钛合金搅拌摩擦焊片层组织的超塑性行为及其在超塑变形中的球化机制

The combination of superplastic forming (SPF) and welding techniques has great application prospects in producing integrated components of titanium alloys. It is not possible to achieve uniform forming of the integrated components of the fusion welds because the nugget exhibits no or low superplasticity, resulting from the fact that the coarse lamellar microstructure in the nugget is difficult to be spheroidized. Fine lamellar microstructure can be obtained by friction stir welding (FSW) and is easily spheroidized. As a result, the weld nugget can exhibit a comparable superplasticity to the base material. Therefore, FSW/SPF is a sound method to produce integrated components of titanium alloys. However, there is no report on superplastic behavior and microstructure evolution of the lamellar microstructure of FSW titatnium alloy joints during superplastic deformation. In this study, Ti-6Al-4V (TC4) will be welded by FSW, obtaining a lamellar microstructure in the weld nugget. The statistic data of thickness，aspect ratio and misorientation distribution of the lamellae at different FSW parameters will be obtained by TEM and EBSD techniques. The superplastic behavior of the weld nugget will be studied by the tensile test. By microstructural analysis on the deformed specimens, the relationship between thickness, aspect ratio, misorientation, phase transformation, and spheroidization will be built. Through these detailed studies, the mechanism and influential factors of spheroidization of the lamellar microstructure will be clarified. Further, a more effective way to control the lamellar microstructure by controlling FSW parameters and improve their superplasticity will be proposed. All these investigations will provide a guidance for the FSW/SPF process in producing integrated components of titanium alloys.

超塑成形（SPF）与焊接技术组合用于生产钛合金整体构件具有广阔应用前景，然而熔焊焊核的粗大片层组织不易球化，通常不具有超塑性或超塑性较低，无法实现整体构件的均匀成型。搅拌摩擦焊(FSW)可得到细小片层组织，易于球化，可获得与母材接近的超塑性，因此FSW/SPF成为制备钛合金整体构件的理想选择。然而关于FSW片层组织的超塑变形行为以及微观演变机理尚无公开报导。本项目拟采用FSW对TC4合金进行焊接，得到焊核为片层组织的接头。采用TEM和EBSD技术获得不同FSW参数下片层组织的厚度与长径比及片层取向分布的统计数据；通过拉伸实验获得焊核的超塑性性能；对超塑变形样品进行微观组织分析，获得片层厚度、长径比、取向及相变与球化的关系。通过这些研究，揭示钛合金片层组织球化的微观机制及影响因素，建立从片层组织获得良好超塑性的途径，实现FSW片层组织的可控制备，为钛合金整体构件的FSW/SPF提供参考。

钛合金超塑性成型与搅拌摩擦焊（FSW）组合用于制备大型钛合金整体构件具有巨大的应用前景，然而目前要实现钛合金FSW接头的整体均匀超塑性变形极具挑战性，主要原因是焊接工具和工艺的限制及搅拌区（SZ）很难获得与母材相似的超塑性变形能力。本研究通过优化焊接工具和工艺，实现高质量Ti-6Al-4V的焊接，并通过FSW工艺调节片层组织的厚度和长径比，得到了平均片层厚度和长径比分别为～175 nm和～8的SZ。这种特殊的细小片层组织在925oC、3×10-3s-1时实现>700%的良好超塑性，明显高于熔焊焊缝 (<400%)，且显示出良好的低温超塑性。在800oC、3×10-4s-1时，SZ的超塑性和流变应力均与母材相当。SZ的良好超塑性主要归因于片层组织在静态退火和超塑性变形过程中的球化。片层组织在静态退火过程中的球化机理是，两个相邻的α片两端首先发生合并，然后通过α/β相界面向β相移动，β相消失，两相邻的α片层合并为粗大的方形晶粒，最终通过末端迁移实现球化。片层组织在超塑性变形过程中的球化机理是，α片层组织在变形过程中发生逐渐偏转，同时通过晶界转动使得片层内部的相邻亚晶的错配角增加，并在β相渗入的辅助下，将片层打断，最后实现球化。在此基础上进一步优化FSW工艺，使得整体接头在900oC、1×10-3s-1时实现均匀超塑性变形。本研究揭示了钛合金片层组织在静态退火和超塑性变形过程中的球化机理，并实现了整体接头均匀超塑性变形，为实现钛合金FSW接头整体超塑性成型的工业化应用提供了重要的参考依据。

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