A Raman and Multinuclear 29Si, 27Al, and 19F NMR Study on the Structural Roles of CaF2 in SiO2–CaO–Al2O3-based Welding Fluxes

The efficacy and efficiency of submerged arc welding (SAW) are largely dictated by the physicochemical properties of the employed welding fluxes, including melting point, viscosity, and activity, etc., which are inherently rooted in the microstructure of the fluxes. SiO2–CaO–Al2O3-based fluxes are widely employed for welding high strength low alloy (HSLA) steels. In the present study, the structural roles of CaF2 on SiO2–CaO–Al2O3-based welding fluxes have been systematically investigated through Raman and magic angle spinning-nuclear magnetic resonance (MAS-NMR) techniques. The results showed that originally intact flux structures depolymerized upon increasing content of CaF2. Raman and 29Si NMR results demonstrated that Q2 (Qn, n is the number of bridging oxygens (O0) in one [SiO4]-tetrahedron) increased at the cost of Q3(1Al) (Qn(mAl), m means the number of neighboring aluminate groups), which could be well explained by the interruption of the Si-O-Al linkages. Moreover, as confirmed by 27Al and 19F NMR results, the proportion of AlF6 species increased from 9.9 to 34.2 pct, indicating the substitution of Al–F for Al–O bonds and the depolymerization of the flux structure. The unique behavior of F may offer a theoretical basis for fine-tuning structural unit mobility and element transfer behaviors during the actual welding process.