Study on Fe reduction in AZ91 melt by B2O3

This article studies the effects of B2O3 on the reduction of Fe content in scrap AZ91. The Fe concentration in a scrap can be decreased to less than 0.001 wt.% when the scrap melt is refined by a B2O3-containing flux. Experimental results suggest that the best effect can be got when the B2O3 addition is 0.61 wt.% of the melt. Optimal process temperature and holding time are 735 °C and 2400 s, respectively. Microstructures of the B2O3-processed samples were investigated by OM, SEM, EDS. Immersion corrosion tests were conducted to study the corrosion resistance. Homogeneous precipitations of a kind of Mg–Al–C–O particles were observed in the refined samples. These particles can refine grains and in turn improve the mechanical properties of the alloy. B2O3 processing reduced the immersion corrosion rate from 15 to 0.3 mg cm−2 per day. The mechanism of Fe reduction by B2O3 processing is analyzed thermodynamically. Formation of FeB and settling of it into melting sludge are suggested to be the primary mechanism of Fe reduction by B2O3 processing.