Amauri Garcia

ABSTRACT Al–In monotectic alloys are potential alternatives for application in the manufacture of wear-resistant automotive components, such as cylinder liners and journal bearings. The comprehension regarding the development of distinct microstructures of monotectic alloys and their interrelation with wear behavior are challenges of prime importance. The present study aims to contribute to a better understanding of the relationship between the scale of the minority phase of the monotectic microstructure and the corresponding wear behavior. Transient directional solidification experiments were carried out with an Al–5.5 wt% In alloy with a view to provide samples with significant differences in the microstructural scale along the casting length. The results of wear tests permitted an experimental quantitative expression correlating the wear volume (V) with both the interphase spacing between indium droplets (λ) and time of wear tests (t) to be proposed. The increase in λ is shown to improve the wear resistance. The effect of λ on V becomes more significant as the sliding distance (or time) is increased.

ABSTRACT This present study is focused on the evaluation of the effect of the cell spacing of as-cast Al-2 wt.% Bi alloy formed by droplet-like bismuth particles distributed in the Al matrix, on the corrosion response. EIS parameters and polarization curves obtained in a 0.5 M NaCl solution at room temperature are discussed. It was found that with the decrease in the cell spacing, the corrosion current density and polarization resistance increases and decreases, respectively. This corrosion behavior was attributed to strains that are generated at the borders between the Al-matrix and droplet-like Bi particles.

ABSTRACT Immiscible Al-based alloys of monotectic composition have a particular feature of minority phases embedded into the Al-rich matrix. The disseminated particles may act as in situ self-lubricating agents due to their lower hardnesses compared with that of the Al-rich matrix, favoring good tribological behavior. There is a lack of systematic fundamental studies on the microstructural evolution of monotectic alloys connected to application properties. In the present investigation, the monotectic Al-1.2wt%Pb and Al-3.2wt%Bi alloys have been chosen to permit the effect of microstructural parameters on the wear behavior to be analyzed. Directional solidification experiments were carried out under transient heat flow conditions allowing a large range of cooling rates to be experienced, permitting a representative variation on the scale of the microstructure to be examined. Samples of the monotectic alloys having different interphase spacing, λ, have been subjected to microadhesive wear tests, and experimental laws correlating the wear volume with the microstructural interphase spacing and test time are proposed. It was found that microstructural features such as the interphase spacing and the morphology of the minority phase play a significant role on the wear process and that for the alloys examined λ exhibits opposite effects on the corresponding wear volume.

Despite the importance of a complete characterization of dendritic patterns in castings, the availability of studies on the development of tertiary dendrite arms is scarce in the literature. In the present study, the tip cooling rate, local solidification time, primary and tertiary dendrite arm spacings have been determined in Pb–Sb alloys castings directionally solidified under unsteady-state heat flow conditions. The alloys compositions experimentally examined are widely used in the as-cast condition for the manufacture of positive and negative grids of lead-acid batteries. The initial growth of tertiary dendritic arms from the secondary branches was found to occur only for a Pb–3.5 wt% Sb alloy at cooling rates in the range 0.4–0.2 K/s, with no evidence of this spacing pattern for Pb–Sb alloys having lower solute content. Tertiary dendritic branches have been observed along the entire casting lengths for alloys of the Pb–Sb hypoeutectic range having compositions higher than 4.0 wt% Sb. It is shown that a power function experimental law with a characteristic −0.55 exponent is able to characterize the tertiary spacing evolution with the solidification cooling rate for alloys compositions ≥4.0 wt% Sb. The only exception was the Pb–3.5 wt% Sb alloy for which λ3 exhibited significant lower values when compared with the experimental values obtained for the other Pb–Sb alloys for a same solidification cooling rate.

During the non-equilibrium solidification typical of DC (direct chill) castings a range of cooling rates occur from the surface to the casting center, and can cause the formation of metastable intermetallic phases (AlmFe, Al6Fe, etc.) in addition to the stable Al3Fe phase. The extensive presence of the plate-like Al3Fe phase in the as-cast structure adversely influences the mechanical properties of