Ultrasound assisted low-temperature synthesis of TiB2 and Al3Ti particulates in molten aluminum

In situ formed TiB2 and Al3Ti are two typical representatives of ceramic and intermetallic reinforcements in the in situ particulate reinforced Al composites. TiB2 particulates can be synthesized in molten Al via the mixed-salts reaction by adding mixed K2TiF6 and KBF4 salts into the Al melt at high temperatures. Al3 Ti particulates can be produced by the direct-melt reaction between solid Ti powders and liquid Al at high temperatures. Generally, a high reaction temperature is always needed to obtain both reinforcements. Some issues, however, such as high cost and burning loss of alloying elements in Al alloys, are usually associated with high manufacturing temperatures. Specifically, a higher temperature can lead to the formation of larger-sized particulates, which severely degrades the mechanical properties of composite materials. Therefore, exploring the low-temperature synthesis of TiB2 and Al3Ti particulates is meaningful for practical productions. Also, the formation mechanisms for both particulates are still unclear, especially at lower synthesizing temperatures. This research is developed the following studies based on the above topics. The influences of reaction temperature and time on the mixed-salts reaction were studied. An ultrasound assisted technique was applied to the reaction at 700°C. Results show that a higher temperature can lead to a higher yield of TiB2 with a shorter reaction time, but the size of TiB 2 produced becomes larger. With a 10-min reaction time, the yield of TiB2 can reach 89.50 % and the size of most of TiB2 is in the range of 300-800 nm at 900 °C; Most TiB2 synthesized at 700 °C are smaller than 300 nm, but the yield is just 28.10 %. By using ultrasound, a high yield (90.40 %) of TiB2 particulates with smaller size (smaller than 300 nm) can be obtained at 700 °C. The formation mechanism of TiB2 was studied through a static experiment. At a higher temperature (900 °C), the synthesis of TiB2 mainly follows the precipitation-growth process at the reaction interface. At a lower temperature (700 °C), the precipitation-growth process and dissolution reaction between AlB2 and Al 3Ti both contribute to the formation of TiB2. As the reaction time is prolonged, TiB2 particulates with a smaller size can be formed. Ultrasound has remarkable effects on the formation of TiB2 particulates, which can lead to accelerated mass transfers of [Ti] and [B] from salts to reaction interface and a high nucleation rate of TiB2. These two effects contribute to the low-temperature synthesis of TiB2. For the direct-melt reaction, a reaction-peeling model is proposed to explain the formation of small blocky Al3Ti particulates. Ultrasound is able to effectively accelerate the reaction-peeling process. The reaction time for a completed synthesis of Al3Ti can be shortened significantly. Most importantly, the formation of inclusions containing solid Ti powders can be avoided in the ultrasonic fields, allowing for the realization of a lower-temperature synthesis of Al3Ti at 700 °C