Some Consequences of Thermosolutal Convection: The Grain Structure of Castings

Hansen, G. P.; Hellawell, A.; Lu, Shu-Zu; Steube, R. S.

doi:10.1007/bf02648947

Cited by 33 publications

(23 citation statements)

References 21 publications

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“…The calculations show that k 2 V m is constant for a given alloy composition, where V is the growth tip velocity and m is a factor ranging from 1/3 to 1/2. Hansen, Hellawell et al [19] developed a six-step physical model in order to explain equiaxed crystal formation, and consequently, columnar-to-equiaxed transition (CET). The model considered remelting as a source for the equiaxed grains and the thermosolutal convection an essential transport mechanism in the mushy zone and in the open bulk.…”

Section: Introductionmentioning

confidence: 99%

On the Coupling Mechanism of Equiaxed Crystal Generation with the Liquid Flow Driven by Natural Convection During Solidification

Stefan-Kharicha

Kharicha

et al. 2018

Metall Mater Trans A

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The influence of the melt flow on the solidification structure is bilateral. The flow plays an important role in the solidification pattern, via the heat transfer, grain distribution, and segregations. On the other hand, the crystal structure, columnar or equiaxed, impacts the flow, via the thermosolutal convection, the drag force applied by the crystals on the melt flow, etc. As the aim of this research was to further explore the solidification-flow interaction, experiments were conducted in a cast cell (95 * 95 * 30 mm 3 ), in which an ammonium chloride-water solution (between 27 and 31 wt pct NH 4 Cl) was observed as it solidified. The kinetic energy (KE) of the flow and the average flow velocity were calculated throughout the process. Measurements of the volume extension of the mush in the cell and the velocity of the solid front were also taken during the solidification experiment. During the mainly columnar experiments (8 cm liquid height) the flow KE continuously decreased over time. However, during the later series of experiments at higher liquid height (9.5 cm), the flow KE evolution presented a strong peak shortly after the start of solidification. This increase in the total flow KE correlated with the presence of falling equiaxed crystals. Generally, a clear correlation between the strength of the flow and the occurrence of equiaxed crystals was evident. The analysis of the results strongly suggests a fragmentation origin of equiaxed crystals appearing in the melt. The transition from purely columnar growth to a strongly equiaxed rain (CET) was found to be triggered by (a) the magnitude of the coupling between the flow intensity driven by the equiaxed crystals, and (b) the release and transport of the fragments by the same flow recirculating within the mushy zone. Graphical AbstractMIHAELA STEFAN-KHARICHA, ABDELLAH KHARICHA, MENGHUAI WU, and ANDREAS LUDWIG are with the

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Section: Introductionmentioning

confidence: 99%

On the Coupling Mechanism of Equiaxed Crystal Generation with the Liquid Flow Driven by Natural Convection During Solidification

Stefan-Kharicha

Kharicha

et al. 2018

Metall Mater Trans A

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“…Several authors have investigated the effect of the flow behaviour on the solidification process using analogue systems [16][17][18][19][20]. Paradies et al [19] showed that an imposed velocity in the melt dramatically changed the rate and total amount of crystal fragmentation developed.…”

Section: Introductionmentioning

confidence: 99%

Cooling slope casting to obtain thixotropic feedstock I: observations with a transparent analogue

2008

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New Rheocasting (the NRC process) is a recently developed semisolid processing route. There are two versions of this route. In one, molten alloy is poured directly into a tilted mould and through careful temperature control during cooling a spheroidal semisolid microstructure is achieved, before the material in the mould is upended into a shot sleeve and hence forced into a die. Alternatively, the molten alloy is poured onto a cooling slope and thence into a mould before processing. The aim of the work described in this paper, and its companion, was to develop understanding of the microstructural development during the initial stages of this process i.e. in the mould before processing and with the cooling slope/mould combination. In this first paper, an analogue system based on aqueous ammonium chloride has been used to visualise what happens when an alloy is poured into a tilted mould with a chill wall. In the companion paper, the results for pouring A356 aluminium alloy directly into a mould, and also via a cooling slope into a mould are presented. Keywords Cooling slope; thixoforming; semisolid processing; thixotropic; transparent analogue; ammonium chloride; New Rheocasting. IntroductionIn the early 1970s, a group at MIT discovered that when a solidifying alloy was continuously stirred the viscosity fell to a much lower value than for the static case [1]. This was associated with the fact that, during stirring in the semisolid range, the dendritic microstructure was destroyed and the semisolid consisted of spheroids of solid in a liquid matrix. In this state, the material behaved thixotropically i.e. when sheared it flowed but when allowed to stand it thickened again; the viscosity was a function both of time at temperature and of shear rate. The behaviour was associated with the dynamic development of microscopic 'welds' between solid spheroids (agglomeration) and the break up of these welds under shear (disagglomeration) [2]. It became the basis of a new range of forming technologies termed collectively semisolid processing. The advantages over conventional die casting including less mould attrition, reduced porosity and hence higher part integrity, and near net shape (because there is less solidification shrinkage). A range of reviews are available [2][3][4][5][6].

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“…Fig. 1 shows a dendrite grown [2] in the transparent analogue system NH 4 Cl-H 2 O. Many of its secondary arms display the familiar geometry characteristic of alloy systems, being highly constricted at the base where they join the primary 2 dendrite trunk.…”

Section: Introductionmentioning

confidence: 99%

“…This geometry is usually attributed to the initial growth phase of the secondary arm as it grows through the solute rich boundary layer adjacent to the primary trunk. These features appear delicate in the extreme, yet it is well established that in quiescent conditions they may persist for long periods of time as relatively stable features [3,4]. However, if perturbed by motion of the parent melt detachment may occur.…”

Section: Introductionmentioning

confidence: 99%