The Effects Of Ageing Condition On Mechanical Property And Stress Corrosion Behavior Of Aluminum Alloy7N01Welded Joints

Recently, aluminum alloy with light weight, high strength has become one of the essential materials of construction in the high-speed train manufacturing process. However, the failure mode changed frequently. Subject to the erosion of the surrounding medium and the periodic force in the material during service, then, fatigue cracking and corrosion cracking has become the main reason for the failure of the aluminum alloy. Thus, it is significant that stress corrosion tests for aluminum alloy, as well as study on the evolution of microstructure and its affecting to the corrosion behavior.In this paper, performance testing for the7N01P-T4aluminum alloy with laser hybrid welded joints of CRH2high-speed train. Use of different conditions which post-weld heat treatment and the cyclic immersion stress corrosion tests. The observation of macro corrosion morphology、testing of micro-crack growth and weight loss, to study its mechanism of resistance to stress corrosion behavior of welded joints.The corrosion test results show that the tensile strength and hardness of the specimen under natural aging were higher than that of the sample under state T6and T73, this was due to natural aging make the GP zone and second phase were in the form of anti-deformed coherent strain, these made the strength and hardness of the alloy increases. Found by TEM observation, alloys formed the transition phase η’(MgZn) formed under T6state, the η’ changed into the η(MgZn2) and grew up with further high-temperature aging, which made the strength and hardness of the alloy to further reduce and soften. It was the weld joint treated with system T73that had the best corrosion resistance property, the worst was natural aging. The average crack growth rate of base metal (da/da) was3.1597×10-3mm/h, the heat affected zone was1.73745×10-3mm/h and weld joint crack nothing. This is due to containing the Mg-rich compounds (Mg2Zn3), the metastable phasen(MgZn2) and stable phase precipitated and raised under T73aging system, thereby the free Mg reducing, hydrogen atom in the grain boundary segregation reducing and balance discontinuous phase precipitates on grain boundaries at the same time, thereby improving the corrosion resistance of the alloy, but the strength reduced to a certain extent.