Fatigue crack propagation characteristics of high strength aluminum alloy welded joint used by high speed train

An experimental set-up was designed for measuring the fatigue crack propagation rate of A7N01 aluminum alloy welded joint using fatigue test system.The microstructure of welded joint was analyzed by an optical microscope,and the morphology of fatigue fracture was studied by an election scanning microscope.The results show that the microstructure of parent metal is the aging rolled structure by the α(Al) matrix,coarse first phase,secondary disperse strengthening phase and some inclusions.The microstructure of HAZ is the α(Al) matrix with some un-soluble microstructure.The microstructure of weld seam is the α(Al) matrix with divorced eutectic phase.The fatigue crack propagation characteristic is different due to different microstructures and different scale second phase precipitates of welded joint.The fatigue crack propagation rate increases with the increase of stress ratio.The discordant plastic deformation between the second phase precipitate and matrix in the crack tip plastic region can promote the propagation of fatigue crack and affect growth rate of it.