Al-Mg-Zn Alloy Design with Combined Effects of Solid Solution and Precipitation Strengthening

Abstract

Al-Mg-Zn alloy was designed for an aluminum alloy with single composition that can realize the mechanical properties of Al-Mg 5xxx series, Al-Mg-Si 6xxx series, and Al-Zn-Mg 7xxx series alloys. The strengthening mechanism of aluminum alloy can be largely divided into solid solution hardening and precipitation hardening. Solid solution hardened alloy is a non-heat treatable alloy, and precipitation hardened alloy requires heat treatment. Al-Mg 5xxx series aluminum alloys, which are representative solid-solution strengthening alloys, exhibit excellent elongation, but exhibit low mechanical properties, so they cannot be applied as structural materials for vehicles. On the other hand, 6xxx and 7xxx aluminum alloys are used as structural materials for vehicles and aircraft due to their excellent workability and high mechanical properties, respectively. However, as the carbon-neutral era approaches, heat-treated alloys are expected to be subject to stricter regulations on carbon emissions, and recyclability for all aluminum alloys is essential. Therefore, in order to reduce carbon emissions and secure recyclability, we tried to design an Al-Mg-Zn-based alloy that exhibits both solid solution/precipitation strengthening effects and can be applied as both non-heat treated and heat-treated alloys. As a result of peak stress analysis, the peak stress was confirmed in the range of Zn and Mg = 5.5 ~ 6 wt.%. At this time, the yield strength was about 240 ~ 260 MPa, the tensile strength was 440 ~ 460 MPa, and the elongation was confirmed to be more than 35%. It was confirmed that both strength and elongation were superior to Al-Mg 5xxx series alloys, which are solid solution hardening alloys. After that, T6 heat treatment was performed to confirm the precipitation strengthening effect, and at this time, peak stress was also confirmed in the range of Zn and Mg = 5.5 ~ 6 wt.%. These results were confirmed as meta stable 𝜂 (MgZn2) phase and T (AlMgZn) phase, which were formed after T6 heat treatment. It means that Mg and Zn solid solution in aluminum matrix were sufficiently precipitated during the T6 heat treatment. The peak stress of T6 heat treated specimens was confirmed as yield strength of 520 to 540 MPa and tensile strength of 600 to 610 MPa, and the elongation at this time was confirmed to be about 17%. It was confirmed that the mechanical properties were similar to those of the Al-Zn-Mg 7xxx series aluminum alloy, which is a high-strength precipitation-strengthened alloy, and the elongation was better. In addition, for the analysis of natural aging behavior, specimens were stored at room temperature for up to 3,000 h after solid solution treatment. As a result, the natural aging specimens exhibited stress over 90% of the T6 heat treated specimens, and the elongation was confirmed to be over 1.6 times. Al-xMg-yZn-0.5Cu-0.2Fe-0.1Ti (wt.%, x, y=3.5~6) alloy was designed and specimens were fabricated through gravity casting and extrusion. In order to analyze the mechanical properties of designed alloys according to the solid solution strengthening, a tensile test was conducted after solid solution treatment. Therefore, it has been confirmed that the designed Al-Mg-Zn alloy for this study is an environmentally friendly alloy that can be used as a non-heat-treated alloy and its mechanical properties continue to improve over time. To analyze the correlation between mechanical properties and microstructures, microstructure analysis was carried out using OM, SEM-EDS, and TEM. Al3Ti and Al3Fe phase were observed as high-temperature precipitates and AlCuMgZn phase was confirmed resulted in slow cooling after extrusion. After solid solution treatment, only alpha aluminum was observed except for high-temperature precipitates. After T6 heat treatment, micro-sized AlCuMgZn precipitates, tens of nanometer-sized meta stable 𝜂 (MgZn2) phase, and T (AlMgZn) precipitates were formed. To predict the service life of the Al-6Mg-6Zn T6 material, which exhibited the best combination of solid solution and precipitation strengthening effects among the designed alloys, stress corrosion cracking (SCC) test was conducted. SCC was carried out by tensile testing in a 6M NaCl solution at a rate of 10-6mm/s, and the results showed that it was equivalent to AA7075, a representative precipitation strengthened aluminum alloy. To analyze the productivity of the designed alloy, a processing map using Dynamic material modeling was constructed. Al-6Mg-6Zn alloy, Al-6Mg-3.5Zn and Al-3.5Mg-6Zn alloys were subjected to hot compression tests, and their formability was evaluated. As a result, the optimal composition Al-6Mg-6Zn alloy was found to have similar formability to Al-6Mg-3.5Zn alloy, and it is expected to have a very narrow range for plastic deformation.