Effects of rapid heating on solutionizing characteristics of Al-Si-Mg alloys using a fluidized bed
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I. INTRODUCTION
INCREASING demands for lightweight metals as structural components in the automotive and aerospace industries have led to the extensive use of Al-Si-Mg–based foundry alloys. One advantage of using lightweight materials is to increase the payload capacity. Al-Si-Mg alloys are candidate materials for such applications due to their good castability characteristics.[1] The addition of Mg makes the alloy heat treatable,[2–6] which helps to tailor mechanical properties by selecting a suitable temper subsequent to casting. The T6 temper is a widely accepted heat-treatment process for aluminum alloys to increase strength. The T6 temper is comprised of solution heat treatment, quenching, and then artificially aging. The solution heat treatment increases ultimate tensile strength and ductility, while aging increases yield strength at the expense of ductility. In the case of the T4 temper (solution heat treatment followed by quenching), the increase in strength is through the solute solution strengthening mechanism (Mg and Si atoms retained in the postquenched state), and the increase in ductility is due to the spherodization of eutectic Si particles. However, the increase in yield strength on aging after T4 temper is through precipitation strengthening owing to the precipitation of Mg2Si particles. The resultant mechanical properties of the heat-treated alloy depend on its chemical composition. In addition to Si and Mg, cast Al-Si-Mg alloys usually contain Fe as an impurity element. Depending upon the Mg content, Fe may exist as phase (Al8Mg3FeSi6), as -Fe S.K. CHAUDHURY, Research Associate, and D. APELIAN, Director, are with the Metal Processing Institute, Worcester Polytechnic Institute, Worcester, MA 01609. Contact e-mail: [email protected] Manuscript submitted July 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
needles (Al5FeSi), as Al8FeSi, or as a combination of the preceding phases. The solidus temperature of the alloy depends on the iron-rich phase(s). The type and morphology of ironrich phase(s) that form in Al-Si-Mg alloys depend on the Mg content. It has been observed[3] that iron-rich phase(s) have a different morphology in Al-Si-Mg alloys containing 0.4 pct Mg as compared to those containing 0.7 pct Mg. The former with 0.4 pct Mg, after solution heat treatment, contains only small -phase plates, whereas the latter (0.7 pct Mg) contain large phase and a small number of plates. The morphology of the iron-rich phase has a significant effect on the ductility of these alloys. The reduction in ductility and fracture toughness in alloys with low Mg content is associated with the formation of plate-shaped phase. In alloys with higher Mg content, the predominant intermetallics are phase and their effect on ductility is found to be dependent on whether the alloy is Sr modified. It is observed[3] that in Sr-modified alloy containing 0.7 pct Mg, the fraction of cracked phase is greater than Si, since phase is much coarser than Si. However, in unmodified alloys, the fraction of cracked Si
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