MACROSTRUCTURAL DEVELOPMENT IN ALUMINUM-ALLOYS SOLIDIFIED VERTICALLY DOWNWARDS

Citation
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
Citations number
8
Categorie Soggetti
Material Science
ISSN journal
09215093
Volume
173
Issue
1-2
Year of publication
1993
Pages
123 - 127
Database
ISI
SICI code
0921-5093(1993)173:1-2<123:MDIASV>2.0.ZU;2-H
Abstract
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.