Numerical Simulation And Performance Analysis Of Laser Polishing Process Of Ti6Al4V Alloy

As a surface finishing technique for rapid remelting and resolidification,laser polishing can effectively eliminate the asperities so as to approach the feature size.It has been widely used in precision surface processing of metallic and non-metallic materials.Nevertheless,there is a lack of in-depth study on the mechanism of laser polishing.In this paper,the model coupled heat transfer field and flow filed through a nonisothermal flow interface by means of COMSOL Mulitphysics were presented,which describes the evolution of polished surface morphology and transient molten pool dynamics in surface over melting and surface shallow melting mechanism of laser polishing.In addition,the Ti6Al4 V alloy is used as the research object as well as the main research results are as follows.(1)A surface shallow melting model of the laser polishing was developed based on stationary heat source,and the evolution of the molten pool morphology of the Ti6Al4 V surface peaks melting and flowing to the waves was simulated.It reveals that the evolution mechanism of physical processes involved in heat transfer,heat radiation,heat convection,melting and solidification during the polishing process.Particularly,the capillary and thermocapillary regimes play a key role in smoothing the free-form surface of molten pool.Furthermore,the depth and width of molten pool expand with the laser radiation duration increasing.Meanwhile,the polished surface topography becomes smoother.(2)A transient two-dimensional model was developed based on moving heat source to demonstrate the molten flow behavior for different surface morphologies.The results show that the uniformity of the distribution of surface peaks and valleys can improve the molten flow stability and obtain better smoothing effect.The high cooling rate of the molten pool leads to a shortening of the molten lifetime,which prevents the peaks from being removed by capillary and thermocapillary forces.Furthermore,the double spiral nest Marangoni convection extrudes the molten to the outsides.It resulting in the formation of expansion and depression,corresponding to nearby the starting position and at the edges of the polished surface.The aforementioned results illustrated that the predicted surface profiles agree reasonably well with the experimentally measured surface height data.(3)A two-dimensional axisymmetric model was developed which describes the evolution of polished surface morphology and transient molten pool dynamics in surface over melting mechanism of laser polishing.The results illustrated that the melt flows are dominated by thermocapillary and capillary forces while the surface temperature is lower than the evaporating ones.On the other hand,the local evaporated material generated a recoil pressure that dominated the molten material is extruded to the periphery,resulting in surface morphology significant asperities.Additionally,a model based on moving heat source was performed so as to bring the simulation results closer to the experimental ones.The results demonstrated that obvious bulge and depression are formed at the initial and final positions of the polished surface respectively.The aforementioned results indicated that the prediction of the numerical model agree reasonably well with the experimental data and micrographs.(4)To further understand the laser polishing,the influence of process parameters on the surface quality was investigated.The results show that the surface roughness is arranged in a "U" shape with the laser energy density,and the polishing mechanism transitions from surface over-melting to surface shallow melting.By polishing the milled surface,it is verified that the capillary force is mainly along the normal direction to eliminate the surface with large curvature as well as the thermocapillary force is along the tangential direction of the surface for smoothing.In further,the superposition of the two forces will improve the smoothing effect,which verifies the correctness of the model.In addition,the surface inspection results show that the the grain refinement in the shallow melt region led to a substantial increase in the nanohardness of the polished surface.Meanwhile,the EDS results show that the distribution of the elements in surface shallow melt polished surface is consistent with the unpolished area as well as no surface oxidation occurs.