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.