Thermodynamic Role of SiO2 in Controlling Weld Metal Composition During Submerged Arc Welding: A Multi-Zone Modeling Approach

The thermodynamic effect of SiO2 on the composition of the weld metal in submerged arc welding is analyzed by employing the basicity index model, the slag–metal model, and a multi-zone thermodynamic framework. A CaF2-SiO2 binary flux system is employed to isolate the intrinsic effect of SiO2. The results show that the basicity index model captures the overall decrease in weld metal O content with increasing flux basicity index but fails to resolve variations in the high-basicity region. The slag–metal equilibrium model provides a thermodynamic description of interfacial reactions yet remains limited to the weld pool zone. In contrast, the multi-zone model incorporates reactions in the droplet and weld pool zones, revealing pronounced O enrichment in the droplet due to flux decomposition and arc–plasma interactions, followed by redistribution under gas–slag–metal equilibrium. By accounting for droplet-stage evaporation and cross-zone interactions, the multi-zone model improves the predictive accuracy of Si and Mn contents and explicitly captures their cross-zone transfer behavior compared with conventional prediction approaches.