Study On Stress Corrosion Cracking Of 7N01 Aluminum Alloy Used In Rail Transportation

7N01 aluminum alloy used in rail transportation was originally developed by Japan. It is commonly used as a kind of important load-bearing structural material applied on the bottom frame beams of train. Although it has high strength, good hot working performance and excellent welding property, it is easy to produce stress corrosion crack (SCC) that usually brings serious consequences.The SCC mechanism of 7N01 aluminum alloys and the factors of SCC have been researched in this paper. By optimizing the composition and heat treatment process, the microstructures can be changed to improve the SCC performance of 7N01 aluminum alloys.It has been analyzed about the microstructure and chemical composition of domestic and imported 7N01 aluminum alloy in the article by using optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray fluorescence spectrometer. By using X-ray diffraction (XRD) and differential scanning calorimeter (DSC), the phases and phase transition have been respectively researched. And slow strain rate testing (SSRT) was experimented by slow strain stress corrosion testing machine. The electrical conductivity and polarization curves were tested by digital conductivity meter and electrochemical workstation. Rheological mechanical properties and microstructural evolution rule was studied by thermal simulator in the deformation process at high temperature.The experimental results show that MgZm is the main strengthening phase in 7N01 aluminum alloy. When the content of Si is too high, Mg and Si can form the Mg2Si phase, which will reduce the tensile strength and hardness of the alloy. Less impurity phases (including Fe and Si) and larger interphase spacing make the resistance to SCC better. The grains are fiber-like along the direction of deforming after thermal processing and heat treatment resulting from the higher recrystallization temperature of the alloy, which helps to improve the resistance to stress corrosion of the 7N01 aluminum alloy.The experimental results of double cantilever beam (DCB) test show that it is mainly intergranular cracking in the imported 7N01 aluminum alloy, however, anode channel and intergranular cracking cause fracture in the domestic one. SSRT results show that the domestic and the imported 7N01 aluminum alloy in (3.5%NaCl+0.5% H2O2) solution have higher SCC sensitivity than in 3.5% NaCl solution. The SCC sensitivity of the domestic one is higher than that of the imported one in 3.5% NaCl neutral solution or (3.5%NaCl+0.5%H2O2) solution. The SCC sensitivity of transverse is higher than that of longitudinal direction in the two kinds of 7N01 alloys. Within the range of sensitive stress corrosion speed (10-6～10-7 s-1), the SCC sensitivity of the alloys decreases with the increase of the initial strain rate. In (3.5%NaCl+0.5%H2O2) solution and the temperature is above 35℃, the SCC sensitivity reduces and the time of fracture extends with the temperature rising.High temperature pre-precipitation process (470℃/lh+470℃/30min+120℃/24h) can obviously improve the stress corrosion resistance of 7N01 aluminum alloy, at the same time, improve the ductility, but reduce the tensile strength about 10%.The impurities including Fe and Si have important influence on the stress corrosion cracking of 7N01 aluminum alloy. It is an important measure to improve the stress corrosion resistance by changing the distribution way of the coarse second phases and reducing their content. For the imported 7N01 aluminum alloy plate, under the action of tensile stress, the pitting corrosion crack sources come from the coarse impurity phases. The combination of crack tip extension and intergranular cracking leads to the occurrence of SCC. For the domestic one, the coarse second phases distribute along the grain boundary and the particle distance is small. The occurrence of SCC results from coarse second phases which are distributed along grain boundary and the space among them is short in domestic 7N01 aluminum alloy, and the sources of crack caused by pitting under the action of tensile stress in the intergranular rapidly expand and link up together, and the channels connecting the anode corrosion come into being, then big cracks begin to take shape and the fracture occurs. The SCC mode of extrusion profiles is not the same as that of domestic and imported plate, and it is slip dissolution model.Nb element can change the precipitation form of the coarse impurity phases at the grain boundary and inhibit the formation of Mg2Si phase. It mainly exists as Al3Nb and Fe2Nbo.3Zro.7 in 7N01 aluminum alloy. The electrical conductivity of the 7N01 aluminum alloy increase with the increase of Nb. According to the relationship between conductivity and stress corrosion, the stress corrosion resistance of the 7N01 aluminum alloy can be improved by adding Nb element.The results of hot compression deformation simulation experiments (temperature: 400℃, deformation amount:50%, strain rate:0.1 s-1) show that obvious dynamic recrystallization process happens in the domestic 7N01 aluminum alloy during the hot deformation, on the contrary, the imported one has higher ability to resist recrystallization.