| High speed train manufacturing technology in our country has developed very fast. Currently, it has been on the advanced ranks in the world. Southern Vehicle Group has engaged in developing fabrication technology by which the railway vehicles can travel at a speed of 350 km/h. The vehicle chassis of high speed trains which generally adopts A7N01 aluminum alloy welding structures is the main bearing position when railway travels. With advantages of high comprehensive capability , A7N01 aluminum alloy was firstly developed for high speed trains by Japan. So far the properties of A7N01 aluminum alloy car-body structure made by our country is not very stable in dynamic load. The welding positions of the vehicle chassis are likely to initiate fatigue rift under dynamic load, as a result the reliability and safety of the whole structure is threatened.After extensive analysis, it has revealed that stress concentration is the key reason which leads to the vehicle chassis structure fatigue failure in a short time. Therefore, declining the weld joint stress concentration is determined to be the study idea and main approach to increase fatigue strength effectively. The scheme employed was as follows: firstly the stress concentration situation of several typical welding joints was calculated, and the influence of different kinds of geometric parameters on stress concentration was analysed by finite element method. And the fatigue strength optimization design proposal of welded joints was carried out. Then the feasibility validity of the optimization method was verified by a series of experiments. Finally, reasonable measure and theoretical basis were provided for raising the fatigue strength of A7N01 aluminum alloy welded assembly. Main works and research achievement of this study are as follows:The stress concentration situation about three kinds of typical butt joints from vehicle chassis of aluminum alloy high speed trains was calculated. Then the stress distribution was analysed by finite element method after changing the joint geometry parameters, such as weld toe angle. The corresponding relationship between stress concentration ratio and geometry shape was summarized, and the fatigue strength optimization design proposal of A7N01 aluminum alloy welded joints was put forward. It is also revealed that eliminating the angular deformation by three point bending techniques can decrease the additional stress largely, and generate compressive residual stress which is helpful to fatigue performance.The statics property and fatigue property of the joints in different kinds of groove forms were investigated. It had showed that the asymmetric V-notch of A7N01 aluminum alloy welded joint exhibited poor fatigue performance, and the fracture positions were mostly in weld toes. It is mostly because of the stress concentration, weld defects and additional stress caused by angular deformation.The geometric parameters are optimized by two methods: one is shaping the weld by manual TIG welding; the other is processing circle arc on weld toes by milling. By comparing the experimental results, it is found that using manual TIG welding is difficult to obtain the favorable weld toe angle to decrease the stress concentration. Accordingly, it revealed that the fatigue property was lower than that of initial joints by fatigue testing . On the contrary, it can easily get favorable circle arc on weld toes by mechanical processing. And the fatigue life of the weld joint can be increased remarkably in this way. This is mainly due to the stress concentration was reduced obviously by rounding off the weld toe. |
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