## 4 Conclusions  
The different a ZM6 (Mg\({}_{2.6}\)Nd\({}_{0.4}\)Zn\({}_{0.4}\)Zr) aging processes were designed, and the phase precipitation and microstructure were analyzed after various heat treatments. Moreover, the mechanical properties that were achieved after various aging processes were discussed. The following results were obtained:  
1. As-cast ZM6 alloys consist of \(\alpha\)-Mg and Mg\({}_{12}\)Nd phases. The Mg\({}_{12}\)Nd phase is mainly distributed along the grain boundaries. Mg\({}_{12}\)Nd is distributed in the crystal and the grain boundary in the form of intersecting sheets, which are formed by the aggregation of the point-precipitated phases. The order of precipitation that occurs during the SSAT and TSAT processes was verified to be: supersaturated solution\(\rightarrow\)G.P. zones\(\rightarrow\)\(\alpha\)(hcp)\(\rightarrow\)\(\beta\)\"(D0\({}_{19}\))\(\rightarrow\)\(\beta\)\"(fcc)\(\rightarrow\)\(\beta\)(bct). 2. As-compressed ZM6 alloys formed an \(\alpha\)-Mg solid solution at 636.54 \({}^{\circ}\)C, and when it was cooled to 586.90 \({}^{\circ}\)C, it produced a Mg\({}_{12}\)Nd phase until the end of the eutectic reaction at 567.91 \({}^{\circ}\)C. There were no obvious differences in the microstructures of the as-cast and as-compressed ZM6 alloys; however, there was an obvious increase in the number of twins. Moreover, the twins that were inside and outside of the grain boundary were different in the as-compressed samples that did not undergo heat treatment. 3. After pre-aging at 275 \({}^{\circ}\)C for 12 min, a large number of small and dispersed G.P. zones were formed in the \(\alpha\)-Mg matrix. After the two-stage aging treatment, the homogeneous lamellar Mg\({}_{3}\)Nd phase formed along the grain boundary, and the continuous precipitated fine Mg\({}_{12}\)Nd phase formed in the grains. During the second aging process at 200 \({}^{\circ}\)C for 8 h, the G.P. zone was used as the core of the \(\beta\)\" phase; thus, the size of the \(\beta\)\" phase was refined, and the density and dispersion of the \(\beta\) phase were increased at the end of the process. 4. When the pre-aging time increased, the elongation decreased slightly, and the tensile strength demonstrated an obvious increase. When the pre-aging process was at 275 \({}^{\circ}\)C for 12 min and when the secondary aging process was at 200 \({}^{\circ}\)C for 8 h, the tensile strength reached 294.46 MPa, which was 26.11 MPa higher than that obtained for the single-stage aging process at 200 \({}^{\circ}\)C for 8.25 h.
5. Compared to the T6 treatment, the tensile strength after the two-stage aging treatment improved great, showing values of up to 294.46 MPa. For the two-stage aging process, the best mechanical properties were obtained at 275 \({}^{\circ}\)C for 12 min + 200 \({}^{\circ}\)C for 8 h, where hardness values of 82.56 HV and tensile strength of 294.46 MPa were obtained compared to the values of 78.9 HV and 268.35 MPa that were achieved during the T6_SSAT process (aging at 200 \({}^{\circ}\)C for 8.25 h). In brief, this basic investigation of the microstructure, phase precipitation, and mechanical properties after various aging treatments helped to reveal the strengthening mechanisms of ZM6 aircraft wheel work pieces. The optimized deformation parameters and crack propagation mechanism will be further investigated in the future. **Author Contributions:** Conceptualization, J.F. ; methodology, J.F. ; validation, J.F. ; validation, S.C.; formal analysis, J.F. ; data curation, S.C.; writing--original draft, S.C.; writing--review and editing, [PERSON]; funding acquisition, J.F. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Natural Science Foundation of Shaanxi Province (No. 2020Q-769), the National Natural Science Foundation of China (No. 51905427), the Graduate Innovation Program of Xi'an Shiyou University (YCS21211070), and the Natural Science Basic Research Program of Shaanxi Province (No. 2021 JM-403).