Revealing unique dual interfacial pathways and mechanical-electrical properties in Al/Cu hybrid conductors: Intermetallic-forming and interdiffusion-hindering regions

Roll-bonded Al/Cu multilayer composites are promising materials for electrical applications owing to their low cost, lightweight, high strength, and good conductivity. This study investigated the effects of post-annealing treatment on the interface evolution, microstructure, and resultant mechanical and electrical properties. The interfacial affected zone (IAZ) of Al exhibited a gradient structure, which progressively disappeared as the annealing temperature increased. In contrast, the IAZ of the Cu layer evolved from dislocation cells to recrystallization at 250 °C and pronounced grain growth at 350 °C. The interfacial extruded Cu regions (IECRs) and fine-grained zone at the edge of the Cu layer adjacent to the interface exhibited remarkable microstructural stability during annealing, maintaining their refined grain structure. Additionally, post-annealing treatment triggered contrasting interfacial behaviors: direct Al/Cu bonding areas promoted diffusion and intermetallic compound (IMC) formation (Al₂Cu at 250 °C; Al₂Cu, AlCu, and Al₄Cu₉ at 350 °C), while the IECR/Al interfaces remained structurally stable, presumably due to the presence of an oxide layer that hindered interdiffusion. The yield strength (YS) and ultimate tensile strength (UTS) decreased as the annealing temperature increased from 129 MPa and 159 MPa in the as-rolled state to 117 MPa and 130 MPa at 250 °C, and further to 90 MPa and 112 MPa at 350 °C, respectively, while elongation improved. Electrical conductivity exhibited a slight reduction after annealing (from 73.4% IACS in the as-rolled state to 72.4% after annealing at 250 °C and 71.1% at 350 °C), primarily owing to the resistive effect of IMCs. These findings provide a design-relevant understanding of Al/Cu hybrid conductors by showing that the strength–ductility–conductivity balance can be tailored through control of local interfacial pathways, particularly by limiting excessive IMC formation.