Investigation On Corrosion Behavior Of Ti-6Al-4V Alloy Fabricated By Electron Beam Melting

Titanium and titanium alloys are considered ideal metallic implant materials due to their high strength,low elastic modulus,good corrosion resistance and high biocompatibility.Electron beam melting(EBM),as one of the important techniques of additive manufacturing(AM),has significant advantages in producing titanium alloys with high melting point and complex structures.At present,most investigations on EBM-produced titanium alloys have been focused on processing parameters optimization,microstructures,mechanical properties and biocompatibility.There is a lack of study on the corrosion behavior.In this thesis,dense and cellular structured Ti-6Al-4V alloys,which have been extensively used in the medical field,are fabricated by EBM.The effects of the EBM processing orientation,thickness of samples,and pore characteristics on the corrosion performance of the EBM-produced Ti-6Al-4V alloys were investigated,and the corrosion mechanisms were discussed.The results showed that the microstructure of EBM-produced Ti-6Al-4V consisted of lamellar a and ? phases.Compared with wrought Ti-6Al-4V alloy containing similar microstructure,the lamellar a of EBM-produced Ti-6Al-4V was much refined with a higher fraction of ? phase due to the faster cooling rate and preheating during fabrication.The fine lamellar a and ? phases homogenized the alloying elements distribution in the two phases,resulting in the decreased galvanic effect between a and ? phases.The high fraction of ? phase increased the resistance of charge transfer through double layer and reduced the rate of metal dissolving reaction.Meanwhile,the high density of uniform grain boundaries provided short channels for the transfer of electrons and the diffusion of atoms.Thus,the increased the growth rate in progressive nucleation of passive film during the initial stage led to rapid formation of a stable and protective passive film.These factors contribute to the improved corrosion resistance in comparison with the wrought Ti-6Al-4V in phosphate buffer saline(PBS).The orientation and sample size also affected the corrosion performance of EBM-produced Ti-6Al-4V alloy.The horizontal plane exihibited better corrosion property than vertical plane because of the high fraction of(0001)? oriented grains.The cooling rate increased with the decreased thickness of sample,resulting in high fraction of ?' martensile retained,corresponding to an inferior corrosion property.The effect of the pore characteristics on the corrosion behavior of cellular structured Ti-6Al-4V alloy was related to the solution.In PBS solution,both the dense and cellular structured Ti-6Al-4V alloys exhibited superior corrosion resistance.But the passive film on cellular structured alloys was less stable and protective,which was attributed to the influence of larger exposed surface area involving an increased interaction between the electrolyte and alloy surface.Meanwhile,the passive current density decreased and the charge transfer resistance increased with the increase of pore size.Because the large pore was conducive to the flow of electrolyte,weakened the compositional change of micro-environment inside the pores,thereby reducing the probability of localized corrosion.However,in acidic PBS containing fluoride ions(pH=2,F-=0.05 M,PBS-H-F),the pore characteristics did not play an important role in the corrosion resistance.It was considered that the protective film broken down such that the corrosion performance of cellular structured alloys was comparable to each other in this harsh environment.The variation of electrochemical corrosion behavior with pore depth in EBM-produced cellular structured Ti-6Al-4V alloy was studied in-situ by a simulation device in PBS-H-F solution.The results showed that the corrosion rate decreased with the increase in pore depth due to buffering of pores.There was a fluoride ion concentration gradient along the direction of the pore depth during diffusion into the pores.The higher concentration of fluoride ions at shallow pore depth led to near-complete dissolution of protective film and steady-state pitting occurred on the surface.While the fluoride ion concentration decreased with the increase of pore depth,which reduced the damage of the protective filmand slowed the corrosion rate of cellular structures.