Structure and properties of hypoeutectic Al-Si-Mg alloys modified with pure strontium
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I.
Table I.
INTRODUCTION
M E C H A N I C A L properties of A1-Si casting alloys, especially elongation, depend on the alloy structure and on the eutectic silicon which may have an acicular or lamellar form. It is well known that elongation is improved when sodium is added to A1-Si alloys ~ to form a fibrous eutectic structure. The major drawback to this practice is that the effect of sodium disappears rapidly by evaporation or oxidation during the holding of the molten metal. 2 Strontium is also used as a modifier for A1-Si-Mg alloys.3 It possesses certain advantages over sodium in that it is easier to add to the melt, and it offers a semipermanent modifying effect. There have been relatively few studies on strontium modified AI-Si-Mg casting alloys, and in the present paper we give the results of a study into the variation of mechanical properties of A356.0 alloys with the levels of both strontium and certain other elements (Fe, Mg). It is worth noting at the outset that all of the experiments described here were done with pure metallic strontium. This is a departure from normal foundry practice where strontium is added as a master alloy. This work forms part of a larger investigation 4 aimed at developing pure strontium as an acceptable modifying agent.
II.
EXPERIMENTAL
All the experiments in this work were conducted on A356.0 alloys having the composition given in Table I. The experimental procedure involved the preparation of 10 kg melts using SiC crucibles in a gas fired furnace. Strontium was added to the melt in the form of an extruded bar 15 mm in diameter using a graphite plunger. Iron and magnesium additions when required were made in various forms, usually as powders or compressed tablets. All additions were made at temperatures in the range 730 to 750 ~ All melts were degassed with either of two agents: pure nitrogen or a nitrogen-freon 12 mixture (95 pct N2-5 pct CC12 F2). The degassing agent was bubbled into the melt at a rate of 1.75 liters per minute until the hydrogen content was reduced to an acceptable level. 4
B. CLOSSET, Research Associate, and J.E. GRUZLESKI, Chairman, are both with the Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7. Manuscript submitted October 6, 1981. METALLURGICAL TRANSACTIONS A
Chemical Analysis of A356.0 Alloy
Element Si Fe Cu Mn Mg Ni Zn Ti B A1
Wt Pct 6.9 pct 0.10 pct 0.004 pct 0.003 pct 0.33 pct 0.008 pct 0.01 pct 0.14 pct 0.01 pct Balance
After degassing the melt was poured between 715 to 725 ~ into sand molds at room temperature to produce both separately cast test bars and a specially designed casting. The test bars were produced according to Canadian Standard Association specification HA 1. The special casting, shown in Figure 1, was designed to produce a variety of cooling rates, and hence it contains regions of both thin and thick section. The results presented here were obtained from both the cast test bars and from samples machined from the locations shown in Figure 2.
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