Mechanism of ZrB₂ 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 (AlB₂/AlB₁₂) master alloys are used for this process. Literature review showed that ZrB₂ are more stable compared to VB₂ and AlB₂ in the temperatures ranging from 948 K to 1173 K (675 °C to 900 °C). As a result, ZrB₂ will form preferentially in molten aluminum during boron treatment process. In this study, a systematic investigation on formation of the ZrB₂ and VB₂ borides in Al-V-Zr-B alloys was carried out at 1023 K (750 °C). Experiments showed that the mechanism of ZrB₂ and VB₂ 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 Al₃Zr, AlB₂/AlB₁₂ in the molten aluminum. Scanning electron microscopy, Energy-dispersive X-ray, and electron backscattered diffraction analyses revealed the preferential formation of ZrB₂ in the Al-Zr-V-B alloys at 1023 K (750 °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 _m) for Zr was 1.15 × 10⁻³ m/s which is within the range of those values reported in the literature.