Experiments were carried out to analyze the effect of growth rates (VL) and cooling rates (TR) on both secondary dendritic arm spacings (λ2) and Vickers microhardness (HV) of an Al-9wt.%Si alloy during the horizontal directional solidification under transient heat flow conditions. A water-cooled solidification experimental apparatus was developed allowing a wide range of TR (from 0.2 to 3.5 ºC/s) to be experienced. Five computer guided thermocouples were connected with the metal, and the time-temperature data were recorded automatically. The solidification path was also calculated by Scheil model in Thermo-Calc software. Casting samples were characterized by the combined analyses of optical microscopy (OM) and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS) revealing a complex arrangement of phases including α-Al dendrites, Si particles and Fe-rich phases within interdendritic regions. It was observed that power law functions characterize the variation of λ2 as a function of VL and TR with exponents of-2/3 and-1/3, respectively. Finally, experimental laws of power and Hall-Petch types are proposed relating the resulting HV to the λ2. According to these results, it was found that, for increasing values of λ2, the results of HV decrease.

This article analyzes the effects of Cu content, thermal and microstructural parameters on the mass loss and electrochemical corrosion resistance of two directionally solidified Al-XCu (X=3wt.% and 6wt.%) hypoeutectic alloys. The investigated solidification parameters include the growth rates (VL), cooling rates (TR), secondary dendrite arm spacings (2) and Al2Cu intermetallic phase. Open circuit potential (EOC), anodic and cathodic polarization curves with the respective corrosion potential and current (ECORR and ICORR) and H2 release volume evolution (H2Vol) corresponding to corrosion kinetics were used to study the corrosion resistance in a 0.2 mol.L -1 solution of HCl with an immersion time of 10 min at 25 º C. The corrosion tests were performed at two different positions in relation to cooled interface of the studied alloys. It was found that the corrosion resistance is higher at position closer to the metal-mold interface, in which the VL and TR values are higher and the 2 value is smaller. The results have shown, for both investigated alloys, that the Al2Cu intermetallic particles are more susceptible to electrolytic corrosion for higher secondary dendrite arm spacings. On the other side, considering a same value for 2 these particles are less resistant to corrosion in the eutectic matrix of the Al-3wt.%Cu alloy. Thus, the results have indicated that the Al-6wt.%Cu alloy has better electrochemical corrosion resistance than the Al-3wt.%Cu alloy.

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