ANOMALOUS GRAIN COARSENING BEHAVIOUR IN GRAIN-REFINED ALUMINIUM ALLOYS CAST USING LOW SUPERHEAT

摘要

Chemical grain refinement of aluminium alloys has been commercially practiced for many years. Low superheat casting has also been recognised as a means of inducing grain refinement in aluminium alloys. However, the present study indicates that when both techniques are sued in combination, the result is not always a further improvement in overall refinement, in fact, even grain coarsening can occur. Grain sizes were measured in test castings made from 356, 6063 and 2024 alloys poured at low superheats using various grain refiners (Al-5Ti-1B, Al-6Ti and Al-4B) at a range of addition levels. High addition levels of the Al-Ti-B and Al-Ti grain refiners sometimes yielded marginal improvements in grain size compared to non grain-refined low superheat samples. However, grain coarsening was also observed particularly at the lower addition levels of the Al-B master alloy. The mechanism proposed for this latter phenomenon carries implications for impurity control and recycling.