Crystallization Products and Structural Characterization of CaO-SiO(2)-Based Mold Fluxes with Varying Al(2)O(3)/SiO(2) Ratios

During the casting of high aluminum steel, the dramatic increase in the Al(2)O(3)/SiO(2) ratio is inevitable, resulting in significant changes of the crystallization behavior, which would result in heat transfer and lubrication problems. Crystallization products and structure characterization of glassy CaO-SiO(2)-based mold fluxes with different Al(2)O(3)/SiO(2) ratios were experimentally investigated using a differential scanning calorimetry technique and Raman spectroscopy. With increasing Al(2)O(3)/SiO(2) ratios, the following results were obtained. The crystallization temperature and the crystallization products are changed. With increasing Al(2)O(3)/SiO(2) ratios from 0.088 to 0.151, the crystallization temperature first increases greatly from 1152 °C to 1354 °C, and then moderately increases. The crystallization ability of the mold flux is strengthened. The species of the precipitated crystalline phase change from two kinds, i.e., Ca(4)Si(2)O(7)F(2) and Ca(2)SiO(4), to four kinds, i.e., Ca(4)Si(2)O(7)F(2), Ca(2)SiO(4), 2CaO·Al(2)O(3)·SiO(2) and Ca(12)Al(14)O(32)F(2), the crystallization ability of Ca(4)Si(2)O(7)F(2) is gradually attenuated, but other species show the opposite trend. The results of Raman spectroscopy indicate that Al(3+) mainly acts as a network former by the information of [AlO(4)]-tetrahedral structural units, which can connect with [SiO(4)]-tetrahedral by the formation of new bridge oxygen of Al–O–Si linkage, but there is no formation of Al–O–Al linkage. The linkage of Al–O–Si increases and that of Si–O–Si decreases. The polymerization degree of the network and the average number of bridging oxygens decrease. Further, the relatively strong Si–O–Si linkage gradually decreases and the relatively weak Al–O–Si gradually increases. The change of the crystalline phase was interpreted from the phase diagram and structure.