Dissolution of Cu/Mg Bearing Intermetallics in Al-Si Foundry Alloys
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INTRODUCTION
IN the last decades, Al-Si-based foundry alloys have been increasingly used in the automotive industry mainly in the fabrication of engine components. High strength-to-weight ratio, high thermal conductivity, and excellent castability are of the major advantages of the Al-Si hypoeutectic alloys. Nevertheless, the increase of operation temperature/pressure of the engines necessitates strengthening of the Al-Si alloys. Magnesium and Cu are the principle alloying elements of the commercial Al-Si-based foundry alloys due to their appreciable solubility and strengthening effects. The large eutectic Cu/Mg bearing phases (h-Al2Cu, Q-Al5Cu2Mg8Si6, and p-Al8FeMg3Si6), which appear at the last stages of solidification, can get dissolved by applying an appropriate solution heat treatment (SHT) and re-precipitated as fine evenly distributed metastable phases to strengthen the alloys. However, they may be insoluble/partially soluble depending of the alloys chemistry (e.g. high Mg/Cu content and fraction) and the SHT parameters used (time and
MOUSA JAVIDANI, formerly Ph.D. Candidate at Laval University, Canada, is now Research Scientist, Casting Group, ARDC, with RioTinto, 1955, Mellon Blvd, Jonquie`re, QC, G7S 4K8, Canada. DANIEL LAROUCHE, Professor, is with the Department of Mining, Metallurgy and Materials Engineering, REGAL-Aluminium Research Centre, Laval University, 1065, Ave de la Me´decine, Que´bec, QC, G1V 0A6, Canada. Contact email: [email protected] X. GRANT CHEN, Professor, is with the Department of Applied Sciences, REGAL-Aluminium Research Centre, Universite´ du Que´bec a` Chicoutimi, Saguenay, QC, G7H 2B1, Canada. Manuscript submitted December 1, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature).[1] If the large eutectic Cu/Mg bearing phases do not dissolve during SHT, the hardening effect of the Cu/ Mg elements will be reduced and the ductility of the alloys will also suffer.[2] It has been reported that a Mg level beyond 0.3 wt pct in 319-type Al alloys does not affect considerably the alloy strength, while it can reduce significantly the alloy ductility.[3,4] In order to minimize/eliminate undissolved Cu/ Mg bearing intermetallics, the alloy chemistry must be optimized. The applicable SHT temperature (TSHT) is generally restricted by the non-equilibrium solidus, which is the melting point of the last solidified phases (Tmp). Al-Si alloys containing both Cu and Mg are generally limited to Tmp~ 780 K (507 C), but for the alloys containing Cu and/or Mg individually, Tmp can be much higher.[5–8] The higher applicable TSHT not only accelerate the dissolution rate of the Cu/Mg bearing intermetallics but also further modify the microstructure (e.g. Si particles) of the alloys.[1] Another strategy is to apply a two-step SHT: the temperature of the 1st SHT step [~773 K (500 C)] is limited by Tmp to avoid incipient melting of the Cu-containing phases (h-Al2Cu and Q-Al5Cu2Mg8Si6); after dissolution of the Cu bearing phases, the 2nd SHT step is applied at a higher temperature [e.g. between
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