Experimental Investigation of Macrosegregation During Vertical Solidification of Lead–Tin Alloys

Thermosolutal convection during solidification of alloys is one of the major causes of macrosegregation. An experimental investigation was carried out on lead–tin alloys to study the evolution of macrosegregation during vertical solidification in laboratory. The emphasis was on accurate measurements of temperatures during solidification as well as segregation measurements and microstructural examination of solidified samples. The nominal compositions of ingots were Pb–35% Sn, Pb–19 wt% Sn and Sn–15 wt% Pb. Experiments were carried out at two superheats and different cooling rates. With decreasing cooling rate, increase in axial macrosegregation was observed in lead rich alloys. No macrosegregation was observed in tin rich alloys. Convection of interdendritic liquid was found to be responsible for macrosegregation. Also, from experimental data, heat flux to cooling water, local solidification times in the melt as well as dendritic arm spacings were determined.     

Dendritic growth of undercooled nickel-tin: Part II

High-speed optical temperature measurements were made of the solidification behavior of levitated metal samples within a transparent glass medium. Two undercooled Ni-Sn alloys were examined, one a hypoeutectic alloy and the other of eutectic composition. Recalescence times for the 9 mm diameter samples studied decreased with increasing undercooling from the order of 1.0 second at 50 K under-cooling to less than 10⁻³ second for undercoolings greater than 200 K. Both alloys recalesced smoothly to a maximum recalescence temperature at which the solid was at or near its equilibrium composition and equilibrium weight fraction. For the samples of hypoeutectic alloy that recalesced above the eutectic temperature, a second nucleation event occurred on cooling to the eutectic temperature. For samples which recalesced only to the eutectic temperature, no subsequent nucleation event was observed on cooling. It is inferred in this latter case that both the α and β phases were present at the end of recalescence. The thermal data obtained suggest a solidification model involving (1) dendrites of very fine structure growing into the melt at temperatures near the bulk undercooling temperature, (2) thickening of dendrite arms with rapid recalescence, and (3) continued, much slower recalescence accompanying dendrite ripening.     

Macrosegregation during solidification resulting from density differences in the liquid

Macrosegregation has been observed in directionally solidified Pb-20 pct Sn alloys, over a range of freezing rates and temperature gradients. The macrosegregation was shown to result from the upward flow of less dense, tin rich, interdendritic liquid during solidification, using radioactive tracer techniques. For comparison, it was shown that macrosegregation occurred in the opposite direction in a Sn-4 pct Pb alloy, where the interdendritic liquid was lead rich, and consequently more dense. Shrinkage trails and pipes were observed in some of the Pb-20 pct Sn ingots, similar to “freckles” observed in directionally cast superalloys. A mathematical model for macrosegregation in vertically solidified ingots is presented, the driving force being the density differences in the interdendritic liquid during solidification. Liquid flow through the dendritic array is estimated by considering the partially solidified alloy as a porous medium of variable porosity. For simplicity, the model neglects backflow due to volume shrinkage (inverse segregation). The experimental results are compared to the model predictions. Formerly Research Associate, Department of Metallurgy, University of British Columbia     

Three‐dimensional Simulation of Thermosolutal Convection and Macrosegregation in Steel Ingots

Thermosolutal convection and macrosegregation formation during the solidification of steel ingots are numerically simulated in three dimensions. The simulation is based on a fully coupled model for mass, momentum, energy, and species conservation equations. The interdendritic flow in the mushy zone is governed by Darcy's law, and the permeability term is discretized using an interpolated liquid fraction method. The numerical results for a benchmark test of macrosegregation in a Pb‐Sn alloy are compared with experimental data taken from the literature. The present model is applied to simulate the solidification of industrial steel ingots. Preliminary predictions are obtained, including the positive segregation in the hot top, and the conically shaped negative segregation zone at the bottom of the ingot. The predicted variation of the segregation ratio in carbon along the vertical centreline of an ingot is compared with measurements, and generally good agreement is observed. Future attention should be paid to the precision of prediction by considering complex solidification issues, such as the sedimentation of free equiaxed grains and the formation of shrinkage cavity.     

Nucleation-controlled microstructures and anomalous eutectic formation in undercooled Co-Sn and Ni-Si eutectic melts

Co-20.5 at. pct Sn and Ni-21.4 at. pct Si eutectic alloys have been levitated and undercooled in an electromagnetic levitator (EML) and then solidified spontaneously at different undercoolings. The original surface and cross-sectional morphologies of these solidified samples consist of separate eutectic colonies regardless of melt undercooling, indicating that microstructures in the free solidification of the eutectic systems are nucleation controlled. Regular lamellae always grow from the periphery of an independent anomalous eutectic grain in each eutectic colony. This typical morphology shows that the basic unit should be a single eutectic colony, when discussing the solidification behavior. Special emphasis is focused on the anomalous eutectic formation after a significant difference in linear kinetic coefficients is recognized for terminal eutectic phases, in particular when a eutectic reaction contains a nonfaceted disordered solid solution and a faceted ordered intermetallic compound as the terminal eutectic phases. It is this remarkable difference in the linear kinetic coefficients that leads to a pronounced difference in kinetic undercoolings. The sluggish kinetics in the interface atomic attachment of the intermetallic compound originates the occurrence of the decoupled growth of two eutectic phases. Hence, the current eutectic models are modified to incorporate kinetic undercooling, in order to account for the competitive growth behavior of eutectic phases in a single eutectic colony. The critical condition for generating the decoupled growth of eutectic phases is proposed. Further analysis reveals that a dimensionless critical undercooling may be appropriate to show the tendency for the anomalous eutectic-forming ability when considering the difference in linear kinetic coefficients of terminal eutectic phases. This qualitative criterion, albeit crude with several approximations and assumptions, can elucidate most of the published experimental results with the correct order of magnitude. Solidification modes in some eutectic alloys are predicted on the basis of the present criterion. Future work that may result in some probable errors is briefly directed to improve the model.     

On The Formation of Macrosegregations in

The effect of the natural convection on the formation of macrosegregations has been experimentally and theoretically analyzed. The Sn-10 pct Pb and Pb-15 pct Sn alloys were unidirectionally solidified. The temperature gradient and the solidification direction were perpendicular to the gravity vector. The concentration and temperature gradients in the samples cause natural con- vection. Large macrosegregations were observed in the sample. Two different convection modes were found in the two alloys caused by different density gradients in the two-phase region. This difference in convection causes an enrichment of Pb in the lower part of the last solidified regions in the Sn-10 pct Pb alloy and an enrichment of Sn in the upper part in the Pb-15 pct Sn alloy. Computer simulation of the convection mode models the convection pattern and the solidification process during which macrosegregation occurs. Formerly with the Department of Casting Formerly with the Department of Casting     

Undercooling and solidification of Al-50 at. pct Si Alloy by electromagnetic levitation

Electromagnetic levitation is applied to achieve containerless solidification of 10-mm-diameter droplets of Al-50 at. pct Si. A maximum undercooling of 320 K is obtained. Phase morphologies on the droplet surfaces and on the deeply etched sections of the samples solidified at different undercoolings are examined by scanning electron microscopy. The primary silicon shows well-developed faceted dendrites at a small undercooling, but a fine granular form at a large undercooling. Stratified deposits of aluminum are found within the primary silicon plates, arising from solute pileup during growth. The microstructural refinement at a large undercooling has its origins in solute restriction of crystal growth and in fragmentation of the primary silicon dendrites. The form of the Al-Si eutectic is also found to be changed into an anomalous form at a large undercooling.     

Microstructures of undercooled Germanium droplets

Small liquid Ge droplets (0.3–0.5 mm diameter) have been undercooled 150–415 ± 20°C below T_m in B₂O₃ flux before solidifying to the diamond cubic phase. A correlation was found between initial undercooling and final grain size. Droplets undercooled <300°C exhibited a coarse grain structure. At greater undercoolings, the grain size became progressively finer. This correlation may be subsidiary to the dependence of grain size on interfacial undercooling. Ge droplets lightly doped with Sn solidified dendritically for undercoolings greater than 250°C. Twinned dendrites have been observed at small undercoolings (~ 10°C) in other experiments. It appears that larger interfacial undercoolings are necessary to grow the twin-free dendrites which we have observed. The correlation between grain size and the presence of dendrites suggests that the grain refinement observed in Ge samples undercooled > 300°C stems from dendritic break-up during solidification.     

Solidification of hypereutectic Al-38 Wt Pct Cu alloy in microgravity and in unit gravity

Solidification in microgravity aboard the space shuttle Endeavour resulted in a dramatic change in the morphology of the primary Al₂Cu phase compared to ground-based solidification in unit gravity. An Al-38 wt pct Cu ingot directionally solidified at a rate of 0.015 mm/s with a temperature gradient of 1.69 K/mm exhibited large, well-formed dendrites of primary Al₂Cu phase. Ingots solidified under similar conditions in unit gravity contained primary Al₂Cu phase with smooth, faceted surfaces. The primary Al₂Cu phase spacing in the microgravity ingot was much greater than that in the unit gravity ingot, 670 μm compared to 171 μm. It is suggested that thermosolutal mixing in the unit gravity ingot reduces the buildup of an Al-rich layer at the solid/liquid interface, which increases the stability of the interface resulting in smooth, faceted particles of Al₂Cu phase. It is also suggested that the large difference in primary phase spacings is due mostly to the difference in morphology rather than changes in parameters that might influence dendrite ripening mechanisms. The presence or absence of gravity had no effect on the interlamellar spacing of the inter-Al₂Cu phase eutectic. The ingot solidified in microgravity exhibited almost no longitudinal macrosegregation, in agreement with the theory of inverse segregation in the absence of thermosolutal convection. The ingot solidified in unit gravity exhibited considerable longitudinal macrosegregation, with the chilled end having about 6 wt pct more Cu than the average composition. It is not clear whether the segregation results from thermosolutal convection during solidification or from sedimentation during melting.     

Gravitational macrosegregation in unidirectionally solidified Lead-tin alloy

Small laboratory samples of binary lead-tin alloys, about 20 mm diameter, 50 mm high, and weighing about 180 grams, were solidified unidirectionally upward, with both cooling rate and thermal gradient being closely controlled. A total of nine ingots were produced; six of these ingots had a nominal composition of Pb-15 wt pct Sn, and the remaining three were Sn-15 wt pct Pb. Detailed thermal measurements, chemical composition measurements (dendrite composition, fraction of interdendritic eutectic formed), and microstructural characterization (primary and secondary dendrite arm spacing measurements) were carried out. Normal macrosegregation was observed in the Pb-rich alloys, with Sn content being highest at the top of the ingot. In the Pb-15 wt pct Sn alloys, macrosegregation was found to increase with increasing thermal gradients, and also with decreasing cooling rates. In some of our experiments with Pb-rich alloys it was possible to induce severe segregation, in the form of deep freckles. The freckles were in the form of a column of tin-rich material which extended over the upper half of the ingot. No significant macrosegregation was evident in the Sn-rich alloy for the growth conditions employed here. Segregation behavior of all ingots was correlated with the measured thermal history, compositional data, and the fraction eutectic phase in the finally solidified samples. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Macrosegregation in Rotated Remelted Ingots

A computer model is presented for predicting macrosegregation in rotated electroslag or vacuum arc remelted ingots. Sample calculations of segregation are carried out for ingots of the model alloy Sn-12 pet Pb in which the liquid density increases during solidification and for two hypothetical alloys; in one, the liquid density decreases during solidification, and in the other, liquid density first increases and then decreases during solidification. In alloys such as Sn-Pb in which liquid density increases during solidification, segregation is positive at the ingot centerline and if solidification is sufficiently slow, “freckles” form near the centerline. Positive segregation and freckles are found at the outer periphery of the ingot when liquid density decreases during solidification. Positive segregation and freckles are found at midradius when liquid density first increases and then decreases during solidification, and when the solidus isotherm changes shape abruptly at midradius (with density increasing during solidification). Ingot rotation, by introducing a radial component to the force field, alters interdendritic flow behavior and therefore macrosegregation. Modest rotation speeds eliminate “freckles” and reduce macrosegregation in all modeling studies conducted. Greater rotational speeds can accentuate the segregation. Experiments were conducted on simulated remelted ingots of Sn-Pb alloy. The ingots were 8 cm diam, rotated at speeds up to 119 rpm and solidified at rates from 5.3 × 10^?3 to 1.36 × 10^?2 cm/s. Segregation behavior obtained agrees qualitatively and quantitatively with theory.     

Phase selection during crystallization of undercooled liquid eutectic lead-tin alloys

During rapid solidification substantial amounts of undercooling are in general required for formation of metastable phases. Crystallization at varying levels of undercooling and melting of metastable phases were studied during slow cooling and heating of emulsified PbSn alloys. Besides the experimental demonstration of the reversibility of metastable phase equilibria, two different principal solidification paths have been identified and compared with the established metastable phase diagram and predictions from classical nucleation theory. The results suggest that the most probable solidification path is described by the “step rule” resulting in the formation of metastable phases at low undercooling, whereas the stable eutectic phase mixture crystallizes without metastable phase formation at high undercooling.     

Modeling of irregular eutectic microstructures in solidification of Al-Si alloys

A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the growth of primary α dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

Modeling of Species Transport and Macrosegregation in Heavy Steel Ingots

In the current study, two significant phenomena involved in heavy steel ingot casting, i.e., species transport and macrosegregation, were numerically simulated. First, a ladle–tundish–mold species transport model describing the entire multiple pouring process of heavy steel ingots was proposed. Carbon distribution and variation in both the tundish and the mold of a 292-ton steel ingot were predicted. Results indicate high carbon concentration in the bottom of the mold while low concentration carbon at the top of mold after the pouring process. Such concentration distribution helps in reducing both negative segregation in the bottom of the solidified ingot and positive segregation at the top. Second, a two-phase multiscale macrosegregation model was used to simulate the solidification process of industrial steel ingots. This model takes into account heat transfer, fluid flow, solute transport, and equiaxed grain motion on a system scale, as well as grain nucleation and growth on a microscopic scale. The model was first used to analyze a three-dimensional industry-scale steel ingot as a benchmark. Then, it was applied to study macrosegregation formation in a 53-ton steel ingot. Macrosegregation predicted by the numerical model was presented and compared with experimental measurements. Typical macrosegregation patterns in heavy steel ingots are found to be well reproduced with the two-phase model.     

Numerical Simulation of Macrosegregation During Vertical Solidification

A continuum formulation based mathematical model has been employed to analyze the evolution of thermosolutal convection, isotherms, liquid fraction profiles and macrosegregation profiles during vertical solidification of lead-rich binary lead?Ctin alloys. The initial conditions, boundary conditions and microstructural parameters of this model have been taken from controlled experiments performed on lead?Ctin system. Numerical simulations were carried out for three chosen experiments on Pb-35?wt?pct. Sn alloy with different cooling rates. The convection was very weak in case of vertical solidification. It was also observed that the nature of the flow field did not show significant effect on thermal field. However, there was substantial effect of nature of flow on macrosegregation. It was found that the choice of permeability model, especially near the liquidus, played a significant role in the evolution of macrosegregation during vertical solidification. Through the use of appropriate permeability model, the predicted profile of average macrosegregation in ingots along the vertical (i.e., axial) direction showed reasonable agreement with experimental data over a wide range of cooling rates.     

Experimental and Modeling Studies of the Lamellar Eutectic Growth of Mg-Al Alloy

Directionally solidified samples of Mg-32.3 wt pct Al eutectic alloy were produced under an argon atmosphere in a vacuum Bridgman-type furnace to study the eutectic growth with different growth velocities. Typical features such as steady-state lamellar eutectic growth, lamellar branching at the quenching interface, and the formation of colony structures due to the impurity of the Mg-Al binary alloy were observed using a JEOL 6301F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The lamellar spacing of the two eutectic phases was measured on the transverse sections of the samples. It was found that the relationship between the measured lamellar spacing and growth velocity agreed well with the prediction of the Jackson-Hunt model. Subsequent studies of Mg-Al eutectic growth were conducted using a numerical model based on the cellular automaton (CA) method. Taking account of the solute diffusion, constitutional undercooling, and curvature undercooling, modeling of steady-state lamellar eutectic growth was achieved. A systematic investigation of the eutectic growth morphology and lamellar spacing of the Mg-Al eutectic was carried out under directional solidification with different undercoolings, initial lamellar spacings, temperature gradients, and growth velocities. The results showed that under the interaction between solute diffusion and surface energy, the adjustment of eutectic lamellar spacing was accomplished by nucleation, lamellar branching, lamellar termination, and overgrowth. The simulated results were consistent with both the experimental results and the Jackson-Hunt eutectic theory.     

Dendritic growth of undercooled nickel-tin: Part I

Solidification of undercooled Ni-25 wt pct Sn alloy was observed by high-speed cinematography and results compared with optical temperature measurements. Samples studied were rectangular in cross-section, and were encased in glass. Cinematographic measurements were carried out on samples undercooled from 68 to 146 K. These undercoolings compare with a temperature range of 199 K from the equilibrium liquidus to the extrapolated equilibrium solidus. At all undercoolings studied, the high-speed photography revealed that solidification during the period of recalescence took place with a dendrite-like front moving across the sample surface. Spacings of the apparent “dendrite” were on the order of millimeters. The growth front moved at measured velocities ranging from 0.07 meters per second at 68 K undercooling to 0.74 meters per second at 146 K undercooling. These velocities agree well with results of calculations according to the model for dendrite growth of Lipton, Kurz, and Trivedi. It is concluded that the coarse structure observed comprises an array of very much finer, solute-controlled dendrites.     

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.