超声表面机械研磨处理对6061铝合金FSW接头疲劳行为的影响

利用超声表面机械研磨处理(USMGT)方法对6061-T6铝合金搅拌摩擦焊(FSW)接头进行了表面晶粒细化处理，采用光学显微镜、显微硬度仪、扫描电镜等测试手段，对接头组织、力学性能及疲劳性能进行了分析。结果表明，USMGT处理后焊接接头表面形成了包含大量细化晶粒的梯度细晶层，表面细晶层的形成使接头表面硬度得到提高;接头的宏观断裂位置在热机影响区(TMAZ)和热影响区(HAZ)之间，滚压处理强化试样表层，FSW接头内部TMAZ和HAZ之间出现的软化现象无法消除，相比其他区域，该部分依然是疲劳薄弱区，承受载荷时微观裂纹源萌生位置由表面向内部迁移;滚压处理会使表层引入残余压应力，从而使得接头的疲劳性能得到提高，当应力幅值为150 MPa时，USMGT后的接头疲劳寿命达到未处理的3．5倍。

Effect of ultrasonic surface mechanical grinding on fatigue behavior of 6061 aluminum alloy FSW joint

The effects of ultrasonic surface mechanical abrasion (USMAT) on the microstructures, mechanical properties and fatigue behavior of friction stir welding (FSW) joints of 6061-T6 aluminium alloy were studied by means of optical microscopy, microhardness tester and scanning electron microscopy. The results show that the gradient nano-layers containing a large number of nanocrystals are formed on the surface of welded joints after USMAT treatment. The formation of fine-grained layers increases the surface hardness of welded joints. The macroscopic fracture location of the joint is between the thermal-mechanical influence zone (TMAZ) and the thermal influence zone (HAZ).As the surface of the sample was strengthened by rolling treatment, the softening phenomenon between TMAZ and HAZ in the FSW joint could not be eliminated. Compared with other regions, this part is still a fatigue weak zone, and the initiation location of micro crack source migrates from the surface to the interior under load. When the residual compressive stress is introduced into the surface layer, the fatigue properties of the joints are improved. Under the condition of low stress, the fatigue life of the welded joints after treatment increases more obviously. At the stress amplitude of 150 MPa, the fatigue life of joints after USMGT is 3.5 times that of untreated joints.