To address the technical challenge of balancing formability and strength in automotive aluminum alloys, this study examined an Al-4.35Mg-3.6Zn-0.2Cu alloy subjected to a combined heat-treatment schedule consisting of a two-step solution treatment (470 °C for 24 h followed by 460 °C for 30 min) and a subsequent two-step aging process (T4P: 80 °C for 12 h, followed by BH: 180 °C for 30 min). Microstructural evolution was characterized using transmission electron microscopy, and uniaxial tensile tests were performed in accordance with the GB/T 228.1-2021 standard at a strain rate of 0.2 mm/min. In the T4P condition, the matrix contained both GPI zones (~0.9 nm) and GPII zones (~1.2 nm), with no detectable T-phase precipitation. The presence of GPII zones enhanced ductility by promoting dynamic recovery after dislocation shearing, resulting in a yield strength (YS) of 178 MPa, an ultimate tensile strength (UTS) of 310 MPa, and an elongation (El) of 9%. After BH treatment, the GPII zones transformed into semi-coherent T′-Mg₃₂(AlZnCu)₄₉ precipitates (~2.4 nm), which strengthened the alloy through their semi-coherent interfaces. The retained GPII zones mitigated the loss of ductility, and the final mechanical properties reached a YS of 275 MPa, a UTS of 340 MPa, and an El of 8.5%, corresponding to a BH response of 97 MPa. Strengthening-mechanism calculations indicated that GP zones contributed approximately 120 MPa to the yield strength in the T4P state, whereas T′ precipitates contributed about 169.64 MPa after BH treatment. The calculated values agreed well with the experimental results, with a deviation of less than 3%. This study clarifies the precipitation sequence in the alloy—supersaturated solid solution → GPI zones → GPII zones → T′ phase—and establishes the relationship between microstructure and strength–ductility behavior. The findings provide theoretical guidance for the design and optimization of high-strength, high-formability aluminum alloys for automotive outer-panel applications.

Al–Zn–Mg–Cu alloys are well known for their outstanding strength, toughness, and corrosion resistance, arising from the balanced addition of Mg, Zn, and Cu. However, conventional casting methods often lead to grain boundary segregation and the formation of coarse Fe-rich phases, which severely limit subsequent heat treatment and plastic processing. To overcome these drawbacks, this study systematically investigates the effects of the Continuous Rheo-Extrusion (CRE) process on the microstructure and mechanical performance of Al–Zn–Mg–Cu alloys using XRD, EBSD, SEM, and TEM analyses. The CRE process refines the average grain size from 53.5 μm to 16.1 μm and raises the fraction of high-angle grain boundaries to 88.8%. Moreover, coarse Fe-rich phases are fragmented to below 5 μm, while the elemental distribution of Zn, Mg, and Cu becomes more homogeneous, effectively reducing grain boundary segregation. The Al₂Cu precipitates are refined from 106.3 nm to 11.7 nm, corresponding to an 88.9% size reduction. These microstructural optimizations yield a remarkable increase in tensile strength (from 204.7 ± 23.7 MPa to 338.0 ± 9.3 MPa) and elongation (from 11.4 ± 2.4% to 13.8 ± 1.3%). Quantitative analysis confirms that dislocation and precipitation strengthening are the dominant contributors to this improvement. Overall, the CRE process enhances microstructural uniformity through the synergistic effects of shear deformation, continuous dynamic recrystallization (CDRX), and dynamic precipitation, thereby providing a solid theoretical and practical foundation for short-process fabrication of high-strength, high-ductility Al–Zn–Mg–Cu alloys.

The Al-Ti-C alloy is the most widely used grain refiner for Al-7Si alloys. However, TiC particles in Al-7Si alloys tend to aggregate and settle, thereby reducing their refinement efficiency. In the present paper, a novel Al-3Ti-4.35La master alloy was developed, and its influence on the stability of TiC particles in Al-7Si alloys was investigated by XRD, SEM, and TEM. The results show that when the Al-TiC alloy is added to the Al-7Si alloy, TiC will accumulate and settle obviously after holding the alloy for a certain time (15 min, 30 min, and 60 min). When the Al-TiC alloy and the Al-3Ti-4.35La master alloy are added to the Al-7Si alloy, the aggregation and settlement of TiC are weakened under the same holding time. Additionally, the refinement effect of TiC is enhanced. The Ti and La elements dissolved by Ti₂Al₂₀La in the Al-3Ti-4.35La master alloy are absorbed on the surface of the TiC particles, which improves the wettability between the TiC particles and the aluminum melt, reduces the agglomeration and sedimentation of TiC particles in the aluminum melt, and improves its stability.