The Research Of Fatigue Performance Of A7n01aluminum Alloy Joints Used In High-Speed Train

With the requirements of raising speed of high-speed train, aluminum alloy material has been applied to the manufacture of high-speed trains. As the special operating conditions of high-speed trains, the fatigue performance of aluminum alloy joints were generally studied by some scholars. In this paper, A7N01 aluminum alloy plates were welded without defects by MIG welding technology in the butt welding, which used in high-speed train with a thickness of 5mm. With the effect of welding thermal cycles, the microstructure changes of A7N01 aluminum welded joints were studied by optical technology, TEM and XRD. Fatigue test were respectively conducted on the specimen of A7N01 aluminum matrix and its welded joints, then part of S-N curves were drown on basis of fatigue test. In order to analyze the fatigue performance fatigue fracture surfaces were also observed by SEM.Our study results show that: good quality aluminum welded joints can be obtained by MIG welding, tensile test and hardness test results show that, the weld zone of A7N01 aluminum welded joints become weak. With the effect of welding thermal cycle, A7N01 aluminum welded joints can be divided into heat affected zone, weld area and base area, different regions contains different microstructure and second phase distribution, the center weld zone is equiaxed, in the edge of the weld, columnar grains was formed. Columnar grains also formed in fusion zone, but fusion zone displays serious chemical and physical heterogeneity. Fatigue results show that, the fatigue fracture of matrix and welded joints can be divided into three parts, namely, the fatigue crack initiation zone, fatigue extension region, and blink-off area, and each region has different fracture appearance. Because of the inhomogeneity of microstructure at the edge of weld, fatigue crack can easily initiated, and then extended to the inner of welds, finally fracture emerged. For the welded joints and matrix, cracks initiation all happens in the vicinage of second phase. Because of the welding thermal cycle, the weld zone of welding specimens shows a coarse grain, as well as the growing of second phase particles at the edge of weld, so, fatigue crack initiation and extension can grow more easily for the welded joints, and the fatigue life of welded joints is shorter than the matrix, fatigue performance of welded joints decline severely. In order to improve the fatigue properties of A7N01 aluminum welded joints, more advanced welding methods or perfect welding process is necessary, so as to reduce the effect of welding thermal cycle on the welded joints.