Eutectic nucleation and growth in hypoeutectic Al-Si alloys at different strontium levels
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I. INTRODUCTION
CONTROL of microstructure formation during solidification is becoming increasingly important for manufacturers of cast aluminum components. Not only does the microstructure determine the mechanical properties, but the evolution of microstructure during the casting process also defines the boundaries of the casting design, the castability, and the susceptibility to formation of defects, and therefore, consistency and quality of the products. Significant development work has been performed to obtain means for monitoring, assuring, and controlling microstructure development. AlSi alloys of hypoeutectic composition are the most widely used aluminum foundry alloys today. Microstructure control in these alloys is often obtained through further alloying with, and/or in the presence of, other elements, notably Ti, Mg, Cu, and Fe. Grain refinement can be obtained by the use of commercial master alloy additions, although these are not as effective as in the wrought alloys.[1] Commercial Al-Si foundry alloys normally contain about 50 to 90 vol pct eutectic. Controlling the eutectic silicon morphology by modification has been used extensively industrially since about the 1970s to improve the mechanical properties of the castings. Modification, obtained by addition of Na or Sr, or by quench modification, changes the morphology of silicon from a platelike or lamellar structure to a fine fibrous structure, while modification by addition of Sb only refines the silicon platelets. Improved mechanical properties A.K. DAHLE, Senior Lecturer, K. NOGITA, Research Fellow, and S.D. McDONALD, Postgraduate Student, are with the Department of Mining, Minerals and Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. J.W. ZINDEL, Senior Technical Specialist, is with the Ford Research Laboratory, Ford Motor Company, Dearborn, MI 48121. L.M. HOGAN, formerly Honorary Research Fellow, Department of Mining, Minerals and Materials Engineering, The University of Queensland, is deceased. Manuscript submitted June 20, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
normally accompany the refined eutectic structure, although the benefits associated with modification can sometimes be outweighed by altered porosity characteristics associated with modification, particularly modification with strontium.[2] Because the formation of eutectic often is the final stage of solidification, it can be expected to have a significant impact on the formation of casting defects, particularly porosity, and castability. Recent work[3–6] has indicated that our knowledge and understanding of this crucial stage in the solidification process is very limited and that there are three different possible eutectic nucleation and growth modes depending on the solidification conditions. Figure 1 is an illustration of the three proposed eutectic growth modes.[3] These are nucleation at or adjacent to the wall and front growth opposite the thermal gradient, (Figure 1(a)), nucleation of eutectic on primary dendrites, (Figure 1(b)), and
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