Desenvolvimento e caracterização térmica, microestrutural e mecânica de uma liga Al-3%cu-0,5%nb solidificada horizontalmente e endurecida por precipitação

Abstract

In this sense, thermal, microstructural and mechanical analyzes were carried out in the present work with the horizontally solidified Al-3wt.%Cu-0.5wt.%Nb alloy, as well as the T6 heat-treatment was applied to some samples of the solidified ingot. Experimental cooling curves for cooling conditions outside thermodynamic equilibrium and very close to this equilibrium, the latter performed by differential scanning calorimetry (DSC), were obtained to thermally characterize the liquidus temperature (TL) of the investigated alloy and thus apply the superheat in liquid metal. Then, horizontal solidification experiment was performed using a water-cooled directional solidification device. A temperature mapping was performed during solidification and the thermal data obtained were used to determine the solidification thermal parameters, such as growth and cooling rates, Gr=2,17(P)-0,3 and CR=82,8(P)-0,7 respectively. The solidified ingot was characterized for microstructural analysis by optical (OM) and scanning electronic (SEM/EDS) microscopes. The DSC analysis have allowed to admit that the value found at 653°C for TL is in accordance with the ranges of values indicated in the phase equilibrium diagram for the binary Al-Nb system, allowing to run the horizontal solidification experiment. The typical solidification microstructure consisted of a dendritic network as an Al-rich primary phase (Alα) plus secondary phases composed of Fe and -Al2Cu intermetallic compounds within the interdendritic regions. For the assumed solidification conditions, the Nb was observed in the investigated alloy completely dissolved in the Al-rich matrix (Alα), not forming the peritectic reaction predicted in the phase equilibrium diagram of the Al-Nb system. The primary phase was quantified using secondary dendritic spacings (2), and a mathematical association between GR- 2, and CR - 2 was proposed, 2=23GR-2/3 -R2=0,8 and 2=54(CR)-1/3 -R2=0,9. Mechanical analysis through the results of the Vickers microhardness test (HV) showed that HV did not vary along the length of the solidified ingot, assuming an average value equal to 55 HV. As expected, higher HV values were noted on the hard interdendritic phases (-Al2Cu+ Fe intermetallic compounds) when compared to measurements on the Alα phase (matrix). It was observed that the T6 heat-treatment applied to the solidified samples, under the assumed conditions in both processes, did not significantly affect the secondary dendritic spacings. It was verified the precipitation hardening after the heat treatment, and an increase in the HV values was observed with the precipitation sequence of the -Al2Cu phase with the aging course, under the assumed conditions. Finally, a theoretical-experimental analysis was carried out in this work on 2 growth models, and the results allowed to admit that although the dendritic network grows as equiaxed dendrites, and the mathematical model, despite being developed for binary alloys, represented very well the experimental data from this work and, thus, it was deduced that the referred model can be used for ternary alloys when the third element is well diluted in the matrix.