Na, Sr and Sb interactions in Al-Si alloy melts and their effects on modification

Na, Sr and Sb are the elements commercially used to modify the silicon phase in Al-Si foundry alloys. Interactive effects which occur during modification resulting from the mixing of modifiers in alloy melts are studied from many different aspects to identify the conditions for the occurrence of negative effects on modification, to study the modifier interaction mechanisms, and to devise ways to eliminate negative effects.Metallographic studies have shown that antimony exerts deleterious effects on sodium or strontium modification of both A356 and A413 alloys. The deleterious effect of antimony has been quantified for various levels of sodium and strontium. The effect of antimony is more severe at higher antimony concentrations in the melt. No negative effects on modification occur in the combined presence of sodium and strontium.Modifier interaction mechanisms were investigated by means of electron beam analysis, and the results were analyzed from the viewpoint of thermodynamics and crystallization. Certain antimony compounds are found in antimony treated Al-Si alloys. These antimony compounds play an important role in modifier interactions. Many interaction mechanisms are possible between antimony and strontium. The interactions may result in the formation of ternary or binary compounds. Duplex particles often form by the heterogeneous nucleation of Al$ sb4$Sr on the substrate of antimony compounds. In the study of sodium and antimony interaction mechanisms, similar duplex particles are also found. Other products from sodium and antimony interactions are a sodium-antimony binary compound and its oxide. When all three modifying elements are mixed together, all of the above interaction mechanisms are possible. Al$ sb4$Sr can heterogeneously nucleate on Na$ sb3$Sb forming an inclusion containing all three of the modifying elements.Negative effects arising from antimony can be eliminated by using a suitable excess of sodium or strontium. Orthogonal tests show that a combination of sodium and strontium is preferable to overcome the influence of antimony on modification. Mechanical property tests have shown that the influence on mechanical properties from the interactions of antimony with other modifiers can be overcome. Improved mechanical behaviour is guaranteed under relatively long periods of melt holding when using the optimum combination of sodium and strontium to treat antimony containing melts