Reseach On Fatigue Property Of A7N01 Welding Jiont Used In High Speed Train

The A7N01 aluminum alloy with good property which beong to the 7×××aluminum alloy is widely used in high speed train and the study on the fatigue property is of great significance to make the high speed train safer. Through the fatigue tests to the A7N01 aluminum alloy welded joint and the observiation of microstructure and the fatigue fracture and morphology of the fatigue damage on surface, we can summarize the following conclusions:The ratchet effect is generated during the fatigue process in the A7N01 aluminum alloy welded joint. Results shows that the ratcheting effect is obvious at the first stage, the plastic deformation is more obvious especially in the first cycle. At the steady state stage, the ratcheting strain rate is almost constant(about 0) and the ratcheting strain remain low. At the tertiary stage, as the material deformation resistance decline, cavity growth and the stress concentration in the material, lead to the ratcheting strain increasing sharply and great plastic deformation and ultimate fracture of the samples.The A7N01 aluminum alloy welded joint mainly shows cyclic softening during the fatigue test. The proportion of cyclic softening on the whole fatigue process is different when the tress is different. The cycle to the stable stage is less when the stress is less. When the stress is smaller than 150 MPa, the whole process keep cyclic softening. When the stress is bigger than 220 MPa, there is cyclic softening in the whole process instead of the cyclic hardening in the early stage.The fatigue plastic deformation of the A7N01 aluminum alloy welded joint mainly occurred in the weld zone whose fatigue damage is mainly slip band cracking. Fatigue damage on the heat affected zone is the fatigue microcrack generated by the hard compound with much element of Fe and Si, which can be easily separated with the matrix under the fatigue stress. When the stress is high, the large plastic deformation in the weld zone and the stress concentration in the fusion zone and the hard compound in the heat affected zone can all lead to the failure of the sample. When the stress is low, the hard compound in the heat affected zone is relatively the greatest damage to the sample and the samples generally fracture in the heat affected zone. Owing to the heating cycle during the welding process, the percentage composition of element Fe and Si in the compound of heat affected zone is three times more than that of base metal. Thus the compound is more hard and brittle, making the fatigue property of heat affected zone is far beyond that of base metal.