Rheological behavior of the CaO-Al2O3-based mold fluxes with different Na2O contents

The CaO-Al₂O₃-based mold flux is expected during the casting of aluminum containing advanced high strength steels. In this study, the rheological behavior of the CaO-Al₂O₃-based mold fluxes with different Na₂O contents was investigated. The results show that the viscosity in the range of 1423–1573 K reduced sharply when the Na₂O content increased from 0 wt % to 10 wt %, but the tendency slowed down with further increasing Na₂O from 10 wt % to 15 wt %. The activation energy for viscous flow also decreased from 237.76 ± 4.88 kJ/mol to 180.37 ± 7.10 kJ/mol with increasing Na₂O. The structure analyses show that the melt networks were mainly constructed by SiO₄]^4--tetrahedral, AlO₄]⁵-tetrahedral and Si-O-Al structural units. These networks were depolymerized with the addition of Na₂O since the charge compensation effect from Na⁺ was relativity weaker comparing with the network breaking effect from O²⁻. In addition, the break temperature of the mold fluxes also decreased from 1406 K to 1198 K due to the more precipitation of low melting point Ca₂Al₂SiO₇, rather than MgAl₂O₄ in the mold flux, during the cooling cycle.