Formation of Fe-based glassy matrix composite coatings by laser processing

Abstract

Coatings of a Fe43.2Co28.8B19.2Nb4Si4.8 alloy on AISI 1020 steel substrates were produced by laser cladding. By properly selecting the processing parameters adherent tracks with negligible dilution could be obtained. For the processing conditions used the clad material presents a graded structure consisting of 5 layers with different microstructures, resulting from the prevalent material solidification path. Solidification starts by epitaxial growth of Fe–Co δ-ferrite on the δ-ferrite resulting from heating of the substrate near the fusion line into the δ-ferrite stability temperature range. This first stage of solidification leads to the formation of consecutive layers consisting predominantly of δ-ferrite and formed by plane front, cellular and columnar dendritic solidification. The solidification of Fe–Co δ-ferrite leads to the segregation of B and Nb first frontally, to the bulk of the liquid, then to the interdendritic regions, resulting in the precipitation of a boride containing eutectic. Initially, the eutectic precipitates in the interdendritic regions, resulting in a layer of material comprised of δ-ferrite dendrites and interdendritic eutectic, then in a layer of material where the eutectic is the bulk alloy constituent. Eventually, a layer of amorphous material with homogeneously dispersed dendrites of Fe–Co δ-ferrite forms in most of the coating thickness. These embedded dendrites form by equiaxed solidification within the supercooled liquid at the trailing edge of the melt pool, from δ-ferrite dendrite fragments and boride particles carried from the growing columnar layer into the liquid bulk by Marangoni convection, driven by the temperature gradients existing within the melt pool. They play a critical role in defining the excellent properties of the coating material: a very high hardness of 1040±16 HV0.5 associated to a reasonable ductility, allowing the formation of crack-free coatings, in contrast with the bulk alloy prepared by spray forming, either in the as-prepared condition or after laser melting.