Simulace proudění kovu ve slévárenské formě z hlediska jeho reoxidace

Abstract

The occurrence of reoxidation is a key aspect of the control of casting quality. The metal entering the mould cavity results in thermal degradation of binders, water and additives. Large quantities of gases and vapours are produced. Air oxygen, mould atmosphere or oxides in the moulding sand may cause secondary oxidation of the metal flow. Imperfect filling of castings induces blending of metal flow with oxygen present in gases and vapours. This might lead to reoxidation of elements in sequence of their affinity to oxygen. The focus of this dissertation is the study of reoxidation processes during casting into green-sand moulds with the utilization of computer simulation, analysis of casting defects and monitoring of changes in oxygen activity during filling of a mould. Reoxidation processes are a frequent cause of foundry defects. They may cause cavities (blowholes, pinholes), macroinclusions and further they negatively affect the morphology of oxides and sulphides discharged as microinclusions. The theoretical part summarizes the nature of reoxidation processes, theoretical foundations of molten metal flow and the application possibility of computer simulation when modelling foundry processes. The practical part contains the results of computer simulation, laboratory tests and an analysis of foundry defects, which occurred during the tests. Laboratory experiments involved continuous measuring of oxygen activity during filling of a mould and metal solidification. Further, a comparative experiment was carried out in which, under otherwise identical conditions (identical composition of cast metal, identical casting temperature), the moisture of moulding sand, the content of aluminium and the manner in which the mould was filled changed. The present dissertation summarizes findings obtained in the study of reoxidation processes and evaluates the influence of dominant factors on the occurrence of defects (pinholes) in castings from GJV.