Upward Unidirectional Solidification Research Articles

Solute segregation affected by transport properties during a bubble entrapment in upward solidification

Abstract The effects of transport properties on segregation of a dissolvable gas, for example, carbon dioxide in water in the presence of an entrapping bubble during upward unidirectional solidification is numerically investigated. Microporosity and inter-related solute segregation are two common defects detrimental to mechanical properties of components, especially to fatigue performance, strength and ductility. In this work, transport processes between the bubble, liquid and solid affected by mass, momentum, energy, and concentration Eqs. are solved by finite element schemes provided from the commercial COMSOL code. The results find that solute concentration in solid and liquid surrounding the pore increases with decreasing Henry's law constant and gravitational acceleration, and increasing surface tension and solid thermal conductivity. Solute gas concentration on the top free surface is insensitive to that in the pore rather than that in liquid and solid away from the pore. Apex cap becomes non-spherical affecting solidification rate and shape of the solidification front as ambient pressure at the top free surface and gravitational acceleration increase. Contact angle versus location of the solidification front predicted from this work and Abel's Eq. agrees well. Solute segregation encountered by pore formation is controllable.

Heat-flow parameters affecting microstructure and mechanical properties of Al–Cu and Al–Ni alloys in directional solidification: an experimental comparative study

Abstract A comparative analysis was carried out of the alloys examined, using the results found for thermal variables, macrostructure, dendrite arm spacing, modulus of elasticity and microhardness. Comparing all the results for variables, a predominance was found of higher values for the Al-5.0 wt.% Cu alloy in relation to those for the Al-5.0 wt.% Ni alloy. Although structural transition has occurred in both alloys, the columnar–equiaxed transition position in castings was not altered during the solidification experiments. Addition of Ni solute into pure aluminum favors a significant decrease in tertiary dendrite spacing, while Cu addition leads to a coarser microstructure. Primary dendrite arm spacing, also, was measured along the castings of both alloys. Theoretical approaches such as the well-known Hunt–Lu, Bouchard–Kirkaldy, and Kurz–Fisher models were used to determine quantitatively this particular microstructural parameter. Good agreement was observed between experimental data and predictions by Hunt–Lu and Bouchard–Kirkaldy models, which assume solidification under a transient heat-flow condition. Finally, empirical equations relating tertiary dendritic arm spacing and mechanical properties to the scale of the dendritic microstructure have been proposed. While no relationship was observed between modulus of elasticity and tertiary dendritic arm spacing, addition of Cu into pure aluminum as an alloying element favored a reduction in the average value of modulus of elasticity when compared with those observed for the Al–Ni alloy. It was found that the microhardness decreases with increasing tertiary dendritic arm spacings. On the other hand, solute addition of Cu into commercially pure aluminum favors a microhardness higher than that verified during upward unidirectional solidification of the Al-5.0 wt.% Ni alloy.

Microstructural Patterns, Microsegregation, Porosity, and Mechanical Properties of Hypoeutectic Al-Fe Alloy, and its Dependency with Solidification Thermal Parameters

Al-Fe alloys are usually used as packaging and structural materials, but in the recent years, there have been considered for possible applications in aerospace field. The solidification sequence in pure aluminum containing 1 wt.% Fe is described in term of the formation of macrostructure, microstructure, microsegregation, porosity and mechanical properties. This material was studied in the upward unidirectional solidification system under transient heat flow conditions. Differences in microstructure, microsegregation, porosity and mechanical properties such as ultimate tensile strength, elongation and microhardness, due to the thermal parameter effects were observed and discussed. Experimental growth laws relating cellular spacing to the cooling rate and solidification speed have been determined, indicating that the increase in thermal parameter have induced a refinement effect on cell morphology. Microsegregation profiles of Fe solute were experimentally determined from the central region of the cell to the intercellular region under different solidification speeds. The Fe microsegregation determined from central region of the cell (FS = 0) to the intercellular region (FS = 1) show a growing profile, in any case considered. However, the profiles move upward with the increase in solidification speed, which indicates that Fe solubility in solid, increases with the increase in solidification speed. The effect of the solidification thermal parameters and cellular spacing on the porosity content were experimentally investigated. The value of porosity content increased along the casting. These results have pointed out that porosity content is affected by solidification parameters and cellular patterns. Further, measurable effects of the thermal parameters, cellular spacing and porosity content on the mechanical properties were experimentally determined. It stands out among experimental results the influence of porosity on the mechanical properties of as-cast material. In any case analyzed, mechanical properties increase with decreasing porosity content.

Three-Dimensional Simulation of Macrosegregation in a 36-Ton Steel Ingot Using a Multicomponent Multiphase Model

A multicomponent multiphase solidification model has been developed to predict macrosegregation of steel ingots in three dimensions. The interpenetrating continua of liquid melt and solid grains is coupled with air for the mass, momentum, concentration, and heat transfer. Interfacial solute constraint relationships are derived to close the model by solving the solidification paths of multicomponent alloy. The upward unidirectional solidification case of a ternary Al-6.0 wt.%Cu-1.0 wt.%Si alloy is taken as a basic validation. Predictions have well captured the inverse segregation profiles induced by shrinkage during solidification. Then, the model is applied to a 36-ton steel ingot, which was experimentally investigated by temperature recording and concentration analysis. Predictions have reproduced the macrosegregation patterns in the measurements. Confidence levels of current predictions compared to the concentration measurements have been presented. General good agreements are exhibited in quantitative comparisons between measurements and predictions of carbon and sulfur variations along selected positions.

Effects of cellular growth on fatigue life of directionally solidified hypoeutectic Al-Fe Alloys

Al-Fe hypoeutectic alloys are a family of casting alloys characterized by cell growth, low cost and appreciable formability. It is well known that fatigue strength is a requirement of prime importance considering the nature of load typically observed during operations involving the risers used in oil extraction. The aim of this study is to examine the influence of cell size and its intercellular phase distribution on the fatigue life (Nf) of the directionally solidified Al-0.5, 1.0 and 1.5wt% Fe alloys. A water-cooled vertical upward unidirectional solidification system was used to provide the castings. Microscopy light and SEM microscopy were used. It was found that fatigue life decreases as cell spacing (λ c) increases. Smaller cell spacing allows a homogeneous distribution of Al-Fe fibers to happen within the intercellular regions, which tends to improve the mentioned fatigue property. Hall-Petch type correlations [Nf= Nf0+A(λc -1/2)-B(λc -1); where A and B are constants] seems to be able to encompass the fatigue life variation along the Al-Fe alloys.

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.

Microstructure and Mechanical Properties of Directionally Solidified Unmodified and Ni-Modified Sn-0.7wt%Cu Lead-Free Solder Alloy

Lead containing solders (SnPb eutectic alloys) are widely used in electronic devices due to their good mechanical properties and low manufacturing cost. However, two European Union regulations (Waste from Electrical and Electronic Equipment and Restriction of Hazardous Substances) banned the use of lead in electrical and electronic equipment because of the toxic effect on human health and the environment. Nowadays, it is particularly important to find replacements for Pb containing solder materials. In that respect, copper is used as an alloying element and the composition Sn-0.7wt%Cu is of particular interest. Small Ni additions can be interesting since they would be included into the composition of the commercial solder SN100 and can also avoid the presence of coarse and deleterious Cu6Sn5 particles for Sn-Cu alloys in the hypoeutectic range of compositions. In the present investigation, growth rate, cooling rate, interfacial heat transfer coefficient (hi), the scale of the microstructure and morphologies, ultimate tensile strength and elongation have been experimentally determined for Sn-0.7wt%Cu and Sn-0.7wt%Cu-0.1wt%Ni alloys solidified in a water-cooled vertical upward unidirectional solidification system. Further, interrelations of thermal parameters, microstructure and tensile properties may be established. A higher time-dependent hi profile was found for the Sn-0.7wt%Cu-0.1wt%Ni alloy which seems to indicate that a higher fluidity was obtained with small Ni addition. Higher fluidity values may characterize better physicochemical affinity between the melt and the mould surface. The modified Sn-Cu alloy propitiates higher ultimate tensile strength values to be obtained. This may be due to the prevalence of eutectic two-phase cells along the casting associated with fine Cu-rich particles close to the water-cooled surface.

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.

Al-Ni 共晶合金の凝固組織に及ぼす重力の影響

The influence of the growth direction on the structure of the Al-Ni eutectic alloy was studied. In order to investigate the solidification phenomena in a gravitational environment, downward unidirectional solidification (in the direction of gravity) and upward unidirectional solidification (in the opposite direction of gravity) were carried out. A zone-melting technique was used for the evaluation of the unidirectional solidification under constant experimental conditions. The upward unidirectional solidification leads to the solid/liquid interface with the morphology of the typical faceted/non-faceted eutectic. On the other hand, in the case of the downward unidirectional solidification, the leading phase of the eutectic solidification transforms from Al3Ni into α-Al with the increasing solidification rate. This must be caused by the concentration distribution of nickel due to the density difference between aluminum and nickel. At the front of the solid/liquid interface upward solidified, the nickel concentration decreases with the increasing distance from the interface even though the liquid phase during the downward solidification has a constant concentration value. Therefore, the result indicates that the interface morphology must be influenced by the amount of nickel supplied to the Al3Ni phase. The supply depends on the nickel concentration corresponding to the solidification direction.

Effect of Flow Field on Formation of Channel Segregation During Upward Unidirectional Solidification of Pb-Sn Alloy

Effect of Flow Field on Formation of Channel Segregation During Upward Unidirectional Solidification of Pb-Sn Alloy

Mechanical properties of Sn–Zn lead-free solder alloys based on the microstructure array

The aim of this study is to develop a comparative experimental study interrelating mechanical properties, solidification thermal parameters and microstructure characteristics of a hypoeutectic Sn–4 wt.% Zn, a hypereutectic Sn–12 wt.% Zn and a eutectic Sn–9 wt.% Zn solder alloys. A water-cooled vertical upward unidirectional solidification system was used to obtain the samples. It was found that a more homogeneous distribution of the eutectic mixture, which occurs for smaller dendritic spacings in hypoeutectic and hypereutectic alloys, increases the ultimate tensile strength. The resulting microstructure of the eutectic Sn-9 wt.% Zn alloy has induced higher mechanical strength than those of the Sn–4 wt.% Zn and Sn–12 wt.% Zn alloys. It was found that the eutectic alloy experiences a microstructural transition from globular-to-needle-like Zn-rich morphologies which depend on the solidification growth rate. It is also shown that a globular-like Zn-rich morphology provides higher ultimate tensile strength than a needle-like Zn-rich eutectic morphology.

Inverse segregation during transient directional solidification of an Al–Sn alloy: Numerical and experimental analysis

In this article macrosegregation phenomenon is investigated by a finite volume numerical modeling technique and by upward unidirectional solidification of an Al–23 wt%Sn alloy casting. The numerical model consists of an explicit/implicit time integration scheme to couple thermal and solutal fields. The experimental solidification set-up allows a vertically aligned macrostructure to be obtained. The concentration profile was obtained by an X-ray fluorescence spectrometry technique carried out in cylindrical slices representing certain position inside the casting from the casting/chill interface. The simulated and experimental temperature and concentration profiles are compared and very good agreement has been observed.

Gravity-driven inverse segregation during transient upward directional solidification of Sn–Pb hypoeutectic alloys

Upward unidirectional solidification experiments were performed with Sn–4.0 wt.%Pb and Sn–12.5 wt.%Pb alloys. Transitory conditions of heat flow extraction were sustained during these solidification experiments, which mean that all solidification variables (cooling rate, tip growth rate, thermal gradient, and metal/mold heat transfer coefficient) have varied freely along solidification. In this work, the macrosegregation phenomenon is experimentally and theoretically evaluated considering both solidification shrinkage and gravity induced flows. The numerical model is based on a one-dimensional solution consisting of an implicit/explicit time integration scheme to couple thermal and solutal fields, which is supported by a finite volume numerical modeling technique to be solved. The applied solidification system permitted columnar growth to prevail along the castings. Since the Pb-rich melt tends to flow downward (in favor of the gravity vector), the bottom of the castings exhibited positive Pb content, mainly induced by gravity. The numerically simulated and experimental temperature and concentration profiles were compared and a very good agreement has been observed.

Determination of unidirectional heat transfer coefficient during unsteady-state solidification at metal casting–chill interface

In this study, the interfacial heat transfer coefficient (IHTC) for vertically upward unidirectional solidification of a eutectic Al–Si casting on water cooled copper and steel chills was measured during solidification. A finite difference method (FDM) was used for solution of the inverse heat conduction problem (IHCP). Six computer guided thermocouples were connected with the chill and casting, and the time–temperature data were recorded automatically. The thermocouples were placed, located symmetrically, at 5 mm, 37.5 mm and 75 mm from the interface. As the lateral surfaces are very well heat isolated, the unidirectional solidification process starts vertically upward at the interface surface. The measured time–temperature data files were used by a FDM using an explicit technique. A heat flow computer program has been written to estimate the transient metal–chill IHTC in the IHCP. The experimental and calculated temperatures have shown excellent agreement. The IHTC during vertically upward unidirectional solidification of an Al–Si casting on copper and steel chills have varied between about 19–9.5 kW/m2 K and 6.5–5 kW/m2 K, respectively.

Analytical, numerical, and experimental analysis of inverse macrosegregation during upward unidirectional solidification of Al-Cu alloys

The present work focuses on the influence of alloy solute content, melt superheat, and metal/mold heat transfer on inverse segregation during upward solidification of Al-Cu alloys. The experimental segregation profiles of Al 4.5 wt pct Cu, 6.2 wt pct Cu, and 8.1 wt pct Cu alloys are compared with theoretical predictions furnished by analytical and numerical models, with transient hi profiles being determined in each experiment. The analytical model is based on an analytical heat-transfer model coupled with the classical local solute redistribution equation proposed by Flemings and Nereo. The numerical model is that proposed by Voller, with some changes introduced to take into account different thermophysical properties for the liquid and solid phases, time variable metal/mold interface heat-transfer coefficient, and a variable space grid to assure the accuracy of results without raising the number of nodes. It was observed that the numerical predictions generally conform with the experimental segregation measurements and that the predicted analytical segregation, despite its simplicity, also compares favorably with the experimental scatter except for high melt superheat.

On macrosegregation in ternary Al–Cu–Si alloys: numerical and experimental analysis

Macrosegregation phenomena in ternary Al–Cu–Si alloys is investigated by a numerical modeling technique and by upward unidirectional solidification experiments. In particular, we consider a vertically aligned casting experiment of a ternary Al8.1wt%Cu3wt%Si alloy sample. The experimental measurements are used for validation of numerical simulations.

Observation of structures feathery of crystals formed during uni-directional solidification of Al-7.08% Mg, Al-0.68% and 1.5% Fe alloys

The feathery crystals of Al-Mg (7.08%) and Al-Fe (0.68 & 1.5%) alloys were formed during upward unidirectional solidification in insulating (Isolite) mold with chill plate at the bottom. The crystals formed were studied by various methods such as macroscopic and microscopic observations, etching pit method for estimating crystal orientation, microscopic observations and surface inspection testings on decanted interface. The following conclusions were drawn.(1) The feathery grains consisted of thin lamination layer of crystals having twin plane of (111), and its growth direction was presumed to be <112>(2) It seemed that as a single feathery crystal began to grow, numerous lamellae of feathery crystals were successively formed in almost parallel direction, which showed laminated structure.(3) As the results of microscopic observations and surface inspection testings on structure of decanted solidliquid interfaces, it was concluded that feathery grains grew prior to dendrites.(4) An excess of iron segregation was recognized at a position, which was assumed to be an original source of feathery crystals in Al-Fe (0.68 & 1.5%) alloys. The segregation would be due to the local accumulation of rejected solute in Al-1.5%Fe alloy, while Al-0.68% Fe alloy caused banded segregation.(5) By the observations of several cross-sections, 3-dimensional growth behavior of feathery crystals was explained.