What is the Process Window for Semi-solid Processing?
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ess window is defined as a collection of process parameters that allow materials to be manufactured and to operate under desired specifications.[1] In semi-solid processing, the process window is related to a stage between the solidus and the liquidus of a metallic alloy. Semi-solid processing from a solid-state starting point (thixoforming) is normally carried out with between 30 and 50 pct liquid as it leads to an optimum slurry viscosity and a good die filling property.[2–4] Kazakov[5] examined the processability from a thermodynamic prediction and Differential Scanning Calorimetry (DSC) view point focusing on the slope of the liquid fraction vs temperature. He defined several critical parameters including the slope (dfL/dT) of the liquid fraction vs temperature curve and noted that at 50 pct fraction liquid, this should be relatively low. Atkinson et al.[6] identified that it is the slope of the liquid fraction vs temperature curve in the region of 30 to 50 pct liquid which is the key parameter. More recently, Liu et al.[7] using the MTDATA thermodynamic package and DSC predicted the liquid fraction sensitivity ðdfL =dTÞfL¼0:4 at 40 pct fraction liquid (mid-way through the process
DUYAO ZHANG, formerly Ph.D. Candidate, and HONGBIAO DONG and HELEN ATKINSON, Professors, are with the Department of Engineering, University of Leicester, University Road, Leicester LE1 7RH, U.K. Contact e-mail: [email protected] Manuscript submitted July 24, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
range of 30 to 50 pct liquid) and argued that it should not change too rapidly with temperature in the process window. As pure metals and the eutectic alloys do not have a freezing (or melting) range, it has been conventionally expected that it is not possible for them to be semi-solid processed because (dfL/dT) is ¥. Based on Liu et al.’s theory,[7] recent studies have been focused on the evaluation of the thixoformability through processing temperature and modifying alloy elements.[8–10] Surprisingly Curle et al.[11,12] recently demonstrated that both high-purity aluminum and binary Al-Si eutectic alloy can be rheocast. Rheocasting involves cooling from the liquid state into the semi-solid range and forming at about 60 pct liquid or higher. This is a different approach from thixoforming but the principles that will be drawn out in this paper are applicable in both cases. Curle et al.[11,12] identify that the processing in the semi-solid state is achievable because of the solidification kinetics during the thermo-arrest, i.e., solidification is not instantaneous. It seems, therefore, that time can be a critical factor during processing and not just temperature. In addition, this will be relevant not only with pure metals, and binaries at the eutectic point, but also with other alloys where there is a freezing range. Those who carry out the process in industry understand that time is a factor because isothermal holds are introduced during the heating process for the billet for thixoforming but the role of time in determining the process window h
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