Recent advances in extrusion-based Mg/Al and Mg/Mg bimetallic composites: Processing, interface control, microstructure evolution, and mechanical properties

Magnesium-based bimetallic composites have attracted increasing attention as lightweight structural materials because they can combine the low density and high specific strength of Mg alloys with complementary properties provided by second metallic constituents, such as improved ductility, strengthening capability, surface stability, and texture-control potential. Among the available fabrication methods, extrusion-based processing is particularly effective because it integrates large plastic deformation, triaxial compressive stress, interfacial bonding, grain refinement, and texture modification within a single forming operation. This review summarizes recent advances in extrusion-based Mg/Al and Mg/Mg bimetallic composites, including conventional and hydrostatic co-extrusion, porthole-die co-extrusion, continuous extrusion, shear-assisted extrusion, asymmetric extrusion, extrusion welding, and stacked extrusion. Particular emphasis is placed on the process–structure–property relationships governing interface formation, microstructural and texture evolution, mechanical properties, and failure behavior. In Mg/Al systems, the interface is the critical region controlling structural reliability. Adequate Mg–Al interdiffusion promotes metallurgical bonding and efficient load transfer, whereas excessive growth or discontinuous formation of brittle Mg–Al intermetallic compounds promotes crack initiation and premature failure. Shear-assisted and asymmetric extrusion can improve interfacial contact, promote dynamic recrystallization, refine Mg grains, weaken basal texture, and enhance deformation compatibility near the interface. Transition layer engineering can further suppress direct Mg–Al reactions, although discontinuous transition layers or localized reaction products may introduce additional failure sites. Mg/Mg bimetallic composites provide an alternative strategy for reducing anisotropy and tension–compression yield asymmetry through alloy-specific texture control and deformation-mode design, while avoiding the brittle Mg–Al reaction layers. Remaining challenges include quantitative control of interfacial characteristics, standardized evaluation of bonding quality, assessment of fatigue and corrosion-fatigue behavior, three-dimensional and in-situ damage characterization, predictive modeling, and the development of scalable processing methods suitable for industrial application.