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A three-dimensional mathematic model based on segmentation that joins the electromagnetic field, melt flow, heat transfer, solidification and solute transport in the whole casting domain has been established to study the solute transport at the final stage of solidification in a bloom continuous casting process. The effects of the final electromagnetic stirring (F-EMS) positions on the macrosegregation degree of the as-cast bloom are compared and analyzed. The results show that the maximum carbon segregation degree, the ratio of the local carbon concentration to the initial carbon concentration, is reduced from 1.292 to 1.254 with the application of F-EMS. In addition, the position of F-EMS is also an important factor. The maximum carbon segregation degrees are 1.254, 1.237 and 1.269 when the installation location of F-EMS is 14 m, 16 m and 18 m away from the meniscus, respectively. The optimal center solid fraction is about 0.1 at the F-EMS center for this study.

Understanding and reducing defects formed during continuous casting of steel are challenging because of the many inter-related, multiscale phenomena and process parameters involved in this complex process. Solidification occurs in the presence of turbulent multiphase flow, transport and capture of particles, superheat transport, and thermal–mechanical behavior. The application of electromagnetic fields provides an additional parameter to control these phenomena to reduce solidification defects. It is especially attractive because the field has the potential to be easily adjusted during casting to accommodate different casting conditions. This article briefly reviews how electromagnetic forces affect solidification defects, including subsurface hooks, particle capture, deep oscillation marks, depressions, cracks, breakouts, segregation, and shrinkage. This includes the related effects on superheat transport, initial solidification, surface quality, grain structure, internal quality, and steel composition distribution. Finally, some practical strategies regarding how to apply electromagnetics to improve steel quality are evaluated.

The effects of applying a 0.2-T transverse magnetic field on a solidifying Ga-25 wt%In alloy have been investigated through a joint experimental and numerical study. The magnetic field introduced significant changes to both the microstructure and the dynamics of escaping high-concentration Ga plumes. Plume migration across the interface was quantified and correlated to simulations to demonstrate that thermoelectric magnetohydrodynamics (TEMHD) is the underlying mechanism. TEMHD introduced macrosegregation within the dendritic structure, leading to the formation of a stable “chimney” channel by increasing the solutal buoyancy in the flow direction. The resulting pressure difference across the solidification front introduced a secondary hydrodynamic phenomenon that subsequently caused solute plume migration.

This work focuses on ultrasonic melt treatment (UST) in a launder upon pilot-scale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650°C and 665°C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density). Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al 3 Zr particle size, and increase in the particle number density. A higher Al 3 Zr particle density was obtained when the alloy was cast at 665°C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided.

The solidification behaviors of high-magnetic-induction grain-oriented silicon steel slabs have been investigated during experiments in industrial strand electromagnetic stirring (S-EMS). The current intensities of S-EMS were 0, 120, 200, and 350 A, and the frequency was 5 Hz. The ratio of the equiaxed crystal was 14.95%, 15.64%, 45.22%, and 66.96%, respectively. Central porosity cannot be eliminated by increasing the current intensity. The number, size, and ratio of segregation spots were markedly reduced by minimizing the equiaxed crystal zone. Carbon in the 0 and 120 A slabs exhibited a lower degree of macrosegregation compared with the 200 and 350 A slabs. Controlling the cooling rate and increasing the total reduction are the directions to further improve solidification structures, defects, and carbon segregation.

The Retained Damage Model successfully predicts the incubation delay during transformation from ferrite to austenite in the presence of an applied external flow field during rapid solidification of ternary stainless-steel alloys. The model incorporates two new features—conservation of the free energy associated with undercooling of the primary metastable phase, and use of a modified Read–Shockley approach to quantify defect energy induced by melt shear. Healing of the microstructure could reduce the amount of free energy retained, but, for the alloys considered in this work, it was found that the model is not sensitive to this phenomenon, and thus 100% of available free energy is retained to provide an additional transformation driving force, significantly shortening the incubation period. Use of a dimensionless approach allows comparison between systems with very different thermophysical properties, and highlights the similarity in response to local flow conditions over a wide range of compositions.

The objective of this work is to investigate the effects of electromagnetic external fields on the spatial distribution of alloying elements and the mechanical properties of Al-Mg-Si alloy prepared using twin-roll casting. The formation mechanism of macro/microsegregation and mechanical properties were examined, and the results indicated that the amount of centerline segregation defects was significantly reduced with the application of an external field. When a pulsed electric field and static magnetic field were applied simultaneously, the distribution of alloying elements in the thickness direction of the twin-roll cast sheet was the most uniform and the segregation was obviously suppressed. By virtue of the Lorentz force, the content of alloying elements in the matrix clearly increased, suggesting that such application of external fields can reduce segregation defects and improve the uniformity of the spatial distribution of alloying elements, ultimately improving the overall mechanical properties of AA6022 aluminum alloy.

The effect of using a pulsed electric current to remove impurity iron from molten polysilicon was investigated. Using optical microscopy observation and area statistics of iron-rich content, it was found that iron tends to accumulate at the bottom of an ingot under the action of a pulsed electric current. A new separation mechanism is proposed, based on the decreased solubility of iron in polysilicon under conditions including a pulsed electric current. Thermodynamic calculations indicate the theoretical possibility of the formation of iron-rich Si clusters. These clusters sink to the bottom of an ingot under the effect of gravity and form iron-rich precipitates with silicon, thereby achieving the iron removal. This technique provides a new method for purification of polysilicon.

Uniformity of composition and grain refinement are desirable traits in the direct chill (DC) casting of non-ferrous alloy ingots. Ultrasonic treatment is a proven method for achieving grain refinement, with uniformity of composition achieved by additional melt stirring. The immersed sonotrode technique has been employed for this purpose to treat alloys both within the launder prior to DC casting and directly in the sump. In both cases, mixing is weak, relying on buoyancy-driven flow or in the latter case on acoustic streaming. In this work, we consider an alternative electromagnetic technique used directly in the caster, inducing ultrasonic vibrations coupled to strong melt stirring. This ‘contactless sonotrode’ technique relies on a kilohertz-frequency induction coil lowered towards the melt, with the frequency tuned to reach acoustic resonance within the melt pool. The technique developed with a combination of numerical models and physical experiments has been successfully used in batch to refine the microstructure and to degas aluminum in a crucible. In this work, we extend the numerical model, coupling electromagnetics, fluid flow, gas cavitation, heat transfer, and solidification to examine the feasibility of use in the DC process. Simulations show that a consistent resonant mode is obtainable within a vigorously mixed melt pool, with high-pressure regions at the Blake threshold required for cavitation localized to the liquidus temperature. It is assumed that extreme conditions in the mushy zone due to cavitation would promote dendrite fragmentation and coupled with strong stirring, would lead to fine equiaxed grains.

The introduction of external fields, such as electromagnetic fields, ultrasonic excitation, and mechanical shearing, to solidification processes has encompassed novel applications to refine grains, homogenize segregation, break up agglomeration of particles, and prevent defect formation. Advances in experimental methods, materials characterization, and computational modeling have led to new insights into controlling the microstructure and defects during solidification. This two-part topic, “Solidification Behavior in The Presence of External Fields,” highlights the most recent investigations into the use of external fields in a broad range of industrial manufacturing processes.