气氛辅助增强激光3D打印纯钛的成型及其强化机理研究

ATMOSPHERE-ASSISTED STRENGTHENING OF THE COMMERCIALLY PURE TITANIUM PRODUCED BY LASER-BASED 3D PRINTING: A RESEARCH ON THE PROCESSING AND MACHANIUSMS

王大为

11749112

硕士

材料加工工程

严明

2019-06-01

2019-07-11

哈尔滨工业大学

深圳

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TiNx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。

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 of ~0.43 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/β dual 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 TiN0.3; however, the subsequent thermal cycles of SLM processing induced the decomposition of TiNx compounds and finally transformed the material into a homogeneous solid solution except the top surface. Nitrogen generally dissolved in the α′-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.

3D打印 纯钛 活性气氛 选区激光熔化 力学性能 微观组织表征

3D printing Commercially pure titanium Active atmosphere Selective laser melting Mechanical properties Microstructure characterization

中文

联合培养

南方科技大学

王大为. 气氛辅助增强激光3D打印纯钛的成型及其强化机理研究[D]. 深圳. 哈尔滨工业大学,2019.