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10/30/03

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Page 2887

Microstructural Effects on the Tensile and Fracture Behavior of Aluminum Casting Alloys A356/357 Q.G. WANG The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (
50 m), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics ( phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (
50 m), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (30 m) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.

I. INTRODUCTION

CAST aluminum alloys A356/357 have widespread applications for structural components in the automotive, aerospace, and general engineering industries because of their excellent castability, corrosion resistance, and, particularly, high strength-to-weight ratio in the heat-treated condition. However, the use of these cast alloys is still limited in comparison with wrought alloys, even though casting would be a more economical production method. It has been reported that aluminum alloys make up about 78 pct of the structural weight of commercial aircraft, of which castings comprise only about 6 pct.[1] Better quantitative understanding of the microstructure-property relationships in cast Al alloys, coupled with improved foundry practice, will allow broader application of reliable castings in low mass structures. The microstructures of alloys A356/357 are comprised of an aluminum matrix, which is strengthened by MgSi precipitates and, to a far lesser extent, by Si precipitates, and a dispersion of eutectic silicon particles and Fe-rich intermetallics. The variables affecting the microstructure mainly include composition, solidification conditio