激光立体成形钛合金全等轴晶均匀组织形成机理及精准调控

Laser solid forming (LSF) process has great potential in rapid fabrication of high performance large and medium-sized titanium alloy components with complex shapes. However, the typical microstructure characteristics of LSFed titanium alloys, such as epitaxial columnar β grains and heterogeneous substructure, usually result in anisotropic, unpredictable and unrepeatable mechanical properties. The fabrication of titanium alloy contains equiaxed β grains with homogeneous substructure is a possible way to solve this problem. In the present project, a novel two-step processing route for achieving equiaxed β grains and homogeneous substructure in LSFed titanium alloy is proposed. Ti17 near β titanium alloy with rich β stabilizers is used as experimental material. The stair-like samples are fabricated to reveal the microstructure evolution during the solidification and solid-state transformation process. Three-dimensional transport models are developed to simulate the melting behavior of the metal powders and the transient evolution of the molten pool, respectively. The microstructures evolution of the samples are also carefully investigated. Particularly, the growth behaviors of epitaxial cellular structure and equiaxed dendrite in the molten pool will be revealed. Effects of the complex thermal cycles and the post heat treatment processes on microstructure evolution will also be studied. A processing-microstructure map will be built. The project is expected to provide a scientific theoretical basis for the accurate control of the microstructure of the LSFed titanium alloys, and will promote its applications in the engineering fields.

激光立体成形技术在大中型较复杂形状钛合金构件的高性能快速制造领域极具应用前景，但成形构件典型的“柱状β晶粒+非均匀晶内亚结构”组织易导致力学性能各向异性等问题。实现“全等轴β晶粒+均匀晶内亚结构”组织的制备是解决这一难题的可行方向。本项目提出大送粉量条件下以部分熔化粉末作为形核核心获得全等轴β晶粒，结合均匀晶内亚结构调控技术，实现钛合金成形件全等轴晶均匀组织精准调控的创新思路。以近β型Ti17合金为研究对象，构造能够反映成形过程典型热循环特征的阶梯状试样。通过对粉末熔化行为和熔池动态演化的数值模拟、成形过程温度监测及宏微观组织特征的完善表征，将凝固参数与凝固组织、往复热循环特征与固态相变组织一一对应并定量描述，明晰粉末熔化-熔池演化-凝固组织特征，及往复热循环-晶内亚结构组织特征两个关键过程的科学机理，建立全等轴晶均匀组织-工艺参数选择图谱，为激光立体成形钛合金力学性能优化奠定基础。

本项目针对激光增材制造钛合金成形构件典型的“柱状β晶粒+非均匀晶内亚结构”组织易导致力学性能各向异性等问题，提出了分两步获得“全等轴β晶粒+均匀晶内亚结构”以改善力学性能的新思路。通过对粉末熔化行为和熔池动态演化的数值模拟、宏微观组织特征的完善表征，明晰了从熔池凝固到固态相变全过程的组织演化规律，并分析了其对力学性能的影响。取得的主要成果如下：（1）合金成分、形核位数、凝固条件等共同决定了激光立体成形钛合金的晶粒形貌，仅调整工艺参数难以持续细化晶粒；（2）在连续沉积过程中，不同沉积层之间的热积累和热历史差异是导致成形试样非均匀晶内亚结构的主要原因。层间暂停不仅可以减少每一沉积层所经历的热历史差异，提高了组织均匀性，还可以弱化热积累的作用，减小了熔池尺寸，避免了已沉积层发生大部分重熔所导致的柱状晶连续生长；（3）力学性能测试结果表明全等轴晶激光立体成形钛合金的拉伸性能明显优于全柱状晶组织，用改进的复合模型可以较好的预测屈服强度和α/β相特征之间的关系。本项目的实施为优化激光增材制造钛合金的力学性能并扩大其应用范围奠定了基础。本研究完成了项目合同书中的主要研究内容，满足了所有的项目研究目标。