Effective joining techniques are urgent and necessary for the rapid developments and widespread applications of aluminum matrix composite (AMC) components. Friction stir welding (FSW) is a very promising method for joining AMCs without clear defects, while double-sided FSW is especially preferred in joining thick or ultra-thick AMC plates. The overlapping region stirred twice by the two welding passes is formed during the double-sided FSW, which has an important effect on the microstructures and mechanical properties of the composite joint. In this work, 120 mm ultra-thick plates of 16 vol % SiCp/2014Al composites were successfully joined by the double-sided FSW, and an overlapping region appearing at the middle thickness of the composite joint is mainly concerned and studied. Compared with the base material and the region stirred once in the SiCp/Al composite joint, the decrease of SiC particle size, improved uniformity of particle distribution and refinement of aluminum grains and precipitates are observed in the overlapping region as a result of repetitive mechanical stirring during the double-sided FSW. The overlapping region exhibits the largest Vickers hardness and overall tensile properties: yield strength of 224 MPa, ultimate tensile strength of 369 MPa and failure elongation of 6.6%. The simultaneous enhancement in tensile strength and ductility in the overlapping region is attributed to the more effective load-transfer of reinforced particles with decreasing particle size, and the increased grain boundary strengthening due to matrix grain refinement. A comprehensive understanding the effect of overlapping region on the microstructural characterization and mechanical properties of the composite joint is positive and important to promote and accelerate the industrial applications of double-sided FSW technology in joining thick or ultra-thick metal matrix composite plates.

有效的连接技术对于铝基复合材料（AMC）组件的快速发展和广泛应用而言是紧迫和必要的。搅拌摩擦焊（FSW）是一种非常有希望的方法，用于连接无明显缺陷的AMC，而双面FSW在连接厚或超厚AMC板时特别优选。在双面FSW期间形成两次焊接道次搅拌的重叠区域，这对复合接头的组织和力学性能具有重要影响。在这项工作中，通过双面FSW成功地结合了120 mm 16vol％SiCp / 2014Al复合材料的超厚板，并且主要关注并研究了复合接头中间厚度处出现的重叠区域。与基础材料和在SiCp / Al复合接头中搅拌一次的区域相比，通过重复机械搅拌，可以观察到SiC颗粒尺寸减小，颗粒分布均匀性提高以及重叠区域中铝晶粒和沉淀的细化。在双面FSW期间。重叠区域表现出最大的维氏硬度和整体拉伸性能：屈服强度为224 MPa，极限抗拉强度为369 MPa，破坏伸长率为6.6％。重叠区域的拉伸强度和延展性的同时提高归因于增强的颗粒在减小粒径的情况下更有效地进行了载荷传递，并且由于基体晶粒细化而增强了晶界强化。