Structure observations related to hot tearing of Al–Cu billets produced by direct-chill casting

Abstract

In this paper, hot cracking in Al–(1–4.5%) Cu direct-chill (DC) cast billet was investigated by means of controlled DC casting followed by structure examination. No hot cracks were found in the billets of alloys containing more than 4% Cu. The hot tear surfaces of the DC cast billets with different compositions were observed in a scanning electron microscope. The porosity fraction measurement was also performed. It was found that decreasing the copper concentration from 3 to 1% caused a decrease in the amount of eutectic observed on the hot tear surface. Special features appeared on the hot tear surface of an Al–1% Cu alloy in which the hot tear predominantly propagated through the solid phase, i.e., bridged grain boundary. A solute-rich (eutectic) path along grain boundaries ahead of the hot-crack tip was observed in billets with hot tears, which could be an evidence of crack healing. The porosity concentrated in the center of the billet and its amount was lower in alloys with high hot tear susceptibility and in alloys not susceptible to hot tearing. Increased casting speed lead to increasing porosity.