The effect of solution heat treatment and quenching rates on mechanical properties and microstructures in AlSiMg foundry
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INTRODUCTION
THE use of light-metal cast components in various applications has increased during the last 10 years, partly as a result of the increased amount of light metals being used for transportation purposes. One advantage is that lightmetal components lead to an overall reduced weight and, thus, to reduced energy consumption. Another advantage, which may be just as important from an environmental point of view, is the fact that aluminum components may be recycled with relatively low energy demands. For such purposes, alloys of the type AlSiMg are found to give good results since they show a good castability, and the addition of Mg makes the alloys heat treatable. The effect of heat treatment on mechanical properties is well known for alloys of the AlSi7Mg type, for example, A356 and A357, and has been described by, among others, Ravi et al.,[1] Kashyap et al.,[2] and Shih and Shih.[3] The main conclusion from these works is that increasing the Mg concentration leads to an increased strength and a reduced ductility, while an increase in the Si concentration mainly reduces ductility. The present work focuses on how changes in the solution heat-treatment time affect the mechanical properties of AlSiMg foundry alloys in the modified and unmodified conditions. The mechanical properties are related to changes in microstructure, primarily to the morphological changes of the silicon crystals in the eutectic areas. The effect of various quenching rates from the solution heat-treatment temperature has also been examined for the modified alloys.
L. PEDERSEN, Process Engineer, is with Elkem Aluminium Lista, 4550 Farsund, Norway. L. ARNBERG, Professor, is with Department of Materials Technology and Electrochemistry, The Norwegian University of Science and Technology, 7491 Trondheim, Norway. Manuscript submitted June 28, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
II. EXPERIMENTS A. Casting of the Tensile-Test Bars The experiments have been based on four main alloy compositions: Al-7Si-0.2Mg, Al-7Si-0.6Mg, Al-11Si-0.2Mg, and Al-11Si-0.6Mg. The Al-7Si-Mg alloys are based on an unmodified commercial alloy with a magnesium concentration of approximately 0.2 wt pct, supplied by Elkern Aluminium Mosjøen (Mosjøen, Norway). Magnesium was added in the form of pure 99.9 wt pct Mg to obtain a Mg concentration of 0.6 wt pct. Hydro Aluminium Sunndalsøra (Sunndalsøra, Norway) supplied the unmodified Al-11Si-Mg alloys. The alloys were modified by adding strontium, in the form of an Al-10Sr rod, 5 minutes prior to casting. Samples for chemical analysis were cast in a steel “scissors mold,” type B, as described in ASTM E-716.[4] Three analyses were taken from each sample, and the concentrations of the main alloying elements are presented in Tables I and II. To obtain reliable measurements of the mechanical properties, it is important to control the amount of porosity in the base material. This requires both removal of hydrogen from the melt and sufficient feeding of the melt into the casting as solidification proceeds. Approximat
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