| In this study, we prepared Al-Zn-Mg-Cu-Zr-Sc Alloy Ingot by squeezing cast. Mechanical properties of tested alloy under different heat treatments have been measured by room temperature tensile test and Brinell hardness measurement, and we also accessed the stress corrosion resistance by electrical conductivity test. The microstructure was studied by optical metallography (OM), scanning electron microscopy (SEM), X-ray diffraction analysis phase, transmission electron microscope (TEM). The effect of heat treatment on mechanical properties and microstructure was investigated. Results of the study are as follows:1) Compared with gravity casting, the density of Al-Zn-Mg-Cu-Sc-Zr alloy billets acquared by squeezing casting was significantly increased. When solidified under the pressure of 100MPA, the density of alloy increase by about 5%, and the influence of pressure to density seems not so distinguish with the pressure continue to increase.2) By composite added Sc, Zr elements to Al-Zn-Mg-Cu alloy, the grain size was greatly refined, reducing from about 100um to 40um and the grain morphology turned to uniformed spherical structure. It has been confirmed the Sc, Zr elements existed mostly in the form of solid solution in casting billets. By transmission electron microscopy, we observed a large number of small dispersed Al3 (Sc,Zr) particles precipitated during the subsequent annealing and heat treatment process.3) For Al-7.8Zn-2.2Mg-1.6Cu-0.2Sc-0.2Zr squeezing cast alloy, the suitable single-stage solution treatment is "470℃/24h+water quenching; the appropriate two-stage solution treatment is "460℃/24h+480℃/10h+water quenching".The tensile strengthσb and elongation 5 of the tested alloy reached 597MPa and 12.5%respectively after the"460℃/24h+470℃/8h+485℃/2h+water quenching+120℃/24h aging treatment.4) After strenghened solution treatment, the coarse eutectics in the matrix dissolved more fully, for which supersaturation increased after quenching and the trend of stress concentration caused by eutectic phase at the grain boundary reduced, so plasticity have been improved significantly after following-up aging treatment.5) After 120℃/24h after single-stage aging treatment, the mechanical properties reached the summit and precipitates in the grain were mainly needle-likeη, and short rod-likeη.During the over-aging treatment process the tested alloy maintained high hardness and stability.After two-step aging and RRA treatment the mechanical properties decreased significantly. Due to lack of dislocations, vacancies and other defects in the matrix without deformation processing,the GP zone andη, phase can not timely dissolve into the matrix, and precipitates further coarsened in the second or third step aging process.6)Squeezing cast alloy after multi-stage solution+T6 aging treatment has good stress corrosion resistance (conductivity 35.6%IACS) and good ductility (elongation of 12.5%).The reason for good resistance to stress corrosion is the grain boundary precipitates were discontinuous, which means the anode was isolated particles during corrosion process and relatively slow down the corrosion process. The width of precipitate free zone at grain boundary after aging treatment was 20nm-30nm.Due to precipitate free zone was soft, the relatively wide precipitate free zone was easy to relax when suffer stress, so alloys with this kind structure have good ductility.
|
PDF Full Text Request