| Al-Zn-Mg-Cu alloy is a key material for large aircraft, which demands high strength, good toughness and superior corrosion resistance when it is used as the primary structural materials in aerocraft field. However, recrystallization induced by deformation and solution treatment and continuous distribution grain boundary precipitates(GBPs) decrease fracture toughness and the resistance to stress corrosion cracking, which limits its potential for application. Therefore, it is very important to study the effect of deformation and heat treatment on the microstructure in order to improve strength and fracture toughness as well as the resistance to stress corrosion.The paper investigated rolling7B50plates provided by Northeast Light alloy Co. Ltd. The effect of deformation, solution treatment and aging treatment on the undissolved particles, recrystallization and precipitates was studied. The mechanism of special microstructure influence on the tensile, toughness and corrosion resistance was discussed. The research results are stated as the following:(1) The local corrosion is pitting corrosion, intergranular corrosion and exfoliation corrosion, respectively; corrosion route develops along the recrystallized grain boundary due to GBPs on the recrystallization boundary are readily dissolved. The recrystallization decreases the EC resistance of rolling surface and SCC resistance of L-T and T-L direction; the recrystallization decreases the tensile property of L direction and fracture toughness of L-T direction while the undissolved particles decrease the tensile property of LT direction and fracture toughness of T-L direction.(2) The greater hot roll-deformation amount is, the less undissolved particles is, the smaller the grain size is, the denser the aging-precipitates are, but the more serious the recrystallization is, resulting in the tensile property increases, EC and SCC resistance get worse, and fracture toughness increases first, then decreases. When deformation temperature decreases, recrystallization during solution treatment is serious, leading to degrade the tensile and corrosion properties.(3) At mediate temperature of solution heating-up treatment(350℃-460℃), phases get coarse and hazards the effect of solution treatment, therefore, increasing the heating rate during the solution heating-up treatment is suggested; high solution temperature and short solution time is suggested by studing the effect of solution temperature and time on the undissolved particles and recrystallization; therefore, combined rapid heating rate and high temperature and short time solution treatments could maximize the undissolved particles dissolve and minimize the recrystallization occur, corresponding to the best comprehensive properties of alloy.(4) When retrogression occurs, intra-grain precipitates dissolve and GBPs become coarse and discrete; Re-aging treatment make intra-grain precipitates re-precipitate and GBPs keep coarse and discrete, leading to keep high strength and superior corrosion resistance. Based on it, the repetitive-RRA treatment is proposed in order to keep high strength and further improved corrosion resistance.. It is found that, within3retrogression time, matrix precipitates coarsening do not occur, while GBPs become more discrete and coarser with the time increasing. The Cu element content increases with the time, inversely for Zn element content. The tensile properties maintain RRA strength, while fracture toughness, EC resistance and SCC resistance increase with time increasing. When the retrogression time reached4, matrix precipitates partly coarsen, still smaller than T76treatment, the tensile properties, fracture toughness and SCC resistance decrease. |
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