DIFFICULTIES IN GRAIN REFINING ALUMINUM LITHIUM ALLOYS USING COMMERCIAL Al-Ti AND Al-Ti-Bor MASTER ALLOYS

High strength aluminum alloys, especially those containing zirconium, are historically difficult to grain refine. Early efforts by Alcoa indicated that the addition of lithium to these alloys increases difficulty with grain refinement. Although grain refining of alloys 2090 and 2091 was possible, consistent results were not obtained. Statistically-designed experiments were used to quickly identify critical parameters in both the casting and grain refining operations. Responses from variables including alloy composition, amount and type of grain refining addition, melt temperature and cooling rate were quantified prior to establishing control limits. Production ingot cast by Alcoa are now consistently grain refined. Future work will simultaneously investigate the impact of important process variables on the fundamental mechanisms of grain refining and establish appropriate targets and ranges for these variables.