Wd. Griffiths et Dg. Mccartney, MACROSTRUCTURAL DEVELOPMENT IN ALUMINUM-ALLOYS SOLIDIFIED VERTICALLY DOWNWARDS, Materials science & engineering. A, Structural materials: properties, microstructure and processing, 173(1-2), 1993, pp. 123-127
To study the role of fluid now in influencing macrostructure, and part
icularly fragmentation processes versus heterogeneous nucleation, thre
e hypoeutectic compositions of Al-Cu and Al-Si alloys were solidified
vertically downwards. This promoted thermally driven convection in the
built liquid. Temperature measurements in the mould showed that, upon
initial cooling, turbulent fluid flow would be expected across the so
lidification front. This then became laminar and finally ceased altoge
ther. At the base of unrefined Al-Cu and Al-Si alloy ingots were found
zones of equiaxed grains; their area apparently independent of compos
ition or pouring superheat. The size of the equiaxed zones was rather
associated with observed porosity and encouraged by the entrainment of
oxide suggesting heterogeneous nucleation was at least partly respons
ible for their formation. In unrefined Al-Cu ingots, cooling curves fr
om the equiaxed zone displayed recalescence typically associated with
equiaxed growth. However, in Al-Si alloys two thermal arrests were rec
orded in this region, possibly indicating two stages in solidification
. Wholly equiaxed structures were obtained in refined Al-Cu and Al-Si
alloys and the cooling curves showed recalescence corresponding to equ
iaxed growth. The microstructures of Al-16wt.%Cu and Al-24wt.%Cu showe
d mixed globular and dendritic particles, the former possibly indicati
ve of fluid now and associated solid transport.