Influence of Solidification Thermal Parameters on the Columnar-to-Equiaxed Transition of Aluminum-Zinc and Zinc-Aluminum Alloys

Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between –0.338 °C/mm and 0.167 °C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

A solutal interaction mechanism for the columnar-to-equiaxed transition in alloy solidification

A multiphase/multiscale model is used to predict the columnar-to-equiaxed transition (CET) during solidification of binary alloys. The model consists of averaged energy and species conservation equations, coupled with nucleation and growth laws for dendritic structures. A new mechanism for the CET is proposed based on solutal interactions between the equiaxed grains and the advancing columnar front—as opposed to the commonly used mechanical blocking criterion. The resulting differences in the CET prediction are demonstrated for cases where a steady state can be assumed, and a revised isotherm velocity (V _T ) vs temperature gradient (G) map for the CET is presented. The model is validated by predicting the CET in previously performed unsteady, unidirectional solidification experiments involving Al-Si alloys of three different compositions. Good agreement is obtained between measured and predicted cooling curves. A parametric study is performed to investigate the dependence of the CET position on the nucleation undercooling and the density of nuclei in the equiaxed zone. Nucleation undercoolings are determined that provide the best agreement between measured and calculated CET positions. It is found that for all three alloy compositions, the nucleation undercoolings are very close to the maximum columnar dendrite tip undercoolings, indicating that the origin of the equiaxed grains may not be heterogeneous nucleation, but rather a breakdown or fragmentation of the columnar dendrites. An erratum to this article is available at .     

Solidification parameters during the columnar-to-equiaxed transition in lead-tin alloys

The columnar-to-equiaxed transition (CET) was studied in lead-tin alloys, which were solidified directionally from a chill face. The main parameters analyzed include the temperature gradients, solidification velocities of the liquidus and solidus fronts, and grain size. The transition was observed to occur when the temperature gradient in the melt decreased to values between −0.8 °C/cm and 1 °C/cm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The comparison with available analytical models and the observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites.     

Resistance-Spot-Welded AZ31 Magnesium Alloys: Part I. Dependence of Fusion Zone Microstructures on Second-Phase Particles

A comparison of microstructural features in resistance spot welds of two AZ31 magnesium (Mg) alloys, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), with the same sheet thickness and welding conditions, was performed via optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). These alloys have similar chemical composition but different sizes of second-phase particles due to manufacturing process differences. Both columnar and equiaxed dendritic structures were observed in the weld fusion zones of these AZ31 SA and SB alloys. However, columnar dendritic grains were well developed and the width of the columnar dendritic zone (CDZ) was much larger in the SB alloy. In contrast, columnar grains were restricted within narrow strip regions, and equiaxed grains were promoted in the SA alloy. Microstructural examination showed that the as-received Mg alloys contained two sizes of Al₈Mn₅ second-phase particles. Submicron Al₈Mn₅ particles of 0.09 to 0.4 μm in length occured in both SA and SB alloys; however, larger Al₈Mn₅ particles of 4 to 10 μm in length were observed only in the SA alloy. The welding process did not have a great effect on the populations of Al₈Mn₅ particles in these AZ31 welds. The earlier columnar-equiaxed transition (CET) is believed to be related to the pre-existence of the coarse Al₈Mn₅ intermetallic phases in the SA alloy as an inoculant of α-Mg heterogeneous nucleation. This was revealed by the presence of Al₈Mn₅ particles at the origin of some equiaxed dendrites. Finally, the columnar grains of the SB alloy, which did not contain coarse second-phase particles, were efficiently restrained and equiaxed grains were found to be promoted by adding 10 μm-long Mn particles into the fusion zone during resistance spot welding (RSW).     

Laser Repair of Superalloy Single Crystals with Varying Substrate Orientations

The casting and repair of single-crystal gas turbine blades require specific solidification conditions that prevent the formation of new grains, equiaxed or columnar, ahead of the epitaxial columnar dendrites. These conditions are best determined by microstructure modeling. Present day analytical models of the columnar-to-equiaxed transition (CET) relate the microstructure to local solidification conditions (temperature gradient and interface velocity) without taking into account the effects of (1) a preferred growth direction of the columnar dendrites and (2) a growth competition between columnar grains of different orientations. In this article, the infiuence of these effects on the grain structure of nickel-base superalloy single crystals, which have been resolidified after laser treatment or directionally cast, is determined by experiment and by analytical and numerical modeling. It is shown that two effects arise for the case of a nonzero angle between the local heat flux direction and the preferred dendrite growth axis: (1) the regime of equiaxed growth is extended and (2) a loss of the crystal orientation of the substrate often occurs by growth competition of columnar grains leading to an “oriented-to-misoriented transition” (OMT). The results are essential for the definition of the single-crystal processing window and are important for the service life extension of expensive components in land-based or aircraft gas turbines. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13–15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Simulation of Solidification Microstructure and Columnar to Equiaxed Transition in Free‐cutting Steel 9SMn28 based on a CAFE Method

The solidification microstructure in 9SMn28 free‐cutting steel is simulated by the finite element – cellular automaton (CAFE) method based on the calculation of convection in a casting. The simulation results are consistent with experimental findings; the simulated crystallisation process conforms to the actual situation. The solidification of 9SMn28 alloy is a volume solidification mode under slow cooling condition. The columnar‐to‐equiaxed transition (CET) is also studied in the CAFE model. The mechanism of the CET in the CAFE model is thermal interaction. The CET is not abrupt but occurs gradually, the long columnar grains are first blocked by elongated grains. The grains become more equiaxed as the thermal gradient is decreased with the development of solidification.     

Using a Three-Phase Deterministic Model for the Columnar-to-Equiaxed Transition

In order to overcome the limitations of previous columnar-to-equiaxed (CET) models, which neglect melt convection and the movement of free equiaxed grains, this article presents a three-phase deterministic CET model. With appropriated multiphase volume-averaging approaches, it is possible to account for nucleation and growth of equiaxed grains ahead of a growing columnar front, the influence of melt convection, and grain sedimentation, and the occurrence of a CET in a casting of engineering scale. Special modeling assumptions ensure that both CET mechanisms, namely, “hard” and “soft” blocking, are tackled. It is highly recommended that both mechanisms should be considered, especially in the situation where grain sedimentation and melt convection are present. Although the current model incorporates almost all the physical variables relevant to a CET event, under special condition of a one-dimensional case, the model still reproduces the results of Hunt’s classical CET approach. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13–15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

The transition from columnar to equiaxed dendritic growth in proeutectic,low-volume fraction copper,Pb−Cu alloys

Lead, 17.1, 11.2, and 5 volume fraction copper (14, 9, and 4 wt pct Cu) alloys have been directionally solidified at constant growth velocities ranging from 1 to 100 μm s⁻¹. Serially increasing the growth velocity within this range results in a graded microstructural transition from fully columnar, albeit segregated, copper dendrites in a lead matrix to one consisting only of equiaxed grains. The imposed velocity necessary to effect fully equiaxed growth is found to drop rapidly as the volume fraction of copper is decreased. Factors which complicate the controlled, directional solidification of these alloys are discussed and the experimental results are interpreted in view of, and seen to be in qualitative agreement with, Hunt’s theory on the transition from columnar to equiaxed growth of dendrites.     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Influence of convection on feathery grain formation in aluminum alloys

The influence of convection on the formation of feathery grains, i.e., of columnar grains made of twinned dendrites growing along 〈110〉 directions, in AA1050 aluminum alloys has been studied. Round billets have been semicontinuously cast in a mold equipped with lateral liquid feeding. The fluid flow pattern in the liquid sump has been modeled using Fidap software. Feathery grains have been observed in the region opposite the mold entrance, i.e., in regions where the change of the velocity field (shearing rate) is the highest. Electron back scattered diffraction (EBSD) maps of two feathery regions, which were symmetric with respect to the liquid flow pattern, showed clear symmetry relationships. Furthermore, the 〈110〉 secondary dendrite arms had grown in directions opposite to the fluid flow. This experimental evidence brings more experimental support to the mechanism of feathery grain formation proposed earlier by Henry et al.     

Seeding of single-crystal superalloys—Role of constitutional undercooling and primary dendrite orientation on stray-grain nucleation and growth

An experimental study, together with a two-dimensional numerical simulation of solute segregation, was conducted to investigate (1) the mechanism for stray-grain nucleation following seed melt-back and initial withdrawal and (2) the role of the primary dendrite disposition of the seed crystal in relation to the mold wall during growth. It is proposed that the factors contributing to stray-grain nucleation during initial withdrawal are (1) the magnitude of local, solute-adjusted undercooling and (2) the rapidly changing curvature of the solidification front close to the mold walls during the initial solidification transient. Based upon the calculated local undercooling and experimentally observed stray-grain morphologies, it was concluded that stray grains nucleate near the mold wall around the seed perimeter and behind the columnar dendrites that advance into the bulk liquid ahead of the melt-back zone. These grains then compete during growth with the dendrites originating from the seed. Therefore, the morphological constraints arising from the inclination of the primary dendrites from the seed crystal with respect to the mold wall (converging/diverging/axial 〈001〉) determines the probability of the stray-grain nuclei developing into equiaxed/columnar grains following competitive growth.     

Morphology Transition from Dendrites to Equiaxed Grains for AlCoCrFeNi High-Entropy Alloys by Copper Mold Casting and Bridgman Solidification

The AlCoCrFeNi high-entropy alloys (HEAs) were prepared by the copper mold casting and Bridgman solidification. X-ray diffraction (XRD) results verify that the main phase was body-centered-cubic (bcc) solid solution by these two solidification processes, indicating its good phase stability. Interestingly, the metallographic photos show a morphology transition from dendrites to equiaxed grains after Bridgman solidification, which was considered to have a strong dependence on the parameter of the G/V (the temperature gradient to the growth rate ratio). Compared to the as-cast sample, the plasticity of alloys synthesized by Bridgman solidification was improved by a maximum of 35?pct.     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Modeling of Dendritic Evolution of Continuously Cast Steel Billet with Cellular Automaton

In order to predict the dendritic evolution during the continuous steel casting process, a simple mechanism to connect the heat transfer at the macroscopic scale and the dendritic growth at the microscopic scale was proposed in the present work. As the core of the across-scale simulation, a two-dimensional cell automaton (CA) model with a decentered square algorithm was developed and parallelized. Apart from nucleation undercooling and probability, a temperature gradient was introduced to deal with the columnar-to-equiaxed transition (CET) by considering its variation during continuous casting. Based on the thermal history, the dendritic evolution in a 4 mm × 40 mm region near the centerline of a SWRH82B steel billet was predicted. The influences of the secondary cooling intensity, superheat, and casting speed on the dendritic structure of the billet were investigated in detail. The results show that the predicted equiaxed dendritic solidification of Fe-5.3Si alloy and columnar dendritic solidification of Fe-0.45C alloy are consistent with in situ experimental results [Yasuda et al. Int J Cast Metals Res 22:15–21 (2009); Yasuda et al. ISIJ Int 51:402–408 (2011)]. Moreover, the predicted dendritic arm spacing and CET location agree well with the actual results in the billet. The primary dendrite arm spacing of columnar dendrites decreases with increasing secondary cooling intensity, or decreasing superheat and casting speed. Meanwhile, the CET is promoted as the secondary cooling intensity and superheat decrease. However, the CET is not influenced by the casting speed, owing to the adjusting of the flow rate of secondary spray water. Compared with the superheat and casting speed, the secondary cooling intensity can influence the cooling rate and temperature gradient in deeper locations, and accordingly exerts a more significant influence on the equiaxed dendritic structure.     

A novel experiment for the study of substrate-induced nucleation in metallic alloys: Application to Zn-Al

In order to investigate the nucleation and growth mechanisms of Zn-0.2 wt pct Al grains in thin coatings deposited on steel sheets by hot dipping, a new directional solidification experiment has been specifically designed with a steel substrate immersed into the melt. The competition (CTG) between regular columnar (C) grains growing parallel to the thermal gradient and transverse grains (TGs) nucleated on the sheet surface and growing transversely has been observed in longitudinal sections of Zn-0.2 wt pct Al (-Sb) alloys. Without Sb, the CTG began at midheight of the casting, whereas with Sb, no CTG was found. A simple CTG criterion similar to that of Hunt for the columnar-to-equiaxed Transition (CET) was derived. The calculated extension of the TGs was successfully compared with that measured on micrographs reconstructed from electron backscattered diffraction (EBSD) measurements. This indicated that the critical nucleation undercooling for the formation of Zn-0.2 wt pct Al grains on the steel sheet is about 0.7 K and that Sb poisons these available nucleation sites.     

Prediction of the As-Cast Structure of Al-4.0 Wt Pct Cu Ingots

A two-stage simulation strategy is proposed to predict the as-cast structure. During the first stage, a 3-phase model is used to simulate the mold-filling process by considering the nucleation, the initial growth of globular equiaxed crystals and the transport of the crystals. The three considered phases are the melt, air and globular equiaxed crystals. In the second stage, a 5-phase mixed columnar-equiaxed solidification model is used to simulate the formation of the as-cast structure including the distinct columnar and equiaxed zones, columnar-to-equiaxed transition, grain size distribution, macrosegregation, etc. The five considered phases are the extradendritic melt, the solid dendrite, the interdendritic melt inside the equiaxed grains, the solid dendrite, and the interdendritic melt inside the columnar grains. The extra- and interdendritic melts are treated as separate phases. In order to validate the above strategy, laboratory ingots (Al-4.0 wt pct Cu) are poured and analyzed, and a good agreement with the numerical predictions is achieved. The origin of the equiaxed crystals by the “big-bang” theory is verified to play a key role in the formation of the as-cast structure, especially for the castings poured at a low pouring temperature. A single-stage approach that only uses the 5-phase mixed columnar-equiaxed solidification model and ignores the mold filling can predict satisfactory results for a casting poured at high temperature, but it delivers false results for the casting poured at low temperature.     

A three-phase model for mixed columnar-equiaxed solidification

A three-phase model for mixed columnar-equiaxed solidification is presented in this article. The three phases are the parent melt as the primary phase, as well as the solidifying columnar dendrites and globular equiaxed grains as two different secondary phases. With an Eulerian approach, the three phases are considered as spatially coupled and interpenetrating continua. The conservation equations of mass, momentum, species, and enthalpy are solved for all three phases. An additional conservation equation for the number density of the equiaxed grains is defined and solved. Nucleation of the equiaxed grains, diffusion-controlled growth of both columnar and equiaxed phases, interphase exchanges, and interactions such as mass transfer during solidification, drag force, solute partitioning at the liquid/solid interface, and release of latent heat are taken into account. Binary steel ingots (Fe-0.34 wt pct C) with two-dimensional (2-D) axis symmetrical and three-dimensional (3-D) geometries as a benchmark were simulated. It is demonstrated that the model can be used to simulate the mixed columnar-equiaxed solidification, including melt convection and grain sedimentation, macrosegregation, columnar-to-equiaxed-transition (CET), and macrostructure distribution. The model was evaluated by comparing it to classical analytical models based on limited one-dimensional (1-D) cases. Satisfactory results were obtained. It is also shown that in order to apply this model for industrial castings, further improvements are still necessary concerning some details.