Research On Fatigue Properties Of Friction Stir Welded AA7050/AA2024Aluminum Alloys

In this paper, the technical principle of Friction Stir Welding (FSW) and research statusat domestic and abroad are studied, and a FSW butt joint between AA7050-T7451andAA2024-T4aluminum alloy is welded under a given set of welding parameters. The grainsizes and grain orientation distributions in weld zone of the FSW are characterized by electronback scattering diffraction (EBSD). Then the fatigue testing of AA7050-T7451, AA2024-T4and FSW of AA7050-T7451/AA2024-T4are conducted at room temperature. The fatigueload is sinusoidal wave form with the frequency of10Hz and the stress ratio of-1. And theS-N curves are fitted based on the Power Function Formula. Finally, the morphologies offracture surfaces are characterized by scanning electric microscope (SEM) to investigate thefracture mechanism.The researches indicate that there is a typical rolling deformation texture C {112}<111>in base material (BM); The weld nugget (WN) consists of equiaxial fine recrystallized grains,the high-angle grain boundaries (HAGBs)(misorientation＞15deg.) are up to92.5%andthere is no preferred orientation; Several weak textures can be observed in thethermo-mechanically affected zone (TMAZ) on both sides. Under the condition of roomtemperature, the fatigue limit is144MPa for AA7050-T7451,95MPa for AA2024-T4and90MPa for FSW of AA7050-T7451/AA2024-T4which is equal to62.5%and94.7%of thetwo base materials respectively. The S-N curves of AA7050-T7451and FSW ofAA7050-T7451/AA2024-T4have the same trend, while the AA2024-T4is steep. The fittingresults show that the Power Function Formula can match the real S-N curves just when thelifetime is no more than106. The fractures of AA7050-T7451/AA2024-T4FSW occur at theregion between the thermo-mechanically affected zone and the heat affected zone at theretreating side (AA2024-T4). The fatigue cracks of AA7050-T7451, AA2024-T4and FSW ofAA7050-T7451/AA2024-T4initiate from the inclusions or welding defects near thesub-surface, and propagate along the dendrite boundaries. There are secondary cracks, tear ridges, holes resulted from debond of particles and fatigue straits in the crack propagationareas of fatigue fracture surfaces. Fracture surfaces show ductile fracture features.