Effects of the two-step ageing treatment on the microstructure and properties of 7B04 alloy pre-stretched thick plates

The effects of the two-step ageing parameters (temperature and time) on the mechanical properties and electrical conductivity of 7B04 (Al-Zn-Mg-Cu) pre-stretched thick plates were studied. The results reveal that the initial T1 ageing contributes a major increase of the tensile strength, and the 0.2% proof stress value reaches 482 MPa after ageing for 7 h at 115°C. Behavioral differences in the tensile properties of the alloy after the two-step ageing treatment were less with the first-step ageing at 115°C for different time periods (7, 14, and 21 h). The effects of the second ageing parameters on the properties and microstructure of the 7B04 alloy were remarkable. TEM analysis of the samples aged at Temper I (7 h at 115°C + 12 h at 160°C) and Temper II (7 h at 115°C + 16 h at 165°C) indicates that two kinds of phases, i.e. η' and η phases, precipitate from the matrix and efficiently improve the tensile strength of the alloy, and the grain boundary precipitates are coarse and discrete. There are obvious precipitate free zones (PFZs) along the grain boundary in the microstructure of the alloy after the two-step ageing treatment.