Solidification thermal parameters and dendrite arm spacings have been measured in hypoeutectic SnPb and Al-Cu alloys solidified under unsteady-state heat flow conditions. It was observed that both primary and secondary spacings decreased with increased solute content for Sn-Pb alloys. For Al-Cu alloys, the primary spacing was found to be independent of composition, and secondary spacings decrease as the solute content is increased. The predictive theoretical models for primary spacings existing in the literature did not generate the experimental observations concerning the Sn-Pb and AlCu alloys examined in the present study. The theoretical Bouchard-Kirkaldy's (BK's) equation relating secondary spacings with tip growth rate has generated adequately the experimental results for both metallic systems. The insertion of analytical expressions for tip growth rate and cooling rate into the predictive model, or into the resulting experimental equations in order to establish empirical formulas permitting primary and secondary dendritic spacings to be determined as functions of unsteadystate solidification parameters such as melt superheat, type of mold, and transient metal/mold heattransfer coefficient is proposed.for instance, the solidification conditions of a body of irregular shape, these variables are interdependent, cannot be controlled, and vary freely with time. The analysis of dendritic structures in the unsteady-state regime is very important, since it encompasses the majority of industrial solidification processes.The measurements of primary and secondary dendrite arm spacings involve looking at the microstructure after complete solidification. Primary spacings do not coarsen with time and can be accurately measured from the microstructure and compared with growth models. On the other hand, secondary spacings are seen to rapidly coarsen during solidification, and the effect of coarsening has to be taken into account by the predictive growth models. [29] Most of the results in the literature, concerning steady and unsteady regimes, pertaining to l 2 in hypoeutectic alloys, indicate a decrease in spacing with increasing cooling rate for a given alloy composition and with increasing solute content for a given cooling rate. [28,29,38,39] The reports in the literature also indicate that for steady or unsteady growth conditions, the primary arm spacings decrease as G L or V L increases. On the other hand, there has been some disagreement in the literature regarding the influence of the initial alloy composition on primary spacings. It has been reported in the majority of cases that l 1 increases as C 0 (for hypoeutetic alloys) increases for both steady and unsteady growth conditions. [28,29] However, Sharp and Hellawell [44] found that C 0 has little effect on primary spacings and Spittle and Lloyd [46] reported that for steady-state growth with low G L , l 1 decreased as C 0 increased and was independent of C 0 for high G L , and for unsteady solidification l 1 decreased as C 0 increased.The present article focuse...

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The roles of growth direction and Si content on the columnar/equiaxed transition and on dendritic spacings of Al–Cu–Si alloys still remain as an open field to be studied. In the present investigation, Al–6 wt-Cu–4 wt-Si and Al–6 wt-Cu alloys were directionally solidified upwards and horizontally under transient heat flow conditions. The experimental results include tip growth rate and cooling rates, optical microscopy, scanning electron microscopy energy dispersive spectrometry and dendrite arm spacings. It was found that silicon alloying contributes to significant refinement of primary/secondary dendritic spacings for the upward configuration as compared with corresponding results of the horizontal growth. Experimental growth laws are proposed, and the effects of the presence/absence of solutal convection in both growth directions are discussed.

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