A thermodynamic prediction for microporosity formation in aluminum-rich Al-Cu alloys
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I.
INTRODUCTION
THE susceptibility of cast alloys to the formation of porosity is well known and is dealt with on a day-to-day basis by foundrymen, ingot-makers, and welders alike. In this paper, it is shown that thermodynamic data combined with dendritic solidification can be used to predict the conditions for which microporosity would be expected to form. A specific alloy system is selected as an example, but the basic approach can be used to study the formation of microporosity in other alloy systems provided enough thermodynamic data and solidification data are available. Here the aluminum-rich alloys of the A1-Cu system have been selected because there are thermodynamic data on the solubility of hydrogen in liquid A1-Cu alloys, and the dendritic freezing of such alloys has been studied extensively. Talbot 1 presented an extensive review of the effects of hydrogen in aluminum, magnesium, copper, and their alloys. His review included thermodynamics, microstructural information, the effects of processing, observations of the effects of hydrogen on mechanical properties, and the relationships among melting-, solidification-, and thermomechanical-process variables and product integrity. Among the many topics discussed by Talbot, 1 this paper addresses what is commonly called microporosity or, perhaps more precisely, interdendritic porosity. This porosity arises because the solubility of hydrogen is less in the solid than in the liquid metal, so that some of the hydrogen is expelled into the interdendritic liquid. If the concentration of hydrogen in the interdendritic liquid rises to a value sufficient to exceed the sum of the local pressure within the interdendritic liquid and the excess pressure attributed to surface tension,
D.R. POIRIER, Professor, and K. YEUM, Research Metallurgist, are with the Department of Materials Science and Engineering, The University of Arizona, Tucson, AZ 85721. A. L. MAPLES is Consultant, Huntsville, AL 35803. Manuscript submitted December 15, 1986.
METALLURGICAL TRANSACTIONS A
then microporosity results. If the melt is refined reasonably well in preparation for the casting process, the gas phase (i.e., microporosity) is constrained to occupy the interdendritic spaces near the end of solidification. Piwonka and Flemings 2 and Flemings 3 give models which illustrate very nicely the interplay between so-called "solidification shrinkage" and gas porosity. The model presented here represents an improvement in their basic approach, mainly because better data are used and the solidification model is more comprehensive and realistic than their models. Turkdogan 4 extended the ideas of solute redistribution in the mushy zone to account for "blow-hole" formation in steel ingots; of course, the major gas considered was carbon monoxide but Turkdogan also showed the interplay of nitrogen and hydrogen along with carbon monoxide on forming the blow-holes. His calculations were conservative in that if the gas pressure exceeded the local pressure (assumed to be 1 to 1.2 atm), the gas bubbles (bl
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