Otávio L. Rocha

Cellular/dendritic transition during unsteady-state unidirectional solidification of Sn–Pb alloys

Abstract Structural parameters such as grain size, dendritic and cellular spacings, segregated products, porosity and other phases are strongly influenced by the thermal behavior of the metal/mold system during solidification, imposing a close correlation between this and the resulting microstructure. Numerous unidirectional solidification studies have been carried out with the objective of characterizing cellular and dendritic spacings and most of the published work has involved solidification in steady-state heat flow conditions. The objective of this article is to determine the thermal solidification parameters affecting the cellular/dendritic transition as well as to compare theoretical models that predict cellular and primary dendritic spacings with experimental results for unsteady-state solidification. Experiments were carried out in a water cooled unidirectional solidification apparatus and dilute alloys of the Sn–Pb system have been used (Sn 1.5 wt.% Pb, Sn 2.5 wt.% Pb and Sn 5 wt.% Pb).

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.

Characterization of the Al-3wt.%Si alloy in unsteady-state horizontal directional solidification

The main purpose of this paper is to investigate both the columnar to equiaxed transition and primary dendritic arm spacings of Al-3wt.%Si alloy during the horizontal directional solidification. The transient heat transfer coefficient at the metal-mold interface is calculated based on comparisons between the experimental thermal profiles in castings and the simulations provided by a finite difference heat flow program. Simulated curve of the interfacial heat transfer coefficient was used in another numerical solidification model to determine theoretical values of tip growth rates, cooling rates and thermal gradients that are associated with both columnar to equiaxed transition and primary dendritic arm spacings. A good agreement was observed between the experimental values of these thermal variables and those numerically simulated for the alloy examined. A comparative analysis is carried out between the experimental data of this work and theoretical models from the literature that have been proposed to predict the primary dendritic spacings. In this context, this study may contribute to the understanding of how to manage solidification operational parameters aiming at designing the microstructure of Al-Si alloys.

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.

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.

On the Natural Convection in the Columnar to Equiaxed Transition in Directionally Solidified Aluminum-based Binary and Multicomponent Alloys

In order to investigate the effect of natural convection in columnar to equiaxed transition (CET), Al-3.0wt.%Cu and Al-3.0wt.%Cu-5.5wt.%Si alloys ingots were obtained during the transient horizontal directional solidification (THDS). Aiming to analyze the effect of superheat in the formation of the macrostructure in ternary Al-Cu-Si alloy, the experiments were conducted with three superheat temperatures above the liquidus temperature of the ternary alloy. A water-cooled solidification experimental device was used. Continuous temperature measurements were made during solidification at different positions in the casting and the data were automatically acquired. Thermal analysis has been applied to determine the thermal parameters such as growth rate (VL), cooling rate (TR) and temperature gradient (GL), whose values have been interrelated with the CET. The observation of the macrostructures has indicated that the resulting thermosolutal convection combined with superheat seem to favor the transition, which did not occur in a single plane, for all ingots obtained, i.e., it has been seen in a range of positions in ingots. The addition of Si element in binary Al-Cu alloy anticipates the CET. A comparison with experimental results for CET occurrence in different growth directions has been carried out.

Solidification thermal parameters and dendritic growth during the horizontal directional solidification of Al-7wt.%Si alloy

O principal objetivo desse trabalho e investigar a influencia dos parâmetros termicos velocidade de solidificacao (VL) e taxa de resfriamento (TR), nos espacamentos dendriticos primarios (λ1) da liga hipoeutetica Al-7%Si, durante a solidificacao direcional horizontal, em regime transiente. Os valores de λ1 foram medidos ao longo do comprimento do lingote e correlacionados com esses parâmetros. A variacao dos espacamentos dendriticos estudados e expressa por meio de funcoes na forma de potencia de VLe TRdadas, respectivamente, por λ1= 55(VL)-1.1e λ1= 212 (TR)-0.55. Um estudo comparativo e realizado entre os resultados encontrados nesse trabalho e aqueles obtidos para a mesma liga quando solidificada direcionalmente nos sistemas verticais ascendente e descendente, sob as mesmas condicoes assumidas. Finalmente, os resultados experimentais obtidos sao comparados com valores fornecidos por alguns modelos teoricos propostos na literatura para analisar espacamentos dendriticos primarios.

The columnar to equiaxed transition of horizontal unsteady-state directionally solidified Al-Si alloys

Experiments were conducted to investigate the influence of thermal parameters on the columnar to equiaxed transition during the horizontal unsteady-state directional solidification of Al-Si alloys. The parameters analyzed include the heat transfer coefficients, growth rates, cooling rates, temperature gradients and composition. A combined theoretical and experimental approach is developed to determine the solidification thermal variables considered. The increasing solute content in Al-Si alloys was not found to affect significantly the experimental position of the CET which occurred for cooling rates in the range between 0.35 and 0.64 K/s for any of three alloy compositions examined. A comparative analysis between the results of this work and those from the literature proposed to analyze the CET during upward vertical solidification of Al-Si alloys is reported and the results have shown that the end of the columnar region during horizontal directional solidification is abbreviated as a result of about six times higher thermal gradient than that verified during upward unidirectional solidification of alloys investigated.

Espaçamentos dendríticos primários da liga Sn-5%Pb solidificada direcionalmente em um sistema horizontal

Este trabalho desenvolve um estudo teorico-experimental sobre a correlacao entre parâmetros termicos e os espacamentos dendriticos primarios da liga Sn-5%Pb solidificada direcionalmente em um sistema horizontal sob condicoes transientes de extracao de calor. Os perfis de temperatura foram medidos em diferentes posicoes do lingote e os dados obtidos foram armazenados automaticamente. Sao apresentados resultados para o coeficiente de transferencia de calor na interface metal/molde o qual foi calculado a partir de uma analise comparativa entre os perfis termicos experimentais e valores teoricos fornecidos por um modelo numerico. Um metodo teorico-experimental e aplicado para determinar as velocidades de deslocamento da isoterma liquidus e as taxas de resfriamento. Os resultados teoricos e experimentais levantados apresentaram boa concordância. Um estudo comparativo e realizado entre os dados encontrados neste trabalho e aqueles apresentados na literatura para os espacamentos dendriticos primarios da liga investigada quando solidificada direcionalmente nos sistemas verticais ascendente e descendente, sob as mesmas condicoes assumidas neste trabalho. A analise das microestruturas indica que os espacamentos dendriticos primarios sao bastante influenciados pelos parâmetros termicos de solidificacao.

Interconnection between microstructure and microhardness of directionally solidified binary Al-6wt.%Cu and multicomponent Al-6wt.%Cu-8wt.%Si alloys

An experimental study has been carried out to evaluate the microstructural and microhardness evolution on the directionally solidified binary Al-Cu and multicomponent Al-Cu-Si alloys and the influence of Si alloying. For this purpose specimens of Al-6wt.%Cu and Al-6wt.%Cu-8wt.%Si alloys were prepared and directionally solidified under transient conditions of heat extraction. A water-cooled horizontal directional solidification device was applied. A comprehensive characterization is performed including experimental dendrite tip growth rates (VL) and cooling rates (TR) by measuring Vickers microhardness (HV), optical microscopy and scanning electron microscopy with microanalysis performed by energy dispersive spectrometry (SEM-EDS). The results show, for both studied alloys, the increasing of TR and VL reduced the primary dendrite arm spacing (l1) increasing the microhardness. Furthermore, the incorporation of Si in Al-6wt.%Cu alloy to form the Al-6wt.%Cu-8wt.%Si alloy influenced significantly the microstructure and consequently the microhardness but did not affect the primary dendritic growth law. An analysis on the formation of the columnar to equiaxed transition (CET) is also performed and the results show that the occurrence of CET is not sharp, i.e., the CET in both cases occurs in a zone rather than in a parallel plane to the chill wall, where both columnar and equiaxed grains are be able to exist.