Experimental study of structure formation in binary Al–Cu alloys at different cooling rates

A series of casting experiments was performed with binary Al–Cu alloys with varying cooling rates, cooling conditions and copper concentration. The quantitative relationships between the cooling rate (Vc) and composition, on one hand and the dendrite arm spacing (DAS) and grain size (Dgr) on the other hand, are clarified in the form of coefficients in equation DAS ( D gr ) = A V c − n , where coefficient A is shown to strongly depend on the copper concentration. Increasing the amount of copper in the alloy causes refinement of both dendrite arm spacing and grain size. The amount of non-equilibrium eutectic depends on the cooling rate in a more complex manner with initial increase in the range of slow cooling (<1 K/s) and then gradual decrease in the range of 1–10 K/s. Some models of microsegregation are tested but fail to reproduce the experimentally observed dependences. The reasons for such a behaviour are discussed in terms of dendrite arm coarsening, eutectic reaction undercooling, structure refinement and homogenization upon cooling in the solid state.