Study of Titanium based Composite Coatings for Resistance against Molten Aluminium Soldering on H13 Tool Steel
The service life of industrial components is limited predominantly by chemical corrosion, mechanical failure or mechanical wear. In the aluminium high pressure die casting industry, liquid aluminium is extremely reactive with the constituents of H13 die steel and has a tendency to form intermetallic layers. This chemical interaction results in sticking of molten metal to the die surface which produces defective castings and also damages the die surface. The use of thermal spray coatings provides protection to the surfaces operating in severe environments. An HVOF thermally sprayed coating has the advantage of having excellent bond strength and very low porosity levels (< 1%). This research work is concerned with producing and evaluating the performance of titanium/alumina based composite coatings to improve the service life of tool steel (H13) used for dies in aluminium high pressure die casting and dummy blocks used in Al extrusion. In this research work, the powder feedstocks for making the composite coatings were produced by high energy mechanical milling of a mixture of Al and TiO₂ powders in two different molar ratios followed by a thermal reaction process. The feedstock powder was then thermally sprayed using a high velocity oxygen fuel (HVOF) technique on H13 steel substrates to produce Ti(Al,O)/Al₂O₃ and TiAl/Al₂O₃ composite coatings. The performance of the coatings was assessed in terms of Al soldering, liquid metal corrosion resistance, thermal shock resistance and wear resistance. In an immersion test, the coated specimens were dipped into molten Al at a temperature of 700 ± 10 °C for different intervals of time. The performance of the coatings was tested in terms of liquid metal corrosion resistance and propensity to Al soldering. The dissolution behaviour of the coatings was evaluated by measuring weight loss after dipping the samples in to molten aluminium. The immersion test results showed that the coated samples have relatively few locations where aluminium soldering (reactive/chemical) occurred, however, an H13 steel surface showed more tendency for aluminium soldering. It was found that composite coatings changed the molten Al attack on H13 tool steel from a generalized to a localized one. No reaction between molten aluminium and a Ti(Al,O)/Al₂O₃ composite coating was identified. The TiAl/Al2O₃ composite coating was found to be attacked by molten aluminium as a result of a reaction between the coating and molten aluminium. The metallic phase TiAl in the composite coating is believed to be attacked by the molten Al. A Ti(Al,O)/Al₂O₃ composite coating was found to be a better protective coating than the TiAl/Al₂O₃ composite coating due its stability against molten aluminium attack. The thermal shock behaviour of the composite coatings was investigated by subjecting the coated coupons to a number of cycles, each cycle consisting of a holding time of 30 seconds in molten aluminium at 700 ± 10 °C followed by quenching into water. The surfaces of the coupons were examined for Al soldering and an evaluation of surface spallation. Any cracks found in the coatings were studied to explain their thermal shock behaviour. A Ti(Al,O)/Al₂O₃ composite coating on H13 tool steel produced from a fine feedstock has better thermal shock resistance than the Ti(Al,O)/Al₂O₃ TiAl/Al₂O₃ composite coatings produced from the agglomerated feedstocks. The study also describes and compares the tribological properties such as friction and sliding wear rate of the composite coatings both at room and high temperature (700.°C) under dry and lubricating conditions. The wear resistance of the coatings was investigated by a tribometer using a spherical ended alumina, flat ended high speed steel and spherical ended hardened steel pins as counter bodies. The experimental results show that the composite coatings look promising for high temperature applications due to their low wear rate at high temperature. However room temperature applications of the composite coatings can be improved under lubricated conditions. Successful trials of a Ti(Al,O)/Al₂O₃ composite coated dummy block revealed that the coating has potential as an industrial coating.
mechanical milling, thermal spraying, HVOF, composite coatings, soldering, H13 tool steel
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