Longitudinal Macrosegregation Research Articles

Growth and transition of dendrites in steel strands under different electromagnetic stirring methods

In the industrial continuous casting process, some central defects such as shrinkage cavity, porosity, segregation and crack are generated especially at the final solidification zone of steel strands because of the poor fluidity and feeding ability of molten steel in the center of strand, especially for the special steel strand. In this paper, a new type of vertical linear electromagnetic stirring (V-LEMS) was proposed to apply in the casting of Incoloy 825 alloy ingot and investigate the growth and transition of dendrites, distribution of Ti in the special steel, and compare its effect with the generally conventional industrial rotary electromagnetic stirring (R-EMS). The results shown that the growth direction of primary dendrite under R-EMS is perpendicular to the mold wall, but the primary dendrite growth under V-LEMS have present an upstream dendrite growth state. It proves that the V-LEMS can generate a longitudinal circulation convection in the height direction of ingot, which can strengthen the temperature and composition mixing of the upper and lower melts of the ingot and promote the homogenization of the overall melt temperature and composition. The radial macrosegregation of Ti under V-LEMS is slightly higher than that of R-EMS, but R-EMS tends to increase the longitudinal macrosegregation of Ti element. The longitudinal macrosegregation degree of Ti element under V-LEMS is less than that without EMS and with R-EMS. V-LEMS makes the longitudinal distribution of Ti element in Incoloy 825 alloy ingot more uniform. The research results give some beneficial suggestions for improving the central quality of special steel continuous casting billet.

The effect of confinement on thermal convection and longitudinal macrosegregation in directionally solidified dilute succinonitrile–camphor alloy

Directional solidification experiments were conducted in a succinonitrile–0.24 wt% camphor alloy with an emphasis on the planar front interface temperature dynamics using different sample thicknesses. The interface temperature was found to depend significantly on the thickness due to non-negligible convection effects in the thicker samples. The results were interpreted with the help of an order of magnitude analysis and a boundary layer model, which permitted estimation of the solute macrosegregation profile. The experiments and corresponding analyses performed in this work constitute an experimental characterization of convection effects as a function of sample thickness.

Hardness and corrosion behaviour of anodised Al-Si produced by rheocasting

The anodised layer of Al-Si alloys produced by rheocasting was studied and compared to anodised traditional liquid casting in this paper. The anodising was performed in 1.0 M H2SO4 solution at room temperature on the as-cast substrates, and anodising voltage and time were optimised as process parameters. This study focuses on understanding the effect of the surface liquid segregation (SLS) layer by rheocasting on the hardness and corrosion protection of the oxide layer. The hardness depends on the anodising parameters and varies along the oxide thickness. The corrosion protection given by the oxide layer was evaluated by electrochemical impedance spectroscopy (EIS) in 3 wt-% NaCl solution, and the results revealed that the longitudinal macrosegregation influences the corrosion protection, with the near-to-vent region showing lower corrosion protection due to a higher eutectic fraction. A comparison between liquid and rheocast samples indicated that the presence of SLS layer by the transverse macrosegregation does not have a significant impact on the corrosion resistance of the oxide layer. Moreover, it was found that an increase of the oxide layer thickness by longer anodising time or higher applied voltage decreases both the hardness and corrosion resistance of the oxide layer.

Effect of macro- and micro-segregation on hot cracking of Inconel 718 superalloy argon-arc multilayer cladding

The hot cracking sensitivity dramatically increases because of the longitudinal macro-segregation of craters and the transverse macro-segregation of welding center in pre-layers. Statistical analysis of the total area, average size and Nb concentration of interdendritic low-melting point Laves phase, the Nb concentration of the γ phase in the dendritic core, and the arm spacing of different sub-regions was used to explore the effects of macro- and micro-segregation of Nb. Solidus and liquidus curves were used to determine the temperature at which solid and liquid phases coexist in different sub-regions, which were combined with Scheil equation curves to identify Nb sources and interlayer transfer, so as to develop a model for the molten pool of subsequent layers. This model explains the effect of transversal and longitudinal macro-segregation in the pre-layer on the Nb concentration of the melt pool of subsequent layers, the dynamic balance of the solidification interface and the relative area of Laves phase in the macro-segregated sub-region of the solidified metal.

Effect of Segregation and Surface Condition on Corrosion of Rheo-HPDC Al–Si Alloys

Corrosion properties of two Al–Si alloys processed by Rheo-high pressure die cast (HPDC) method were examined using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The effects of the silicon content, transverse and longitudinal macrosegregation on the corrosion resistance of the alloys were determined. Microstructural studies revealed that samples from different positions contain different fractions of solid and liquid parts of the initial slurry. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and surface quality.

Macrosegregation Due to Convection in Al-19Cu Alloy Directionally Solidified Through an Abrupt Expansion in Cross-Section: A Comparison with Al-7Si

Hypoeutectic Al-19 wt.% Cu alloys were directionally solidified at two different growth speeds in cylindrical molds that featured an abrupt increase in cross-section, from 3.2 to 9.5 mm in diameter. The effects of thermosolutal convection and shrinkage flow induced by the cross-section change on macrosegregation were investigated. Dendrite clustering and extensive radial macrosegregation were seen, particularly in the larger cross-section after expansion. Negative longitudinal macrosegregation right after the cross-section increase was observed; the extent of macrosegregation, however, decreases with increasing growth speed. Both thermal and flow effects due to cross-section change were seen to influence the radial macrosegregation immediately before, and after the expansion. Radial macrosegregation pattern was found to be changing as the mushy zone enters the larger cross-section region above the cross-section change where the solidification is in its unsteady state. The effect of the solutal expansion coefficient on macrosegregation was studied by comparing the degree of thermosolutal convection in Al-19 wt.% Cu with a previous study in which we investigated Al-7 wt.% Si. A two-dimensional model accounting for both shrinkage and thermosolutal convection was used to simulate the resulting steepling, as well as the axial and radial macrosegregation. The experimentally observed macrosegregation associated with the expansion during directional solidification is well predicted by the numerical simulations.

Effect of a magnetic field on macro segregation of the primary silicon phase in hypereutectic Al-Si alloy during directional solidification

Effect of static magnetic field on longitudinal macro segregation of primary silicon phase has been investigated in directionally solidified hypereutectic Al-Si alloy. Experimental and numerical simulation results indicate that the melt flow of Al-21 wt% Si alloy under magnetic field is the dominant factor leading to macro segregation of primary silicon phase. The intense melt flow, i.e., recirculation loops and microscopic flow, promotes solute Si to the mushy zone and sidewall of the crucible where the temperature is low and the primary silicon is more likely to precipitate, resulting in the formation of the U-shaped interface. The precipitated primary silicon tends to disappear during directional solidification, which induces the transition from the U-shaped interface to the planar interface. The increase of magnetic field intensity accelerates this transition process and increases the macro segregation. Furthermore, the freely drifted fine silicon particles in the bulk melt are transported to mushy zone by the forced flow, which contributes to macro segregation. This work facilitates the understanding of forced flow greatly changing the solidification structures of alloys under the magnetic field during directional solidification.

Microstructure Evolution and Macro Segregation of Directionally Solidified Hypoeutectic and Hypereutectic Lead-Tin Alloys

Unidirectional solidification is preferred to multidirectional solidification for growing crystals in a particular direction. An experimental set-up consisting of Bridgman type of upward directional solidification was employed for the present investigation. The main aim of the present investigation was to assess the effect of unidirectional upward solidification on the segregation of off-eutectic Pb-Sn alloys at different translational speeds of the experimental set-up. Solidification experiments were conducted on hypoeutectic and hypereutectic Lead-Tin alloys. Different combinations of growth rate V and composition Co. were used to investigate their effect on longitudinal macro segregation. Macro segregation along the length of the samples was observed in hypoeutectic Pb-Sn alloys whereas no such macro segregation was observed in hypereutectic alloys. The intensity of longitudinal macro segregation was found to increase with the increase in initial tin content of the alloy, increase in distance from the chill end and decrease in the solidification rate.

Convection and macrosegregation in Al-19Cu alloy directionally solidified through an abrupt contraction in cross-section: A comparison with Al-7Si

Hypoeutectic Al-19 wt. % Cu alloys were directionally solidified in cylindrical molds that featured an abrupt cross-section decrease 9.5 to 3.2 mm in diameter). Thermo-solutal convection and cross-section-change-induced shrinkage flow effects on macrosegregation were investigated. Dendrite clustering and extensive radial macrosegregation was seen, particularly in the larger cross-section before contraction. This alloy shows positive longitudinal macrosegregation near the contraction followed by negative macrosegregation right after it; the extent of macrosegregation, however, decreases with increasing growth speed. The degree of thermo-solutal convection was compared to another study investigating directional solidification of Al-7 wt. % Si [1] in order to study the effect of solutal expansion coefficient on macrosegregation. An interesting change of the radial macrosegregation profile, attributable to the area-change-induced-shrinkage flow, was observed very close to the contraction. A two-dimensional model accounting for both shrinkage and thermo-solutal convection was used to simulate solidification, the resulting steepling as well as axial and radial macrosegregation. The experimentally observed macrosegregation associated with the contraction during directional solidification was well predicted by the numerical simulations.

Macrosegregation in Al–7Si alloy caused by abrupt cross-section change during directional solidification

Hypoeutectic Al-7 wt.% Si alloys were directionally solidified vertically downward in cylindrical molds that incorporated an abrupt cross-section decrease (9.5mm to 3.2mm diameter) which, after 5cm, reverted back to 9.5mm diameter in a Bridgman furnace; two constant growth speeds and thermal gradients were investigated. Thermosolutal convection and cross-section-change-induced shrinkage flow effects on macrosegregation were investigated. Dendrite clustering and extensive radial macrosegregation was seen, particularly in the larger cross-sections, before contraction and after expansion, this more evident at the lower growth speed. This alloy shows positive longitudinal macrosegregation near cross-section decrease followed by negative macrosegregation right after it; the extent of macrosegregation, however, decreases with increasing growth speed. Primary dendrite steepling intensified as solidification proceeded into the narrower section and negative longitudinal macrosegregation was seen on the re-entrant shelves at expansion. A two-dimensional model accounting for both shrinkage and thermo-solutal convection was used to simulate solidification and the resulting mushy-zone steepling and macrosegregation. The experimentally observed longitudinal and radial macrosegregation associated with the cross-section changes during directional solidification of an Al–7Si alloy is well captured by the numerical simulations.

Initial transient behavior in directional solidification of a bulk transparent model alloy in a cylinder

To characterize the dynamical formation of three-dimensional (3-D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile–0.24wt.% camphor model alloy was carried out under low gravity in the Device for the Study of Critical Liquids and Crystallization (DECLIC) Directional Solidification Insert on board the International Space Station (ISS). The present paper focuses on the study of the transient solid–liquid interface recoil. Numerical thermal modeling led us to identify two thermal contributions to the interface recoil that increase with the pulling rate and add to the classical recoil associated with the solute boundary layer formation. As a consequence of those additional contributions, the characteristic front recoil is characterized by a fast initial transient followed by stabilization to a plateau whose location depends on pulling rate. The analysis of comparative experiments carried out on the ground shows the absence of stabilization of the interface position, attributed to longitudinal macrosegregation of the solute induced by convection. This behavior is surprisingly also observed in space experiments for low pulling rates. An order of magnitude analysis of the mode of solute transport reveals that for these conditions, the effective level of reduced gravity on board the ISS is not sufficiently low to suppress convection so that the interface recoils with longitudinal macrosegregation in a similar way as in ground experiments.

Experimental Studies on Microstructure Evolution and Macro Segregation during Upward Directional Solidification of Lead-Tin Alloys

Directional solidification experiments were carried out on hypoeutectic and hypereutectic Pb-Sn alloys. Different combinations of growth rate V and composition Co. were used to investigate their effect on longitudinal macro segregation. Macro segregation along the length of the samples was observed in hypoeutectic Pb-Sn alloys whereas no such macro segregation was observed in hypereutectic alloys. The intensity of longitudinal macro segregation was found to increase with the increase in initial tin content of the alloy, increase in distance from the chill end and decrease in the solidification rate.

Mushy-zone rayleigh number to describe macrosegregation and channel segregate formation during directional solidification of metallic alloys

A recently defined mushy-zone Rayleigh number (RaM) that includes side-branching contributions to the mushy-zone permeability has been examined for its correlation with the longitudinal macrosegregation and channel segregate formation. The Rayleigh number shows (1) a strong correlation between the extent of longitudinal macrosegregation and increase in the mushy-zone convection and (2) a good ability to predict the formation of channel segregates during directional solidification.

Evolution of two phase microstructure in peritectic Fe–Ni alloy

In this paper, two phase microstructure formation of the peritectic Fe–Ni alloy is discussed. Bridgman type directional solidification was carried out on Fe–4.4at.%Ni alloy. Careful metallographic observation showed a sequence of microstructures that began to solidify with the tree like δ morphology surrounded by γ. It changes to the island δ/γ bands, and further to the δ/γ lamella after gradual morphological transition before quenching. Electron probe microanalysis yielded quantitative results of radial composition inhomogeneity and solute enrichment in the quenched liquid. It implies that this microstructure developed under convection with radial and longitudinal macrosegregation. Taking into account these results, simple analyses were made to characterize them and evolution of peritectic two phase growth was discussed briefly.

Effect of thermosolutal convection on directional solidification

The impact of thermosolutal convection during directional solidification is explored via results of numerical investigations. Results from fully transient numerical simulations of directional solidification in a differentially heated cavity under terrestrial conditions and Bridgman cytstal growth in space are discussed. The pivowl role ofboth thermaland solutal convection in the solidifaction process is illustrated by examining these two cases. In particular, radical and longitudinal macrosegregation resulting from this thermosolutal convection is discussed.

Macrosegregation caused by thermosolutal convection during directional solidification of Pb-Sb alloys

Pb-2.2 and 5.8 wt pct Sb alloys were directionally solidified with a positive thermal gradient of 140 K cm−1 at growth speeds ranging from 0.8 to 30 µm s−1, and then quenched to retain the mushyzone morphology. Chemical analysis along the length of the directionally solidified portion and in the quenched melt ahead of the dendritic array showed extensive longitudinal macrosegregation. Cellular morphologies growing at smaller growth speeds are associated with larger amounts of macrosegregation as compared with the dendrities growing at higher growth speeds. Convection is caused, mainly, by the density inversion in the overlying melt ahead of the cellular/dendritic array because of the antimony enrichment at the array tip. Mixing of the interdendritic and bulk melt during directional solidification is responsible for the observed longitudinal macrosegregation.

Suppression of channel convection in solidifying Pb-Sn alloys via an applied magnetic field

Channel convection through the porous, dendritic mushy zone in solidifying alloys results from a nonlinear focusing mechanism, whereby liquid enriched in the solute melts dendrites as it convects away from the solid. The local melting reduces the parameterized (Darcy) viscous force and increases the flow speed to form a convective channel. However, it has been predicted that an applied magnetic field might prevent channels from forming because, as the Lorentz force replaces the Darcy force as the primary resistance to flow, the retarding force becomes less sensitive to the lengthscale of the flow, so that the focusing mechanism no longer operates. In this study, it is found experimentally that, as predicted, an applied horizontal magnetic field can suppress channel convection when Q m , the Chandrasekhar number appropriate to a mushy zone, exceeds an order of one. The nondimensional number Q m is a measure of the strength of the Lorentz force relative to the Darcy force in the mushy zone and, for a given magnetic field, is much smaller than the analogous Chandrasekhar number (Q) for the fluid melt, since the Darcy force in the mushy zone far exceeds the viscous force in the fluid. Previous experimental work failed to find that magnetic fields could suppress channel convection because, although Q exceeded an order of one, Q m did not. For experiments with a smaller cooling rate, and, thus, a larger permeability and larger mushy zone Rayleigh number (Ra m ) a stronger magnetic field is necessary to suppress channel convection. The longitudinal macrosegregation is not affected by the absence of channel convection, suggesting that such channels are not always primarily responsible for the mass flux between the mushy zone and the melt.

Effect of Solute Convection on the Macrosegregation in Pb-Sn Binary Alloys during Upward Directional Solidification.

Upward directional solidification experiments have been carried out on Pb-Sn binary alloys. The macrosegregation along the length of a sample in hypereutectic Pb-Sn alloys is not observed. However, it is observed in the hypoeutectic Pb-Sn binary alloy. The intensity of the longitudinal macrosegregation increases as the solidification rate decreases. It can be found that solute convection resulted from the density profile of interdendritic melt and induced the macrosegregation. The intensity of the interdendritic solute convection responsible for the longitudinal macrosegregation can be represented by the effective partition coefficient of the alloy. The change in the length of a mushy zone affects the intensity of the solute convection during the upward directional solidification even when steady-state dendritic growth is maintained.

Influence of convection on the selection of solidification microstructures at low growth rates

The influence of thermal convection on the selection of microstructures is studied in directional solidification at low growth rates. Using the concept of solute boundary layer, the composition of the alloy at the different transitions between planar, eutectic and cellular fronts is predicted under a stationary regime and also in the presence of longitudinal macrosegregation. Furthermore, the appearance of mixed microstructures owing to radial segregation is discussed. Predictions concerning the planar/eutectic and cellular/eutectic transitions are checked against experimental results obtained with hypo-eutectic Sn-Cu alloys.

The occurrence and periodicity of oscillating peritectic microstructures developed during directional solidification

The layered microstructures that can form during plane-front directional solidification in peritectic systems were characterized quantitatively as a function of growth velocity using a Sn-Cd alloy. Layers were formed for an alloy composition outside of the two-phase peritectic region in the absence of longitudinal macrosegregation. The layers did not extend over the entire sample cross sections, so that the layered regions had a different composition than the alloy. Each of the two solids was found to be interconnected and continuous in three dimensions. The layer lengths and individual layer compositions did not vary with solidification distance. The average layer compositions were not a function of growth velocity and were approximately those at the peritectic temperature. This research was compared to the current model by Trivedi, which is based upon cyclic accumulation and depletion of solute in the liquid ahead of the interface linked to repeated nucleation events. The dependence of layer length on growth velocity predicted by the model was not obtained experimentally. The differences between results and predictions are related to the continuity of the two solids and the nonuniform cross-sectional composition in the Sn-Cd samples, which contradict assumptions of the model. A formation mechanism involving competitive lateral growth between the two solids at the solid-liquid interface would be more consistent with the current research.