Multi-scale gradient strengthening in 6000-series aluminum alloys: Temporal synergistic effects of solution dissolution, deformation dislocation, and precipitation aging

The present study investigates the strengthening sequence effects on the mechanical properties and microstructure of 6000-series aluminum alloys. A comparison was made between two process routes: “solution treatment-torsional deformation-aging treatment” (SDA) and “solution treatment-aging treatment-torsional deformation” (SAD). The SAD process (55° torsional deformation) generates a high density of geometrically necessary dislocations (GNDs, up to 1.43 × 10¹⁴ m⁻²) in the surface layer, creating a “hard surface-ductile core” gradient structure, and yet the SDA reduces GND density (1.04 × 10¹⁴ m⁻²) due to dislocation recovery, weakening the gradient effect. The application of deformation treatment prior to the aging process reduced the activation energy required for β” phase precipitation by 8.85 %. This reduction in energy leads to the coarsening of both the β’ phase and the β phase, results in a concomitant decrease in strength. Conversely, the SAD process exerts no influence on the precipitation of the β” phase. The SAD-55° process has been demonstrated to achieve an optimal balance between strength and ductility (as indicated by yield stress and ultimate tensile strength of 401.7 MPa and 424.03 MPa, which with a 38.82 % and 31.4 %, and 8.14 % and 1.68 % higher than those of single-aging and SDA, respectively; and an elongation of 10.97 %), and has been shown to outperform SDA in terms of elastic modulus (72.6 GPa vs. 71.1 GPa). A microstructural analysis reveals that SAD process enhances the properties mainly through refining dispersed strengthening phase and gradients in dislocation, grain size, texture structure. In contrast, SDA process achieves synergistic interactions between plasticity and ductility through gradients in dislocation density, grain size, texture, and precipitate distribution. The study's findings indicate that solution-aging-deformation sequences facilitate the construction of gradient structures, thereby overcoming the strength-ductility trade-off in 6000-series aluminum alloys. This research provides a foundation for the design of lightweight components.