Research On Electrochemical Polishing Experimental And Performance Of Ni-Ti Shape Memory Alloy

The plastic deformation of Ni-Ti alloy at low temperature can overcome the self-deformation and restore the original shape when heating to a certain set temperature,which to show the shape memory effect on the macro level.Because of its unique shape memory characteristic,ultra-elasticity,low density,non-magnetic and so on,it is widely used in mechanical,electronic,aerospace,energy,transportation,petrochemical and other fields.In addition,it is also used in medical,due to its good biocompatibility and corrosion resistance.Owing to the high elasticity,small volume and complex structure of Ni-Ti alloy implants,the traditional mechanical polishing methods are not applicable.The difficult-to-cut materials,complex surface and small parts can be processed by electrochemical polishing,which mechanical polishing is difficult to process.The workpieces have good surface quality after electrochemical polishing.What's more,the electrochemical polishing has many advantages,such as high efficiency,simplicity of operator,low labor intensity and so on.Therefore,electrochemical machining is used as surface treatment and polishing of Ni-Ti alloy commonly.A new electrochemical polishing liquid of Ni-Ti alloy was developed based on the self-developed electrochemical polishing system.The influence degree and mechanism of current density,polishing time,polishing temperature and electrode gap were analyzed by orthogonal and single factor test,and the best electrochemical polishing process was obtained.The roughness,surface morphology and composition were tested by MicroXAM-100 white-light interferometer and Quanta 250 scanning electron microscope;the microhardness was tested by FM800 microhardness tester,the hydrophobic property was tested by OCA15EC contact angle measurement;the corrosion resistance was tested by CHI670E electrochemical workstation.The results show that the optimum processing parameters are current density i=1.0A/cm~2,electrode gap d=15mm,polishing time t=140s and polishing temperature T=15?.The current density is the main factor that affects the quality of electrochemical polishing,the electrode gap,polishing time and polishing temperature are in turn.Under the optimum process parameters,the surface is smooth and bright and the roughness value is 27.8 nm,which can reflect the steel ruler scales clearly.The effect of electrochemical polishing on the surface properties of Ni-Ti alloy were studied under the optimal polishing process.The EDS tests show that the net int of Ti increased significantly,and the net int of Ni is changed rarely,the content of O is increased to 7.86%,which compared with non-electrochemical polishing.The microhardness tests show that compared with the original Ni-Ti alloy,the microhardness of Ni-Ti alloy can be reduced by acid pickling and electrochemical polishing.The microhardness of Ni-Ti alloy by electrochemical polishing is the lowest,and the cold hardened layer is thicker than the electrochemical polishing.The hydrophobicity tests show that the surface of Ni-Ti alloy specimens treated by sanding,pickling,mechanical polishing and electrochemical polishing are all hydrophobic.But the electrochemical polishing process has the strongest hydrophobicity and the contact angle is 97.7°.The corrosion tests show that compared with sanding,mechanical polishing and acid washing,the Ni-Ti alloy specimens have the best corrosion resistance by electrochemical polishing,which have the self-corrosion potential correction,the weakest cathodic hydrogen evolution reaction,and the most gentle Tafel polarization curve.Compared with mechanical polishing,the corrosion current density is one order of magnitude lower,which have the good corrosion resistance and improved biocompatibility.