Mechanical properties and performance of metallic materials depend on their microstructures. In order to develop engineering materials that match prescribed criteria and to enable design with multifunctional materials, it is essential to be able to predict their microstructural patterns, such as size, shape, and spacing of the dendritic structures observed in solidified metals. In the cases of metallic alloys, which present dendritic structure, the mechanical properties of foundry products depend mainly on the primary-and secondary-arm. Therefore, it is important, in a computational simulation of the solidification processes, to use reliable methods to correlate the thermal parameters with secondarydendrite arm spacing. This study presents a numerical model for prediction of secondary-arm spacing as a function of thermal parameters (cooling rates and local solidification time). Spacing of the arms for a binary alloy is numerically predicted using a phase-field model. Secondary dendrites calculated by phase-field model, they are similar to the ones found in experiments investigation of solidification in Al-Cu alloys. Arm spacing predicted in the present work, when compared with the experimental results, showed good agreement. Its estimation takes place at the late stage of growth. The effect of physical properties (partition coefficient (k e ), diffusion in the liquid (D L ) and diffusion in the solid phase (D S )) on secondary-arm spacing is systematically investigated by phase-field model. With the help of numerical results for Al-4.5wt%Cu alloy, the applicability of the phase-field model to the estimation of secondary-dendrite arm spacing during unidirectional solidification is demonstrated.

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The directional solidification technique allows the study of growth of the solid phase, as-cast structure and, finally, its mechanical characteristics as a consequence of thermal parameters. On the other hand, in the last decade, a process called equal-channel angular pressing (ECAP) has emerged as a widely-known technique in fabrication of ultrafine-grained metals and alloys. Applicability of the ECAP technique affords an excellent potential for changing, in a controlled and beneficial manner, the resulting properties of metals and alloys. For this paper, an experimental research has been conducted to study the effects of solidification parameter (cooling rate) on resulting microhardness in hypoeutectic Al-Si alloys, upon use of an ECAP procedure. The influence of cooling on the scale of the dendritic patterns is presented and discussed with recourse to equations. The resulting microhardness variation with position throughout the as-cast materials and cooling rate were characterized by experimental power laws. Results determined after the solidification experiments have revealed microhardness as a function of both cooling rate and position (P) of the as-cast materials to be dependent on alloy composition. In the ECAP process via route C with three passes, "as-solidified" microstructures have been found to be distorted and fragmented during the severe plastic deformation. This deformation imposed on the billets during the ECAP process facilitated obtaining a fine microstructure and high levels of microhardness were observed. However, even with the ECAP process, it was shown that microhardness is strongly dependent of the cooling rates.

A comparative analysis was carried out of the alloys examined, using the results found for thermal variables, macrostructure, dendrite arm spacing, modulus of elasticity and microhardness. Comparing all the results for variables, a predominance was found of higher values for the Al-5.0 wt.% Cu alloy in relation to those for the Al-5.0 wt.% Ni alloy. Although structural transition has occurred in both alloys, the columnar–equiaxed transition position in castings was not altered during the solidification experiments. Addition of Ni solute into pure aluminum favors a significant decrease in tertiary dendrite spacing, while Cu addition leads to a coarser microstructure. Primary dendrite arm spacing, also, was measured along the castings of both alloys. Theoretical approaches such as the well-known Hunt–Lu, Bouchard–Kirkaldy, and Kurz–Fisher models were used to determine quantitatively this particular microstructural parameter. Good agreement was observed between experimental data and predictions by Hunt–Lu and Bouchard–Kirkaldy models, which assume solidification under a transient heat-flow condition. Finally, empirical equations relating tertiary dendritic arm spacing and mechanical properties to the scale of the dendritic microstructure have been proposed. While no relationship was observed between modulus of elasticity and tertiary dendritic arm spacing, addition of Cu into pure aluminum as an alloying element favored a reduction in the average value of modulus of elasticity when compared with those observed for the Al–Ni alloy. It was found that the microhardness decreases with increasing tertiary dendritic arm spacings. On the other hand, solute addition of Cu into commercially pure aluminum favors a microhardness higher than that verified during upward unidirectional solidification of the Al-5.0 wt.% Ni alloy.