Abstract
In order to study on tensile and fatigue fracture mechanism of friction stir welded (FSW) joints, the tensile and fatigue behavior of FSW joints are studied based on the microstructure and strain distribution. The large plastic deformation and fracture occurred in the thermo-mechanically affected zone (TMAZ) on retreating side in tension tests. High contents of shear texture and small angle grain boundary reduce the tensile mechanical property of TMAZ material. The fatigue weak area for FSW joints is affected by the loading condition. The strain concentration in the welded nugget zone (WNZ) and base material makes the fatigue fracture liable to happen in these areas for the FSW joints under the stress ratios of 0.1 and −0.3. When the fracture occurred in WNZ, the crack initiation mainly occurred in clusters of hardened particles, while when the fracture happened in base material, the crack initiation mainly occurred near the pit. The crack in WNZ propagated in an intergranular pattern and the crack in the other areas extended in a transgranular mode, leading to a higher crack growth rate of WNZ than of other regions.
Highlights
Friction stir welding (FSW) is widely used in aerospace, shipbuilding, automotive, and other industries
The material microstructure has an influence on its mechanical property, crack initiation, and propagation of welded joints
Shou et al noted that the grain size has a certain effect on fatigue crack growth rate of aluminum alloy [15]
Summary
Friction stir welding (FSW) is widely used in aerospace, shipbuilding, automotive, and other industries. The material microstructure has an influence on its mechanical property, crack initiation, and propagation of welded joints. Sun et al [14] found that the initiation and propagation of small cracks are affected mainly by strengthening particles, grain boundaries, and rough pits for 7075 aluminum alloy FSW joints. The initiation and propagation of fatigue cracks are influenced by the evolution of microstructures, hardness distribution, and local strain [23]. To further explore the influence of microstructure and strain distribution on tensile and fatigue properties of FSW joints, the tensile and fatigue behavior for 7075 aluminum FSW joints were studied using electron backscatter diffraction(EBSD) and DIC techniques. The effects of texture, hardness distribution, and strain variation on tensile and fatigue behavior of FSW joints were studied, and the difference of fracture mechanism was revealed
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