| Selective laser melting(SLM),a kind of widely used metal additive manufacturing technology,can well satisfy the requirements of customization and precision of medical implants,promoting its wide application in the field of medical implants.The phase composition of metastableβtitanium alloy is very complex,and the thermal history during SLM has a significant effect on the microstructure and properties of the as-builtβtitanium alloy.Ti15Mo(wt.%)alloy with{332}<113>mechanical twins as the main deformation mechanism has good mechanical properties and relative low elastic modulus,which is a potential medicalβtitanium alloy for implants.In this study,high density biomedical Ti15Mo alloy was prepared by SLM,and the relationship between processing parameters,microstructure evolution and mechanical properties was systematically studied.The results provide theoretical and experimental basis for the effect of thermal history on the microstructure and property evolution of metastableβtitanium alloy prepared by SLM.Firstly,large numbers of attempts have been conducted to figure out the processing window of Ti15Mo alloy manufactured using SLM,and the range of process parameters to obtain the highest density was determined.The internal defects of as-built Ti15Mo alloy are irregular un-melted defects caused by too low energy density and spherical pores under too high energy density.The solidification behavior in the molten pool changes from the planar growth at the molten pool boundary to the cellular growth in the molten pool,accompanied by the micro segregation of Mo element.Secondly,the effect of scanning speed on microstructure and mechanical properties of as-built Ti15Mo alloy with support structure was studied by various analysis methods.The microstructure analysis shows that the phase compositions of Ti15Mo formed by SLM includeβphase and nano-sizeωphases.Further analysis shows that theωphase in the alloy is isothermalωphase formed by in-situ low temperature aging during SLM process.The tensile tests at room temperature show that the Ti15Mo alloys have extremely high yield strength and poor elongation.Its yield strength can reach 1000-1200 MPa,which is significantly higher than that of the Ti15Mo alloy treated by rolled-solution(392 MPa).Strengthening mechanism analysis indicates that the precipitation of isothermalωphase is the main contribution to the high yield strength.The precipitation of isothermalωphase causes the plastic deformation mechanism of as-built Ti15Mo alloy to change from the{332}<113>twinning deformation in the roll-solution state to the dislocation slip.Thirdly,the isothermalωphase in the as-built Ti15Mo alloys was further regulated by the in-situ heat treat effect modified by the processing parameters during SLM process,and then the modulation of mechanical properties of the Ti15Mo alloy were achieved.When the support structure was removed,the laser scanning speed and interlaminar residence time have been proved to play significant effects on the microstructure and mechanical properties of Ti15Mo alloy.With the increase of laser scanning speed and interlaminar residence time,the content and size of isothermalωphase and the yield strength of Ti15Mo alloys decrease,and the plastic deformation ability increases gradually.The plastic deformation behavior of Ti15Mo alloy formed by SLM changes from{332}<113>mechanical twins in Ti15Mo with a low yield strength to{332}<113>twins+stress-inducedωphase transformation in Ti15Mo with a medium strength to dislocation slip in{112}<111>slip system in Ti15Mo with a high strength.In situ heat treatment during SLM process promotes the precipitation of isothermalωphase.The temperature rangeΔT(T_f-T_s)and interaction imeΔt of the in situ heat treatment are the main factors to determine the evolution of isothermalωphase.Finally,the electrochemical corrosion behaviors of as-built Ti15Mo alloys in simulated saliva were investigated through various electrochemical testing methods.The as-built Ti15Mo alloys performed a good corrosion resistance due to the passivation film with high protectiveness formed on the samples.The orientation of samples plays a significant effect on the microstructure and electrochemical corrosion properties of the as-built Ti15Mo alloys.The(001)βplanes of almost all grains in the 0°sample are parallel to the surface of the sample,while the surface of the 45°sample is mainly composed of(101)βand(111)βplanes,and the surface of the 90°sample is mainly composed of(101)βand(100)βplanes.Electrochemical tests show that the corrosion resistance of as-built Ti15Mo alloys has obvious anisotropy in the simulated saliva electrolyte,and the variation trend of the corrosion resistance of three samples with different orientations is 90°>45°≈0°.Further analysis shows that the anisotropy of corrosion properties of as-built Ti15Mo alloys is mainly attributed to the different grain orientations,and the effect of grain boundary density on corrosion properties of Ti15Mo alloy formed by SLM is relatively small. |
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