Numerical Simulation Of Laser Texturing Process Under Millisecond Laser

Surface interactions are ubiquitous in engineering applications,so laser surface texturing is born.Among them,the millisecond laser is widely used in the surface of the mold,roll,wheel-track,cutting tool,etc.for its good economics,high efficiency,and can process a variety kinds of microtextures.However,the mechanism of laser ablation is complex and the morphology of the texture has poor controllability.Therefore,it is necessary to establish a numerical model to research the forming mechanism and evolution law of the microtexture under the ablation of millisecond laser,to realize a self-designed microstructure.In this paper,a numerical model of laser ablation was established by using two surface description methods of deformation geometry and level-set.The influence of laser processing parameters on the crater forming and the formation mechanism together with the evolution law of the central bulge in the W-shaped texture was analyzed.The main contents and conclusions of this paper are as follows:(1)A millisecond laser ablation model was established by using the deformation geometry method and level-set method,coupled with heat transfer and laminar flow physical fields,considering heat conduction,heat convection,heat radiation,latent heat,thermo-capillary stress,Young-Laplace stress,recoil pressure,and volume force.Besides,the Hertz-Knusen equation is used to add ablation velocity to realize evaporation removal.By comparing the results of the two modeling methods,it can be concluded that both the deformation geometry method and the level-set method is suitable for the study of molten pool flow under the ablation of laser,while the accuracy and efficiency of the deformation geometry method are better than that of the level-set method,and the level-set method has wider applicability.(2)The deformation geometry model was used to study the forming law of the crater,and the accuracy of the model was verified by experiments.It is found that with the increase of peak power density or pulse duration the influence of ablation rate on the forming process is more and more significant,and the diameter of the crater and the depth of the central depression increase almost linearly.However,the pulse duration has little effect on the height of the crater bulge.In a crater with the same depth,a larger melted zone can be obtained with a higher laser duration than with higher peak power density.The depression depth of the center increases almost linearly with the increase of the number of pulses,while the growth rate of the diameter slows down gradually.(3)The deformation geometry model was used to study the morphology evolution of the W-shaped texture under the ablation of the laser.The formation mechanism of the central bulge and the influence of laser processing parameters on the morphology of the texture were analyzed through the temperature field,flow rate,flow direction,and the surface stress of the molten pool.It is found that when the peak power density is low(I≤5×10~9W/m~2)and the pulse width is moderate(τ=1 ms),an obvious bulge appears in the center of the texture,and the height of the bulge decreases with the increase of the pulse duration and peak power density,and gradually changes from W-shape to crater shape.In the laser heating phase,the center of the molten pool is mainly affected by the recoil pressure,and the change of the surface profile is mainly affected by it.The tail of the molten pool is mainly affected by thermo-capillary stress,which promotes the acceleration of flow velocity and the generation of circulation.At the initial phase of cooling and solidification,the molten metal reflux rapidly to the middle of the molten pool under Young-Laplace stress.The degree of the bulge in the center of texture is related to the strength of thermo-capillary stress and Young-Laplace stress.With the increase of peak power density and the decrease of pulse duration,the temperature in the center of the molten pool is lower than that in the surrounding area,and the reflux of the molten metal will be inhibited by thermo-capillary stress,failing to form a bulge in the center.With the increase of pulse duration,two abnormal circulations will appear during the cooling phase of the molten pool,which will inhibit the backflow of molten metal and lead to the decrease of the central bulge height.