Features: Inclusions are particles, whose composition is different from that of the casting, formed on the surfaces of the die casting part
or inside the casting due to the coating materials, molding materials and refractory materials getting into the alloy melt.
Analysis of causes: First, other materials is mixed with alloy melt and are poured into the casting mold. Second, the refining effect is bad. Third, the exotic items or molding materials from the cavity surface of casting mold peel off.
Prevention methods: First, carefully refine and slag off. Second, the coating layer of the smelting tool should be firmly adhered. Third, the casting system and the cavity should be cleaned. Fourth, the furnace burden should be kept clean. Fifth, the surface inclusions can be removed by grinding, and welding repairing treatment can be used when necessary.
5. Slag Inclusions
Features: First, the flocculent oxide inclusions exist inside the casting, and the fracture surfaces are yellow or off-white. Second, the flux inclusions are dun dots, and they become holes with smooth surfaces after slag inclusions are removed. Sometimes the corrosion characteristics appear after the oxide inclusions are exposed to air for a period of time.
Analysis of causes: First, the slags are not completely removed after refining modification. Second, there is not enough time for standing after refining modification. Third, the casting system is not reasonable, and the secondary oxide skin is mixed into alloy melt. Fourth, the alloy melt is stirred or polluted after refining.
Prevention methods: First, strictly follow the process requirement of refining modification casting. Second, the metal liquid should be steadily poured into the casting mold in the pouring process. Third, the furnace burden should be kept clean, and the foundry returns treatment as well as application amount should strictly follow the technological procedure specification.
Features: First, cracks are straight lines or irregular curves. Second, the fracture surfaces of cracks have oxidation characteristics without metallic luster, which often occur inside the sharp corner of hot spots, the intersection of thick and thin fracture surfaces together with lose structure. Third, the fractured metal surfaces are clean.
Analysis of causes: First, each part of castings is not evenly cooled. Second, the castings can not freely shrink due to the external obstruction during the process of solidification and cooling, and cracks appear since the internal stress exceeds the alloy's intensity.
Prevention methods: First, try to maintain the sequence or solidify the alloy melt simultaneously to reduce internal stress. Second, refine the alloy structure. Third, choose the proper pouring temperature. Fourth, increase the deformability of molds.