Effect of the Delay in Time Between Cooling and Aging in Heat-Treated Cast Aluminum Alloys
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CAST aluminum alloys are the material of choice in powertrain applications in the automotive industry. The reasons behind this are manifold; among them is the reduction in weight, which translates in the reduction in power consumption and the increase in delivery power that results from higher temperatures in the combustion chamber.[1,2] Most aluminum alloys used in powertrain applications are heat treated to increase their strength. Some alloys rely in the precipitation of Mg2Si particles, whereas other alloys form Al2Cu precipitates.[3–8] Hardening in these alloys is caused by the formation of particles that interact with dislocations. The early stages of precipitation involve clustering of either silicon and magnesium or copper atoms that transform into coherent and incoherent particles with the increase of time and temperature. Peak aging conditions take place when coherent or semicoherent particles are present; overaging takes place once the precipitates become incoherent. A full description of the precipitation reactions in aluminum alloys can be found elsewhere.[3–8] Heat treating of these aluminum alloys involves solubilization of the particle forming elements, either magnesium and silicon or copper. These elements are kept in a supersaturated solution by the rapid reduction in temperature from the solution temperature (normally called quenching), which is achieved by immersing the EFRE´N CARRERA and J. ALEJANDRO GONZA´LEZ, R&D Engineers, and JOSE´ TALAMANTES-SILVA, R&D Manager, are with Nemak, S.A. de C.V., Libramiento Arco Vial km 3.8, 66000 Garcı´ a, Mexico. RAFAEL COLA´S, Director, is with the Centro de Innovacio´n, Investigacio´n y Desarrollo en Ingenierı´ a y Tecnologı´ a, Universidad Auto´noma de Nuevo Leo´n, 66629 Apodaca, Mexico, and Professor with the Facultad de Ingenierı´ a Meca´nica y Ele´ctrica, Universidad Auto´noma de Nuevo Leo´n, 66451 San Nicola´s de los Garza, Mexico. Contact e-mail: [email protected] Manuscript submitted December 28, 2010. Article published online June 7, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
alloys in water or any other cooling medium. The cooling rate is reduced in pieces of complex shape to minimize the risk of distortion.[5,9] The reduction in cooling rate might affect the properties of the material as the elements put in solution can precipitate when the average cooling rate fall below 10 K/s.[10–12] The critical rate for different parts should have to be determined experimentally[13,14]; although a compromise for obtaining the mechanical properties and avoiding distortion is achieved by immersion in water close to the boiling point. Precipitation hardening proceeds by heating the material in furnaces (artificial aging) or by letting them rest at room temperature (natural aging). Modern day production relies on the introduction of furnaces to obtain age-hardened pieces in short periods of time. Aluminum cast alloys are used when treated to either the peak-aged (T6) or to the overaged (T7) conditions. T7 treatment is preferred in Al-Si-Cu alloys, as this co
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