Influence of Tin Addition on the Microstructure and Mechanical Properties of Al-Si-Cu-Mg and Al-Si-Mg Casting Alloys
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INTRODUCTION
WEIGHT reduction in the manufacture of automobiles and trucks has become a key focus area because of the need to meet increased customer expectations for vehicle safety and performance, while complying with regulations on fuel economy. In this respect, aluminum (Al) alloys have great potential for high-performance automotive applications because of intrinsic properties such as low density, high specific strength, and good machinability, which make them highly suitable for such applications. For a specific application, the selection of an alloy depends on its castability, the casting process involved, the required mechanical and physical properties, and the use of the casting.[1,2] Tin (Sn) is a minor alloying element in Al alloys. In the past, it was added to increase the fluidity of casting alloys and presently it is added to alloys for bearings. According to the phase diagram,[3] Al and Sn have mutual solid insolubility, with the maximum solubility of Sn in Al being about 0.1 wt pct at approximately 627 C, decreasing to reach a probable value of 0.05 to 0.07 pct Sn at the eutectic temperature, and considerably lower values at lower temperatures. The eutectic point occurs at 228 C to 229 C and 99.5 pct Sn, which is close to the melting point of Sn. Aluminum-silicon (Al-Si) cast alloys have attracted widespread attention because of their high strengthA.M.A. MOHAMED, F.H. SAMUEL, and A.M. SAMUEL are with Sciences Applique´es Universite´ du Que´bec a Chicoutimi, Chicoutimi, PQ, Canada G7H 2B1. Contact e-mail: [email protected] H.W. DOTY is with GM Powertrain Group, Metal Casting Technology, Inc., Chicoutimi, NH 03055. S. VALTIERRA is with Corporativo Nemak, Research and Development, Garza Garcia, N.L, Mexico. Manuscript submitted June 2, 2007. Article published online January 30, 2008 490—VOLUME 39A, MARCH 2008
to-weight ratio and excellent castability. The microstructure of Al-Si alloys is characterized by two phases, Al and Si. Thus, these alloys are a combination of a high strengthbrittle phase (Si) and a low strength-ductile phase (Al). Obviously, the deformation behavior and mechanical properties are controlled by characteristics of the constituent phases. Regulation of the microstructure may also be obtained through grain refinement and modification, where small additions of Al-Ti-B and Al-Sr master alloys, respectively, provide the necessary impetus for the production of a refined microstructure and the required morphology of the constituent phases.[4–6] Alloy 319 is one of the main alloys of the Al-Si-Cu group.[7–9] Another very important group is comprised of alloys that are age hardenable due to the presence of magnesium (Mg). The most popular alloy of this family is 356 alloy.[10,11] The as-cast microstructure of 319 and 356 alloys consists of the Al matrix (a-Al dendrites, silicon particles, intermetallic phases, nonmetallic inclusions, and porosity) and is controlled by chemical composition, melt treatment conditions, solidification rate, and the heat treatment applied. Most of the heat treatments r
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