| SKD61 die steel is a kind of material widely used in die casting dies of aluminum,zinc and other materials.Laser polishing is a surface reconstruction technology based on rapid adaptation and recuring of materials.It can effectively reduce the roughness of surface parts and make them close to the feature size.Widely used in the precision machining of metals and non-metals.However,laser polishing is easy to produce new marks caused by polishing track while eliminating surface wear marks.At present,there is a lack of in-depth research on reducing the secondary overflow of laser polishing.In this paper,taking SKD61 die steel as the research object,through the multi-physical field simulation software COMSOL,the non-isothermal flow interface is coupled with the heat transfer field and the flow field,and the Lorentz force is coupled in the flow field in the form of volumetric force,and a steady magnetic field-assisted laser polishing melt dynamics evolution model is established.The main research results are as follows.(1)Based on the existing conditions,the orthogonal experiments of laser polishing of SKD61 die steel with or without magnetic field were carried out,and the effects of laser power,scanning speed,scanning distance and magnetic field intensity on the surface roughness were analyzed.In order to reduce the secondary overflow of the surface and reduce the surface roughness,the magnetic field assisted laser polishing is adopted.The experimental results show that under the action of a single laser,the surface roughness can be effectively reduced from 6.1μm to 0.681μm,with a reduction rate of 88.84%.Under the action of magnetic field-assisted laser polishing,the roughness is further reduced to 0.607μm.Roughness reduction up to 90.05%,compared to the original surface.(2)Simulates continuous laser and magnetic field assisted laser polishing.Variation of molten pool flow rate,temperature,capillary force and thermocapillary force are analyzed.At different times and locations,capillary and thermocapillary forces dominate,respectively.In order to obtain a smoother surface,a stable magnetic field is applied to suppress the undulation in the process of laser polishing.The role of magnetic field in the remelting process is analyzed from the aspects of temperature,speed and surface morphology,and experiments are carried out to verify the numerical model.The results show that in the simulation of magnetic field-assisted laser polishing,the simulated molten pool depth is 65μm,and the actual penetration depth is 56.52μm.Compared with the actual working conditions,the error is 15.0%.The experimental results are consistent with the simulation results.The Lorentz force generated by the constant magnetic field is the resistance to the melt flow,which greatly reduces the flow velocity of the melt.Therefore,surface undulations would be suppressed by a stable magnetic field,obtaining a smooth surface after laser polishing.(3)The microstructure and element changes of the subsurface layer of the material after polishing with the aid of steady magnetic field under different energy density are analyzed,and the reason for the increase of hardness in the melting zone under the condition of low energy density is explained,surface corrosion resistance and tribological properties of the materials were analyzed.The corrosion resistance and wear resistance of the material surface were analyzed,the result shows that,the hardness and Young’s modulus in the melting zone with energy density of 1200J/cm~2are increased by 3 times and 1.25 times,respectively.The corrosion resistance of the sample is higher than that of the original sample when the energy density is 2500J/cm~2and 4000 J/cm~2. |
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