| In developed countries,the number of people over 50 years old suffering from different types of bone diseases is increasing every year.The social population is gradually showing an aging trend.The number of joint injuries caused by various reasons is also increasing every year,and the demand for artificial implants is increasing.NiTi alloy with near-equiatomic ratio is widely used in the field of medical implantation due to its excellent shape memory effect,super elasticity,good mechanical properties,good corrosion resistance and biocompatibility.In this paper,the milling electrochemical polishing process is proposed based on Ni50.8at%Ti shape memory alloy.The mechanism of the influence of milling surface integrity,the reaction mechanism and polishing mechanism in the electrochemical polishing process are revealed.The influence of milling electrochemical polishing process on the shape memory effect and corrosion resistance is clarified in order to further promote the application of this processing technology in the manufacture of NiTi shape memory alloy medical products.Firstly,the influence of cutting parameters on surface roughness is studied by combining theoretical analysis and experimental verification,and the theoretical prediction model of surface roughness is established to obtain the optimal process parameters.The influence of cutting parameters on the degree of hardening and the depth of hardened layer is analyzed,and the mapping relationship between the degree of hardening and the depth of hardened layer is clarified,which provides the basis for the subsequent finishing process(electrochemical polishing).The results show that,cutting speed and feed rate are important factors affecting the surface roughness and work hardening.The prediction model of surface roughness is obtained by cutting simulation,theoretical analysis and fitting the experimental data.The surface roughness decreases gradually with the increase of cutting speed or the decrease of feed rate.The microhardness of the machined surface is obviously improved compared with the original material.The hardening degree ranges from 75%to 117.5%,and the depth of hardened layer ranges from 100 ?m to 195 ?m.The higher the content of austenite phase on the surface is,the more severe the hardening degree and the deeper the hardened layer will be.Secondly,the influence mechanism of electrochemical polishing parameters on the surface roughness and work hardening is studied,and the functional relationship between the hardening degree and the grain size is established.The surface roughness was taken as the evaluation index,and the influence mechanism of different electrolyte components was analyzed.Then the special electrochemical polishing fluid based on milling surface of NiTi shape memory alloy was developed.The results show that the effect of electropolishing parameters on surface roughness is higher than that on work hardening and grain size,and there is a linear relationship between hardening degree(N)and grain size(D).The smaller the grain size is,the higher the hardening degree will be.The current density and the distance between anode and cathode can improve the surface roughness and work hardening more significantly.The current density of 1.5 A/cm2 and the distance between anode and cathode of 7 cm can obtain lower surface roughness and work hardening.The electrochemical polishing solution was composed of basic system glacial acetic acid-perchloric acid(HClO4-CH3COOH)and additive anhydrous ethanol-glycerol(CH3CH2OH-C3H5(OH)3).The optimal volume ratio is HClO4:CH3COOH:CH3CH2OH:C3H5(OH)3=82:800:106:55.Then,the kinetic parameters and polarization properties of NiTi shape memory alloy were determined by analyzing the polarization curve,which quantitatively reflected the difficulty of electrochemical reaction.The reaction process and products of NiTi shape memory alloy milling surface in solution were studied by cyclic voltammetry and potentiostatic electrolysis.The electrochemical reaction mechanism was revealed,which provided theoretical basis for the analysis of polishing mechanism.The polishing mechanism was analyzed according to the mapping relationship among reaction products,surface morphology and current voltage,and based on the anodic dissolution theory of electrochemical polishing.The results show that the exchange current density and overpotential of milled samples are smaller than those of NiTi shape memory alloy,which indicates that the electrochemical reaction of milled samples in HClO4-CH3COOH is relatively difficult.The reaction process of the electrode is irreversible.After constant potential electrolysis,the surface of the alloy changes from NiTi to TiO2.The relationship between current and voltage is parabolic,and there are corrosion and pits on the polished surface.Furthermore,the mechanism of electrochemical polishing of NiTi shape memory alloy in HClO4-CH3COOH system is proposed,which is divided into polishing mechanism of viscous film in the front part and polishing mechanism of oxide film in the back part.Finally,the change mechanism of stress and strain under axial loading was clarified based on the influence of processing technology on work hardening,and the shape memory effect of NiTi alloy was characterized;The corrosion resistance of NiTi alloy was analyzed by potentiodynamic polarization test,impedance spectroscopy test and surface corrosion morphology study.The equivalent circuit model of NiTi alloy in electrolyte was established to verify the test results,and the influence of milling electrochemical polishing process on the corrosion resistance of NiTi alloy was clarified.The results show that:the work hardening and plastic deformation caused by processing hinder the martensitic transformation.The smaller the hardening degree and plastic deformation of the alloy are,the smaller the residual strain after axial loading unloading will be,which make the recovery rate of residual strain after heating(shape memory effect)greater.After electrochemical polishing,the surface roughness of milled samples is reduced,and the surface of NiTi alloy with austenite phase(B2)is transformed into TiO2,which reduces the conductivity,increases the polarization resistance and corrosion potential,decreases the corrosion current density,and improves the corrosion resistance of the alloy.There are feed traces on the milling surface,and the roughness is large,so the corrosive liquid is easy to gather in the depression.Pitting corrosion,pitting corrosion and crevice corrosion exist on the surface,and the corrosion resistance of the sample is poor.The surface roughness of electrochemical polishing is smaller than that of milling surface,the oxide film is dense and thick,the surface mainly exists pitting and pitting corrosion,and the sample has good corrosion resistance. |
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