Effect of Composition and Stirring on the Grain Structure of Aluminium Alloys solidified in Ingots

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Abstract
Solidification experiments have been performed on a series of industrial aluminium alloys in a rectangular vertical ingot mould, in which an electromagnetic linear motor activated a recirculating flow. Results include macrographs, grain size measurements, and high resolution temperature records on 14 thermocouples. The refinement effect is clearly demonstrated. It is strong when stirring changes from zero to some moderate level, then its increment decreases asymptotically. In most of the temperature records, the recalescence behaviour due to the formation of equiaxed crystals can be followed in detail. Under good stirring conditions, equiaxed crystallisation starts simultaneously in all the volume elements and all of them follow the same thermal history, which explains the very good uniformity in grain structure. Non-uniform or coarse grain structures which are observed when stirring conditions are poor can be correlated to dissimilarities in the thermal history of the different volume elements. A theoretical model explains these observations and allows the effects of stirring and of composition on the equiaxed grain size to be predicted.