摘要搅拌摩擦焊（FSW）是一种新型的固相连接方法，与传统的焊接方法相比，具有 较好的力学性能，能够降低残余应力，降低焊接缺陷等优点。已被广泛应用于在航空 航天和汽车行业等铝合金的焊接中。尽管 FSW 存在一定优点，但也有缺点，比如单 面焊缝的根部会发生缺陷等。由于使用双轴肩可以避免根部缺陷，所以使用双轴肩是 可以很好的解决以上问题。与常规 FSW 技术相比，BTFSW 有上、下两个轴肩。这 能使 BTFSW 能够焊接中空的工件，例如空心型材等。此外，在常规 FSW 中经常出 现的焊缝根部缺陷，如未焊透等在 BTFSW 技术中是完全可以避免的。本文研究目的 是为高强铝合金在海洋环境中的应用提供依据，同时为优化铝合金 BTFSW 工艺提供 有益的建议。81349

本文采用双轴肩搅拌摩擦焊（BTFSW）技术对 7。8 mm 厚 6061-T6 铝合金进行了 焊接，对焊接接头各部分的微观组织和截面显微硬度进行了分析，并对母材和焊核在 3。5% NaCl 溶液中的腐蚀电化学行为和腐蚀产物膜进行了研究。结果表明，BTFSW 后焊缝表面质量良好，热机影响区晶粒发生再结晶和塑性变形，有部分的再结晶晶粒 发生了伸长和弯曲；热影响区的部分晶粒在热循环的作用发生粗化，焊核呈细小的等 轴晶组织；焊接接头中前进侧热机影响区硬度最低；在 3。5% NaCl 溶液中，6061-T6 铝合金的极化曲线表现为阳极溶解和点蚀，母材和焊核区的电化学阻抗谱均由容抗弧 和感抗弧组成，BTFSW 后 6061 铝合金的腐蚀电流降低；浸泡 480 h 后，呈疖状腐蚀 的特征，腐蚀产物为 Al(OH)3 和 Al2O3，BTFSW 可提高 6061-T6 铝合金的耐蚀性。

毕业论文关键词: 腐蚀，铝合金，双轴肩搅拌摩擦焊，微观组织 

Abstract Friction stir welding (FSW) is a new solid-state joining method which offers several advantages compared with conventional welding methods, including better mechanical properties, lower residual stress and reduced occurrence of defects。 It has already been used forjoining Al alloys in the aerospace and automotive industries。 In spite of the advantages, FSW also has drawbacks, such as the risk of root flaws in single-side welds。 Using a bobbin tool instead is a promising way to solve this problem since the root region is avoided。 Compared with standard (single-side) FSW techniques, the bobbin tool FSW has an extra shoulder attached to the tip of the probe, namely the lower shoulder。 This setup makes BTFSW capable of joining closed profiles like hollow extrusions。 Furthermore, root flaws, such as lack of penetration, which occasionally occurred in standard FSW techiques, can be completely avoided。 The purpose of this paper  is to provide a basis for the application of high strength aluminum alloy in the marine environment, and provides useful suggestions for the optimization of the BTFSW process of aluminum alloy。

In this paper, 6061-T6 aluminum alloy was welded by using bobbin tool friction stir weld (BTFSW)。 The influence of BTFSW on the microstructure development and hardness distribution in the weldment has been investigated。 The corrosion behaviors of the base metal and weld nugget in 3。5% NaCl (mass fraction) solution were investigated using techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical measurements。 The results showed that the weld surface of 6061-T6 welded by BTFSW is of good quality。 No welding defect was detected in the joints。 Three microstructural zones,i。e。, nugget zone, thermo-mechanically affected zone, and heat affected zone were discernible。 The microstructural analysis indicates that the weld nugget region exhibited fine and equiaxed grain structure with an average grain size of ~8µm, indicating the occurrence of dynamic recrystallization due to severe plastic deformation and thermal exposure。 The thermo-mechanically affected zone underwent plastic deformationand recrystallization occured in this zone due to deformation strain  and thermal input。 The low hardness zone, determined by constructing the hardness distribution profile on cross-section of joint, located at thermo-mechanically affected zone of advancing side。 Although 6061-T6 alloys are readily weldable, they suffer from severe softening in the heat affected zone because of the dissolution of Mg2Si precipitates   during the weld thermal cycle。 BTFSW can improve the corrosion resistance of 6061-T6 aluminum alloy in 3。5% NaCl solution。 The corrosion behavior results show that both anodic dissolution and pitting are observed after the immersion test due to the inhomogeneous microstructureof 6061-T6 aluminum alloy。The corrosion products mainly composed of Al(OH)3 and Al2O3。 Furthermore, the corrosion process and mechanism were also discussed。