The Effects of Cooling Rate on Microstructure Formation during Solidification of 319 Alloy

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This work assesses the effect of cooling rate ( Ṫ T ˙ ) on the solidification kinetics and microstructural evolution of a 319 aluminum alloy with and without the addition of Al–5 wt%Ti–1wt%B grain refiner. Directionally solidified samples were produced under various cooling rates (from 0.05 to 40 °C/s). Microstructure coarsening was then quantitatively analyzed by measuring grain size (GS) and primary (λ1), secondary (λ2) and average (λL) dendrite arm spacing. The results show that polarized optical images can be used to identify dendrite fragments between neighboring primary dendrites, allowing a more accurate microstructural analysis after the addition of grain refiner. The results also show that, for a given Ṫ T ˙ , although GS is reduced after the addition of grain refiner, the respective values of λ1, λ2 and λL remain practically the same. In other words, the grain refiner only affects nucleation and not grain growth. Experimental growth equations are proposed to represent this behavior. Furthermore, λL is shown to be a good parameter for evaluating microstructural refinement.