零轴肩压入零减薄搅拌摩擦焊焊缝组织性能特性及形成机理研究

The weld thinning phenomenon occurring in conventional friction stir welding (FSW) has negative effect on the formation quality and serviceable reliability of FSW joints. The existing methods for solving the weld thinning problem in FSW exhibit some universal deficiencies, such as low efficiency, low quality and narrow applicable range. Motivated by this problem, the present project proposes a novel non-thinning FSW process, in which the high quality non-thinning friction stir weld can be one-time produced at a zero shoulder plunge depth. This novel process opens a brand new pathway for the high-efficiency and high-quality non-thinning FSW of structures. To deeply elucidate the characteristics and mechanisms of the novel non-thinning FSW process, the microstructure-property characteristics and formation mechanism of non-shoulder-plunge and non-thinning friction stir weld is investigated in the present project. Firstly, the features and mechanisms of the indirect thermal-mechanical effects of tool shoulder on microstructure evolution and mechanical behavior of FSW joints are explored to illuminate the microstructure-property characteristics of non-thinning FSW. After this, a double-surface frictional heat source model of tool shoulder, which can truly reflect the heat generation features of non-thinning FSW, is developed, and then the thermal and flow fields of non-thinning FSW are simulated. The intrinsic reasons for the process characteristics are therefore illustrated, and the weld formation mechanism of non-thinning FSW is revealed in nature. The project is expected to provide theoretical basis for the optimal control of non-thinning FSW, and is also beneficial to promote the development of accurately shaped welding technique. So this project is significative for both theory and practical application.

常规搅拌摩擦焊（FSW）中出现的焊缝减薄现象对接头成形质量及服役可靠性都存在不利影响。现有消除FSW焊缝减薄现象的方法普遍存在效率低、质量差、可适用范围窄等问题，为此，本项目提出了一种新型零减薄FSW方法，能够在焊具轴肩压入量为零的工艺条件下，一次性获得优质零减薄FSW接头，从而为结构件高效率、高质量的零减薄焊接开辟了新的技术途径。为了深刻阐明这一新型零减薄FSW工艺的特性和机理，本项目开展了零轴肩压入零减薄FSW焊缝组织性能特性及形成机理研究。首先探索焊具轴肩的间接热机作用对焊缝微观组织演变及接头力学行为的影响特征和机制，明晰焊缝组织性能特性，而后建立能够真实反映零减薄FSW产热特征的轴肩双面摩擦热源模型，进行焊接热-流模拟分析，阐明工艺特性的本质原因，揭示焊缝形成机理。研究成果可望为此新型零减薄FSW工艺的优化控制提供理论依据，促进保形精确焊接技术的发展，具有较大的理论与实际意义。

常规搅拌摩擦焊（FSW）中出现的焊缝减薄现象对接头成形质量及服役可靠性都存在不利影响。现有消除FSW焊缝减薄现象的方法普遍存在效率低、质量差、可适用范围窄等问题，为此，本项目提出了一种新型零减薄FSW方法，能够在焊具轴肩压入量为零的工艺条件下，一次性获得优质零减薄FSW接头，从而为结构件高效率、高质量的零减薄焊接开辟了新的技术途径。为了深刻阐明这一新型零减薄FSW工艺的特性和机理，本项目开展了零轴肩压入零减薄FSW焊缝组织性能特性及形成机理研究。工艺特性研究表明，零减薄FSW焊缝的轴肩作用区几乎消失，变形区尺寸显著减小；零轴肩压入条件抑制了焊具轴肩的热作用，导致变形区晶粒得到了更高程度的细化，且在焊缝厚度方向分布更为均匀；同时热影响区的晶粒粗化、位错多边形化以及强化相溶解程度都得到了抑制。零减薄FSW接头的软化区宽度仅为常规接头的一半左右，强度系数由常规接头的89%提高到了97%。基于轴肩双面摩擦热源机制及CEL方法，构建了零减薄FSW几何模型、产热模型、材料流变应力本构模型和焊具/工件摩擦接触模型，利用Abaqus有限元软件，进行了零减薄FSW全过程多场耦合模拟分析。结果表明，常规FSW搅拌针周围材料存在显著的竖直方向流动，轴肩驱动材料流动行为显著；对于零减薄FSW而言，轴肩热力驱动作用减弱，焊具搅拌针呈现出梯度变化的层流方式，焊缝材料流动汇聚点聚集在前进侧焊具边缘。本研究成果可望为此新型零减薄FSW工艺的优化控制提供理论依据，促进保形精确焊接技术的发展，具有较大的理论与实际意义。