Solidification behavior and morphological evolution in laser surface forming of AlCoCrCuFeNi multi-layer high-entropy alloy coatings on AZ91D

The AlCoCrCuFeNi high-entropy alloy (HEA) coatings after one, two and three layers were laser cladded on the AZ91D substrate. Towards the coating interface, the substrate is composed of Mg dendrites and interdendritic Mg + Q eutectics. The formed coatings can be categorized into a low narrow composite region and a top thick dense HEA region. The dominated microstructures in the dense HEA region are columnar dendrites. Nevertheless, the average primary spacing of columnar dendrites became smaller when the coating grew thicker mainly due to the temperature gradient decreasing slowly. Moreover, the planar morphology at re-melted boundary of each clad layer is absence because of the local conditions being suitable for epitaxial growth. Additionally, it is found that the columnar to equiaxed transition (CET) had occurred in the top area of the three-layer HEA coating. This evolution of the microstructure can be explained by the CET theory combined with the sluggish diffusion kinetics and the abnormal variation of the thermal conductivity with the temperature of the AlCoCrCuFeNi HEA. Additionally, the problem associated with dilution by Mg can be completely suppressed by the HEA coating after three layers.