Pedro R. Goulart

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (hi), secondary dendrite arm spacing (λ2), corrosion potential (ECorr), corrosion rate (iCorr), polarization resistance (R1), capacitances values (ZCPE), ultimate tensile strength (UTS, σu), yield strength (YS, σy), and elongation. It is shown that σU decreases with increasing λ2 while the CR increases with increasing λ2, for both alloys experimentally examined. A combined plot of CR and σU as a function of λ2 is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.

Dendritic Microstructure Affecting Mechanical Properties and Corrosion Resistance of an Al-9 wt% Si Alloy

It has been reported that the mechanical properties and corrosion resistance of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve desired final properties. The aim of the present work is to investigate the influence of dendritic microstructural parameters of an Al-9 wt.% Si alloy on mechanical properties and corrosion resistance. The experimental results establish correlations between secondary dendrite arm spacings (λ2) and ultimate tensile strength (σ u ), yield strength (σ y ), corrosion potential (E Corr ), and corrosion rate (i Corr ).

Correlation between dendrite arm spacing and microhardness during unsteady-state directional solidification of Al–Ni alloys

Hypoeutectic Al–Ni alloys are characterised by the formation of as-cast structures composed by a dendritic aluminium-rich matrix surrounded by a eutectic mixture (α + β), where α is the Al-rich phase and β is the Al3Ni intermetallics. The morphology, size and distribution of the intermetallic particles decisively affect the mechanical properties of these alloys. This study aims to determine experimental laws relating the primary dendritic arm spacing (λ1) and Vickers microhardness (HV) of hypoeutectic Al–Ni alloys. Upward unidirectional solidification experiments were performed in order to obtain castings with significant differences in the scale of microstructural parameters along the casting length. It was found that the microhardness values were directly influenced by both the amount of solute and λ1 and Hall–Petch-type equations relating the microindentation and λ1 are proposed.

Steady and unsteady state peritectic solidification

Abstract In the present study, Pb–Bi as a model of alloys from peritectic systems was directionally solidified under transient heat flow conditions, which is the class of heat flow encompassing the majority of industrial solidification processes. Some experiments were also carried out in a vertical tube furnace under cooling rates closer to equilibrium during solidification in order to broaden the range of solidification parameters. Thermal parameters such as the tip growth rate (VL) and the cooling rate were experimentally determined and correlated with the primary (λ1) and secondary (λ2) dendrite arm spacings by experimental growth laws. It is shown that the proposed growth laws are able to encompass also the growth of dendritic branches during steady state growth from the melt.

Microstructural development during transient directional solidification of a hypomonotectic Al–In alloy

Upward directional non-steady-state solidification experiments have been performed on a hypomonotectic Al–5.5 wt%In alloy. The alloy developed cellular as-solidified microstructure for tip growth rates, V L, higher than 0.95 mm/s. The casting regions associated with V L < 0.95 mm/s were shown to be characterized by a microstructure formed by In droplets disseminated in the Al matrix. Tip growth rate and microstructural features, such as cell spacing and interphase spacing, have been experimentally determined. The experimental cell spacings have been compared with theoretical predictions furnished by the Hunt–Lu model. It was found that the experimental scatter lies below the minimum range of values theoretically predicted. Moreover, the experimental cell spacing evolution with V L is characterized by a −1.1 power law. The droplets’ interphase spacing, λ, is related to the growth rate by the Jackson–Hunt relationship (λ 2 V L = constant).

Microstructural Development in a Ternary Al-Cu-Si Alloy during Transient Solidification

Macrosegregation and microporosity formation numerical models are dependent on microstructural parameters, such as primary and secondary arm spacings to provide the permeability coefficient in the mushy zone. As can be observed in the literature, growth models for ternary alloys for unsteady solidification are rarely found. In this paper, the primary (λ1) dendrite arm spacing was measured along the length of an Al-Cu-Si alloy casting and correlated with transient solidification thermal variables. A combined theoretical and experimental approach has been carried out to quantitatively determine such thermal variables, i.e., transient metal/mold heat transfer coefficient, liquidus isotherm velocity and cooling rate ahead the liquidus front.

Cellular Microstructure and Mechanical Properties of a Directionally Solidified Al-1.0wt%Fe Alloy

Upward directional transient solidification experiments have been carried out with an Al-1.0wt%Fe alloy. Tensile tests were carried out with samples collected along the casting length and these results have been correlated with measured cell spacings, since cellular growth has prevailed along the directionally solidified casting. The resulting mechanical properties include ultimate tensile strength, yield tensile strength and elongation. The used casting assembly was designed in such a way that the heat was extracted only through the water-cooled system at bottom of the casting. During non-equilibrium solidification, typical of DC (direct chill) castings, different cooling rates occur from the casting cooled surface up to the top of the casting, causing the formation of metastable intermetallic phases (AlmFe, Al6Fe, etc) in addition to the stable Al3Fe phase. The extensive presence of plate-like Al3Fe phase in the as-cast structure adversely influences the mechanical properties of Al-Fe alloys, since this morphology is more likely to induce microcracks than the fibrous Al6Fe phase. In order to permit an appropriate characterization of these intermetallic phases, they were extracted from the aluminum-rich matrix by using a dissolution technique. These phases were then investigated by optical microscopy and SEM techniques. It was found that the ultimate tensile strength, the yield strength and the elongation increase with decreasing cell spacing and experimental laws correlating cell spacing and these mechanical properties have been established.

An artificial immune system algorithm applied to the solution of an inverse problem in unsteady inward solidification

Abstract The numerical simulation and optimization in solidification involving geometries beyond one dimension normally requires expensive computational approaches in memory and data processing, generating consequently high cost associated with runtime execution. When boundary conditions are unknown, such as how heat is extracted from the casting surface, which is a typical inverse heat transfer problem (IHTP), the search for such conditions by trial-and-error makes the whole process less feasible. In this sense, this work introduces the application of an artificial immune system (AIS) algorithm as a possible meta-heuristic alternative, with a view to returning acceptable objective values to converge on a set of acceptable solutions. In the present work, the search process of the heat transfer coefficient (hg) at the casting surface during two-dimensional inward solidification of Al-1.5 wt.%Fe alloy castings of Cartesian and Cylindrical geometries in chilled molds is optimized. Two water-cooled solidification apparatuses were designed to in situ melt the alloy by a set of electrical resistances, in which heat is extracted only through the chilled faces of the molds. The thermal history during solidification was obtained via thermocouples placed at different positions in the castings. A Finite Difference heat transfer model integrated to an optimized version of the artificial immune network algorithm, which uses the experimental thermal profiles as inputs, has been applied to solve the IHTP through the search for acceptable values of heat transfer coefficient. It is shown that fast convergence of the developed algorithm can be achieved for a relatively small number of iterations. The mean relative errors associated with differences between simulated and experimental temperatures are shown to be 1% and 0.07% for Cartesian and Cylindrical geometries, respectively and expressions relating hg to time have been determined for both geometries.