Crack initiation and propagation behavior of dissimilar interface with intermetallic compound layer in Al/steel joint using coupled multiscale mechanical testing

We demonstrate multiscale mechanical testing using miniature and microscale tensile tests to elucidate the crack initiation and propagation behaviors corresponding to interfacial nanostructures in dissimilar joints of 6061 aluminum alloy and 304 stainless steel. The initial nucleation and subsequent growth of intermetallic compounds (IMCs) can be controlled by heat treatment after friction stir lap welding. The miniature tensile tests exhibited the relationship between the joint strength and IMC layer thickness with changes in the fracture location. Microscale tensile testing using a single edge-notch tension geometry initiates the crack and subsequent propagations near the interface, which helps identify the crack initiation site and understand the dependence of the local strength on the type of IMCs. Crack deviation behavior toward the Al side, attributed to the difference of local strength, is confirmed by in situ observations, which indicated that continuous failure can easily occur within the grown IMCs, particularly in macroscale joints. It was concluded that the crack propagation is controlled by the local crack deviation toward metal matrix, which tends to occur for the dissimilar interface with thin IMC layer. This miniature/microscale tensile testing approach is expected to clarify the essential relationship between the interfacial microstructure and the fracture behavior.