| A small leak will sink a great dam. The failure of great mechanical structure usuallyoriginated from the micro-crack. Therefore, it is important to understand the fatigue damagemechanism and the life evolutions of metal materials.Friction stir welding (FSW) is a new welding technology which can weld aluminumalloys and widely used in aviation. Even though a lot of research about FSW has been done, alittle research works has been found on fatigue property, especially on crack initiation andmicro-crack growth of FSW.The fatigue testing of friction stir welded7050-T7451aluminum alloy at roomtemperature were carried out, the fatigue loading form is sinusoidal wave, the frequency is10Hz and the stress ratio is-1. The mechanisms of fatigue crack initiation and micro-crackgrowth were investigated using phase microscope and SEM to observe the specimen surfaceand fracture morphology, respectively.The fatigue experiment results show that the FSW joints have good welding performanceat given welding factors. The fatigue cracks initiated from the specimen surface orsub-surface because of surface quality and the internal welding defects. The fatigue cracksinitiate in the region between thermo-mechanical affected zone (TMAZ) and heat affectedzone (HAZ) at the advancing side, and multi-cracks initiation can be found. Some initiatedcracks are stable, i.e. no propagation, while others propagate at the same time, but main crackleads fracture. The micro-crack propagation is linear, and the fracture surface shows ductilefeature, multi-initiate and multi-growth.The results are very important to utilization of FSW. |
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