The performance of thermally sprayed titanium based composite coatings in molten aluminium

The service life of die and tool steels in contact with molten aluminium is important in the aluminium processing industry. This article describes an investigation of the efficiency of thermally sprayed Ti(Al,O)/Al2O3 and TiAl(O)/Al2O3 composite coatings in resisting aluminium soldering on H13 tool steel. The powder feedstocks for making the composite coatings were produced by high energy mechanical milling of a mixture of Al and TiO2 powders using different molar ratios followed by a thermal reaction process. The feedstocks were then thermally sprayed using a high velocity oxygen fuel (HVOF) technique onto an H13 steel substrate to produce composite coatings. The performance of the coatings was assessed in terms of Al soldering and liquid metal corrosion resistance after immersing in molten aluminium at a temperature of 700 ± 10 °C. The results of this study showed that composite coatings have a potential to act as a physical barrier to prevent aluminium attack on H13 tool steel. The nature of Al attack on H13 tool steel changed from generalised to more localised attack with the thermally sprayed coating. A Ti(Al,O)/Al2O3 composite coating was not attacked by molten aluminium, however localised attack on a H13 tool steel substrate was found to be caused by molten metal penetration through defects in the coating. Composite coating TiAl(O)/Al2O3 was attacked by molten aluminium resulting in dissolution of the coating. Composite coatings showed significantly lower dissolution in molten aluminium compared with the uncoated H13 steel. A Ti(Al,O)/Al2O3 composite coating was found to be a more effective protective coating than a TiAl(O)/Al2O3 composite coating due its greater resistance to molten aluminium attack.

Research highlights
► Self propagating combustion was used to produce two composite feedstock powders.
► Thermal spraying produced dense metal ceramic composite coatings on a H13 die steel.
► These coatings were resistant to attack from molten aluminium.
► A Ti and alumina coating prevents Al soldering to the die steel.
► A TiAl and alumina coating is not as good as the former.