(1) Strength and stiffness of body structure. The closed section cylindrical or box structure shall be selected as far as possible. The component form of high-speed train components has a great relationship with the final service performance of components. For example, the car body structure adopts the cylindrical overall bearing structure of large extruded hollow profile, which increases the stiffness in all directions, especially the torsional stiffness by more than 100 times, while the assembly and welding of the structure is easy to realize automation and the welding deformation is easy to be controlled. In particular, the overall welding of side wall and NC machining of window holes can further simplify the welding process and reduce deformation. Similarly, box welded structures with high torsional stiffness can also be widely used in bogie structures.
(2) Arrange welds reasonably. Due to the low efficiency of welded joints, the general welds are not used as the main force transfer welds, but the hollow extruded profiles are made into components with the same length as the vehicle body, so as to avoid the welded joints and welds perpendicular to the stress direction. Instead, the full-length hollow profiles are connected into the vehicle body with the connecting welds. By comparing the two kinds of welds, the longitudinal connecting welds can make full use of the strength of the aluminum alloy base metal, greatly reduce the welding deformation and reduce the influence of residual stress, The effect of stress concentration is also reduced to very small, and the softening zone of low strength weld and heat effect is reduced. Working welds must be used in the underframe or traction beam, and shall not be arranged in the high stress area of the structure. At the same time, the minimum welding deformation must be considered to minimize the adverse effect of welding residual stress. For example, the side beam of some foreign bogies is changed into a straight side beam, two seamless pipes are used to penetrate the side beam web, the middle part is a cross beam, and the side beam web is welded with multiple layers around the pipe. In this way, when the weld is in the low stress area, the influence of welding deformation and welding residual stress can be minimized.
(3) Reasonable design of welded joints. Butt joints shall be used as far as possible, but due to the easy collapse of aluminum alloy during fusion welding, the joints are extruded into lap butt joints, which can prevent collapse and greatly simplify the welding and assembly work. The corner joint shall also be fully considered to prevent collapse.
(4) Safety and reliability of welded structure. The safety and reliability of the structure mainly depends on the structural strength, especially the fatigue strength and brittle fracture strength, while the safety and reliability of the welded structure mainly depends on the welded joint, because the structural performance variation, residual stress, welding deficiency and stress concentration caused by the weld shape of the welded joint will affect its strength, especially the fatigue strength and brittle fracture strength. The performance of welded joints of aluminum alloy is very different from that of steel. Extrusion strengthening and heat treatment strengthening are difficult to make the weld equal to the base metal. In order to reduce the tendency of welding hot cracking, low matching welding materials must be used to cause low strength in the weld zone. For high-strength aluminum alloy with work hardening and heat treatment strengthening, a softening zone is formed due to welding heating, which reduces the strength of the joint. In addition, the welding joint is the welding residual stress and stress concentration part. Therefore, it is necessary to optimize the matching of welding materials, reasonably design and arrange welds and welded joints in the design to obtain reasonable stress conditions and residual stress distribution, use reasonable welding methods and processes in the manufacturing to obtain the least defects and reasonable shape of welds and joints, and optimize welding parameters to obtain high toughness and low welding process stress and residual stress. The influence of load form and environment on the safety of welded structure is also very important. The impact toughness of aluminum alloy base metal and welded joint is far lower than that of steel, but it is less sensitive to low temperature and impact load.
Pure aluminum and aluminum alloy with low alloy have good cavitation resistance, but in the process of using a7n01 high-strength aluminum alloy components, stress corrosion has a great impact on their strength, which is often the main reason for their failure. The main strengthening phase in a7n01 high strength aluminum alloy is mgzn2 (N phase). Increasing the content of Zn and Mg within the solid solution limit can greatly improve the strength of the alloy, but its resistance to stress corrosion cracking (SCC) will be reduced. Therefore, the contradiction between the sensitivity and strength of stress corrosion cracking (SCC) is still a major problem for the industrial application of a7n01 high strength aluminum alloy. The Welding Research Institute of Southwest Jiaotong University cooperated with CRRC in the national high-speed train scientific research project, and conducted extensive and in-depth research on the welding residual stress, joint strength, especially fatigue strength, corrosion and stress corrosion of aluminum alloy welded joints of high-speed trains. In the study of failure analysis with CRRC Qingdao Sifang locomotive and rolling stock Co., Ltd. on traction beam, buffer beam, vehicle end mounting seat and other parts, it is found that the main failure form of a7n01 high-strength aluminum alloy welded joint is stress corrosion. Combined with the development of stress corrosion and corrosion behavior, this paper studies the relationship between stress and corrosion behavior, and studies the influence of aluminum alloy welded joints on residual stress distribution under different depths, different weld positions, different joint forms and aging treatment. At the same time, on this basis, the measures to control the corrosion behavior of a7n01 aluminum alloy welded joints of high-speed train are put forward, which provides basic data support for the safety and reliability research of high-speed train.