EFFECT OF DISPERSED PHASE COMPOSITION AND DISTRIBUTION AFTER AGING ON FORMABILITY OF D16 ALUMINIUM ALLOY SHEETS

Abstract

The relevance of this paper is connected with rising accuracy requirements to stamped parts made of aged aluminium alloys applied also for layered composite making. These requirements can be met by controlling the sheet blank structure, and particularly its phase composition. The paper provides experimental results obtained when studying the effect of aging modes on the composition, dispersed phase distribution pattern and formability of sheet samples made of D16 (AA2024) aluminium alloy. Heat treatment consisted in quenching from a temperature of 500 °С into room temperature water and further aging: natural aging during 7 days, artificial aging at temperatures of 50, 100, 150 and 200 °С and holding at each temperature during 15, 30, 60, 120 and 240 minutes. The quantitative method is proposed to evaluate the dispersed phase distribution pattern by microstructure pictures. Formability was evaluated using the stamping number, i.e. a proof/ultimate factor. It was found that the stamping number rises as the aging temperature and holding time are increased, which shows the lower alloy applicability for sheet stamping operations. No dispersed phase was formed when aged at t = 50 °С in both optical metallography and scan electron microscopy cases. The non-uniformity of dispersed phase distribution inside a grain rises at initial aging stages with a holding time less than 1 hour at 100, 150 and 200 °С and decreases with a further increase in the holding time up to 4 hours. No correlation was observed between the uniformity of phase distribution and the stamping number. The chemical composition of phases has a greater effect on the formability and changes depending on a heat treatment mode: annealing and natural aging primarily lead to the θ and S phase precipitation; aging at temperatures below 150 °С with a holding time less than 1 hour lead to the θ, S and T phase precipitation; θ phase appears after aging at temperatures over 150 °C with long holding times.