Structural and Mechanical Properties of Directionally Solidified Al-Si Alloys
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JMEPEG https://doi.org/10.1007/s11665-019-04414-3
Structural and Mechanical Properties of Directionally Solidified Al-Si Alloys S.P. Nikanorov, V.N. Osipov, and L.I. Regel (Submitted April 19, 2018; in revised form September 20, 2019) This review covers research aimed at finding the optimum composition and growth rate to obtain a highly modified Al-Si alloy using directional solidification. Investigations of microstructure and mechanical properties as a function of Si content and growth rate are analyzed. These works show that the composition yielding a eutectic microstructure changes considerably with increasing solidification rate in the range of 102-104 lm/s. The increase in ultimate tensile strength with increasing Si content up to that giving a completely eutectic microstructure is explained by a redistribution of volume content of a-Al and eutectic. The increase in tensile strength with increasing rate is explained by a decrease in microstructural scale accompanying the transformation of flake-to-fiber eutectic microstructure. The optimal fine fiber structure without any primary crystals of Al-Si alloy at a given Si content is obtained at the solidification rate giving a completely eutectic microstructure at that composition. Hypereutectic alloys can be fully modified using rapid cooling at such solidification rate that causes coupled growth of the eutectic for given composition of the alloy. Additional Sr modification results in a super-modified structure, high tensile strength and record high elongation. Keywords
Al-Si alloy, directional solidification, elongation, eutectic point, microstructure, tensile strength
1. Introduction The binary Al-Si system is the basis of aluminum-silicon casting alloys in which silicon is the major alloying element. These alloys are called ‘‘silumins.’’ The manufacture of these alloys constitutes over 90% of all aluminum-based castings. AlSi alloys are used as construction materials for different purposes, as well as solder for aluminum like Al-Ge. Heatresistant aluminum alloys are used for car engine parts such as connecting rods, rocker arms, cylinders, pistons and valve retainers (Ref 1, 2). Construction alloys usually have a hypoeutectic content of silicon. Alloys for other applications are eutectic or hypereutectic in Si. This is connected primarily to ductility decreasing with increasing Si content. Other elements are added to these alloys to increase the strength and improve some other properties. The search continues for optimal additions and optimal Si content for different applications of cast alloys based on the Al-Si system. This search will be aided by analysis of published information on the dependence of structural and mechanical properties on Si content, solidification rate and modification of Al-Si alloy. The effects of Si content in hypoeutectic alloys and low rates of solidification have been well studied. There are also many results for very high rates of solidification, 105-106 lm/s. Presented here are the results of such investigations obtained S.P. Nikanorov and V.N. O
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