3D激光立体成形典型合金的高温动态拉伸变形及失效特性研究

In the manufacturing fields of sophisticated components applied to aeronautics, aerospace and engine, 3D laser solid forming technology has shown the incomparable advantage over the traditional process. In this application, two types of 3D laser lowly particles cladding-forming material widely used in engines, Titanium alloy (TC4) and Ni-based superalloy (GH4169), have been focus on and researched. Based on the characteristics of epitaxial grown grain columnar or dendrite grain along the deposition direction formed by the shapely cooling process, two groups of experiments will be performed, the tensile tests with large spectrums of temperatures (77K~1200K) and strain rates (0.001/s ~5000/s) will be performed firstly. The plastic flow behavior and deformation mechanism will be researched, especially the relation among special texture of alloys, temperature and strain rates, and the anisotropy of grain, combined with typical experimental skills, such as coupled effect of temperature and strain rate, separation between isothermal and adiabatic, strain rate jump, etc. In addition, utilized the SEM and TEM microstructure analysis technique. And then the physically based constitutive relation will be developed. Then considered the effect of stress triaxiality in these two alloys, the dynamic tensile experiments at elevated temperature and strain rates will be performed. What`s more, combined with SEM and TEM microstructural analysis, the damage failure/failure mechanism caused by micro-porous/micro-crack aggregation toughness fracture in these deposited alloys. The development of this project will provide scientific basis for shock resistance design and containment design of 3D laser solid formed aircraft components, engine components, etc.

对于航空航天及发动机等领域精密复杂构件的制造，3D激光立体成形技术显示出传统工艺无法比拟的优势。针对在飞机及发动机应用最广的3D激光粉末熔覆成型钛合金（TC4）和镍基高温合金（GH4169）材质，基于成形时高温熔融急速冷却所形成的沿沉积方向呈外延性柱状和枝状晶特征，首先进行在温度77-1200K、应变率从0.001-5000/s 拉伸试验。采用高温高应变率耦合、等温与绝热、等温和绝热分离、应变跳跃等试验技巧，结合SEM和TEM等的微观分析，研究这种特殊组织合金与温度、应变率和组织的关系以及组织中晶粒各向异性特性和塑性流动机理，进而发展本构关系。然后对不同应力三轴度的试样进行高温动态拉伸试验，结合微观分析，研究这些熔覆沉积合金组织以及微损伤等缺陷的形核与增长所导致的损伤失效/破坏机制，研究失效判据。本项目为我国开展激光立体成形飞机结构件、发动机部件等的抗冲击和包容性设计提供科学依据。

利用激光沉积技术制备的合金一般具有组织不均匀性、各向异性和多发原始缺陷等问题。本项目选择两种激光沉积(LMD)典型发动机合金：TC4钛合金和GH4169镍基高温合金。针对发动机合金在服役中面临的高温高应变率受载的极端环境，对两种合金的高温高应变率力学行为进行了系统研究，对微观组织和初始缺陷对动态失效行为的影响进行了全面评估。.研究内容：1）系统测试了两种合金在宽温度和应变率范围内的单轴拉/压/剪塑性应力应变响应，揭示力学各向异性、温度和应变率敏感性、拉压不对称性以及工艺参数和热处理制度对塑性流动行为的影响规律；2）基于微观结构、温度和应变率耦合影响的塑性流动规律，分别建立了针对两种合金的基于物理概念的本构模型；3）对不同加载方向、温度、应变率、以及应力状态的失效模式进行微观分析，研究初始微观组织和缺陷对失效模式的影响；4）研究了LMD TC4的振动疲劳特性，揭示微观组织和缺陷对振动疲劳性能的影响。.重要研究结论：.1）LMD TC4中初始微观缺陷在拉压载荷下的变形机制的不同，加剧了拉压不对称性。随着应变率提高，不对称性程度增大。.2）随着激光沉积中的热输入密度增大，LMD GH4169的一次枝晶间距增大，压缩流动应力降低。流动应力随温度升高出现动态时效引起的反常应力峰现象，同时表现出突变的应变率敏感性。建立了考虑微观组织各向异性、反常应力峰现象和突变的应变率敏感性的基于物理框架的本构模型。.3）根据失效微观分析，原始缺陷成为高应变率加载下的应力集中和热失稳区域，诱导裂纹和断裂，降低激光沉积合金的塑性；由于微观组织各向异性，LMD GH4169在不同方向压缩加载下产生的裂纹扩展路径具有差异。LMD TC4的失效行为对于Lode角系数较为敏感。随着应力三轴度增大，试样断口由光滑斜断裂转变为垂直受力方向的韧性断裂。.4）由于LMD TC4中粗大的微观组织和原始缺陷，其疲劳寿命比锻造TC4短23％-44％，且数据分散性更大。β柱状晶的各向异性导致裂纹在不同方向上传播速度不同，引起疲劳寿命的各向异性。