Plasma Electrolytic Polishing Process and Performance for TC4 Made by Selective Laser Melting
Plasma electrolyte polishing(PEP)is an emerging technology which aims to polish,clean,deburr and smooth the surface of metal samples formed by selective laser melting(SLM),with advantages including high polishing rate,environmental friendliness,and wide application range,which possesses incomparable advantages in processing samples with complex shape compared with other techniques.Titanium alloy,with excellent tensile strength,outstanding biocompatibility and corrosion resistance,has been widely used and developed in the manufacturing of aeroengine impellers,medical implants and prostheses,and auto parts.However,prob-lems like"step effect","spheroidization"and"powder adhesion"still generally exist in SLM-formed TC4 samples,leading to poor surface quality of SLM-formed samples with surface roughness(Ra)in the range of 8～15 μm.In order to improve the surface quality and solve the dificult problems of surface polishing of SLM-formed TC4 samples,PEP was carried out to find the best combination of process parameters in this paper,so that it could be applied in industrial production.TC4 spherical metal powder provided by Sino-Eu-ro Materials Technologies Co.,Ltd.was used as the original material,and ProX DMP 320 SLM equipment manufactured by National Innovation Institute of Additive Manufacturing was applied to make experimental samples with the size of 30 mmx20 mmx5 mm.Pre-treatment including sandblasting,ultrasonic cleaning,blowing and drying was made after the samples were formed so that unified sur-face quality of the samples could be achieved before PEP experiment.Experiments were designed to explore the effects of electrolyte temperature,polishing voltage and immersion depth on surface quality and polishing rate,and PEP was applied to samples with com-plex shape to testify the rationality of the optimal process parameters.Electrolyte concentration was set as 4％(mass fraction)and pol-ishing time at 15 min.Surface roughness,surface morphology,three-dimensional contour and elemental composition of the samples were characterized by roughness meter,optical microscope(OM),confocal microscopy and scanning electron microscope(SEM).The results indicated that the optimal combination of process parameters of using PEP to reduce the surface roughness of SLM-formed TC4 samples was as follows:polishing voltage of 280 V,electrolyte temperature of 90 ℃ and immersion depth of 40 mm,and the opti-mal combination of process parameters for material removal rate was:polishing voltage of 280 V,electrolyte temperature of 80 ℃ and immersion depth of 20 mm.After 15 min of polishing with the electrolyte concentration set at 4％,Ra was reduced to the bottom of 1.906 µm,while the material removal rate was up to 5.98 μm·min-1.The surface topography of the samples was smooth and defect-free,the peak of the three-dimensional contour was 37.99 µm,and the surface quality was good.The polishing voltage had the most significant influence on the polishing effect.At low voltage,the plasma gas layer could not be formed,leading to passivation of the sample,and thick TiO2 layer was generated,which hindered the polishing.When the voltage exceeded 280 V,thick and stable gas layer between the samples and the electrolyte was not conducive to form discharging channel,leading to decreased contact rate be-tween the samples and the electrolyte,discontinuous electrochemical reaction,material removal mainly through spark discharge,as well as gradually forming electrolyte corrosion pits with diameter exceeded 100 μm.The electrolyte temperature had a certain effect on the polishing effect,and the ideal temperature of stable gas layer was around 80 ℃.At low temperature,solubility of electrolyte was relatively low and produced thin and unstable gas layer,leading to the probability of generating discharging channel not merely related to microscopic fluctuation on the surface of the material.Microscopic convex wouldn't be removed preferentially,failing to achieve ide-al smoothing function and surface quality.The influence of immersion depth was weak under the interaction,which made the gas layer experience different liquid pressure,affecting the stability and flow process of the gas layer.For samples with relatively simple shape and uniform surface roughness,a smaller immersion depth could be selected during polishing to reduce the current density and improve the efficiency.However,for some samples with complex shape,during the polishing process,as the randomness of the gas layer flowing on the surface of the sample increased,certain current fluctuations existed inevitably.If the immersion depth was too small,the gas lay-er would become more disordered,which was not conducive to the polishing.According to the experiment,using PEP to process SLM-formed TC4 impeller with the optimal process parameters could significantly improve the overall gloss(measured at 253 GU),Ra de-creased from 9.534 to 1.987 μm and Rz reduced from 71.436 to 10.193 μm,with decrease rate of 79.16％and 85.73％respectively.SEM images showed that after PEP treatment,many tear patterns caused by sandblasting impact were completely removed on the surface of TC4 samples,and only a few tiny pores generated by additive manufacturing process existed.As it could be seen from the 3D contour,surface bumps and pits were completely removed,the color change was more uniform,and the peak was only 37.47 μm.Energy disper-sive spectrometer(EDS)result showed that the impurities covered on the surface of the samples were removed,and no residual elements of electrolyte components were detected.By optimizing PEP process parameters,the surface defects of SLM-formed TC4 samples were ef-fectively removed,and the surface quality and polishing rate of SLM-formed TC4 samples with complex shape were improved.
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