热力极端非平衡条件下两相钛合金线性摩擦焊接头形性控制研究

It is of significant importance to know the joint formability and microstructure evolution during linear friction welding (LFW), and to realize the control of weld microstructure and mechanical properties of joints. In this project, based on the periodic feature during LFW, a multi-factor coupling model for weld deformation conditions will be established by measuring and analyzing the thermal and mechanical information in a single vibration cycle of the friction process. The grain refinement mechanisms of two-phase titanium alloys under the extremely non-equilibrium conditions during LFW will be investigated according to the proposed deformation control method. And then the interaction between dynamic recrystallization and solid-state phase transformation and their influence on weld microstructure will be examined. Finally, the relationship among the welding parameters, joint microstructure and properties will be established. The novelty of this project lies in: (1) a “differentiation” approach is proposed based on the analysis of deformation information in a single vibration cycle of LFW, and the quantitative representation of deformation conditions is achieved; (2) through the control of weld temperature, the weld microstructure is adjusted; (3) the formation theory of two-phase titanium alloy welds under the extreme deformation conditions during LFW is proposed to guide the optimization of welding parameters. The findings of this project will also enrich the metal plastic forming theory under extremely non-equilibrium conditions.

掌握线性摩擦焊极端变形条件对两相钛合金焊缝成形与组织演变的影响规律，实现接头组织性能调控，对于推动钛合金重要构件的线性摩擦焊连接制造意义重大。本项目拟利用线性摩擦焊周期性工艺特点，通过摩擦过程单个振动周期的热、力测试与变形条件分析，构建焊缝变形条件的多因素耦合关系模型。基于提出的线性摩擦焊变形调控焊接工艺开展相关试验，研究极端非平衡条件下两相钛合金在微观不均匀形变过程中的晶粒细化规律及机制，揭示动态再结晶与固态相变交互作用对焊缝显微组织的影响规律及机制，并建立焊接工艺与接头组织及力学性能的相关性。项目特色与创新：1)提出基于振动周期的焊缝复杂形变过程“微分”研究方法，实现变形条件的定量表征；2)通过提出的定温区变形调控焊接工艺实现焊缝组织控制；3)建立线性摩擦焊极端变形条件下两相钛合金的焊缝成形理论，指导焊接参数的匹配组合设计与优化，同时也推动极端非平衡条件下金属塑性成形理论的发展。

本项目以热物理模拟、数值模拟及焊接试验研究相结合，阐明了线性摩擦焊(LFW)变形条件对两相钛合金接头成形与组织演变的影响规律，获得了接头形性调控理论与方法。通过热压缩试验建立的焊接过程本构模型实现了LFW过程模拟仿真，并基于正交试验焊接过程基本变量的检测与分析，揭示了焊接参数多因素耦合对钛合金LFW过程变形条件的影响规律。基于设计的帽形结构热压缩试验，实现了钛合金LFW过程热物理模拟，获得了变形参数对界面变形行为的影响；由流变应力曲线特征确定了钛合金LFW过程的连续动态再结晶(CDRX)机制。TA和TB单相钛合金LFW接头EBSD分析表明，焊缝区通过亚晶旋转发生CDRX。TA+TB接头LFW试验结果表明，因母材热物性、合金元素成分及含量等不同引起的焊缝变形条件差异，导致TA侧主要发生CDRX，而TB侧主要发生动态回复(DR)，且焊缝成形、晶粒形态与尺寸及相分布等产生显著差异。以TC17钛合金开展的LFW过程固态相变与动态再结晶行为研究结果表明，调整焊接参数可改变焊接过程的热平衡及界面金属塑性流动行为，使焊缝处于不同温区并引起不同的固态相变行为，造成β、α、α′及二次α的含量、尺寸及分布差异；同时温度与应变速率不同使焊缝发生不同程度的DR和CDRX，晶粒细化程度不同。以TC11钛合金进行的LFW接头组织性能相关性研究发现，焊缝组织无“母材遗传性”完全相同，表明LFW过程“再结晶-相变”行为仅受变形条件影响。焊缝与热力影响区性能的提高分别主要受细晶和形变强化影响。再结晶与相变都使焊缝晶界数量增加，但二者以不同机制对焊缝产生“细晶强化”。增大摩擦压力能够有效减小焊缝宽度，细化晶粒，同时提高接头强度与延伸率；而提高剪切速度也能细化晶粒提高强度，但需适当增加摩擦压力才能提高延伸率。因此，最差与最优的参数组合分别是低摩擦压力与高剪切速度组合和中等压力与高剪切速度组合。