【作者】 王大为；

【导师】 严明；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2019， 硕士

【摘要】 纯钛（CP-Ti）因其优越的耐腐性能与生物相容性,成为了应用最早、最为广泛的Ti/Ti合金材料之一,但硬度、强度不足始终是纯Ti拓展其应用范围的重要掣肘。本课题提出了一项新颖、高效的加工方法,将Ar-N₂复合活性气氛引入激光3D打印（选区激光熔化,SLM）过程,在3D打印纯Ti构件的同时使材料与气氛中的N₂原位反应,成功产出了近乎完全致密（相对致密度可达99.5%以上）、综合力学性能优异的气氛辅助增强SLM纯Ti。相对于惰性（纯Ar）气氛中打印的纯Ti,活性气氛强化的纯Ti在硬度和强度方面获得了40-50%甚至更高比例的提升,同时在氮含量不超过0.43 wt.%的前提下可保持优良的塑性。最优试样的屈服/抗拉强度达807/1037 MPa,同时断裂延伸率达19.15%;强度接近锻态的Ti-6Al-4V双相Ti合金但塑性高出其一倍以上。本课题运用背散射电子衍射（EBSD）、三维原子探针（3D-APT）、透射电镜（TEM）、激光原位加热同步辐射X光衍射（SXRD）等多种先进表征手段,对活性气氛强化的纯Ti进行了详尽、系统的微观组织研究,揭示了Ar-N₂活性气氛中SLM打印纯钛过程的微观组织演变:每一“层”材料打印时,Ti与气氛中的N₂反应生成Ti-N间隙固溶体与TiN_x化合物颗粒;但此类化合物会在接下来的打印热循环中分解,并使N均匀的固溶于细小的针状马氏体（α′-Ti）晶粒基体内。N的溶入为SLM纯Ti带来了固溶强化与细晶强化两大强化作用,其中N原子进入α′-Ti晶体八面体间隙的固溶强化为作用的首要因素。在微观组织与强化机理研究的基础上,本课题提出了基于Mechanical Threshold Stress理论的本构模型,可合理描述具有不同N含量与晶粒尺寸的活性气氛强化纯Ti的拉伸变形行为,并能够通过流变方程准确预测强化纯Ti的屈服点与完整的应力-应变曲线。本研究成功实现了激光3D打印纯Ti强化与成型的一体化,制备了不含有害合金元素（如Al、V、Cr等）的高性能纯Ti,阐明了活性气氛原位强化的机理以及材料性能、组织的控制原理与方法。为开发高性能、无毒害生物医用Ti材料及其他高性能金属材料提供了一项崭新的技术工具,同时加深、拓展了对SLM这一新型3D打印技术所带来的材料基础问题的理解。更多还原

【Abstract】 Commercially pure titanium（CP-Ti）is one of the earliest and most widely used Ti-based materials.CP-Ti is well-known for its excellent corrosion resistance and biocompatibility,but the extensive application of CP-Ti is hindered by the lack of strength and hardness.In this thesis,we designed a novel and efficient manufacturing approach by introducing the Ar-N₂ reactive atmosphere into laser-based 3D printing（selective laser melting,SLM）processing.CP-Ti reacted with the atmosphere simultaneously during the SLM processing and yielded almost full-dense（relative density≥99.5%）components.CP-Ti strengthened by the reactive atmosphere exhibited outstanding comprehensive mechanical properties.Its hardness and strength have been enhanced by 40-50%or even higher compared to those of CP-Ti printed in the inert（pure Ar）atmosphere.CP-Ti’s ductility remains unharmed up to a N content of⁰.43 wt.%.The optimal specimen showed yield/ultimate tensile strength of 807/1037 MPa,with a strain-to-fracture of 19.15%.The strength of this strengthened CP-Ti is close to that of wrought Ti-6Al-4V（α/βdual phase）alloy,while the ductility is more than twice as Ti-6Al-4V’s.Electron backscatter diffraction,atom probe tomography,transmission electron microscopy,and laser-heated in-situ synchrotron X-ray diffraction analysis were employed to reveal the mechanism underlying the in-situ reaction between CP-Ti and the reactive atmosphere.The microstructure and its evolution were systematically studied and the features were described in detail:Ti reacted with N₂ during melting,forming a Ti-N solid solution as well as compound particles such as TiN and TiN_0.3;however,the subsequent thermal cycles of SLM processing induced the decomposition of TiN_x compounds and finally transformed the material into a homogeneous solid solution except the top surface.Nitrogen generally dissolved in theα′-Ti matrix and occupied the octahedral vacancy sites as interstitial solute atoms.N solute exerted solid solution strengthening and grain refinement on the as-printed CP-Ti,while the former was the major contributing factor of the total strengthening effect.A constitutive model was developed to describe the tensile deformation behavior,such as the yield point and stress-strain curve,of the CP-Ti printed in the reactive atmosphere.The model provides a rational description of the flow behavior of the as-printed CP-Ti over various solute concentrations and grain sizes.This study has achieved the integration of 3D printing and in-situ strengthening of CP-Ti,yielded a high-performance CP-Ti which is free of detrimental alloying elements（such as Al,V,Cr,etc.）.The strengthening mechanisms have been illuminated along with the control methods of microstructure and mechanical properties.This thesis has demonstrated a promising methodology for the production of nonhazardous biomedical Ti and other high-performance metallic materials.Additionally,it has also extended the fundamental understanding of the SLM process under the reactive atmosphere.更多还原