Effect of Cooling Rate on Microstructure and Hardness during Solution Treatment and Aging Process of Ti-6Al-4V Alloy for Aerospace Components

The microstructure and hardness of Ti-6Al-4V alloys were evaluated before and after aging to investigate the effect of the cooling rate at the solution treatment stage of the solution treatment and aging (STA) process for aerospace components. The cooling rate distribution across a Ti-6Al-4V alloy sample representing practical STA process of an aerospace fan disk component was estimated using finite element analysis. The Ti-6Al-4V alloy specimens were solution-treated with the obtained cooling rates, and aged. Experimental WQ of the Ti-6Al-4V alloy specimen in the solution treatment facilitated the retention of the primary α phase, while the β phase was transformed to α′. Controlled cooling at slower rates compared to WQ led to the formation of coarser globular α and the transformation of β to α+β lamellae. Subsequent aging of the specimens led to the observation of α+β lamellae in all specimens regardless of the cooling conditions, where the average thickness of the plates in lamellae was higher at slower cooling rates. The specimens exhibited higher hardness values when cooled more quickly at the solution treatment, which was attributed to the formation of thinner α plates and/or a higher α′ fraction. The hardness value of all specimens increased further after aging, regardless of the cooling conditions, due to precipitation of Ti3Al in the α phase and/or fine α plates from the α′ phase.