Quantitative Evaluation Of Fatigue Fracture In Laser-MAG Hybrid Welding Of SMA490BW

Laser-arc hybrid welding is one of emerging methods which can obtain the high effec-tive and excellent quality joint. The property of joints of high-speed bogie located in high speed train is critical for the safety,especially the dynamic performance. SMA490BW is one of common anti-corrosion steels. The process of semi-automatic and automatic MAG welding are used to joint the material with low efficiency. Maybe the Laser-MAG hybrid welding will replace the technology in the future. The researches pay less attention on dynamic performance about the hybrid welding joint in domestic and foreign literature. Thus it is valuable to explore the fatigue property in detail in some way. In this study,the law of fatigue life and the quantitative analysis of hybrid welding joint fracture were studied. Basic datas which benefit for understanding the mechanism of crack propagation were acquired.In the study,optimal technology was used to acquire the welding process. Basic mech-anical property, microstructure, hardness and law of the fatigue life were tested and analyzed. The fatigue stress life curve and the law hidden at stress ratio zero were discussed. Then fracture appearance characterized through fractal was considered adequately. At the same time, the distribution of fractal in the area adjacent to crack source was explained as well as the crack propagation zone. The further discussing about the relationship between the propagation process and the change law of fractal was realized. Graphical User Interface was designed to calculate the fractal through the Matlab at last.The results of basic mechanical properties satisfied the relevant standard. The fatigue limit analyzed through the fatigue life at stress ratio zero was 385MPa. Crack initiation was identitied due to the presence of defects located in the surface or in the interior. Quasi cleavage morphology gradually disappeared in the process of crack propagation. The boun-dary of propagation zone showed quasi-circle or local trajectory in quasi oval. The results of fractal characterization of fracture implied that the average fractal and coefficient of dispersion were comparatively lower values with uniform features in propagation zone. Large irregularity with nonuniform morphology was found in the area adjacent to crack source. The fractal in the area adjacent to crack source increased with consideration of the stress concentration when the stress level was increasing, but an inverse law in the propagation zone was exposed that the fractal decreased. In the process of crack propagation, Quasi cleavage morphology gradually disappeared firstly, then secondary crack initiated and tear ridge appeared and they disappeared in the following process at the distance between 500μm and 2000μm. The fractal showed combination law of an normal ’V’ and an inverse’V’ which verified morphology of propagation process. Then the fractal values were obtained when the quasi cleavage morphology disappeared. The spatial dimension was 2.50 and the information dimension was 1.741. Graphical User Interface can realize calculation of fractal and storage of pictures.