激光增材制造钛合金α相取向选择机制及其控制方法研究

Coaxial powder laser additive manufacturing technology has obvious advantages in the field of rapid and low-cost preparation of large, integral and high-performance titanium alloy components. Due to the special solidification and cyclic unsteady solid phase transformation in the additive process, the content of α phase in some specific orientations was significantly higher than that in other orientations, which significantly affected the mechanical properties of the alloy and the dimensional accuracy in the heat treatment process. To make clear the characteristics and evolution of α phase orientation in micro heat cycle and subsequent heat treatment of additive manufacturing is the precondition and foundation of regulating micro-structure orientation, improving mechanical properties of alloy and ensuring dimensional accuracy of components in heat treatment process. This project aims to reveal the main factors and control mechanism that affects the orientation of α phase, and to clarify heat treatment temperature, holding time and cooling mode on the influence law of α orientation, by study the α orientations in different deposited layers and evolution in different heat treatment process of laser deposited TC4 alloy. The exploration of the active control method of α orientation by changing the deposition and heat treatment process, would provide the theoretical basis to the new property and size control technique of laser additive manufactured titanium alloy which based on the theory of the α orientation control method. In this meaning, the project has important theoretical meaning and engineering value.

同轴送粉式激光增材制造技术在大型、整体、高性能钛合金构件快速、低成本制备领域技术优势明显。增材过程中特殊的凝固和循环非稳态固态相变过程致使钛合金中某些特定取向的α相含量明显高于其他取向，显著影响了合金的力学性能和热处理过程中的尺寸精度。搞清楚增材制造微热循环及后续热处理过程中α相取向特征及其演变行为，是调控微观组织取向、改善合金力学性能、保证构件热处理过程中尺寸精度的前提和基础。本项目通过对TC4钛合金沉积态不同沉积层内α相取向特征及其在不同热处理工艺中演变行为的研究，揭示沉积过程中控制和影响α相取向的主要因素和作用机制，阐明热处理温度、保温时间及冷却方式对α相取向的影响规律，探索通过改变沉积及热处理工艺主动控制增材制造钛合金α相择优取向的方法，为形成基于α相取向主动控制理论的激光增材制造钛合金“形、性”控制技术提供科学依据和理论基础，具有重要的理论意义和工程价值。

受增材制造过程中的温度场及应力场影响，激光增材制造双相钛合金构件沉积态微观组织往往表现出明显的α和β织构，在热处理过程中，织构的取向及强弱也在发生变化。由于BCC结构的β相和HCP结构的α相对称性的差异，在固态相变过程中α板条转变为β相时在不同方向的体积变化也不同，织构取向及强弱的变化导致构件宏观上出现了各向异性的尺寸变化。本项目以TC4钛合金为研究对象，首先分析了不同扫描策略下激光增材制造TC4钛合金沉积态的组织特征，发现不同扫描策略下增材制造TC4钛合金的织构特征基本一致，仅强弱略有差别；然后对比了不同热处理制度下激光增材制造TC4钛合金微观组织形态及取向的变化，分析了增材制造TC4钛合金热处理过程中的尺寸变化特性；之后通过高温保载冷却实验，通过实验方法证明在钛合金发生固态相变时，近屈服强度的应力可以促使特定取向α板条的析出概率大幅提升，并通过理论计算推导了在定向凝固的β晶粒情况下，α织构的形成特性，及其对宏观尺寸的影响；最后基于理论分析和多批次数据积累，确定了大尺寸钛合金构件增材制造过程中的全流程尺寸精度控制方法，并已在部分大型钛合金构件的研制过程中得到应用。