Thermal-spray deposition of enamel on aluminium alloys

Traditional enamelling requires high temperatures, which soften high-performance aluminium substrates and limit their application in engineering. Thermal spraying can, in principle, overcome this problem by enabling low substrate temperatures during deposition but it was found that its rapid cooling conditions produced enamel with high residual stress and poor adhesion. The research shows that thermal-spray enamelling of aluminium can be successfully achieved by introducing a preheating step prior to spraying, so that the enamel can flow for long enough to relax the residual stress. In this study, preheating was undertaken by flame scanning as a means of raising the enamel above its glass transition temperature and allowing sufficient flow. Furthermore, it was found that in the Al-Mg alloy substrate under investigation, the magnesium segregated to the substrate-enamel interface and this also adversely affected adhesion. The preheating temperature therefore needs to be optimized to give adherence but avoid excessive substrate softening and magnesium segregation. The results showed that optimally flame-sprayed enamel successfully increased the wear resistance of the Al-Mg alloy by a factor of ten. The thermal-spray enamelling of generic aluminium alloys is discussed.