Ivaldo L. Ferreira

Growth direction and Si alloying affecting directionally solidified structures of Al–Cu–Si alloys

Abstract The roles of growth direction and Si content on the columnar/equiaxed transition and on dendritic spacings of Al–Cu–Si alloys still remain as an open field to be studied. In the present investigation, Al–6 wt-%Cu–4 wt-%Si and Al–6 wt-%Cu alloys were directionally solidified upwards and horizontally under transient heat flow conditions. The experimental results include tip growth rate and cooling rates, optical microscopy, scanning electron microscopy energy dispersive spectrometry and dendrite arm spacings. It was found that silicon alloying contributes to significant refinement of primary/secondary dendritic spacings for the upward configuration as compared with corresponding results of the horizontal growth. Experimental growth laws are proposed, and the effects of the presence/absence of solutal convection in both growth directions are discussed.

Theoretical and experimental analysis of inverse segregation during unidirectional solidification of an Al–6.2 wt.% Cu alloy

Abstract An analytical solidification model has been coupled with the local solute redistribution equation proposed by Flemings and Nereo, permitting a complete analytical description of the positional variation of the segregation as a function of solidification thermal parameters. The theoretical predictions were compared with the experimental inverse segregation profile of a vertical directionally solidified Al–6.2 wt.% Cu ingot.

Modeling and experimental analysis of macrosegregation during transient solidification of a ternary Al–6wt%Cu–1wt%Si alloy

The aim of this article is to provide a numerical tool, which is able to predict macrosegregation profiles during the solidification of ternary alloys. The solute profiles during the transient directional solidification of an Al–6wt%Cu–1wt%Si alloy were calculated taking into account secondary phase transformations, which occur along the corresponding solidification path. The simulations were compared with experimental segregation profiles determined along the length of a directionally solidified casting through X-ray fluorescence spectrometry measurements. A very good agreement between predictions and experiments has been observed.

The Growth of Secondary Dendritic Arms in Directionally Solidified Al-Si-Cu Alloys: A Comparative Study with Binary Al-Si Alloys

Experiments of vertical unsteady-state directional solidification were carried out in order to permit the influence of copper alloying to Al-Si alloys on the scale of secondary dendritic arm (λ2) to be investigated. The microstructures of Al-nSi-3wt%Cu alloys, with “n” equal to 5.5wt%Si and 9.0wt%Si, were characterized and correlated with solidification thermal parameters: the growth rate (VL), the tip cooling rate (Ṫ) and the local solidification time (tSL). A comparative analysis between the present results and those from the literature related to the secondary dendrite growth during directional solidification of Al-nSi alloys is also conducted. It is shown that the addition of Cu to both Al-nSi alloys decreases λ2, and experimental growth laws relating λ2 to VL and ṪL are proposed for the ternary alloys examined. The experimental scatter of λ2 is also compared with the only theoretical dendritic growth model from the literature for multicomponent alloys, and it is shown that the theoretical predictions overestimate the present experimental results.

2D Phase-Field Simulation of the Directional Solidification Process

The microstructure evolution during the directional solidification of Al-Cu alloy is simulated using a phase field model. The transformation from liquid to solid phase is a non-equilibrium process with three regions (liquid, solid and interface) involved. Phase field model is defined for each of the three regions. The evolution of each phase is calculated by a set of phase field equations, whereas the solute in those regions is calculated by a concentration equation. In this work, the phase field model which is generally valid for most kinds of transitions between phases, it is applied to the directional solidification problem. Numerical results for the morphological evolution of columnar dendrite in Al-Cu alloy are in agreement with experimental observations found in the literature. The growth velocity of the dendrite tip and the concentration profile in the solid, interface and liquid region were calculated.

Study of the Interaction of Copper Nanoparticles with Titanium in Landfill Soils Layers

The nanotechnology has spread out for every field of the sciences and engineering with applications covering the variety of human and life activities. The TiO2 nanoparticles are of special interest due to the wide range of applications from cosmetics to paint and new applications have continuously been searched. However, the effect of these particles into the environment need detailed investigation since some deleterious effects on the ecosystems have been observed. Among the nanoparticles found in natural soils the copper plays important role due to their strong interactions with the other materials. In this paper, the influence of nanoparticles of copper (NPCu) in the behavior of NPTiO2 were investigated. A normalized experimental procedure on the soil column was carried out with soil impregnated by copper nanoparticles and compared with the natural soil. Stable solutions of water and NPTiO2 were prepared and used to carried out the column percolation experiments with the concentrations of NPTiO2 measured at the inlet and outlet of the solutions. The results indicated that the TiO2 nanoparticles can mediate transport of Cu in some soil environments. Suggested that decreases in Cu transport were mainly due to increased desorption.

Application of Computational Thermodynamics to the Evolution of Surface Tension and Gibbs-Thomson Coefficient during Multicomponent Aluminum Alloy Solidification

Numerical simulation of multicomponent alloy solidification demands accuracy of thermophysical properties in order to obtain a numerical representation as close as possible to the physical reality. Some alloy properties are only seldom found in the literature. In this paper, a solution of Butler’s formulation for surface tension is presented for Al-Cu-Si ternary alloys, allowing the Gibbs-Thomson coefficient to be calculated as a function of Cu and Si contents. The importance of the Gibbs-Thomson coefficient is related to the reliability of predictions furnished by predictive microstructure growth models and of numerical computations of solidification thermal variables that will be strongly dependent on the values of the thermophysical properties adopted in the calculations. A numerical model based on Powell hybrid algorithm and a finite difference Jacobian approximation was coupled with a ThermoCalc TCAPI interface to assess the excess Gibbs energy of the liquid phase, permitting the surface tension and Gibbs-Thomson coefficient for Al-Cu-Si hypoeutectic alloys to be calculated. The computed results are presented as a function of the alloy composition.

Simulation of the Microstructural Evolution of Pure Material and Alloys in an Undercooled Melts via Phase-field Method and Adaptive Computational Domain

The phase-field methods were developed mainly for studying solidification of pure materials, being then extended to the solidification of alloys. In spite of phase-field models being suitable for simulating solidification processes, they suffer from low computational efficiency. In this study, we present a numerical technique for the improvement of computational efficiency for computation of microstructural evolution for both pure metal and binary alloy during solidification process. The goal of this technique is for the computational domain to grow around the microstructure and fixed the grid spacing, while solidification advances into the liquid region. In the numerical simulations of pure metal, the phase-field model is based on the energy and phase equations, while, for binary alloy, the said model is based on the concentration and phase equations. Since the thermal diffusivity in the energy equation is much larger than the diffusivity term in phase equation in pure metal system, about twenty eight times the difference between them. The computational domain growth around the microstructure is controlled according with the thermal diffusivity for pure material in the liquid region. In the numerical simulation of dendritic evolution of Fe-C alloy, the idea is similar, i.e., the solute diffusivity in concentration equation is larger than the diffusivity term in phase equation in the liquid region, in this case eleven times the difference in Fe-C alloy system. The computational domain growth is controlled via solute diffusivity in the liquid region. Hence, phase-field model is proposed with an adaptive computational domain for efficient computational simulation of the dendritic growth in a system for both pure metal and binary alloy. The technique enables us to reduce by about an order of magnitude the run time for simulation of the solidification process. The results showed that the microstructure with well-developed secondary arms can be obtained with low computation time.

Influence of Metal/Mold Heat Transfer Coefficient on the Inverse Macrosegregation Profile of an Al - 6.2wt% Cu Alloy Unidirectionally Solidified

The present work focuses on the influence of transient metal/mold heat transfer coefficients (hi), affected by melt superheat, on the positional variation of inverse segregation during upward solidification of Al 6.2wt% Cu alloy. A numerical model which permits the theoretical predictions of inverse segregation and consists of an implicit/explicit time integration scheme for coupling solutal and thermal fields, has been modified to take into account different thermo-physical properties for the liquid and solid phases, time variable metal/mould interface heat transfer coefficient, and a variable space grid to assure the accuracy of results without raising the number of nodes. The numerical simulations were compared with the experimental segregation profiles and a good agreement can be observed in both cases experimentally examined.

Mathematical method to characterize the inward solid state diffusion in cylindrical parts

This work presents an analytical method for the study of the solid state diffusion process in binary systems of two phases with cylindrical radial atomic flux. The method is developed from the differential equation that describes Ficks second law that is modified by geometric function and suitable changes of variables. The modified differential equation is solved by using a well-known closed form solution based on the error function, and then analytical equations are obtained to analyze the diffusion interface position as a function of time, and the concentration profiles as a function of time and position. The predictions provided by the analytical method are compared with numerical results.