Interfacial microstructure and strengthening mechanism of dissimilar laser al/steel joint via a porous high entropy alloy coating

A porous high entropy alloy (HEA) coating was prepared on a steel surface by vacuum sintering. The coating was then used as a transition layer during dissimilar laser joining of Al to steel. Compared with the uncoated laser joints, the liquid alloy spread and infiltrated into the porous structure, the contact angle of the weld reduced from 65.8° to 56.7°, and the brazed width increased from 5.1 mm to 5.9 mm, which improved the wettability and spreadability of the molten filler wire on the substrate. In the case of the uncoated steel, the fusion zone/steel interfacial microstructure consisted of laminated Al7·2Fe1·8Si and Fe(Al,Si)3, while it changed to a composite-like structure containing a soft HEA skeleton and hard IMCs which included Al7·2Fe1·8Si, Al3Ni, and (Al,Si)2Cr. In addition, due to the sluggish diffusion effect of HEAs, a layer of gradient nanocrystalline composed of Al7·2Fe1·8Si was generated, which significantly strengthened the dissimilar laser joints with improvements in both the fracture load (∼26.5%) and the displacement (∼101.8%). The fracture mode changed from brittle to ductile failure when the porous HEA coating was applied, with fracture propagating through the HEA skeleton. This work provides a novel solution for the strengthening of hard-to-join dissimilar combinations.