What is the Process Window for Semi-solid Processing?

Abstract

Abstract.In semi-solid processing, the liquid fraction versus temperature is commonly used to define the process window.Conventionally it is assumed a freezing range is required for semi-solid processing but recently both high-purity aluminium and binary Al-Si eutectic alloy have been rheo-processed. Here, the kinetics during melting and solidification of pure metal are analyzed and a comparison between the liquid fraction versus temperature and the liquid fraction versus time presented. It is found that liquid fraction versus time is a significant criterion for semi-solid processing.Keywords: Process window; Aluminium; DSC; Semi-solid processing Process 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% liquid as it leads to an optimum slurry viscosity and a good die filling property [2,3,4]. Kazakov [5] examined the processability from a thermodynamic prediction and Differential Scanning Calorimetry (DSC) view point focusing on the slope of the liquid fraction versus temperature. He defined several critical parameters including that the slope ( / ) of the liquid fraction versus temperature curve and noted that at 50% fraction liquid this should be relatively low. Atkinson et al. [6] identified that it is the slope of the liquid fraction versus temperature curve in the region of 30-50% liquid which is the key parameter. More recently, Liu et al. [7] using the MTDATA thermodynamic package and DSC predicted the liquid fraction sensitivity ( / ) =0.4 at 40% fraction liquid (mid-way through the process range of 30-50% 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 ( / ) is ∞. Based on Liu et al. 's theory [7], recent studies have been focused on the evaluation the thixoformability through processing temperature and modifying alloy elements [8][9][10].Surprisingly Curle et al. [11,12] recently demonstrated that both high-purity aluminium 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% 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 rele...

Ab initiolocal-energy and local-stress analysis of tensile behaviours of tilt grain boundaries in Al and Cu

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“…Processability of semi-solid alloys are affected by the size and morphology of primary particles as well as liquid fraction [169][170][171], so a sufficiently expanded solidus-liquidus interval and optimal liquid/solid ratio of the metal slurry are required traditionally.…”

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Current Progress in Rheoforming of Wrought Aluminum Alloys: A Review

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