置氢钛合金搅拌摩擦焊片层组织的超塑性行为及氢的作用机制

The combination of superplastic forming (SPF) and friction stir welding (FSW) has great application prospects in producing integrated components of titanium alloys. However, tool wear easily takes place during FSW of Ti alloys, which limits the developing of FSW of Ti alloys. Besides, it is difficult to obtain the similar superplastic deformation ability of the nugget with the base material (BM), which hinders the uniform SPF of the entire FSW joints. Hydrogen is the temporary element in Ti alloys, which can adjust the superplastic deformation ability of the nugget to obtain the similar superplastic deformation ability with the BM. Therefore, the SPF of FSW joints of hydrogenated Ti alloys is a sound method of producing integrated components of titanium alloys. However, there is no report on superplastic behavior of the lamellar microstructure of FSW titanium alloy joints. In this study, Ti-6Al-4V (TC4) will be hydrogenated with different contents of hydrogen, and then be welded by FSW, obtaining a lamellar microstructure in the weld nugget. The detailed information of hydride contents, distributions and martensite boundaries 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 hydrogen contents, hydride contents, misorientation, martensite boundaries, phase transformation, and spheroidization will be built. Through these detailed studies, the mechanism and influential factors of superplastic deformation and spheroidization of the lamellar microstructure with hydrogenation will be clarified. Further, a more effective way to control the lamellar microstructure with hydrogenation by controlling FSW parameters and improve their superplasticity will be proposed. All these investigations will provide guidance for the FSW/SPF process in producing integrated components of titanium alloys.

超塑成形（SPF）与搅拌摩擦焊（FSW）技术组合用于生产钛合金整体构件具有广阔应用前景，然而在钛合金FSW过程中易发生工具磨损，且焊核很难得到与母材相近的超塑变形能力，制约了接头整体成型。置氢可减轻工具磨损，大范围调节钛合金超塑变形能力，是实现钛合金FSW接头均匀成型的理想选择。本项目拟采用FSW对置氢（不同含量）TC4合金进行焊接，得到片层组织焊核。采用TEM和EBSD获得不同FSW参数下片层组织中氢化物含量及分布和马氏体界面关系信息；通过拉伸实验获得焊核超塑性性能；对超塑变形样品进行微观组织分析，获得氢含量、马氏体界面关系、氢化物含量及分布、相变与超塑性性能及片层组织球化的关系。通过这些研究，揭示置氢钛合金片层组织超塑性变形机制及氢诱导片层组织球化的微观机制及影响因素，建立从置氢钛合金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。

钛合金搅拌摩擦焊（FSW）与超塑性成型组合技术用于制备大型钛合金整体构件具有巨大的工业应用前景，然而目前要实现钛合金FSW接头的整体均匀超塑性成型极具挑战性，除了焊接工具和狭窄工艺窗口的限制外，焊核区很难获得与母材相似的超塑性变形能力（其中焊核流变应力和超塑性变形温度过高）是制约其应用的重要因素。本研究通过焊前对钛合金进行置氢处理，得到不同氢含量的钛合金母材，并将置氢钛合金进行FSW后的接头进行超塑性变形研究。研究表明，焊前合适的置氢有利于降低焊接工具的磨损，并扩大FSW工艺窗口。通过FSW工艺优化和超塑性工艺优化，在优化的氢含量下，得到600-700%的良好超塑性，流变应力较未置氢时大幅下降，且焊核与母材可实现相似的超塑性。通过对优化氢含量的钛合金焊核片层组织在超塑性变形过程中的球化行为研究发现，氢的存在诱导β相在α片层内部形核，且氢含量的增加，导致β相变的增加。β相的增加一方面有利于α片层长径比降低，但另一方面，β相的增加导致α片层的变形主导作用发生改变，α片层的变形量降低，从而导致α片层的球化程度降低。氢化物与位错交互作用导致球化作用受阻。整体而言，氢的存在导致整体的片层球化程度降低。但是氢对流变应力降低及变形温度降低具有积极促进作用。置氢钛合金FSW焊核片层组织的超塑性变形机制主要是界面滑移。本研究阐明了氢对钛合金FSW焊核组织超塑性行为的作用，揭示了氢对钛合金片层组织在超塑性变形过程中球化的作用，弄清了置氢钛合金片层组织的超塑性变形机制。为降低钛合金接头超塑性成型成本、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。