Fatigue Behavior of Friction Stir Welded Joints of 6061-T6 Aluminum Alloy

Abstract

In this study, the fatigue behavior of friction stir welded (FSW) joints of 6061-T6 aluminum alloy was investigated. 6061-T6 plates were joined with the welding speeds of 100 and 200 mm/min and the rotation speeds of tool of 1200 and 1800 rpm. The microstructure of the weld zone was classified into three regions : stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). In the SZ, fine equiaxed grains were observed resulting from dynamic recrystallization, and TMAZ was recognized as the microstructural transition zone between SZ and HAZ. Vickers hardness measurement revealed softening inside the weld zone, which was attributed to the dissolution of precipitates due to temperature rise during the FSW process. Fatigue tests were conducted at stress ratio R = -1 under axial loading. It was found that the fatigue strength of FSW joints was lower than that of the parent metal and the location of fatigue fracture was dependent on stress level. In the high stress region, fracture occurred at the TMAZ, while at the HAZ in the low stress region. Macroscopic observation revealed that the localized plastic deformation at the TMAZ was responsible for the fatigue fracture at the TMAZ. Such deformation at the TMAZ was not recognized when the applied stress was low and fracture occurred at the HAZ. Based on the hardness measurements before and after fatigue tests, the fatigue fracture at the HAZ was attributed to both the grain refinement at the SZ and the dynamic aging at the SZ and TMAZ by cyclic loading. The microstructures, hardness profiles, tensile strength and fatigue strength were hardly affected by the welding condition.