Microestrutura dendrítica, macrossegregação e microporosidade na solidificação de ligas ternárias Al Si Cu

Abstract

Aluminum casting alloys have properties which are of great industrial interest, such as low density, good corrosion resistance, high thermal and electrical conductivities, good combination of mechanical properties, good workability in machining processes and mechanical forming. Currently, these alloys are produced in various systems and dozens of compositions. The literature presents several studies, both theoretical and experimental, focusing on the microstructural evolution of binary aluminum base alloys. Theoretical and experimental cellular and dendritic growth laws have been proposed and validated. Macrosegregation and pore formation during solidification of binary alloys of aluminum have been the focus of several recent studies. However, there are few studies in the literature addressing important families of multicomponent aluminum base alloys. Accordingly, this study aims to analyze Aluminum Silicon Copper alloys (series: A319.1 and A333.1) [Al 5.5wt%Si 3.0wt%Cu and Al 9.0wt%Si 3.0wt%Cu] with respect to the evolution of the dendritic microstructure, porosity formation and macrosegregation during solidification. For the production of the ternary alloys commercially pure aluminum and silicon and electrolytic copper have been used. Solute macrosegregation and microporosity formation are investigated both experimentally and through numerical simulations. The dendritic microstructure is quantified by their primary, secondary and tertiary arm spacings, which are correlated with solidification thermal parameters. The solute macrosegregation profiles, theoretical and apparent densities have been determined along the castings lengths. The solute segregation profiles were obtained by X ray fluorescence spectrometry and the simulations were performed taking into account secondary phase transformations that occur during solidification. Microporosity measurements were carried out by the picnometry technique. Experimental laws are proposed for the evolution of dendrite arm spacings as a function of cooling rate (Ṫ) and the rate of displacement of the liquidus isotherm (VL), given by (λ1, λ3) = C (T) 0.55andλ2 = C (VL) 1/3, respectively. The experimental values of dendrite arm spacings were compared with other experimental studies of dendritic growth for binary Al Si alloys,as well as with the only theoretical growth model existing in the literature for multicomponent alloys. The ternary phase diagram, the solidification paths of both alloys analyzed, and thermo physical properties required for numerical simulations were determined by the software Thermo Calc®. The results have shown that the theoretical growth model fits well the experimental scatter for the alloy with lower Si content, overestimating that of the alloy with higher Si content.The volumetric fraction of pores showed an upward trend from the bottom to the top of the casting. It was also found that the presence of silicon in the alloy acts as an inhibitor of inverse segregation of copper.