Microstructures And Properties Of Titanium Alloy For Spinal Fixation Devices

Ti6Al4V(TC4)alloy due to its high specific strength and good biocompatibility is widely used as spinal fixation devices consisting of pedicle screw(U-shaped nail),rod and nut,which are used to treat vertebral and thoracic vertebrae fractures and malformations.When TC4 alloy is processed to U-shaped nail and rod for spinal fixation devices,it will occur to necking and springback,respectively.It can not be used to treat patient when U-shaped necking size is larger than 0.05 mm,and it does not convenient for surgery when rod with large springback behavior.In addition,TC4 alloy contains potentially elements of Al and V,while β-type titanium alloy,for example Ti15 Mo,is nontoxic and its modulus is lower than TC4 alloy.It has outstanding prospect in spinal fixation devices.Microstructures and mechanical properties of TC4 alloy bar for medical U-shaped nails with different necking size were investigated,and them of bar after different procedures and annealing procedures were inquired.Meanwhile,springback and mechanical properties of rods for spinal fixation devices with different springback behavior were explored by three-point bending test,and them of wires after different annealing procedures were investigated concurrently.In addition,elastic modulus of Ti15 Mo and TC4 alloys were measured by ultrasonic velocity test.Their corrosion property in Ringer’s human simulated body fluids was explored by three-electrode system.Meanwhile,springback behaviors of Ti15 Mo and TC4 alloys plates were compared.U-shaped nails with different necking size and rod with different springback behaviors are found to be caused by the difference of recrystallization degree leading to the difference of microstructures and mechanical properties.Comparing with microstructure and mechanical property of bar after different procedures,recrystallization microstructure is found to be affected significantly by high temperature annealing,while mechanical property is affected by hyperbola straightening and low temperature annealing.The recrystallization degree of hot drawing bar is sufficient after annealing at 730℃-750℃ for 2 h.The optical microstructure of straightening bar does not change significantly,but the Vickers hardness decreases evidently after annealing at 580℃-700℃ for 2 h.When the high and low temperature annealing are performed at 730℃-750℃ and 580℃-640℃,respectively,mechanical property of the bar is suitable for the medical TC4 alloy based on microstructure control.In addition,the necking size of U-shaped nail processed from the bar is desirable to be lower than 0.05 mm,and the rod with high strength and low springback ratio of 34% will be obtained after annealing 2 h at 730℃.Elastic modulus and springback of Ti15 Mo alloy after solid solution treatment are found to be lower than them of TC4 alloy.Its corrosion resistance is stronger than that of TC4 alloy in Ringer’s human simulated body fluids,while its flexural strength is lower.It can be obtained the alloy for spinal fixation devices with a good combination of strength,modulus and springback after controlling deformation microstructure and precipitated phase of Ti15 Mo alloy.