高强铝合金厚板变摩擦系数双轴肩FSW焊缝组织性能均匀性研究

To solve the problems of nonhomogeneity of microstructure and properties along thickness direction by the single side friction stir welded (FSW), which has become a trouble for lightweight aerospace manufacturing and need to solve the key scientific and technological issues. Double shoulder friction stir welding, which presents relatively uniform thermal cycle and mechanical stir, was performed on high strength aluminum alloy thick plate for aerospace using the telescopic welding tool with the variable friction coefficient, different screw thread direction of pin and double shoulder. In the present research, microstructure evolution, macro and micro texture, second phase particles along thickness direction were investigated. Tensile testing of the whole joint as well as three slices through the plate thickness was also conducted. Texture analysis was calculated using the results of the orientation distribution function and Schmid factor, revealing the start of the weld material and quantitative analysis of tensile failure location and fracture mechanism. The systematic study was able to charify microstructure evolution law, recrystallization mechanisms, distribution of second phase particles and strengthening mechanisms, and also establish change laws for tensile properties, joint efficiency, strain hardening behavior and strain-rate sensitivity at different temperature and strain rates. The interaction relation mechanism between microstructure or mechanical properties and double-side friction stir welding were also obtained. An attempt was made to develop the associated control technology and the mechanism of homogenization of microstructure and mechanical properties through the plate thickness. These results may provide the supporting technology and fundamentals for the homogenization of microstructure and properties along thick direction of friction stir welded thick plate. The theoretical result is also helpful for expanding the applied range of FSW in the modern equipment manufacturing.

当前高强铝合金厚板单面搅拌摩擦焊（FSW）焊缝组织性能的不均匀性，已成为困扰航空航天领域轻量化复杂构件连接制造发展的难题和亟待解决的关键科学技术问题。为此，本项目提出采用变摩擦系数、不同螺纹旋向和双轴肩可伸缩搅拌头，开展沿板厚方向热力耦合相对均匀的双轴肩FSW研究。通过研究焊接工艺参数对焊缝成形的影响机制，沿板厚方向晶粒尺寸、第二相粒子形貌分布特征和织构演变，接头整体和分层切片拉伸性能以及断裂失效的定量分析。阐明双轴肩FSW焊缝沿板厚方向组织演变规律、再结晶机制、第二相粒子强化机制；揭示不同温度和应变速率时接头拉伸性能、硬化行为、应变速率敏感性的变化规律；获得双轴肩FSW工艺与焊缝组织结构分布特征、力学行为之间的内在相关性，及焊缝组织性能沿板厚方向均匀化机制，探索实现焊缝组织性能均匀化的控制技术途径。相关研究成果可为铝合金厚板FSW接头的均匀化制备提供理论依据、技术支撑，并拓展其应用范围。

针对高强铝合金厚板单面搅拌摩擦焊接（FSW）过程中沿板厚方向存在的上高下低温度梯度和不均匀塑性变形，形成组织结构不均匀的FSW焊缝，导致接头沿板厚方向力学性能存在较大的差异，进而降低其服役可靠性，本项目创新性地提出了热力耦合相对均匀的双轴肩FSW，实现高强铝合金厚板组织性能均匀的FSW焊缝制备。项目研究了双轴肩FSW用搅拌头设计制备、焊接工艺参数、温度分布特征、焊缝成形机理，沿板厚方向焊缝不同区域晶粒尺寸、再结晶机制、取向分布、织构组分、第二相粒子形貌分布等组织结构特征，接头整体和局部分层切片力学性能变化规律、拉伸应变、断口形貌和显微硬度分布云图，揭示了铝合金厚板双轴肩FSW焊缝热历史特征、成形机理与微观组织结构演变规律，接头整体和局部分层切片拉伸性能变化规律及其内在相关性、拉伸应变分布与演变规律、断裂机理和断裂失效位置的量化分析；阐明了双轴肩FSW工艺与焊缝热循环分布特征、焊缝成形、微观组织结构特征和力学行为的内在关联机制及沿板厚方向组织性能的均匀化机制，进而提出强化铝合金厚板FSW接头组织与力学性能均匀性的技术路线。项目的开展为铝合金厚板高质量、高寿命复杂结构件的先进固相焊接制备与工程应用提供新的思路和科学依据，丰富了FSW工艺相关数据和理论，有助于拓展FSW工艺的工程应用范围。本项目的研究成果先后发表在包括Metallurgical and Materials Transactions A，Journal of Materials Science & Technology在内的国际材料加工著名SCI期刊上，共计论文8篇；受邀国际和国内学术会议报告各1次；利用本项目的研究成果和基础，孵化了陕西省自然科学基金面上项目和入选了2016年人社部“香江学者计划”，前往香港理工大学从事两年的大塑性变形FSW铝合金厚板接头组织演变与力学行为响应合作研究。