Flame-sprayed MgAl2O4- and calcium aluminate-based coatings for application in the aluminium industry

Abstract

The aluminium industry is a major consumer of refractories with increasing wear by fluxes, the use of oxy-fuel burners, higher furnace temperatures, higher amounts of alloying Mg and recycled Al metal as well as a requirement for higher efficiencies. Today, most furnaces are lined with aluminosilicate-based materials. The reactions between the aluminosilicate and the metal lead to the formation of a mixture of Si, Al, and Al2O3 with unreacted refractory material, which has to be removed as they negatively affect the thermal properties of the refractories and the efficiency of the furnace. To reduce the penetration of Al into the refractory material, non-wetting additives such as barium sulphate, different types of fluorides, and CaO were used. The aim of this study was the investigation of different calcium aluminate-based coatings to reduce the wettability of a metallic substrate by molten aluminium. Feedstock composed of calcium magnesium aluminate CMA, calcium di-aluminate CA2, and CA/CA2 with and without TiO2 were characterised and processed via powder and flame spraying to thin coatings on metal substrates. The spraying parameters have been optimized and the phase composition, roughness, macro- and microstructure, porosity, and bending strength of the coatings have been analysed. Additionally, a finger test, as well as a casting test, have been performed by pouring 1.5 kg AlSi7Mg alloy at 800 °C over coated half-pipes using pure alumina as reference coating. The amount of residual Al sticking on the substrates after the casting test indicates the degree of wetting of the coatings. The thermally sprayed CMA and CA2 powder feedstock showed the lowest degree of interaction with the aluminium melt.