Bond Characteristic-Dependent Viscosity Variations in CaF₂-SiO₂-Al₂O₃-MgO Welding Fluxes

Viscosity is critical in welding fluxes as it could affect weldability and alloying element transfer behaviors during submerged arc welding. It is widely accepted that viscosity is intrinsic to bonding structures. However, it is unclear how such structures could modify viscosity in commercial CaF2-SiO2-Al2O3-MgO fluxes. In this study, structure-viscosity correlations have been revealed by employing molecular dynamics calculations coupled with viscosity measurements. The results show that viscosity at 1300°C reaches maximum (1.99 Pa·s) and minimum values (0.10 Pa·s) at the highest SiO2 and MgO levels, respectively. Furthermore, bond stability and distribution could indicate the structural degree of polymerization. Specifically, the proportion of stabilized bridging oxygen and bonded F (Si-F and Al-F) correlates positively with the activation energy of viscous flow. Due to the dilution effect of free F and the depolymerization effect of bonded F, the activation energy of viscous flow in the targeted system (60–130 kJ/mol) is significantly lower than in fluorine-free ones (160–240 kJ/mol). The current investigation establishes a relationship between bond stability and viscous flow activation energy, demonstrating a stronger correlation than considering O-related structural parameters alone.