Improving The Corrosion Degradation Behavior Of AZ91D Magnesium Alloy By Ultrasonic Burnishing Surface Strengthening

Biomaterials have excellent mechanical strength and mechanical properties,so they are widely used in clinical medicine as medical implants.Magnesium and magnesium alloys have similar elastic modulus and tensile strength to human bone,which can effectively alleviate the stress shielding phenomenon caused by the large elastic modulus gap between implants and human bone.Meanwhile,magnesium alloys have good biological activities and properties,and can be spontaneously degraded in human physiological environment.Therefore,magnesium alloys as medical implants have great application potential in the medical field.Due to the low standard electrode potential of magnesium and magnesium alloys,the passive film cannot effectively protect magnesium alloys.Moreover,a large number of chloride ions in human physiological environment will destroy the passive film and accelerate the corrosion rate of magnesium alloys,resulting in poor bone bonding,which limits the extensive medical use of magnesium alloys in the medical field.It was proposed that the surface strengthening process of AZ91D magnesium alloy by ultrasonic burnishing was used to improve its corrosion resistance.The influence of ultrasonic burnishing processing parameters on the surface properties of magnesium alloy and the in-depth stress and strain inside the material,the influence of surface properties of magnesium alloy on its corrosion resistance,and the influence of ultrasonic burnishing processing parameters on the corrosion rate of magnesium alloy are studied.The main research contents and conclusions include:(1)Based on the orthogonal experiment and the full-factor experiment research method,the surface morphology,surface roughness,and surface residual stress are used as surface performance characteristic parameters to study the effect of ultrasonic burnishing surface strengthening process parameters on the surface performance of AZ91D magnesium alloy.Experimental results show that static pressure,burnishing pass,and spindle speed are significant factors affecting the surface integrity of AZ91D magnesium alloy.The prediction models of the influence of ultrasonic burnishing processing parameters on the surface roughness and residual stress of AZ91D magnesium alloy were established respectively.The results of the full-factor ultrasonic burnishing experiment show that the surface roughness and residual stress increase with the increase of static pressure,and the optimal value is obtained when the static pressure is 900N.When the burnishing pass takes the best value,the surface roughness and residual stress get the best results.Before the burnishing pass takes the best value,the two indexes increase with the increase of the burnishing pass.After the best value,it will decrease as the number of burnishing pass increases.The spindle speed has no significant influence on the two indicators.When the spindle speed is set to the lowest level of 120r/min,the surface roughness and residual stress reach the optimal results.The internal stress and strain of the material are analyzed through the finite element analysis method.The results show that the greater the static pressure,the greater the residual stress and plastic strain inside the material,and the greater the strain depth.Burnishing pass has different effects on residual stress and plastic strain.Plastic strain increases with the increase of burnishing pass.Residual stress first increases with the increase of burnishing pass,and then decreases with the increase of burnishing pass.Both residual stress and plastic strain decrease with the increase of spindle speed.(2)The effect of ultrasonic burnishing on the electrochemical corrosion resistance of AZ91D magnesium alloy was studied by electrochemical corrosion experiments.The results of electrochemical experiments show that the open circuit potential of the sample after ultrasonic burnishing is higher and more stable than that of the turned sample and the polished sample after turning.The polarization curve shifted in the positive direction,the self-corrosion potential increased from-1.3956 V to-1.3042 V,and the corrosion current density decreased from 260.62μA/cm~2 to 141.53μA/cm~2.After the ultrasonic burnishing process,the radius of the capacitor arc of the sample is significantly increased,which is much larger than that of the turned sample and the polished sample after turning.After increasing the static pressure,the open circuit potential,self-corrosion potential,corrosion current density and capacitance arc radius of the sample further increase.The effect of ultrasonic burnishing strengthening on the corrosion resistance of AZ91D magnesium alloy in simulated body fluids was studied through simulated body fluid immersion corrosion experiments.The simulated body fluid immersion corrosion test results show that after the turning sample and the polishing sample after turning are immersed and corroded,obvious pitting pits and corrosion marks appear on the surface,while the surface of the sample after ultrasonic burnishing is relatively complete and only some minor corrosion marks appear.The corrosion rates of the three groups of samples were 10.98 mg/cm~2·h,7.29 mg/cm~2·h,and 0.43 mg/cm~2·h.After ultrasonic burnishing,the corrosion rate of AZ91D magnesium alloy in simulated body fluids was significantly reduced.After increasing the static pressure,the corrosion rate of the sample decreased from 0.43mg/cm~2·h to 0.13mg/cm~2·h.By analyzing the influence of the surface morphology,residual stress and microstructure of the four groups of samples on the corrosion performance of the material,the influence mechanism of the ultrasonic burnishing surface strengthening process on the corrosion resistance of AZ91D magnesium alloy was discussed.(3)A simulated body fluid immersion corrosion experiment was performed on the samples after the full-factor ultrasonic burnishing experiment,and a prediction model between the ultrasonic burnishing processing parameters and the corrosion rate was established.The corrosion rate results show that the static pressure and burnishing pass has a significant effect on the corrosion rate.When the static pressure,burnishing pass and spindle speed are respectively 900N,3,and 120r/min,the corrosion rate obtains the optimal value.