Formation of Non-metallic Inclusions in Castings

Sources of inclusions and gases in molten metal mainly include several aspects: residues and gases from thermal decomposition and vaporization of patterns, slag and gases generated during molten metal smelting, oxide slag formed by oxidation of molten metal, dissolution of specific gases by high-temperature molten metal, and external impurities entering the sealed mold cavity.

Among these slags and gases, slag produced in molten metal smelting accounts for the largest proportion, followed by slag generated from reactions between dissolved gases in high-temperature molten metal and the metal itself, as well as oxides formed by molten metal oxidation. In fact, residues from pattern vaporization and thermal decomposition occupy a relatively small proportion. Nevertheless, poor sealing of pattern cavities allows external impurities to invade, which may induce severe massive inclusion defects.

These slags, gases and externally mixed solid impurities feature low density. They slowly float upward during molten metal filling and cooling before solidification, and drift toward areas with lower lateral pressure under negative pressure. If such slags and gases escape from castings while the metal remains liquid, neither inclusion nor blowhole defects will occur.

In lost foam casting enterprises, negative pressure chambers of sand box negative pressure systems are mostly arranged at the bottom and surrounding walls of sand boxes. The direction of negative pressure extraction is perpendicular to the natural upward floating direction of slags and gases, which hinders the upward floating and discharge of impurities and gases.

Thermodynamic and kinetic factors governing the formation and removal of inclusions in molten metal facilitate the occurrence of inclusion defects in lost foam castings, while impeding their elimination. Due to diverse raw and auxiliary materials adopted by domestic foundries, and under pressure of continuous cost reduction, the purchased scrap steel and pig iron contain complicated compositions and abundant impurities.

Most foundries melt scrap steel in medium-frequency induction furnaces for casting production, resulting in high impurity element content in molten metal. Meanwhile, pattern materials used by domestic lost foam casting enterprises are mostly polystyrene foam for construction and packaging. Complex physical and chemical reactions between molten metal filling and pattern materials produce abundant pyrolysis products, further increasing inclusions in molten metal.