A comprehensive analysis of a pseudo-brittle fracture at the interface of intermetallic of η and steel in aluminum/steel joints made by FSW: Microstructure and fracture behavior

Intermetallic compounds (IMCs) formation during friction stir welding (FSW) of dissimilar metals such as aluminum (Al) and steel (St) is known to cause a reduction in the joint strength. IMCs are brittle and act as the preferential sites for fracture propagation. In the present study, a modified FSW was used to join a commercially pure Al to a mild carbon St. Several characterization techniques were performed to analyze the joint interface and the results revealed that a nanostructured IMC layer made of Fe4Al13 (theta) and a micrometer IMC layer made of Fe2Al5 (eta) were formed adjacent to the Al side and St side, respectively. During the tensile test, the crack propagated through the interface of q-St and not through the IMCs layer. It was shown that the interface of q-St was prone to brittle debonding under normal load, but it showed a slight plastic deformation at the triple junctions with the eta grain boundaries. At the triple junctions of boundaries, the loading state was shear and no debonding occurred during the tensile test. This promoted a pseudo-brittle fracture surface. The results of the present study showed that the grain refinement of the IMCs can improve the joint strength of the dissimilar materials through increasing the fracture toughness of the IMC and increasing the number of triple junctions at the interface of metal-IMC.

Automated summary

The study focused on using modified FSW to join aluminum and steel, analyzing the joint interface and identifying the formation of different IMC layers on the Al and St sides. The results showed that improving the grain refinement of IMCs can enhance joint strength by increasing fracture toughness and triple junctions at the metal-IMC interface.