Microstructures and Intergranular Corrosion Properties of 7055 Alloy Plate with Cooling Pre-Treatment AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY21100025

Chinese Library Classification Number:TG133+4

Citation Information:

7055 aluminum alloy treated by traditional peak ageing （T6） had the highest tensile strength and a poor corrosion resistance. The high alloying element content and extremely complicate precipitation behavior during heat treatments of 7055 aluminum alloy led to the diversity of micro-area composition，which showed a great difference of corrosion resistance. High Cu content at grain boundaries could improve the corrosion resistance 7055 aluminum. In order to improve the corrosion resistance of 7055 aluminum alloys treated by traditional peak ageing （T6），7055 aluminum alloy plates prepared by snake rolling were used as experiment materials for different heat treatment processes. The equilibrium phase diagram of 7055 aluminum alloy was calculated by JMatPro software and the precipitate temperature of the main precipitates of MgZn₂ （η phase），Al₂CuMg （S phase） and Al₃Fe were 430，454 and 570 ℃，respectively. According to the precipitate temperature of S phase，three cooling pretreatment temperatures （465，455 and 445 ℃） were selected to heat-treat 7055 aluminum alloy plates，respectively. The solid solution （480 ℃×1 h） and cooling pretreatment of 7055 aluminum alloy plate were carried out on Gleeble-3500 thermal simulation test machine，and ageing process were performed in the air circulating furnace. EIectrochemical Impedance Spectroscopy （EIS） of different heat-treated samples was measured by Zennium electrochemical workstation. According to GB/T 7998-2005 test standard，the accelerated corrosion test of the aged samples for 12 h was carried out. The corrosion environment was neutral corrosion solution （35 g NaCl+10 ml H₂O₂+100 ml H₂O） at （35±3） ℃ in aqueous solution. The corrosion depths of the samples were observed and measured by Axio imager microscope. The effects of cooling pre-treatment on the precipitate behavior，micro-zone composition of 7055 aluminum alloy plates were studied by means of scanning electron microscopy （SEM），scanning transmission electron microscopy （STEM）. The cooling pre-treatment could cause the local enrichment of alloying elements，and the precipitation of S phase was closely related to the content of Cu elements at the grain boundary. After water quenching，the content of solute atoms at grain boundaries firstly increased and then decreased with the decrease of pre-treatment temperature. When the pretreatment temperature was 455 ℃，the solute content of grain boundary was the highest，and the contents of Zn，Mg and Cu elements were 3.97%，3.63% and 3.15%，respectively. When the pre-treatment temperature was 445 ℃，S phase precipitated，the solute content of Mg and Cu at grain boundary decreased and the contents of Zn，Mg and Cu elements were 4.13%，2.83% and 2.47%，respectively. The sample pretreated by 455 ℃was aged by peak ageing process （Sample CPt455+T6），and then was compared with sample treated by traditional T6 （Sample SS+T6）. Compared with Sample SS+T6，the size of intragranular precipitates of Sample CPt455+T6 slightly increased，while the precipitates at grain boundary was obviously different. In the cooling pretreatment stage at 455 ℃，alloying elements quickly segregated to grain boundaries，which shortened the diffusion time of alloying elements gathering to grain boundaries during aging. Therefore，the discontinuous η phase formed at the grain boundaries of Sample CPt455+T6，whileSample SS+T6 had a continuous η phase at grain boundaries. Micro-area composition analysis presentedη-MgZn₂ phases at grain boundary in Sample CPt455+T6 had higher Cu content than that of Sample SS+T6. After intragranular corrosion test，the corrosion depth of Sample SS+T6 was （68±5） μm，while the corrosion depth of Sample CPt455+T6 was （154±5） μm. The larger the diameter of circular arc in electrochemical impedance spectroscopy （EIS）curves，the greater the impedance and the better the corrosion resistance. The circular arc diameter of Sample CPt455+T6 was larger than that of Sample SS+T6. Therefore，the corrosion resistance of Sample CPt455+T6 was better than that of Sample SS+T6. η-MgZn₂ phases at the grain boundaries were regarded as an anodic during corrosion. For continuous η-MgZn₂ phases in Sample SS+T6，the formed corrosion channel could continuously expand along the grain boundaries. The discontinuous η phase in Sample CPt455+T6 could cut off corrosion channels，which made the corrosion only occur in the local area of grain boundaries and the integrity of grain boundaries were retained. Thus，Sample CPt455+T6 had better intergranular corrosion resistance thanSample SS+T6. the probability of Cu atoms replacing Zn atoms in grain boundaries η of Sample CPt455+T6 was larger than that of Sample SS+T6，it led to the positive shift of the corrosion potential of η phase，which reduced the potential difference between the grain boundaries η phase and the precipitate free zone. Based on the test and characterization results，the conclusion we could get were the content of grain boundaries solute atoms increased with the cooling pretreatment temperature decreased. When the cooling pretreatment temperature was 445 ℃，S phase precipitated and the solute content at grain boundary decreased. Compared with SS+T6 process，CPt455+T6 process caused discontinuous η phase with high Cu content to precipitate at grain boundaries of 7055 aluminum alloy. Furthermore，compared with SS+T6 process，CPt455+T6 process could enhance AC impedance spectroscopy and intergranular corrosion resistance of 7055 aluminum alloy.