Research On Laser Additive Manufacturing And Degradation Regulating Of Iron-Based Bone Implants

Iron（Fe）is a potential metal bone implant material due to its excellent mechanical properties,good biocompatibility,and degradability.However,the degradation rate of Fe is too slow to meet the actual requirements of the clinical application of degradable bone implants.The slow degradation rate is caused by the low mass transfer rate,slow electron transfer rate,and high standard electrode potential.In this work,Fe-based bone implants were developed using mechanical alloying and selective laser melting technologies.The degradation rate of implants was accelerated by applying a magnetic field to improve the mass transfer rate,introducing a composite cathode to catalyze the electrochemical reaction,and alloying with low electrode potential elements to reduce the electrode potential.The preparation process,microstructure characteristics,degradation performance,and biocompatibility of implants were studied.The main research contents are as follows:（1）An external magnetic field was introduced to improve the mass transfer rate of charged particles or magnetic particles during the degradation of Fe-based materials.Fe-Ga bone implants were prepared by mechanical alloying and selective laser melting technologies.Among them,the alloying of gallium（Ga）with Fe can reduce the electrode potential of Fe and improve the response-ability of Fe to the magnetic field.The external magnetic field accelerates the mass transfer rate in the degradation process through Lorentz force or magnetic field gradient force.The experimental results showed that the corrosion potential of Fe-Ga alloy was lower than that of Fe,about-0.72 V;the corrosion current density of Fe-Ga alloy under a parallel magnetic field was about 28.12μA/cm²,and the electrochemical corrosion rate was 0.25 mm/year.The immersion degradation rate under a vertical magnetic field was 0.17 mm/year.（2）Manganese dioxide（Mn O₂）with high work function and oxygen reduction catalytic activity and titanium dioxide（Ti O₂）with high conductivity were introduced as the composite cathode to accelerate the electron transfer rate during Fe degradation.Fe-Mn O₂-Ti O₂ composite bone implants were prepared by mechanical alloying and selective laser melting technologies.The experimental results showed that Mn O₂ had a higher work function than Fe,and it was an N-type semiconductor,which can form ohmic contact when in contact with Fe.After adding Mn O₂-Ti O₂ composite cathode into Fe,the limiting current density,half-wave potential,and electron transfer number were 5.32m A·cm^-2,-767.4 m V,and 2.9,respectively.The electrochemical corrosion rate of Fe-Mn O₂-Ti O₂was 0.33 mm/year,and the immersion degradation rate was 0.19 mm/year.（3）Manganese（Mn）and silicon（Si）elements were introduced to alloy with Fe to improve the corrosion sensitivity of Fe.Among them,Mn has a lower electrode potential,which can shift the potential of the matrix negatively and improve the corrosion sensitivity after alloying with Fe.At the same time,Mn can transform the ferrite phase of the Fe matrix into the austenite phase.The Si element can reduce the stacking fault energy of austenite and transform part of the austenite phase into the martensite phase.The galvanic corrosion effect between austenite and martensite can further accelerate the degradation of Fe.Biphasic Fe30Mn6Si bone implants were prepared by mechanical alloying and selective laser melting technologies.Degradation experiments showed that the corrosion potential of Fe30Mn6Si was about-0.79V,lower than that of Fe.The electrochemical corrosion rate was about 0.29 mm/year.The immersion degradation rate was 0.25 mm/year.In addition,in vitro cell experiments showed that Fe30Mn6Si implants had good biocompatibility.