Abstract: Casting Al-Si alloy has become one of the most important structural materials in the foundry industry due to its light weight, good fluidity, good gas-tightness, small shrinkage and low thermal expansion coefficient. The mechanical properties of the as-cast Al-Si alloy mainly depend on defects, morphology, size and distribution of the α-Al phase, the eutectic silicon, the primary silicon and the intermetallic compounds, among which the shape, size and distribution of the silicon phases and the Fe-rich phase have prominent influence upon alloy properties. Needle-like eutectic silicon and acicular β-Fe phases severely segregate the matrix in the undeformed Al-Si alloy, meanwhile the tips and corners of coarse bulk primary silicon and needle-like eutectic silicon are prone to stress concentration, which leads to deterioration in the properties of the alloy, especially plasticity, strength and wear resistance. In addition, the wear of machined tools will be aggravated by hard and brittle primary silicon, and the machined parts will also have poor surface finish, which are unsatisfactory for actual production.
Although some special techniques such as rapid solidification, electromagnetic stirring, ultrasonic treatment and mechanical vibration, etc. can improve the microstructure and properties of cast Al-Si alloy to a certain extent, their applications are limited by the dependence on particular equipment and specific conditions. The modification treatment, with no device requirement, only involving a small amount of modifying elements, can achieve effective regulation of α-Al phase, eutectic silicon, Fe-rich phase and primary silicon, and thus has become the most effective approach to structure improvement and performance promotion for Al-Si alloy with the characteristics of low cost, simple process and remarkable effect. However, the modification mechanism of various elements and the interactions of them are extremely complicated, and hence have proved to be the research focus in this field.
The cognition about single modifying elements’ action mechanism has been undergoing rapid progress in recent years, as Na, Sr, Sb, P, Ti, Mn and RE, etc. have been discovered to be moderately effective upon the modification of cast Al-Si alloy, and the research over their deterioration, timeliness and cost is exhaustive. Moreover, many researchers have turned to the multi-element modification and have made considerable strides in the study of binary elements interaction such as Sr-X, P-X, Mn-X and Ti-X, endowing the modification technology with greater cost efficiency, effectiveness and diversity.
In this paper, the characteristics, mechanisms and limitations of single modifying elements for Al-Si alloy are depicted. The interaction rules of Sr, P, Ti, Mn and RE with other elements are systematically summarized. We illuminate some binary or multiple modifying elements which display synergistic modifying effect, and expect to benefit further exploration for the binary/multi-element interaction mechanism and provide reference for developing more efficient, diverse and green composite modifiers and the adapted modification methods.
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2018, Vol. 32 
