Achievement of superior strength in Al–Ni dissimilar joints through designing high-entropy alloy interlayer and post weld heat treatment

The formation of brittle intermetallic compounds (IMCs) at the interface during Al–Ni dissimilar joining deteriorates joint strength, thereby limiting their applicability in high-performance engineering structures. This study investigates the effects of a high-entropy alloy (HEA) interlayer, CoCrFeMnNi, and post weld heat treatment (PWHT) on the interfacial microstructure and mechanical performance in dissimilar Al–Ni friction stir lap welding (FSLW). Without the interlayer, a brittle Al3Ni IMC layer formed at the interface and grew significantly after PWHT, leading to interfacial fracture and reduced joint efficiency (∼20 %). In contrast, the application of an HEA interlayer suppressed IMC formation and led to the formation of a nanoscale amorphous layer under as-welded conditions, which effectively prevented interfacial fracture and enhanced joint efficiency (∼63 %). After PWHT, a favourable Al13M4 IMC layer formed, wherein the M-site was co-occupied by Co, Cr, Fe, Mn, and Ni. This IMC exhibited no brittleness and effectively prevented interfacial fracture. Moreover, superior joint strength was achieved after PWHT which promoted the re-precipitation of strengthening precipitates that had previously been dissolved during FSLW owing to thermal exposure of the Al matrix. Joint efficiency thus improved to ∼93 % which is comparable to the base metal. The synergistic combination of an HEA interlayer and PWHT is a promising strategy for suppressing the formation and growth of brittle IMCs and for enhancing the reliability of dissimilar Al–Ni joints.