Effect of Sc on the microstructure and properties of 7056 aluminum alloy

Al-Zn-Mg-Cu alloys are widely used due to their excellent properties. For the 7056 aluminum alloy developed on the basis of 7055 aluminum alloy, exploring its aging characteristics and the effects of rare earth elements on its microstructure and mechanical properties has a great significance to promote the use of the alloy. In this paper, the as-cast alloy is subjected to homogenization treatment, extrusion, solution treatment, and aging treatment. The effect of adding 0.2% Sc to the 7056 aluminum alloy on the microstructure and properties of the alloy was investigated by analyzing the chemical composition of the alloy, observing the microstructure of the alloy in different states, observing the precipitated phase by transmission electron microscopy (TEM) and testing the hardness and tensile properties of the alloy after heat treatment. The experimental results show that the addition of Sc significantly refines the microstructure of the grains, and the as-cast grains decrease from 100-500 μm to about 50 μm. The addition of Sc element greatly improves the plasticity of the alloy. After the aging treatment, the elongation after fracture of the alloy increased from 10.82% to 13.60%, but the yield strength reduced from 668 MPa to 657 MPa. By comprehensively calculating the grain size and precipitation phase strengthening, the reasons for the decrease of the yield strength of the peak-aged 7056 aluminum alloy were analyzed in detail. Theoretical calculations show that when 0.2% of Sc is added to the alloy, after peak aging treatment, the strength of the alloy will decrease by 12.005 MPa, which is close to the test value of 11 MPa. Through the research, the best single-stage aging system condition for the 7056 aluminum alloy was found to be 120℃+16 h, and the corresponding peak hardness and strength were 195.2 HV and 714 MPa, respectively. At this time, the main strengthening phase of the alloy was a disk-shaped and short rod-shaped MgZn₂ phase, which was about 4 to 6 nm in size, and the alloy also had a spherical Al₃Zr phase with a size of about 20 nm.