Research On Corrosion Fatigue Crack Propagation Behavior Of Aluminum Alloy Welded Joints From High-speed Train Underframe

Compared with traditional materials,aluminum alloy materials are widely used in key parts such as aluminum alloy body and pillow beam of high-speed trains because of their light weight and high strength properties.High-speed train will be affected by the interaction of alternating load and corrosive medium in the actual service process,and,as the service life of high-speed train increasesthe risk of corrosion fatigue failure of aluminum alloy and the welded joints will increase greatly.Therefore,it is of great engineering value to study the corrosion fatigue fracture behavior of A7N01 aluminum alloy welded joint of high-speed train after serving 1.8 million kilometers.The high-speed train pillow beam is made of A7N01P-T4 aluminum alloy plate and A7N01S-T5 aluminum alloy profile through MIG welding butt.Therefore,this paper studies the corrosion fatigue crack propagation behavior of the MIG welded joints of A7N01P-T4 and A7N01S-T5 aluminum alloy pillow in air and 3.5w.% Na Cl solution after serving for high-speed train for 1.8 million kilometers.The results show that the combined action of corrosion environment and fatigue can reduce the fatigue properties of the joint.Thereinto,thecorrosion fatigue crack growth rate is the slowest in the weld zone,the fastest in the base metal and heat affected zone of A7N01S-T5 aluminum alloy,and the corrosion fatigue crack growth rate of A7N01P-T4 aluminum alloy is in the middle.The corrosion fatigue crack propagation process in each region of the joint has two mechanisms: hydrogen embrittlement and anodic dissolution,and the crack path propagates through the mixed mode of transgranular and intergranular.The weld and A7N01S-T5 aluminum alloy corrosion fatigue crack path is straight,A7N01P-T4 aluminum alloy base metal and heat affected zone crack bifurcation phenomenon,it is probable that A7N01P-T4 aluminum alloy fatigue damage during service and the distribution of the second phase.A7N01P-T4 aluminum alloy under different mechanical damage of parent metal corrosion fatigue crack propagation test showed that the mechanical damage will change the microstructure and mechanical properties of materials,compared with the mechanical damage of the sample,corrosion fatigue crack propagation rate of plastic damage specimen showed a trend of decrease the increase,then the plastic damage degree of the largest sample fatigue crack propagation threshold value is higher than that of the samples without plastic damage,this is due to the internal elastic interaction between dislocation led to corrosion fatigue crack growth resistance increase.After the fatigue damage of specimens corrosion fatigue crack propagation rate increases with the increase of damage degree,and after fatigue damage A7N01P-T4 aluminum alloy material and service after 1.8 million km A7N01P-T4 aluminum alloy corrosion fatigue crack propagation path is evident in the secondary crack and crack bifurcate,plastic damage after sample fatigue crack no crack bifurcation.Considering the corrosion fatigue crack propagation path and fracture surface of different samples with mechanical damage,the anodic dissolution mechanism is more likely to occur in the corrosion fatigue crack propagation process of the samples after fatigue damage,while the corrosion fatigue crack propagation process of the samples with plastic damage is mainly through hydrogen embrittlement mechanism.