Predicting the Effect of Pouring Temperature on the Crystallite Density, Remelting, and Crystal Growth Kinetics in the S
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INTRODUCTION
DESPITE the wide use of gravity casting for aluminum (Al)-based materials and some generally welldeveloped solidification theories,[1–4] there are still some areas where the impact of processing conditions on the microstructural evolution of gravity cast alloys remains unexplained; for instance, though it has been established that the primary and secondary dendrite arm spacing in a certain Al alloy (i.e., Al-4.5Cu alloy) depends on the square root of the solidification time,[1] predicting the grain size of this alloy (or any alloy for that matter) under different processing conditions has until now not been accomplished. Yet grain size is one of the most important factors in resistance to hot tearing[1] as well as mechanical properties including yield stress, ultimate tensile stress, fatigue strength, and others.[2] Being able to predict the grain size that results from processing conditions is, therefore, essential. Previous research to predict grain size in Al alloys from gravity casting has primarily focused on the role of grain refining agents such as TiB2. Stefanescu[5] has J.B. FERGUSON, Postdoctoral Research Associate, MEYSAM TABANDEH-KHORSHID, Graduate Research Assistant, JOHN C. MANTAS, Undergraduate Research Assistant, PRADEEP K. ROHATGI, Professor, and CHANG-SOO KIM, Assistant Professor, are with the Materials Science and Engineering Department, University of Wisconsin-Milwaukee, 3200 N. Cramer St, Milwaukee, WI 53211. Contact e-mail: [email protected] KYU CHO, Senior Engineer, is with the U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD 21005. Manuscript submitted October 29, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
reviewed models that predict grain size based on the undercooling that is achieved during solidification. Easton and St. John[6] and Murty et al.[7] have reviewed the many nucleant- and solute-based theories that have been proposed for the two widely studied systems of Al-TiB2 and Al-Ti-TiB2. Easton and St. John[8] have proposed a solute-based theory to describe the effect of Ti and TiB2 on grain size. This theory takes into account dendrite growth in promoting nucleation in the constitutionally undercooled zone ahead of the solidifying interface. There have also been attempts to predict the final grain size of Al alloys with TiB2 additions based on the size distribution of TiB2 particles.[9–11] These models attempt to show that only certain sizes of TiB2 particles can actively influence the grain size and that grain refining efficiency can be enhanced by limiting particle size ranges to only those that are active. However, the quantitative effect and fundamental basis of pouring temperature are not generally considered in the previous studies. Also, it is unclear if these previous theories are applicable to alloys in which grain refining agents are not present. In the nucleation-based models, nucleation rate is a critical parameter. However, it is immeasurable and must be determined a posteriori based on the assumed equival
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