On the performance of commercial coatings for die casting

Soldering of core pins is one of the major issues limiting the productivity of aluminum castings using high pressure die casting process. This issue is associated with the failure of the ceramic coating, chemical reactions between the molten aluminum and the matrix metal and the dissolution of the pin matrix metal. Methods that prevent coating failure, retard dissolution rates of the core pin materials in molten aluminum can be used for reducing soldering tendency of core pins. As a result, a ceramic coating is usually applied to the core pins in order to reduce soldering tendency, because the coating prevents severe reactions between the core pin material and the molten alloy. Therefore, the coating lifetime governs the soldering process and the core pin lifetime. The purpose of this study is 1) the mechanism of soldering formation, 2) the performance of coatings when contacting with molten alloys, 3) the in-depth analysis of the starting up coating failure, and 4) an ultrasonic method that is used for accelerating testing processes. Results show that the soldering process starts from a local coating failure, involves a series of intermetallic phase formations from reactions between molten aluminum alloys and the H13 steel pin, and accelerates when the aluminum-rich phase is formed between the intermetallic phases. It is the formation of the aluminum-rich phase in the reaction region that joins the core pin with the casting, resulting in the sticking of the casting to the core pin. When undercuts are formed on the core pin, the ejection of castings from the die will either lead to a core pin failure or damages to the casting be ejected. High intensity ultrasonic vibration was applied on H13 coupons to accelerate the experiments. Results obtained using the accelerated testing method were then validated using in a high pressure die casting machine under production conditions. These results indicate that coating detachment from the steel substrate is the main failure mechanism of the coatings when tested in water, oil, molten salt, or molten zinc at low temperatures. Coating failure accelerates as the temperature of the melt is increased. At high temperatures, coating fragmentation, as well as detachments, occurs on coupons tested in molten salt. Therefore, the coverage percentage and quality of coatings play an important role in retarding coating failure. As soon as coating failure occurs, the dissolution rate of H13 steel is extremely high. The affinity, between the core pin material and the molten alloy, plays an important role in the later soldering process. The dissolution rate of H13 steel in molten zinc is much higher than that in molten A380 at the same experiment conditions. The dissolution rate of niobium in molten aluminum is at least 300 times slower than that of H13 steels. It is suggested that Nb alloys have good potential to be used as soldering resistant core pin materials