Effects of La and Ce Addition on the Modification of Al-Si Based Alloys

Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2016, Article ID 5027243, 13 pages http://dx.doi.org/10.1155/2016/5027243 Research Article Effects of La and Ce Addition on the Modification of Al-Si Based Alloys Emad M. Elgallad,1 Herbert W. Doty,2 Saleh A. Alkahtani,3 and Fawzy H. Samuel1 1 Université du Québec à Chicoutimi, Chicoutimi, QC, Canada G7H 2B1 General Motors Materials Engineering, Pontiac, MI, USA 3 Industrial Engineering Program, Mechanical Engineering Department, College of Engineering, Sattam Bin Abdulaziz University, Al Kharj, Saudi Arabia 2 Correspondence should be addressed to Fawzy H. Samuel; Received 29 December 2015; Revised 14 February 2016; Accepted 30 March 2016 Academic Editor: Jörg M. K. Wiezorek Copyright © 2016 Emad M. Elgallad et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This study focuses on the effects of the addition of rare earth metals (mainly lanthanum and cerium) on the eutectic Si characteristics in Al-Si based alloys. Based on the solidification curves and microstructural examination of the corresponding alloys, it was found that addition of La or Ce increases the alloy melting temperature and the Al-Si eutectic temperature, with an Al-Si recalescence of 2-3∘ C, and the appearance of post-𝛼-Al peaks attributed to precipitation of rare earth intermetallics. Addition of La or Ce to Al-(7–13)% Si causes only partial modification of the eutectic Si particles. Lanthanum has a high affinity to react with Sr, which weakens the modification efficiency of the latter. Cerium, however, has a high affinity for Ti, forming a large amount of sludge. Due to the large difference in the length of the eutectic Si particles in the same sample, the normal use of standard deviation in this case is meaningless. 1. Introduction Nafisi et al. [1] studied the nucleation mechanism of eutectic grains in hypoeutectic Al-Si alloys. They proposed that, following nucleation of eutectic Al on the primary 𝛼-Al dendrites, fine Si particles form at the solidification front upon which the eutectic Si flakes and fibers could develop. The formation of small Si particles is attributed to Si enrichment of the remaining melt due to the formation of eutectic Al (aluminum spikes) at the eutectic temperature. It has been proposed that modification of eutectic silicon by trace additions occurs due to a massive increase in the twin density caused by atomic effects at the growth interface [2]. Although the discovery of the modification effect dates back to 1921 [3], the exact mechanism of eutectic medication is not completely understood. The high twin density in Si fibers would require Sr atoms to be distributed rather uniformly within the Si phase. Besides the effect of Sr addition on the growth of eutectic Si, recent studies have confirmed that Sr also changes significantly the nucleation behavior of the eutectic phases [4]. The elements causing modification have an atomic radius ratio close to 1.65 from the theory of impurity induced twinning (IIT), and the fine fibrous eutectic modifications of hypoeutectic Al-7%Si alloys containing Sr, Na, Ca, and Ba have all been investigated. The addition of rare earth elements La, Sm, and Ce was also reported to cause eutectic modification [5]. Strontium, sodium, calcium, and antimony are added to eutectic or hypoeutectic Al-Si casting alloys to modify the morphology and microstructure of the eutectic silicon phase from its usual relatively coarse continuous network of thin platelets in the unmodified as-cast structure [6]. Modification with one of these elements can change or “modify” the eutectic silicon into a fine fibrous or lamellar structure [7]. The hypothesis that Sr additions cause modification by increasing the growth rate of the eutectic has been tested by directionally solidifying unmodified and Sr-modified alloys [8]. It was found that Sr-modified alloys have a smaller eutectic spacing than unmodified alloys, even when both are grown at identical velocities [9]. 2 The modifying action of many rare earth metals in an Al-10 mass% Si alloy was investigated by Nogita et al. [11], where all rare earth elements had some effect on the eutectic silicon. Among them, europium was the only element which produced a fully modified, fine fibrous silicon structure. The other elements provided only a minor refinement of the plate-like silicon morphology. Pourbahari et al. [12] studied the effects of La addition and heat treatment on the microstructure and tensile properties of A357 aluminum alloy. Their results showed that the addition of La obviously reduced the size and area of eutectic silicon particles after heat treatment. New intermetallics were detected at higher La levels (>0.5%). The effects of Ce and La on microstructure and properties of a 6xxx series type aluminum alloy were investigated by Hosseinifar and Malakhov [13]. They found that, in alloys containing 0.1–0.2 wt.% of La, the fraction of 𝛽-Al5 FeSi particles was pronouncedly less than that in the base alloy. In addition to this advantage, much smaller grains were seen in the alloy containing 0.2 wt.% La. On the other hand, Ce addition neither modified the microstructure nor noticeably affected the grain size. According to Hosseinifar and Malakov [13, 14], the intermetallics formed during the solidification of an AlMg-Si alloy containing La may be proposed as follows: the addition of La results in the formation of the La(Al,Si)2 phase and a depletion of Si in the remaining melt. It is hypothesized that the decreased Si/Fe ratio in the melt caused by the presence of La favors the formation of the 𝛼-AlFeSi phase, which is less detrimental to the formability of the alloy than the 𝛽-AlFeSi phase. The work of Tsaia et al. [15] on the effect of trace Ce additions on the microstructure and mechanical properties of A356 (Al-7Si-0.35 Mg) aluminum alloy indicates that modification efficiency and hence mechanical properties of A356 alloy are greatly enhanced by adding 1.0 wt.% Ce. Two kinds of intermetallic compounds were reported in their study, including Ce-23%Al-22%Si and Al-17%Ce-12%Ti-2%Si-2%Mg (all percentages are in wt.%) phases. In addition, Ce improved the thermal stability of the Ω phase by decreasing the diffusion velocity of Cu atoms and increasing the energy barrier of the thickening ledge nucleation, thus improving the strength of the Al-Cu-Mg-Ag alloy at both room and elevated temperatures [16]. In the present work, A356 and 413 alloys were selected for study, being two commercially popular Al-Si alloys. These alloys are also in high demand in the automotive industry which is the largest consumer of cast Al-Si alloys. The 356 alloy is a hypoeutectic alloy, whereas the 413 alloy is a eutectic alloy. As the microstructure of Al-Si alloys, in particu (...truncated)