Mechanism of ZrB sub(2) Formation in Molten Al-V-Zr Alloy During Boron Treatment

Smelter-grade aluminum cannot be used for electrical conductor applications without the removal of transition metal impurities including zirconium (Zr) and vanadium (V). These impurities are removed by treating molten aluminum with boron bearing substances in the casthouse. Such procedures are called boron treatment. Transition metal impurities form their borides and settle at bottom of the furnace. Industrially, Al-B (AlB sub(2)/AlB sub(12)) master alloys are used for this process. Literature review showed that ZrB sub(2) are more stable compared to VB sub(2) and AlB sub(2) in the temperatures ranging from 948 K to 1173 K (675 degree C to 900 degree C). As a result, ZrB sub(2) will form preferentially in molten aluminum during boron treatment process. In this study, a systematic investigation on formation of the ZrB sub(2) and VB sub(2) borides in Al-V-Zr-B alloys was carried out at 1023 K (750 degree C). Experiments showed that the mechanism of ZrB sub(2) and VB sub(2) borides formation in molten aluminum is complex. It involves the possible phenomenon of chemical reactions, mass transfer in liquid phase, diffusion through borides layers, and the dissolution of Al sub(3)Zr, AlB sub(2)/AlB sub(12) in the molten aluminum. Scanning electron microscopy, Energy-dispersive X-ray, and electron backscattered diffraction analyses revealed the preferential formation of ZrB sub(2) in the Al-Zr-V-B alloys at 1023 K (750 degree C). Moreover, ICP-AES analysis showed the higher rate of Zr removal compared to V from Al-Zr-V-B alloys. Overall it was suggested that the kinetics of Zr removal was controlled by the mass transfer of Zr through liquid phase. The calculated mass transfer coefficient (k sub(m)) for Zr was 1.15 10 super(-3) m/s which is within the range of those values reported in the literature.