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 O-2(-). 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.