Dissimilar laser joining of aluminum to steel via a porous FeCoCrNi high entropy alloy coating: Interfacial microstructure, wettability, and mechanical properties

Dissimilar laser joining of AA6061 aluminum alloy and low carbon steel was performed with the aid of a porous FeCoCrNi high entropy alloy (HEA) coating. At low laser power, the interfacial reaction zone mainly consisted of η-Zn solid solution, Zn-Al eutectoid structure, and extremely thin AlNi. Because of the poor wettability of filler metal on the steel substrate, the joint strength was low (218.3 N/mm) and the joint fracture mode was a brittle fracture. At high laser power, in the interfacial reaction zone, the amount of η-Zn solid solution gradually decreased and the AlNi became thickened. Enhanced wettability of filler metal improved joint strength. The maximum joint linear load of 297 N/mm was obtained at a laser power of 3000 W. The fracture mode of the joint was a mixture of brittle and ductile fractures. Moreover, the thermodynamic calculations predicted that the Ni elements were preferred to be enriched toward the Al elemental zone, which revealed the formation mechanism of AlNi. The joint enhancement mechanism was also studied using the edge-to-edge matching model. This study elucidates the microstructure-regulating mechanism of HEA porous coatings in Al/steel laser joining, offering new ways to enhance dissimilar Al-steel bimetallic component quality.