激光增材制造钛合金非平衡亚晶结构及β柱状晶等轴化机理研究

The laser additive manufacturing (LAM) technology has developed rapidly in recent years, and the controlling of microstructure are the basis to achieve excellent properties of LAM titanium alloys. The complicated non-equilibrium solidification behaviors and subsequent thermal cycling could result in a special microstructures, e.g formation of large number of sub-grains, when compare to conventional process. The high density of sub-grain have an important influence on the solid phase transformation behavior such as the nucleation and growth of α and the columnar to equiaxed transition of prior β grains, resulting the change of structure stability during service. In this project, the TC11 α+β titanium alloy and TB7 β titanium alloy are selected for studying the formation mechanisms of non-equilibrium subgrain structure, and the complex internal stress effect and evolution behavior during the annealing treatments. Then the transition mechanism of the columnar βgrains to equiaxed grains will be revealed. The special emphasis will be put on the studying the mechanism of the abnormal grain growth of small equiaxed grains growing to large columnar grains during the phase transition. The project will illuminate the effect of subgrain structure on the phase transformation and microstructure of the titanium alloy, and it will also offer an important basic theory for controlling the structure and modifying the mechanical properties for laser additive manufacturing key titanium alloys components.

激光增材制造(LAM)技术快速发展，组织演变规律与调控研究是实现LAM钛合金优异性能的基础。激光增材制造复杂的非平衡凝固行为和循环热影响作用导致LAM钛合金组织与传统制造技术明显不同，甚至形成大量亚晶结构，高密度亚晶结构对α相形核生长及β柱状晶等轴化等固态相变行为具有重要影响、甚至会影响构件服役的组织稳定性。本项目以激光增材制造α+β型TC11和全β型TB7钛合金为对象，研究LAM钛合金构件中非平衡亚晶结构特征，揭示熔池非平衡快速凝固和逐层沉积复杂内应力作用下亚晶结构形成机制及其在后续退火热处理过程的演变行为；研究LAM钛合金在β单相区退火原始β柱状晶转变成粗大等轴晶的动力学过程及物理模型，阐明小等轴晶长大吞并原始柱状晶的反常晶粒长大机制和β柱状晶等轴化的机理。项目将揭示亚晶结构对LAM钛合金固态相变行为及构件组织的影响规律，为激光增材制造关键钛合金构件组织和力学性能精确调控提供理论基础。

定向能量沉积激光增材制造制造技术以其突出的技术优势为高性能钛合金大型复杂整体关键构件的低成本、短周期制造提供了变革性技术途径，近20年来在航空航天等领域先进装备制造中获得了广泛应用。激光增材制造技术是一个先进材料制备和构件成形一体化的制造技术，具有微区小熔池冶金和超高温度梯度超快冷却速率的非平衡快速凝固条件，从而能够获得与传统制造技术完全不同的显微组织和力学性能。开展激光增材制造钛合金显微组织和固态相变行为研究，对于实现组织性能优化和工程应用具有重要意义。本项目针对激光增材制造双相钛合金在单相区保温时原始β柱状晶转变成粗大的等轴晶这一特殊固态相变特征，制备了激光增材制造TC11和TB7钛合金构件样品，并采用金相、扫描、EBSD等技术对各种状态样品组织特征进行了详细的表征。研究了激光增材制造TC11和TB7钛合金构件的沉积态凝固组织和非平衡亚晶结构特征，揭示了熔池非平衡快速凝固和逐层沉积复杂内应力作用是亚晶结构形成的主要机制及其在后续退火热处理过程的演变行为；研究激光增材制造钛合金在β单相区退火原始β柱状晶转变成粗大等轴晶的动力学过程并建立了晶粒长大的物理模型，阐明小等轴晶长大吞并原始柱状晶的反常晶粒长大机制和β柱状晶等轴化的机理。项目研究成果为激光增材制造关键钛合金构件组织和力学性能精确调控提供理论基础。