Solidification Thermal Variables Research Articles

MACHINABILITY ANALYSIS OF AN AL-1,2% Pb ALLOY SOLIDIFIED IN A HORIZONTAL DIRECTIONAL DEVICE

In general, the binary monotectic alloys are characterized by the limited solubility in the liquid state, which gives them a benefited tribological behavior such as wear resistance. Researches regarding the development of monotectic alloy microstructures during the unsteady-state heat flow conditions are fundamental, as it encompasses most of the solidification industrial processes. However, the microstructural relationship between the mechanical properties of monotectic alloys is little explored and practically nil. In this context, the present study consists of investigating and correlating solidification thermal variables and structural parameters such as microhardness and machinability (cutting temperatures and tool wear) of Al-1.2wt% Pb alloy, in a horizontal directional device. It was observed that the cutting temperature and tool wear results complement each other when correlated with position and interphase spacing, indicating that for smaller interphase spacings the addition of lead harms machinability.

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.

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.

The Application of Computational Thermodynamics for the Determination of Surface Tension and Gibbs-Thomson Coefficient of Aluminum Ternary Alloys

Casting simulation requires high quality information about the thermophysical properties of the alloy, but the number of alloys for which such information is available is limited. In this paper, a solution of Butler’s formulation for surface tension is presented for Al-Cu-Si ternary alloys and consequently, permitting the Gibbs-Thomson coefficient to be determined. 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, which will be strongly dependent on the values of the thermophysical properties adopted in the calculations. The Gibbs-Thomson coefficient for ternary alloys is seldom reported in the literature. A numerical model based on Powell hybrid algorithm and on 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.

Experimental Investigation and Simulation of Al–Si Casting Microstructure Formation

The aim of the present study is to investigate the influence of thermal variables on the as-cast microstructure of hypoeutectic and eutectic Al–Si alloys and to establish correlation between the solidification thermal parameters and interparticle spacing for Al–12%Si alloy during the unidirectional solidification. New relationships are suggested based on the image processing programs in order to accurately quantify the microstructure evolution. A numerical approach is developed to quantitatively determine solidification thermal variables such as growth rate (interface velocity), temperature gradient, cooling rate, and local solidification time. The experimental microstructure measurements are combined with numerical solutions using finite difference (FD) and control volume (CV) methods, and models are obtained for this structure with solidification parameters. Applying image processing program with relationships suggested in this study gives more accuracy to compute interparticle spacing compared with conventional methods. For Al–12%Si alloys, which are widely used, the dispersion of the phases must reflect both decreasing interface velocity and temperature gradient as the microstructure is examined from the chilled surface towards the riser, thus, increasing coarseness of the microstructure caused by both factors.

The application of computational thermodynamics and a numerical model for the determination of surface tension and Gibbs–Thomson coefficient of aluminum based alloys

In this paper, a solution for Butler's formulation is presented permitting the surface tension and the Gibbs–Thomson coefficient of Al-based binary alloys to be determined. The importance of Gibbs–Thomson coefficient for binary alloys is related to the reliability of predictions furnished by predictive cellular and dendritic growth models and of numerical computations of solidification thermal variables, which will be strongly dependent on the thermophysical properties assumed for the calculations. A numerical model based on Powell hybrid algorithm and a finite difference Jacobian approximation was coupled to a specific interface of a computational thermodynamics software in order to assess the excess Gibbs energy of the liquid phase, permitting the surface tension and Gibbs–Thomson coefficient for Al–Fe, Al–Ni, Al–Cu and Al–Si hypoeutectic alloys to be calculated. The computed results are presented as a function of the alloy composition.

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.

The columnar to equiaxed transition during the horizontal directional solidification of Sn–Pb alloys

Experiments have been carried out to analyze the columnar to equiaxed transition during the horizontal directional solidification of Sn–Pb hypoeutectic alloys as a function of solidification parameters, heat transfer coefficients, temperature gradients, growth rates, cooling rates and composition C o. For this purpose, a water-cooled solidification experimental apparatus was developed, and specimens were solidified under unsteady state heat flow conditions. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal variables considered. The results have supported a criterion proposed based on a critical value of cooling rate which for a particular binary alloy system is independent of the solute concentration in the hypoeutectic range. The observation of the macrostructures has indicated that the columnar-to-equiaxed transition occurred in a zone rather than in a sharp plane parallel to the chill wall and that resulting thermo-solutal convection seems to favor the structural transition which occurs for a critical cooling rate of about 0.048 K/s and temperature gradients in the range between 0.25 and 0.90 K/mm for any of four alloy compositions examined. A comparative analysis between some results of this work and those from the literature proposed to analyze the CET during upward and downward vertical solidification of Sn–Pb hypoeutectic alloys is conducted.

Microstructural Development in Al-Ni Alloys Directionally Solidified under Unsteady-State Conditions

Three Al-Ni hypoeutectic alloys were directionally solidified under upward unsteady-state heat-flow conditions. Primary (λ1) and secondary (λ2) dendrite arm spacings were measured along the castings for all alloys and correlated with transient solidification thermal variables. A combined theoretical and experimental approach was used to quantitatively determine such thermal variables, i.e., transient metal/mold heat-transfer coefficients, tip growth rates, thermal gradients, tip cooling rates, and local solidification time. The article also focuses on the dependence of dendrite arm spacings on the alloy solute content. Furthermore, the experimental data concerning the solidification of Al-1.0, 2.5, and 4.7 wt pct Ni alloys are compared with the main predictive dendritic models from the literature.

Microstructural development in Al–Sn alloys directionally solidified under transient heat flow conditions

Despite the wide use of Al–Sn alloys for engineering applications studies on the microstructural development of such materials are rare. Optimized microstructures during the solidification stage of processing can be fundamental for final properties. In the present study, three Al–Sn hypoeutectic alloys were directionally vertically solidified under upward unsteady state heat flow conditions. Primary ( λ 1) and secondary ( λ 2) dendrite arm spacings were measured along the alloys castings and correlated with transient solidification thermal variables. A combined theoretical and experimental approach has been used to quantitatively determine such thermal variables, i.e., transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time. The article also focuses on the dependence of dendrite arm spacings on the alloy solute content. Furthermore, the experimental data concerning the solidification of Al 20, 30 and 40 wt% Sn alloys are compared with the main predictive dendritic models from the literature.

PARÂMETROS TÉRMICOS, MACROESTRUTURA E MICROESTRUTURA NA SOLIDIFICAÇÃO DIRECIONAL DA LIGA Al-20%Sn

Al-Sn alloys are widely used in tribological applications. Aluminium and tin form immiscible alloys and the type of the microstructural arrangement causes a good wear resistance. In this study, thermal and microstructural analysis were carried out along an Al-20wt%Sn alloy ingot produced by vertical upward directional transient solidification. The experimental thermal readings were fitted to numerical predictions in order to determine the global heat transfer coefficient (h g ) and were also used to calculate the evolution of thermal variables (tip growth rate and cooling rate) during solidification. The growth rate and the cooling rate play a key role in the microstructural formation. The dendritic microstructure has been characterized by secondary dendritic arm spacings (λ 2 ) which were experimentally determined and correlated to solidification thermal variables. The behavior presented by the Al-20wt%Sn alloy during solidification was similar to that of other aluminum alloys, i.e., the dendritic network became coarser with decreasing cooling rates, indicating that the immiscibility between aluminum and tin has not a significant effect on the relationship between the secondary dendritic arm spacing and cooling rate. An experimental dendritic growth law has been

Design of mechanical properties of a Zn27Al alloy based on microstructure dendritic array spacing

The present work focuses on the influence of the as-cast dendritic microstructure of a ZA27 alloy (Zn–27 wt%Al) on tensile mechanical properties. A low carbon steel chill was used in a unidirectional solidification experimental set-up in order to permit a wide range of dendritic spacings to be obtained along the casting. Experimental results include transient metal/mold heat transfer coefficients ( h i ), tip growth rate ( V L), secondary dendrite arm spacing ( λ 2), ultimate tensile strength ( σ U) and yield strength ( σ y) as a function of solidification conditions imposed by the metal/mold system. Experimental laws relating σ U and σ y with secondary dendrite arm spacing are proposed. It was found that both tensile properties increase with decreasing λ 2. A predictive theoretical dendritic growth model has been compared with the present experimental observations. Expressions correlating tensile properties, dendritic spacing and solidification thermal variables have been established. Such expressions permit the control of as-cast microstructures by manipulating casting variables, such as the cooling rate and the tip growth rate and can be used as an alternative way to design mechanical properties.

The effect of solidification thermal variables on surface quality of Al–Cu ingots

A combined theoretical and experimental approach was undertaken to quantitatively determine thermal variables during solidification, i.e., transient metal/mold heat transfer coefficients, cooling rates and a driving force factor for the solidified shell distortion, given by the gradient of the cooling rate. Such thermal variables were correlated with the geometric features of the observed surface marks of Al–Cu alloys ingots in the range of pure Al and the eutectic composition. It was shown that the thermal distortion factor increases with alloy Cu content, and it is strongly dependent on the initial values of the metal/mold heat transfer coefficient. A correlation with the resulting experimental surface marks has indicated that larger depth and pitch are associated with higher thermal distortion driving forces.

Mechanical properties as a function of microstructure and solidification thermal variables of Al–Si castings

The aim of the present study was to investigate the influence of solidification thermal variables on the as-cast microstructure of hypoeutectic Al–Si alloys and to establish correlations with the casting mechanical properties. Experimental results include transient metal/mold heat transfer coefficients, tip growth rate, local solidification time, secondary dendrite arm spacing, ultimate tensile strength and yield strength as a function of solidification conditions imposed by the metal/mold system. It was found that the ultimate tensile strength increases with increasing alloy solute content and with decreasing secondary dendrite arm spacing. Yield strength seems to be independent of both alloy composition and dendritic arrangement. Such results have permitted general expressions correlating dendrite spacing with transient solidification processing variables to be established. The correlation of such expressions with experimental equations relating the ultimate tensile strength and dendrite spacing provides an insight in the preprogramming of solidification in terms of mechanical strength of Al–Si castings. Predictive theoretical and experimental approaches for dendritic growth have been compared with the present experimental observations.

Cellular growth during transient directional solidification of Pb–Sb alloys

Cellular spacings and solidification thermal variables were determined for three dilute Pb–Sb alloys, which were solidified under unsteady-state heat flow conditions. The experiments have been carried out by using a casting assembly, which was designed in such way that the heat was extracted only through the water-cooled system at the casting bottom, promoting vertical upward directional solidification. An experimental approach, based on thermal readings collected at six different positions from the casting surface, was used in order to quantitatively determine the solidification thermal variables: tip growth rate and cooling rate of Pb–0.3 wt%Sb, Pb–0.85 wt%Sb and Pb–1.9 wt%Sb alloys castings. After grinding, polishing and etching procedures, both longitudinal and transverse sections of the castings were characterized by a typical cellular microstructure which prevailed along the entire casting. The cell spacing variation with cooling rate and tip growth rate was characterized by −0.55 and −1.1 experimental power laws, respectively. The experimental cell spacings were compared with the theoretical predictions furnished by cellular growth models.

Thermosolutal convective effects on dendritic array spacings in downward transient directional solidification of Al–Si alloys

Solidification thermal variables, such as tip growth rate, tip cooling rate, local solidification time and dendrite arm spacings, have been measured in Al–Si hypoeutectic alloys directionally solidified under downward transient heat flow conditions. It is a well-known fact that the dendritic spacings can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A reduced number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction on dendritic arm spacings. In this work, an experimental approach is used to quantitatively determine the solidification thermal variables. The work also focuses on the dependence of dendrite arm spacings on these solidification thermal variables. The experimental data concerning the solidification of Al 5, 7 and 9 wt.% Si alloys are compared with the main predictive dendritic models from the literature. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.

Factors affecting solidification thermal variables along the cross-section of horizontal cylindrical ingots

During solidification the mathematical analysis of heat flow depends on the transient heat transfer coefficient at the metal/mold interface. The analysis of heat transfer behavior along the cross-section of radial geometries is necessary for a better control of solidification in conventional foundry and continuous casting processes. For this purpose, a water-cooled solidification experimental apparatus was developed, and experiments were carried out with Sn–Pb alloys with different melt superheats. The transient metal/mold heat transfer coefficients were determined by a numerical-experimental fit of casting thermal profiles based on inverse heat transfer calculations. The results have shown a significant variation in heat flow conditions along the cylinder cross-section provoked by the simultaneous action of solidification thermal contraction and the gravitational effect. Experimental equations correlating heat transfer coefficients as a power function of time along the cross-section of cylindrical horizontal castings of Sn–Pb alloys are proposed.

Influence of melt convection on dendritic spacings of downward unsteady-state directionally solidified Al–Cu alloys

The dendritic spacings are important microstructural parameters involved in solidification processes. They can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A small number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction in the dendritic arm spacings. In this work, a combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time during downward unsteady-state solidification of hypoeutectic Al–Cu alloys. These solidification thermal variables are correlated with dendritic spacings, which have been measured along cross and longitudinal sections of ingots solidified under downward unsteady-state heat-flow conditions. Predictive theoretical models for dendritic spacings have been compared with experimental observations. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.

Influence of melt convection on dendritic spacings of downward unsteady-state directionally solidified Al–Cu alloys

Abstract The dendritic spacings are important microstructural parameters involved in solidification processes. They can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A small number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction in the dendritic arm spacings. In this work, a combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time during downward unsteady-state solidification of hypoeutectic Al–Cu alloys. These solidification thermal variables are correlated with dendritic spacings, which have been measured along cross and longitudinal sections of ingots solidified under downward unsteady-state heat-flow conditions. Predictive theoretical models for dendritic spacings have been compared with experimental observations. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.

Macrostructural and microstructural development in Al–Si alloys directionally solidified under unsteady-state conditions

Experiments were conducted to analyze the upward unsteady-state directional solidification of Al–Si hypoeutectic alloys. A combined theoretical and experimental approach is developed to quantitatively determine solidification thermal variables such as transient metal/mold heat transfer coefficients, tip growth rates, thermal gradients, tip cooling rates and local solidification time. The observation of the macrostructures has shown that the columnar to equiaxed transition (CET) occurred essentially at the same position from the casting surface for any alloy experimentally examined. The solidification thermal variables were very similar at the CET and it was not possible to identify a structural transition criterion based on a particular variable. The article also focuses on the dependence of dendrite arm spacings on the solidification thermal variables and alloy solute content. The experimental data concerning the solidification of Al with 3, 5, 7 and 9 wt.% Si alloys are compared with the main predictive dendritic models from the literature.