Study On Surface Polishing Of Biomedical Ti-6Al-4V Alloy Through Additive Manufacturing By Electron Beam Melting

Titanium alloys are widely used in the medical implant due to excellent mechanical properties,corrosion resistance and biocompatibility.However,its elastic modulus is much higher than that of human skeleton,causing the"stress shielding"phenomenon,which would led to the loosening of implants.The porous Ti-6Al-4V alloy through additive manufacturing by electron beam melting(EBM)can effectively reduce elastic modulus.But the surface will inevitably generate powder adhesion and spheroidization effect,resulting in the increase of surface roughness.In this paper,the effects of chemical polishing,electrochemical polishing and chemical abrasive flow polishing on surface roughness,thickness reduction and material(weight)removal rate(MRR)of EBM Ti-6Al-4V alloy were investigated.The electrochemical and fatigue properties of Ti-6Al-4V alloy before and after chemical polishing were compared.Hydrofluoric acid has a good polishing effect because of its high corrosiveness to Ti-6Al-4V alloy.By adding an appropriate amount of nitric acid in hydrofluoric acid solution,the thickness reduction and MRR can be reduced,and the occurrence of hydrogen embrittlement can be prevented.With hydrofluoric acid and nitric acid as the polishing solution,the surface of the alloy after chemical polishing is flat and smooth,and no unmolten particles exists.The roughness can reach 1.651μm.The suitable chemical polishing process is the solution of 5%hydrofluoric acid,10%nitric acid and 85%water for 5.5h.The suitable electrochemical polishing process is the solution of 4%hydrofluoric acid,8%nitric acid and 88%water for 3h,and the roughness can reach 5.515μm.The suitable chemical abrasive flow polishing is the solution of 5.5%hydrofluoric acid,11%nitric acid and 83.5%water for 4h,and the roughness can reach 3.545μm.The roughness,thickness reduction and MRR of Ti-6Al-4V alloy after chemical polishing are lower than those of other polishing processes.The influence of chemical polishing on electrochemical behavior of Ti-6Al-4V alloy in simulated body fluids was studied by polarization curves and ac impedance.The results show that the polarization resistance of Ti-6Al-4V alloy after chemical polishing is higher than that of original alloy.The corrosion current velocity also decreases.Therefore,chemical polishing is conducive to the improvement of corrosion resistance of EBM Ti-6Al-4V alloy in human body fluids.The fatigue behaviors of porous Ti-6Al-4V alloys before and after chemical polishing were investigated.The results show that the S-N curves trend is basically the same,and the fatigue strength for all alloys under the 10~7 cycles is about 8MPa.Chemical polishing has almost no effect on fatigue performance of porous Ti-6Al-4V alloy.Obvious fatigue stripes and secondary cracks can be found in the fatigue fracture.There is no dimple feature and the fracture is brittle.