Microstructure and Wear Behavior of Solidification Sonoprocessed B390 Hypereutectic Al-Si Alloy
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HYPEREUTECTIC Al-Si alloys with contents ranging from 14 to 25 pct Si exhibit excellent wear resistance, high hardness, and low thermal expansion. Because of these properties, these alloys find applications in some critical components such as internal combustion engine parts, cylinder bodies of compressors and pumps, brake systems, etc. They are usually cast by low-pressure casting, high-pressure die casting. and permanent mold casting methods, and rarely by sand casting. The prominent properties of these alloys depend mainly on the morphology and size of the primary Si particles and the eutectic Si. In conventional casting solidified at a moderate cooling rate, the primary silicon crystallizes in the form of hexagonal plates joined together at the center into star-shaped particles. These coarse platelet particles have deleterious effect on the mechanical properties of the alloys. These microstructural features can be controlled by the addition of phosphorus as a refiner for the primary Si and modifiers such as Sr or Na for the eutectic Si and/or by controlling cooling rate encountered during solidification.[1,2]
WALEED KHALIFA, Associate Professor, is with the Department of Mining, Petroleum and Metallurgical Engineering, Faculty of Engineering, Cairo University, Giza, 12613, Egypt. SHIMAA EL-HADAD, Researcher, is with the Central Metallurgical Research& Development Institute, P.O. Box 87, Helwan-El-Tebbin, Cairo, Egypt. Contact e-mail: [email protected]; [email protected] YOSHIKI TSUNEKAWA, Professor, is with the Toyota Technological Institute, 2-12-1, Hisakata, Tempaku, Nagoya, 468-8511, Japan. Manuscript submitted September 10, 2012. Article published online October 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Phosphorus forms AlP particles which serve as nucleation sites for the primary Si, so that its addition can refine and distribute well the primary Si, but unfortunately, it leaves behind coarse eutectic Si. The level of phosphorus required to obtain the optimum size of primary Si (15 to 20 lm) depends on the Si level of the alloy and the cooling rate, and is ranging from about 0.003 to 0.015 pct. However, the efficiency of phosphorus refiner is limited at high Si levels, so that complete refining of alloys containing 30 pct Si or higher is difficult to achieve.[3] This drawback needs to be overcome. Concerning the effect of cooling rate, the rapid cooling conditions assist in refining the primary Si particles, while during slow cooling, the phosphorus refiner is not efficient and large Si particles form and float up because of the density differences.[4] This is why sand casting is not a method of choice for these alloys. This detracts from the technological importance of these alloys and constitutes another challenge for the modem industry. Another aspect of refining with phosphorus is that the AlP nuclei have better distribution at higher temperatures rather than low temperatures, leading to treatment and casting at the highest feasible temperature.[3] To overcome such problems, novel refining technique is re
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