Analysis of transition metal (V, Zr) borides formation in aluminium melt

Smelter grade aluminium can be used to replace costly copper for electrical conductor applications, if the concentration of impurities (in particular transition metals V, Ti, Zr) is controlled efficiently. Industrially, boron treatment (addition of Al-B alloys) has been used to remove V, Ti and Zr impurities by converting them into boride particles in the aluminium melt. The detailed mechanisms of reactions between boron and these transition metals are not fully understood. The current study focuses on the analysis of V and Zr-boride formation in Al melts. Thermodynamic analysis of V-Zr-B systems at temperature range 675 to 950 °C has been carried out. Equilibrium analyses suggest that when boron is introduced into Al melts, it preferentially combines with Zr to form ZrB2, and with increasing levels, will associate with V to form VB2. Experimental work has been performed at the laboratory scale, supported by optical microscopy, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analyses to study the morphology and chemical composition of the boride phases. The microscopy revealed that the V and Zr from the melt formed a ring of borides around AlB12 particles (which were present in the initial Al-B master alloys). It was further observed that the dissolution rate of AlB12 was very sluggish even after one hour at 750 °C. The results showed that the process of the boride transformation is complex, especially when multiple transition metal elements are present.