Effect of defocusing distance on interfacial reaction and mechanical properties of dissimilar laser Al/steel joints with a porous high entropy alloy coating

Aluminum alloy and low carbon steel were successfully laser weld-brazed when using a porous high entropy alloy (HEA) transient layer. It is shown that the joint wettability increased with the decreasing defocusing distance and the best wettability was obtained at 0 mm defocusing distance with the contact angle of 59.8° and wetting length of 5.57 mm. A fine and dense layer of τ5-(Fe,Co,Cr,Ni)1.8Al7.2Si phase was observed wrapping around the HEA spheres, and its thickness changed in a pronounced way with the defocusing distance, where a maximum thickness of 3.20 μm was obtained at 0 mm. A non-coherent interface and a composite-like structure was formed between the τ5 phase and the HEA spheres. However, the τ5 phase, consisting of a large number of nano-grains, aided in realizing the residual stresses at the joint interface through dislocation slip, leading to an enhanced interfacial bonding and joints failure near the fusion zone. An outstanding fracture load and displacement were achieved, with 333.9 N/mm and 4.07 mm respectively, at the optimized condition of 0 mm defocusing distance. And the mechanism of diffusion and metallurgical evolution was also revealed by thermodynamic calculations and diffusion kinetic analysis.