Numerical And Experimental Study On Femtosecond Laser Polishing Of Titanium Alloy And Stainless Steel

Femtosecond laser has the advantages of high peak power,small thermal effect and precise control,which provides a new method for precision processing of materials.Because the femtosecond laser interacts with the material for a very short time,it is difficult to use the instrument to directly detect the temperature of the material during this period.Therefore,it is necessary to develop a numerical model that can accurately simulate the material temperature and ablation.At present,most numerical simulations are based on the two-temperature equation to calculate the changes in electron and lattice temperature,but there are few studies on material removal.In this paper,the removal model of femtosecond laser ablation of titanium alloy is established based on the two-temperature equation.Titanium alloy and stainless steel are ablated and polished under different laser parameters,the method and mechanism of femtosecond laser polishing of metal surface are explored.It provides a reference for the application of femtosecond laser material surface treatment in industrial production.The research summary of this paper are as follow:The electron-lattice temperature model was established using COMSOL Multiphysics.Based on the electron-lattice two-temperature equation,Introducing a body heat source with Gaussian distribution in both time and space,and dynamic thermal properties of materials such as heat capacity and thermal conductivity.The temperature of the electron and lattice system of Ti6Al4V titanium alloy under the action of femtosecond laser is obtained.The radius and depth of the removal and the ablation topography are obtained by using the phase indicator.And the interaction process between femtosecond laser and metal materials is analyzed in detail.Calculations proves that:The depth of the heat-affected zone of titanium alloy processed by femtosecond laser is on the order of nanometers.When the energy density is 2.0 J/cm~2,the range of the heat-affected zone is larger than the spot diameter.Increasing the pulse energy density from 0.19 J/cm~2to 0.41 J/cm~2,when the electron-lattice system reaches thermodynamic equilibrium,the temperature increases by about 10,000 K,and the coupling time increases by 4 ps.The ablation depth and radius increase with the energy density,which stabilizes when the energy density reaches 8.0 J/cm2,which is 0.12 nm and 5.7 nm,respectively.Experiments of line scan,rough polishing,and fine polishing were conducted on the surface of the titanium alloy under different laser parameters.The evolution law of the surface microstructure observed in the experiment under different laser parameters is analyzed.Experimental results show that:Keeping other processing parameters unchanged,with fine polishing 60 times by 0.19 J/cm~2and fine polishing 25 times by0.27 J/cm~2,the roughness is reduced about 30%.But longer polishing time(about 60%)is required by the former.The main reason for the increase of surface roughness is explained based on the surface texturing mechanism and the surface protrusion growth mechanism.Single factor experiments on the surface of stainless steel were carried out by adjusting laser processing parameters to conduct.The changing law of surface roughness under different processing parameters is obtained.Compared with titanium alloy,under the action of the femtosecond laser,the surface of the stainless steel forms a micro-nano structure,which increases the roughness.During the polishing process,when the overlap rate of the spot diameter is similar to the overlap rate of the scanning track,it is easier to obtain a uniform and flat surface.