Investigation on In-situ Fabrication of High-Performance Heterostructured Aluminum Alloys with Continuously Graded Interfaces and Their Synergistic Strengthening and Toughening Mechanisms

To address the contradictory relationship between strength and ductility in 7075 aluminum alloy, this study used friction stir processing (FSP) to in situ create heterogeneous surface-layer structures with continuous gradient interfaces. Combining microstructural characterization with crystal plastic finite element method (CPFEM) simulations, we systematically explored the microstructure evolution and the mechanisms responsible for concurrent strengthening and toughening. The results show that heterogeneous-structure aluminum alloys with different layer depths can be produced by varying the stir pin length (L). When L is 3 mm, the sample exhibits the best overall performance, with tensile strength (Rm) of 488.5 MPa, elongation (A) of 23.8%, and a strength–ductility product (Rm·A) of 11627.2 MPa·%, surpassing other gradient-structure samples and 7075 alloys produced by conventional processing. The heterogeneous structure displays a V-shaped hardness profile, contains a softened interface zone about 1.5 mm thick, and is dominated by Cube texture. Using CPFEM simulations and the K-M model, we show that a softened layer promotes “asynchronous deformation” during tensile loading and induces stress redistribution, which enhances the material's strain-hardening capacity. A cooperative strengthening model, built on the principle of load equivalence, quantifies the strengthening contribution of the heterogeneous structure. This work provides theoretical support and a technical reference for the structural design and process optimization of high-performance 7075 aluminum alloy.