Precipitation of Sr-rich intermetallic particles and their influence on pore formation in Sr-modified A356 alloy
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I. INTRODUCTION
A major concern with the refinement of the Al-Si eutectic phase in Al-Si alloys by Sr modification is the increased tendency to microporosity formation.[1,3–7] This phenomenon has been widely investigated and some of the common hypotheses have been summarized by Fuoco et al.[8] and Anson et al.[9] Some of these hypotheses are listed and discussed critically, as follows. It has been suggested that Sr modification increases the susceptibility of a melt to absorb hydrogen by formation of hydrogen containing compounds, not necessarily taken into account by hydrogen measurement techniques, and that these compounds subsequently decompose during solidification to release hydrogen.[10,11,12] Alternatively, Miresmaeili et al.[13,14] has shown that there are no hydrogen containing compounds (such as hydrides and hydroxides), which can form in aluminum melts, and such compounds, if introduced, cannot continue to exist in the melt because they are unstable at these temperatures. It has further been suggested that modification treatments make interdendritic feeding more difficult,[15,16] leading to porosity. However, it has also been demonstrated that when only shrinkage porosity occurs (i.e., with low hydrogen contents), there is generally much less porosity in Sr-modified alloys than unmodified alloys[9,17,18] demonstrating better, not worse, interdendritic feeding. Other studies suggest that the observed porosity might be due to the effect of Sr on the surface tension of Al-Si melts. This proposal is seen to be irrelevant if the proposition is accepted that nucleation of pores cannot occur, and that pores form only from bifilms S.M. MIRESMAEILI, Lecturer, is with the Department of Metallurgy and Materials, Faculty of Engineering, Rajaei University, Lavizan, Tehran, 16788, Iran. Contact e-mail: [email protected] J. CAMPBELL, Professor, is with the Department of Metallurgy and Materials, School of Engineering, University of Birmingham, Birmingham, B15 2TT, United Kingdom. S.G. SHABESTARI, Associate Professor, and S.M.A. BOUTORABI, Professor, are with the Department of Materials and Metallurgical Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran. Manuscript submitted July 10, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
or other entrained (i.e., nonwetted) interfaces. Such processes do not involve surface tension.[1] In addition, strontium oxide has been held to be responsible for the increase in porosity observed in Sr-containing Al-Si alloys.[6,9–12,17] Anson et al.[9,17] reported that Sr modification might affect pore formation by an increase in inclusion content, although there has been no thorough investigation of this, probably due to the difficulties associated with the quantitative measurement of inclusions. However, from observation of pores in Sr-modified castings, Miresmaeili et al.[5,13] suggested that the oxides Al2O3 (alumina) and Al2O3MgO (spinel) films and particles were associated with pore formation rather than SrO-containing oxides. It was true that Sr
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