Mechanism of toughness enhancement of brittle fracture by intermittent η-intermetallic in Al/Cu joint made by FSW

It is well documented that the mechanical failure mechanism of various compounds of aluminum 1050 and commercial pure copper (Al/Cu) involves a combination of brittle and ductile fracture processes. Brittle fracture occurs through the intermetallic compounds (IMCs), while ductile fracture occurs through the base materials, either in the stir zone or the heat-affected zone. In this study, a clean and uniform Al/Cu interface was created using the Friction Stir Welding (FSW) technique, which allowed the investigation of the mechanism of pure brittle fracture in Al–Cu IMCs, using a notched tensile specimen. Fracture surfaces were analyzed in detail using scanning electron microscopy (SEM) and microstructural characterization techniques. SEM analysis of the Al/Cu interface revealed the presence of continuous Al2Cu, intermittent AlCu (η), and continuous Al4Cu9. Fractography results showed that the brittle fracture was multilayered and multi-faceted, with an intergranular fracture observed between the Al2Cu and Al4Cu9 IMCs. This crack was stopped by the intermittent AlCu IMC, resulting in transgranular fracture through AlCu. Cracks in the AlCu phase were deflected toward the Al/Al2Cu interface, resulting in delamination of the Al/Al2Cu interface. It was found that this multilayer fracture, induced by the intermittent AlCu between Al2Cu/Al4Cu9, provided the high tensile strength of the compound (173 MPa) compared to the values reported in the literature. A comparison of the Al/Cu joints with Al/steel joints also confirmed the contribution of this fracture deflection to the improved joint strength.