Experimental and modeling studies of the thermal conditions and magnesium, iron, and copper content on the morphology of
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7/1/04
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Experimental and Modeling Studies of the Thermal Conditions and Magnesium, Iron, and Copper Content on the Morphology of the Aluminum Silicon Eutectic in Hypoeutectic Aluminum Silicon Alloys G. HEIBERG, Ch.-A. GANDIN, H. GOERNER, and L. ARNBERG Hypoeutectic aluminum silicon alloys without and with additions of magnesium, copper, iron, and strontium have been cast in a mold, giving directional solidification from a chill. Detailed temperature measurements have been carried out. Solidification modeling based on front tracking of the microstructure growth fronts allowed identification of the time and temperature at which the dendrite tips and the first eutectic pass the thermocouples. The undercooling, growth rate, and thermal gradients at the dendritic and eutectic growth fronts were derived. The effect of varying thermal parameters and alloy compositions on the microstructure was investigated. Compared to the binary alloy, a coarser eutectic was observed in the alloys with magnesium, iron, and/or copper. The coarsening is explained as a result of the transition from a eutectic forming at one specific temperature, to a eutectic forming over a temperature range. The former is likely to grow as a plane front, whereas the latter is likely to form an interdendritic eutectic mushy zone.
I. INTRODUCTION
HYPOEUTECTIC aluminum-silicon alloys such as A356 and 319 are extensively used in automotive applications, and it is becoming more and more common to use them in critical load-bearing components. Controlling the microstructure and the formation of defects during solidification is, therefore, very important. The aluminum silicon eutectic forms during the final stage of the solidification. How the eutectic nucleates and grows have been shown to have an effect on the formation of defects such as microporosity.[1] Both the defects and the morphology of the eutectic itself have an important effect on the ultimate mechanical performance of the casting. How the eutectic solidifies and the resulting morphology depends on many parameters. The effect of the thermal gradient and growth rate is important and has been studied extensively in the past using directional solidification experiments with Bridgman-type furnaces.[2–6] Several models have been proposed to quantify the effect of the cooling conditions on the eutectic microstructure and growth temperature.[7,8] A second parameter that plays an important role in determining the morphology of the aluminum silicon eutectic is chemical modification. By the addition of small amounts of certain elements like sodium or strontium, the morphology of the eutectic silicon is changed from one consisting of platelets to a much finer fibrous structure, thereby usually increasing the ductility of the material.[9] Chemical modification is used extensively in commercial aluminum-silicon castings. Other alloying elements such as magnesium and copper also have an effect on the morphology of the eutectic microstructure. G. HEIBERG is with Det Norske Veritas, 1322 Høvik,
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