| Friction Stir Welding (FSW), as a solid state joining technology, has much more significant advantages than traditional fusion welding technology in light alloys,such as aluminum alloys. And its application in the fields of aviation, aerospace, shipping and so on is wider and wider. At present many issues of FSW technology have been studied by researchers home and abroad, such as welding parameter, welding defects, welding joints'microstructure and mechanical properties. But the subjects on fatigue properties of FSW joints are so scarce, especially on fatigue crack growth rates (FCGR). Moreover, building a reasonable fatigue assessment norm is necessary for the popularization and application of FSW, so it's essential to carry out researches on FSW joints'fatigue behavior.In this thesis, fatigue properties of the aeronautic aluminum alloy 7075-T6 FSW joints with prefabricated crack through weld seam center, weld advancing side (AS) and weld retreating side (RS) were studied in high-frequency fatigue experiments, and then compared with those of 7075-T6 aluminum alloy base metal. The fatigue experiments of specimens with prefabricated crack through weld seam center and base metal were divided into two groups, i.e. R=0.1 and R=0.5. Microstructure observation, Vickers hardness tests and fatigue fracture SEM analysis were also operated as complementary tests, in order to explain the joints' fatigue performances better. The metallographic tests demonstrated that FSW joints were composed of nugget zone (NZ), thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ).The NZ was formed of fine equiaxial recrystallized grains. The grains in the TMAZ were elongated after being stirred, while those in the HAZ just grew bigger because of friction heat. The boundary between HAZ and base metal in AS was clear, but the one in RS was dim. And the hardness tests told that hardness profile looked like a"W". The hardness value in the WN was the second high, lower than that in the base metal, and the lowest value was in the HAZ of AS which was the weakest part of the FSW joints. The hardness values varied as the microstructure varied in the joints.The fatigue fracture SEM observation showed that fatigue fractures in the NZ were smooth while fatigue fractures in other areas were rather crude. The fracture mode in the NZ was almost intergranular failure but the fracture modes in other areas were intergranular failure at the beginning and intracrystalline failure in the later phase. The fracture surface of base metal was clearer than those of FSW joints. There were few fatigue striations in the WN but many in AS and RS besides tire-like patterns. Fatigue striations in base metal were tinier.The analysis of FCGR of different specimens indicated that when△K is low the FCPR in the HAZ of RS was the lowest and that of AS was the highest; when△K is high the FCPR in the HAZ of AS was the lowest and that of NZ was the highest. In both cases of R=0.1 and R=0.5, the FCGR of base metal was higher than that of NZ, and it decreased as△K increased until it equaled to that of NZ. Both the FCGRs of base metal and NZ were higher in the case of high stress ratio, and the difference between high and low stress ratio of the latter one was more obvious.Comparisons between the experimental results and the computer simulations were done. The calculations were in very good agreement with the fatigue tests. So a conclusion can be drawn that AFGROW simulation is reliable to predict FCPRs of aluminum alloy FSW joints and base metal. The more simulations were conducted to obtain fatigue lives of different specimens based on AFGROW, and△max△N curves were drawn. It can conclude that 7075-T6 aluminum alloy FSW joints have better fatigue performances than base metal, because they can bear larger stress in the same condition. |
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