| 7xxx series aluminum alloy, especially the 7075 alloy has been widely used in the aerospace industry due to its desirable mechanical properties with general acceptable tensile strengths between 500-600 MPa. In order to further improve the mechanical properties, the spray formed method was used and the strength over 700 MPa was obtained for 7075 alloy. In recent years, the comprehensive performance of the materials is demanded higher and higher, which makes the traditional preparation and heat treatment difficult to meet the requirements.Therefore, the effects of heat treatment, especially the retrogression and re-aging (RRA), on micro structure and properties of spray formed 7075 aluminum alloy were investigated and. the RRA treatment suited for spray formed 7xxx series alloy was built.The traditional heat treatments for 7xxx series alloy have peak aging (T6), over aging (T73) and traditional RRA treatments. With comparing the above three kinds of heat treatments on microtructure and properties of spray formed 7075 alloy, it can be found that the UTS is obtained because the tiny matrix precipitates (MPt) are homogeneously distributed in the matrix after T6. While the close-connected grain boundary precipitates (GBP) and the narrow precipitate free zones (PFZ) cause an inferior corrosion susceptibility of the alloy. For T73, disconnected GBP and wide PFZ can improve the corrosion resistance of the alloy. However the UTS of the alloy decreases with the volume fractions of the precipitates. After RRA, UTS up to 758MPa is obtained because the tiny MPt are homogeneously distributed in the matrix, which is comparable to those by T6 treatment. The IGC depth, EXCO rating and SCC index (ISSRT) are 16.8 μm, EA and 0.138 respectively, which is comparable to those obtained in T73.The pre-aging of traditional RRA is peak aging. After pre-aging, coarsened precipitates have been formed and hard retrogressed in matrix, which will consume large amounts of strengthening elements and make the strength of the alloy after RRA as not good at T6. The effects of of RRA on microstructure and properties of spray formed 7075 alloy pre-aged at 120 ℃ for 8,16,24 and 32 h were investigated. The results show that under aging at 120 ℃ for 16 h as pre-aging is more beneficial for the re-dissolution of MPt, more conducive to interrupted distributions of GBP and existed a certain extent of PFZ. The tensile strength, IGC depth, EXCO rating and ISSRT are 782 MPa,27.9 μm, EA and 0.143 respectively.With adjustment of the temperature and time, the effect of RRA on microstructure and properties of the alloy has been investigated. The results show that MPt can be dissolved by high retrogression temperature, which also make GBP intermittent and PFZ wide. But the MPt are coarsened in a long retrogression time and properties of the alloy are influenced. During re-aging, MPt are precipitated again and GBP are further discrete. The strength of the alloy is enhanced by low re-aging temperature and lost by high re-aging temperature. The corrosion resistants of the alloy with different parameters of retrogression and re-aging are similar, which present superior to T6 and approach T73 level.The size and volume fraction of the precipitates during RRA are analyzed, due to the relationship between the properties and precipitated morphology. The evolution model of platelet precipitates are established in nucleation, growth and coarsening. At early stage of pre-aging, the size of precipitates has a linear relation with the square root of aging time. After coarsening, the size and cubic root of aging time are linear relationship. And the volume fraction of precipitates and aging time are accorded with JMAK equation. The early stage of retrogression can be viewed as the inverse process of high temperature aging and the size and volume fraction of precipitates are respectively decreased with the relationship of the square root and exponent of retrogression time. Retrogressed to a certain extent, the volume fraction of precipitates will be stable in a degree and the size increases quickly on coarsening way. During re-aging, the evolution of the size and volume fraction of precipitates can be regarded as in a single-stage aging, but the bases of the size and volume fraction are different.The expression of yield strength of the alloy during RRA is established with solution strengthening, grain boundary strengthening, dislocation strengthening and precipitation strengthening, and the strength models in the three stages of RRA all are good fitting. Compared to the model of traditional RRA, the model of the RRA with an under pre-aging is more accorded with the actual measurement. After traditional RRA, some big size precipitates are formed in matrix during pre-aging, those precipitates are hard retrogressed in matrix and coarsened, which will lose the strength of the alloy. |
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