| Because of excellent mechanical properties,low elastic modulus,high biocompatibility and excellent corrosion resistance,?-type titanium alloys are widely used as hard tissue substitutes or restorations for bone trauma products and artificial joints.Nevertheless,the elastic modulus of these materials is needed to further reduce in order to match with human bone by reducing the stress shielding effect.Forming porous structure is one of the most effective methods to reduce the elastic modulus.Besides,porous structure also favours cell adhesion,proliferation,and bone regeneration.The implant and tissue get a greater biological combination,which improve the success rate of implantation and has great application potential.However,the working state of human bone is not considered in the current design and manufacture of porous medical?-titanium alloy and the effect of pore structure on mechanical and biological properties is also unclear.In this study,different types of porous structure models were designed by topology optimization method according to different stress states of human bone.These models were fabricated by selective laser melting(SLM)technology with Ti-35Nb-7Zr-5Ta(TNTZ)titanium alloy powder.The effects of SLM process parameters on forming quality,microstructure,and properties were systematically studied.The effects of geometric parameters on mechanical and biological properties were investigated.The specific work and conclusions are summarized as follows:(1)Based on the stress state of human humerus,cervical vertebrae and lumbar vertebrae,three kinds of porous structures(Compress,Shear and Torque)with different porosities were designed by variable density topology optimization model.Through the statics and compression process simulation,it is found that the optimized porous structure has more uniform stress distribution and less stress concentration uder corresponding load conditions,which verifies the effectiveness of topology optimization design herein.In addition,the yield strength and elastic modulus of Compress,Shear and Torque porous structures decrease in turn at the same porosity.With the increase of porosity,the yield strength and elastic modulus of different porous structure decrease gradually.(2)The effects of SLM processing parameters on the forming quality,microstructure,and properties of porous?titanium alloy were systematically studied.The results show that with the increase of laser energy density,TNTZ alloy displays the decrease of porosity and forming quality,and increase of the size deviation,the yield strength and elastic modulus.When the scanning speed is 1400 mm/s and the laser power is 180 W,the SLMed porous TNTZ alloy has the least internal defects and optimal comprehensive properties.The microstructure analysis shows that the porous sample is composed of single?-Ti phase with a columnar or cellular shape,showing{110}<001>Goss texture in the architectural direction.With the increase of scanning speed,the microstructure of the sample is refined,and the texture is weakened.The normalized strength and Vickers hardness rule reveal that the mechanical properties of porous TNTZ alloy are mainly strengthened by the second phase.Finally,the prediction model of process parameters structural porosity and yield strength is established:=-0.26((1)+67.77,=551.5+43.7.(3)Based on the optimized processing parameters,porous samples with different porosities and pore structure were produced.The effects of porous geometry parameters on mechanical and biological properties were systematically studied by compression test and cell culture in vitro,respectively.The results show that the lower the porosity results in the higher the mechanical properties and cell proliferation level,and 70%porosity is optimal cell adhesion.At the same porosity,the optimized Compress porous structure has better mechanical properties,cell adhesion and proliferation ability than the corresponding ones of traditional porous structures like Cubic and BCC structures. |
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