Study Of Ca Removal In Al-Si Piston Alloys

Al-Si alloys are widely used for power-transmission parts such as the pistons in engines. In order to improve the mechanical properties, it is necessary to modification, and P modification is widely used in Al-Si alloys in which A1P particles act as the nuclei of primary Si. However, Ca impurity will poison the P modification efficiency in Al-Si piston alloy. Moreover, it makes the flowability of Al-Si melt worse, and increase inspiratory and shrinkage porosity. The generation of segregation brittle compounds increases the rejection ratio of castings. So solving the Ca removal problems in Al-Si alloys is a meaningful work.The rich-Ca compounds in near eutectic Al-12Si alloys were systematically studied using optical micrograph (OM), electron-probe micro-analyzer (EPMA), differential scanning calorimeter (DSC) and Scanning electron micro-analyzer (SEM) etc. in this work. By discussing the interaction of F and B with Ca in A1-Si melt, two new methods different from traditional addition of [C1] are put forward. The major research efforts of the present study are as follows:(1) Reaction of rich-Ca compounds with P in near eutectic Al-Si alloysIt is important to study the rich-Ca compounds and its formation rules in A1-Si alloys for Ca removal. The study shows that when the Ca containing is high, part of Ca modifies eutectic Si from flake-like to coral-like, the other act with A1 and Si forming CaA12Si2 compounds. There is no particular influence of addition of P and heat treatment on the bloke CaA12Si2 compounds, element P mainly solutionized in CaAl2Si2 compounds. There is great influence of Ca on the P modification in Al-Si alloy. Ca reacted with P and Si, which supresses the formation of A1P, makes P modification inefficiency. Extension of holding time can reduce the Ca containin in Al-Si alloy, addition of Carbon can also recuce the Ca containin in Al-Si alloys, but there is no particular influence of re-melt on the Ca removal in Al-Si alloys.(2) Study of Ca removal by B in Al-Si alloysB can improve the P modification efficiency in hypereutectic A390 alloy. The primary Si can be refined to less than 40μm in A390 alloy with combined B and P addition. The addition of B can also improve the mechanical properties of Al-Si alloys. The strength of A390 alloy with combined B and P addition can reach to 289.37 MPa, 11% higher than that with P addition.B can recover the P modification efficiency in Ca "poisoned" Al-Si alloys, can promote the primary Si precipitated again. The addition of B can decrease the Ca containin in Al-Si alloy, but there is no particular influence on the P containing. There are two reasons for why B can recover the P modification efficiency in Al-Si alloy. O n the one hand, B reacts with Ca and reduce the Ca containing; on the other hand, addition of B improve the P modification efficiency, which makes the primary Si finer, eutectic Si rounder.(3) Study of Ca removal by fluoride in A1-Si alloysCa poison P modification efficiency in near eutectic Al-Si alloys. Priamry Si can not precipitate when the Ca containing is high, so does the P modification efficiency. The Ca containing in Al-Si alloy is sharply decreased with addition of K2TiF6 and AIF3. The Ca containing in Al-Si alloy can be decreased from 1100ppm to 50ppm with 1.5% K2TiF6 addition. While the Ca containing in A1-Si alloy can be decreased to 7ppm with 1% A1F3 addition. The addition of K2TiF6 and A1F3 can recover the P modification efficiency of the "poisoned " Al-Si alloy, primary Si can precipitate again, and AIF3 is the key element. So it is feasible method to remove Ca by K2TiF6 and A1F3 in near eutectic Al-Si piston alloy.In a word, by studying Ca removal in Al-Si piston alloys, two new techniques different from the traditional Ca removal by [C1] are put forward in this work. It provides the referencable ideas for the studies of strength of Al-Si piston alloys, and will provide the more important theoretical basis and a new way for the strengthing of Al-Si alloys.