异种钛合金真空电子束焊接界面第二相调控及强化机理研究

The strengthen and plasticity of combining interface of high temperature titanium/Ti2AlNb joined with vacuum electron beam welding (EBW) are always weakened due to a mount of brittle second phase in the interface. To solve this issue, isothermal forging will be employed to improve the EBW interface of Ti600/Ti2AlNb. The effects of process parameters of isothermal forging and heat treatment on the size, volume fraction, shape and location of second phase will be investigated. Then the globularization kinetics model of second phase and age-strengthening model will be established, respectively. In additional, the effect of different second phases on the spread mechanism of cracks in the interface will be investigated with in-situ tensile experiments. Then second phase strengthening mechanism and disabled mechanism of the combining interface will be concluded. The room temperature and high temperature tensile properties of Ti600/Ti2AlNb specimens will be tested. And the response laws of mechanical properties to microstructure will be analyzed and the inherent relationship between them will be researched. Through the project research, the control approach of brittle second phase within the Ti600/Ti2AlNb combining interface at interaction of heat and force will be obtained and the essence of interface strengthening mechanism through isothermal forging will be revealed. The joint with excellent comprehensive properties can be achieved. The necessary theoretical basis will be established for the interface strengthening mechanism of high temperature titanium/Ti2AlNb applications in aviation and aerospace field.

高温钛合金和Ti2AlNb基合金的真空电子束焊接界面存在大量脆性第二相，严重弱化了界面的强度和塑韧性。为解决上述问题，本项目提出采用等温锻造的方法对Ti600/Ti2AlNb真空电子束焊接界面进行强化。深入研究Ti600/Ti2AlNb等温锻造及热处理工艺参数对第二相尺度、含量、形态、分布的影响，建立第二相的球化动力学模型和时效工艺-强度量化模型；进行原位拉伸试验，研究混合第二相对焊接界面裂纹扩展机理的影响，揭示第二相强化机理和接头强度失效机理；测试室温拉伸及高温拉伸性能，分析力学性能对微观组织的响应规律，探索界面组织和性能的内在联系。通过本项目的研究，获得热力耦合作用下Ti600/Ti2AlNb焊接界面第二相调控方法，揭示等温锻造强化焊接界面本质，得到综合性能优良的焊接接头，为高温钛合金/Ti2AlNb基合金焊接构件在航空航天方面的应用奠定必要的理论基础。

双钛合金双性能构件在航空航天领域有广阔应用前景。异种钛合金构件制造技术可以灵活地组合不同合金以满足构件不同部位的性能需要，但所形成的焊缝组织和性能通常较差。项目通过真空电子束焊接技术对Ti600和Ti2AlNb合金进行连接，并采用等温锻造和热处理方式对该异种钛合金焊接界面进行强化，研究了Ti600/Ti2AlNb合金焊接界面微观组织调控规律，揭示了异种钛合金电子束焊接界面强化本质，得到如下主要结论：Ti600/Ti2AlNb合金焊后晶粒粗大，变形时呈脆性断裂。等温锻造过程能较好的破碎焊缝粗大柱状晶，细化界面组织。变形温度、变形量和变形速度均对焊接接头微观组织和性能有显著影响。等温锻造时焊接界面第二相球化规律满足Avrami模型，第二相球化体积分数曲线开始时上升较慢，随之逐渐加快随着变形的进行快速增加。在随后的热处理过程中接头组织发生静态球化和粗化行为，两者本质都是高温下溶质原子迁移过程。建立了时效制度与析出相尺寸、体积分数的定量关系，发现第二相长宽比和体积分数与时效时间之间的定函数关系均为指数衰减型函数。构建了时效工艺-强度量化模型，发现合金应力增量除了受到相结构尺寸、体积分数等微观组织参数的影响，还受到时效时间、时效温度等工艺参数的影响。建立了界面微观组织与宏观性能之间的定量关系。根据试样断裂位置的不同，分别对950-1000℃和1010-1050℃两个阶段进行拟合发现Ti600合金中等轴相的体积分数和焊缝α2/α的体积分数与抗拉强度之间基本成线性关系。通过原位拉伸实验研究了调控后不同类型组织焊缝的变形和断裂行为。Ti600/Ti2AlNb焊接界面裂纹扩展与等轴α2相分布和片层α2/α相取向有关，当裂纹穿过等轴α2相分布较多区域或沿片层α2/α相长轴方向扩展时，扩展速率较慢，合金发生较大塑性变形。项目研究结果可为异种钛合金焊接界面强化技术提供理论依据，为实现双钛合金双性能构件在航空航天领域的应用奠定基础。