The effect of partial decanting on the chemical composition of semi-solid hypo-eutectic Al–Si alloys during solidification

Abstract

Liquid enrichment was studied for hypoeutectic Al–7% Si alloys using a new innovative process. A portion of the residual liquid within the semisolid state was decanted and the chemical composition of the decanted billet and liquid was investigated by the spectroscopic analysis. The measured silicon content of the decanted Al–Si binary alloy is in good agreement with the theoretical calculations. For commercial 356 alloy, a multi-component alloy system, the calculated Si content showed about ∼2–3% difference to that of the experimental trials. The Si, Mg, and Fe contents in the drained liquid were always higher than the cast piece; a clear indication of segregation of these elements within the last liquid. For Ti however, the trend was reversed and the reduced Ti% in the decanted liquid was attributed to the peritectic reaction and the presence of Ti-based intermetallics as the nucleation sites for the formation of the primary α-Al within the mush. The comparison of the chemical compositions of the cast pieces and drainages with those of the standard value indicated that even with 15% drainage, the composition of the alloy is still within the targeted range, i.e. standard value. Consequently, the billets so produced are appropriate feedstock and the remaining scrap is fully recyclable.