Study On Fatigue Crack Propagation Properties Of Al-Zn-Mg Alloy Welded Joints

7N01 aluminium alloy is specially developed for high speed trains for body bearing in Japan. It is one of the highest strength aluminium alloy, belongs to Al-Zn-Mg alloy system, with good weldability. We are doing the research of high speed train materials and manufacturing technology and have been starting to develop the manufacturing process of above 350km/h railway. The train chassis is one of the main bearing components which adopted welding structure of Japanese materials 7N01 aluminium alloy. Higher requirement was put forward to the bearing capacity of structure after speed acceleration of the train, and fatigue is the main failure form. Therefore, in order to further accurately describe the fatigue performance of 7N01 aluminium alloy welding joints, fatigue crack propagation properties of high-speed train body materials 7N01 aluminium alloy welding joint was studied, which based on the fracture mechanics theory and part of national standards .The rule of welding residual stress relaxation under cyclic load was studied by the test technology. Results show that stress relaxation occurs when the superposition values of applied load and residual stress greater than the yield strength, and mainly in the first cycle of cyclic loading.Using the welded joints after eliminating stress processing, fatigue crack propagation rate of weld zone, heat affected zone and the parent metal was studied by M(T) samples. Results showed that when the stress ratio were 0 and 0.3, fatigue crack propagation rate of each area of welded joints, the sequence was the parent metal, weld zone and heat-affected zone from small difference to big. When the stress ratio was 0.5 and 0.8, the sequence was the parent metal, heat-affected zone and the weld zone. By fatigue fracture, there were some bloeholes in the welded sample when stress ratio was 0.8, and some of these were big. Those reflected that the microstructure is one of the main factors to fatigue crack propagation rate. The crack growth rate of the weld zone will be obviously accelerated with the number and the size of the gas pore increase.The rule of redistribution with fatigue crack propagation residual stress was also studied by M(T) samples. The effective stress ratio and effective stress intensity factor amplitude were obtained, as well as the parameters on the fatigue crack propagation were revised. Fatigue crack propagation rate was revised under the various treatments of residual stress. With regard to the results, it showed that the fatigue crack propagation rate was higher under the average stress processing method, and this was conservative. The crack growth rate was lower when the residual stress was not considered, which was not safe. When the residual stress redistribution with fatigue crack propagation was considered, the value was between the former two.