Study On The In-situ Preparation And Properties Of Biomedical Titanium And Zinc Alloy Porous Structures Using Selective Laser Melting

With the fast growth of the aging population,the number of orthopedic operations in our country is rapidly increasing.Metallic bone implants currently in clinical use are prepared using traditional casting or machining methods.The high elastic modulus and bio-inert of the implants can cause osteoporosis and displacement.Selective laser melting（SLM）,as one of the additive manufacturing technologies,is capable to fabricate metal with favorable properties and complex structures,which can meet the requirements of high-performance metallic implants.Titanium-tantalum（Ti-Ta）alloys and Zn alloys have emerged as attractive bone substitute materials.Ti-Ta alloys and Zn alloys manufactured by conventional processes such as cast and forging exhibited coarse grain,inhomogeneous element distribution and poor mechanical properties.Considering the difficulties in the preparation of Ti-Ta alloys and Zn alloys,this study aims to in-situ fabricate Ti-Ta alloys and Zn alloys porous structures using SLM.The convection and rapid solidification in the melt pool could benefit the chemical homogeneity of the elements and grain refinement,realizing the integrated preparation of low modulus,high strength and bioactivity implants.The research mainly focused on in-situ alloying,microstructure evolution,mechanical properties and corrosion resistance.The main contents are as follows:An in-situ preparation method of Ti-Ta alloy porous structure with high strength-modulus ratio,bioactivity and excellent bone regeneration ability was proposed.The in-situ preparation mechanism,microstructure,mechanical properties,bioactivity and osteogenesis ability of Ti-Ta alloy were systematically studied.The alloy composition was close to the element ratio of the original design through process optimization.We successfully prepared Ti-Ta alloy porous structure without defects,which had high yield strength,low elastic modulus and excellent elastic admissible strain（EAS）.The mechanisms of the high EAS were ascribed to the formation ofβ（Ti,Ta）solid solution,ultrafineβgrains accompanying with nanocrystallineα’grains,and the existence of dislocations and stacking faults.Bone-like apatite was spontaneously induced on the surface of the printed Ti-Ta alloy due to the generation of a self-passivating Ta₂O₅ film,indicating a good biomineralization ability.Compared to pure Ti,the printed Ti-Ta alloy exhibited an enhanced expression of vinculin,earlier cell extension,increased nuclei density,better cell proliferation,and the up-regulated expression of osteogenesis genes.Animal studies further validated that the printed Ti-Ta porous structures were capable to reinforce bone integration and accelerate bone regeneration.An in-situ preparation method of high-strength Zn-Mg and Zn-Mg-Cu alloy porous structure was proposed.We systematically studied the microstructure,mechanical properties,electrochemical properties and degradation properties of Zn-Mg and Zn-Mg-Cu alloys.Zn-Mg and Zn-Mg-Cu alloys were composed of HCP-Zn phase and Mg₂Zn₁₁ with the dendritic and cellular grain.The grain sizes of Zn-Mg and Zn-Mg-Cu alloys were 1.66μm and 1.21μm,respectively.Compared with pure Zn,the addition of Mg and Cu elements promoted the grain nucleation and inhibited the formation of coarse grain and preferred orientation.The addition of Mg and Cu led to a corrosion rate suitable for bone repair.Before and after the dynamic degradation experiments,the porous structures exhibited favorable mechanical properties that match the cancellous bone.The structures showed a self-reinforcing effect based on corrosion degradation.The strengthening mechanisms of Zn-Mg alloys included Orwan strengthening resulted from Mg₂Zn₁₁,grain refinement strengthening,and dislocation strengthening.We systematically studied the corrosion fatigue behavior of the Zn-Mg alloy porous structure and the effect of the fatigue test on the microstructure.A fatigue self-reinforcement mechanism driven by corrosion degradation was found.Compared with pure Zn porous structure（fatigue strength 9.59 MPa,strain 2.82%）,Zn-Mg alloy porous structure had higher fatigue strength and lower strain（fatigue strength 31.4 MPa,strain2.35%）.Zn-Mg alloy porous structure possessed the better ability to resist deformation during the cyclic loading process.For Zn and Zn-Mg porous structures,the dominating contributor to fatigue failure was cyclic ratcheting.Tensile stress in the porous structures led to crack nucleation and growth.Zn O/Zn（OH）₂ corrosion products formed during the corrosion fatigue process could improve the fatigue performance of Zn and Zn-Mg alloy porous structure.Zn-Mg alloy porous structures could effectively inhibit the formation of coarse grain and preferred orientation and reduce the internal residual stress of the porous structure during fatigue tests.