Picosecond Laser Based On Phase Field Model

According to the high processing efficiency,fine accuracy and relatively low cost,picosecond laser has gained much attention in the precision processing field.During picosecond laser micro/nano fabrication,the pulse duration is comparable to the electron-phonon coupling relaxation time.Thus,the thermal effect is inevitable,and the ablation mechanism is neither thermal equilibrium ablation of nanosecond laser nor cold machining of femtosecond laser.Due to the relatively low cost,simple structure and high laser beam quality,picosecond laser shows a wide application prospect in industrial processing.For the above reasons,it is necessary to study the influence of parameters of picosecond laser on the microstructures of processed materials.In this thesis,the microstructure morphology of different locations during resolidification is studied using phase field method,and the longitudinal section of copper film is selected as the research area.In addition,the influence of laser parameters on dendritic growth is discussed,and the relation between cooling rate and dendritic arm spacing is obtained,which is of great significance for improving the processing quality of picosecond laser.The specific details and conclusions are as follows:(1)The two dimensional temperature model and phase field model was established based on two-temperature model(TTM)and phase field model(PFM)for pure metal,respectively.Then,based on the macro-micro coupling method,the TTM was nondimensionalized and then incorporated into the phase field model to build the macro-micro coupled model.Through conducting the thermal analysis,the melting pool characteristics and solidification conditions were obtained.After this,different small areas in melting pool were selected for microstructure simulation.A series of simulations with different laser influence,pulse duration and pulse number are executed.The results show that the solidification conditions vary a lot in different areas of melting pool,a larger temperature gradient and a smaller cooling rate was observed at the bottom area of melting pool compared with the top area.By reducing the laser fluence,pulse duration or pulse number,a thinner melting pool will be obtained which causes a higher cooling rate and a larger temperature gradient.(2)Different areas of melting pool were selected as the micro simulation areas,and the temperature distribution was also taken into account.The results show that the microstructure morphology varies a lot in different micro calculation areas,and the maximum temperature gradient has a significant effect on the local dendritic growth.The dendritic structures which grow along the temperature gradient has an obvious priority during the competition growth.(3)The microstructure morphology during picosecond laser machining was predicted based on the coupled TTM-PFM model using different laser parameters.Besides,the relationship between the cooling rate and dendritic arm spacing was also obtained.The results show that elongated columnar crystals were obtained in picosecond laser microfabrication,and the secondary dendritic arm spacing(SDAS)in each case is smaller than 1 ?m.The dendritic structure will be finer when reducing the laser fluence,shortening the pulse time or decreasing the number of pulses,and the SDAS obtained a small value of 0.2 ?m if the cooling rate is very high.In this condition,the growth of some secondary arm spacing will be stopped and the microstructure showed an obvious dendrite-to-cellular transition.In addition,the quantitative simulation results were found in good agreement with formal studies,thus,the established macro-micro-coupled model was verified can be used for predicting the influence of picosecond laser processing on microstructure morphology,which can be used for optimize the laser parameters in order to obtain a better processing quality of picosecond laser machining.