1. INTRODUCTION
7xxx-series aluminum alloys are essential materials in the aerospace industry due to their high strength, low density, and excellent fracture toughness. These alloys are widely used in aircraft structures and critical components such as machine body beams, horizontal stabilizers, upper wing skins, keel beams, ejection seats, and rails. Additionally, they have significant potential applications in small aircraft missiles, anti-tank missiles, and military pontoon bridges [1].
Among high-strength aluminum alloys from the 3xxx to 7xxx series, the most common feedstocks for subsequent deformation processing—such as extrusion, rolling, and stamping—are round-section billets for extrusion, and rectangular or square slabs and blooms for rolling or stamping, respectively [2]. The casting of rectangular or square shapes, typically using a semi-continuous casting method, involves short water-cooled sliding molds. In the case of 7xxx-series alloys, which have a broad crystallization temperature range, this process can lead to temperature segregation, central porosity, and structural heterogeneity, necessitating post-cast homogenization annealing [3]. Strong thermal stresses are particularly prevalent in alloys with Zn content exceeding 5~7%, such as 7068 (over 8%) [4].
During slab casting, temperature gradients arise due to uneven cooling, potentially causing longitudinal cracking during and after the process [5]. The primary challenge in the semi-continuous casting of slabs, blooms, or billets is to minimize thermal inhomogeneity in the crystallization zone, ensuring efficient heat and mass transfer while controlling thermal influences on crystal growth. Various methods, such as mechanical actuators, impellers, ultrasonic treatments, and low-frequency direct contact techniques, have been employed to manage heat transfer within the liquid sump. However, non-contact electromagnetic fields - including high, medium, and low-frequency electromagnetic stirrers— are among the most effective external control forces [6-12].
The semi-continuous casting of ingots requires the precise control of heat and mass transfer processes to ensure highquality and homogeneous metal structures. Among the available techniques, electromagnetic casting (EMC) has emerged as a promising method, utilizing Lorentz forces generated by alternating magnetic fields (2.0~20 kHz) to regulate melt flow and solidification dynamics. This technique effectively achieves smooth slab surfaces and prevents surface segregation. However, industrial application is limited by the need for high machine stability, component overheating, and electromagnetic interference [13,14].
Electromagnetic stirring (EMS) has also been employed to refine microstructures and reduce segregation. Lowfrequency magnetic fields (15~60 Hz) enhance surface quality by modifying primary cooling intensity [15]. However, high-frequency magnetic fields involve challenges including safety risks, unpleasant noise, and harmonic distortions in the electric network that negatively impact other industrial equipment [16].
For rectangular or square ingots, traveling magnetic field systems are particularly effective. These systems employ a three-phase asynchronous motor stator to generate a magnetic field along the slab’s long sides, promoting melt mixing along the crystallization front and improving heat and mass transfer. However, the penetration depth and mixing efficiency depend on the frequency of the alternating magnetic field [17]. Lower frequencies enhance penetration but weaken force intensity, while higher intensities may cause porosity and structural heterogeneity [18].
Pulsed electromagnetic stirring methods, including amplitude and frequency modulation, have been explored to intensify melt mixing and ensure a uniform microstructure. Superimposing two electric currents of slightly different frequencies (e.g., 60 Hz and 60.03~60.25 Hz) produces pulsating mixing, both reducing negative segregation and improving homogeneity [19,20]. However, these methods often involve technical complexity and efficiency challenges, especially for large-scale applications [21].
Recent advancements in microstructural refinement have focused on modulating the vector direction and intensity of electromagnetic forces to generate hydrodynamic vortices. For example, applying pulsed traveling magnetic fields within the liquid core of a slab has been shown to homogenize temperature and chemical composition while simultaneously disrupting dendritic growth at the crystallization front [22]. This approach holds significant potential for mitigating segregation and stress formation in semi-continuous casting [23].
This study builds upon previous advancements, and proposes a novel electromagnetic control method and device for the semi-continuous slab casting of 7xxx-series aluminum alloys. The proposed system utilizes horizontal multi-zone traveling magnetic field inductors to achieve both unidirectional and reversible pulsating electromagnetic stirring. This approach enhances temperature homogenization, microstructural refinement, and magnetohydrodynamic stability in the semi-continuous slab casting process.
The key advantage of employing amplitude-modulated pulsating magnetic fields in the electromagnetic processing of 7xxx-series aluminum billets and slabs is an at least a twofold increase in the penetration depth of the electromagnetic waves. This is achieved by creating conditions for super positioning electromagnetic fields in the inter-pole gap, in contrast to conventional harmonic AC magnetic fields [20-23].
Additionally, the formation of magnetic field lines between adjacent electromagnet poles induces a repelling effect, leading to the periodic sequential repulsion of magnetic force lines. This mechanism facilitates deeper penetration of intense magnetic fields [21]. Furthermore, the superimposed pulsed magnetic field generates magnetohydrodynamic forces and micro-vortices in the billet’s liquid sump, forming a system of interconnected hydrodynamic flows. These flows significantly influence melt mixing dynamics, extending well beyond the effective magnetic field zone. This innovation expands the possibilities for the electromagnetic and magnetohydrodynamic processing of large ingots (100~ 1000 mm) [20,24,25].
Traditional electromagnetic methods for controlling the crystallization of 7xxx-series aluminum alloys suffer from limited penetration depth, often failing to reach the liquid metal zone in slabs. This leads to non-uniform microstructures and defect formation. This study introduces a novel approach utilizing pulsating electromagnetic fields in semi-continuous slab casting to enhance field penetration by positioning the electromagnetic device 100 mm below the metal meniscus in the crystallizer.
For the first time, this research compares the proposed method with traditional DC techniques and evaluates its impact on microstructure formation. Additionally, we assess the method’s influence on chemical segregation, crystallization front stabilization, post-casting stress reduction, gas porosity minimization, and overall slab quality improvement.
2. MATERIALS, METHOD AND EXPERIMENTAL
2.1 Material
A high-strength aluminum alloy 7075, with the chemical composition of Al-5.5%Zn-2.5%Mg-1.6%Cu-0.2%Cr and the addition of titanium diboride within ~0.05% by weight, was used as the alloy. The alloy was melted in a tilting-type resistance crucible furnace with a capacity of 700 kg and degassed using a GBF rotary impeller device while blowing argon gas at a flow rate of 10~15 l/min for 20 minutes.
2.2 Semi-continuous slab casting installation
Implementation of the pulse traveling electromagnetic control proposed in this study method was carried out in a sliding type semi- or continuous casting unit, (see Figs 1, 3), which consisted of a horizontal casting table, with a tundish on top for feeding liquid metal. During the casting process it feeds into a water-cooling casting mold (crystallizer) thru a feeding nozzle (spout) to a floating melt level control valve, to maintain a fixed melt level inside.
2.3 Electromagnetic stirring system
To create conditions for a unidirectional pulsating traveling magnetic field (U-PTMF) below the crystallizer of the primary slab crust (as shown in Fig 1), we used two 4-zone traveling magnetic field inductors with a three-layer coils decoupling and a shift relative to each other by 1/3 part. Fig 2 presents the scheme for the pulsating unidirectional counterclockwise electromagnetic stirring of melt in the liquid sump of the slab’s crystallized crust in a horizontal plane, using two pulsating traveling magnetic fields created by the left and right inductors. The application of pulse mode melt stirring can create volumetric micro vortex hydrodynamic mass transfer processes in the melt. In addition, the stirring method should create compressive/rarefaction wave effects in the liquid sump of the slab, known as acoustic impact.
2.4 Creation of the unidirectional pulsating traveling magnetic field (U-PTMF)
The creation of a pulsed mode was carried out by connecting coils in layers to three-phase power sources, where the inputs of the coils of the first inductor (left for example) were connected to a three-phase voltage with a frequency of 20 Hz, and the inputs of the coils of the second inductor (right for example) were connected to a three-phase voltage of 25 Hz.
Both inductors’ coils outputs are sequentially connected to each other. This makes it possible to generate a traveling magnetic field with a unidirectional clockwise direction, pulsating with a frequency of 5 Hz for pulse mixing of the melt inside the slab liquid sump at creation conditions algebraic superposition for two 3-phases harmonic electrical currents with different frequencies but equal intensities at modulation ratio M=1, amplitude modulation envelope 5 Hz and carrier frequency 22.5 Hz.
The current in the EMS inductor coils was 275~315 Amperes, when powered by a voltage of 12~14 Volts at frequencies of 20 and 25 Hz. Accordingly, this created a magnetic field intensity with a peak value of up to 0.4T on the pole.
Fig 4a shows the theoretical time diagram of the pulsating amplitude-modulated magnetic field, indicating the tangential component (shown in blue) and the normal component (shown in red), and also a screenshot (Fig 4b) from an oscilloscope with a magnetic induction sensor and two measuring coils, respectively aligned with the axis parallel to the electromagnet and perpendicular to it.
2.5 Casting conditions
The comparative experimental casting included casting at least two slabs with dimensions of 350×150×1800 mm under DC and U-PTMF conditions. The casting speed for both conditions was 62 mm/minute with cooling water (~20°C) supplied to the crystallizer and a secondary cooling zone at a rate of 150 l/minute. The melt temperature for pouring the 7075 alloy was maintained in the range of 710~720°C.
The casting speed of 62 mm/min falls within the critical low-speed range (50~70 mm/min) for 7075 aluminum alloy. This range is determined by the solidification interval (477~635°C) and the structural features of the mold with internal cooling. This speed was deliberately selected to evaluate the effectiveness of forced electromagnetic stirring (EMS) in the liquid sump, compared to conventional DC casting. The primary objective was to assess the feasibility of stabilizing the slab casting process at very low speeds, while preventing chemical and structural segregation.
A lower casting speed leads to enhanced heat extraction, which promotes a finer microstructure under both DC and EMS casting conditions. This effect has been previously confirmed in studies on low-speed continuous casting, where grain refinement is directly linked to controlled solidification rates [26].
In addition, such very low speeds are commonly used to narrow the solidification range of low-alloy aluminum alloys, particularly in the 6xxx series (Al-Mg-Si). The developed casting equipment and EMS system for 7075 will be applicable to these alloys in future industrial applications [27]. The study’s findings can be extended to other alloy systems, particularly those in the 6xxx series, where similar EMS technology can be utilized. Future research should investigate the optimal EMS parameters for achieving further microstructure enhancement at low casting speeds.
Further, several cross-sectional plates with a thickness of 15~20 mm were cut from the cast slabs at no less than 450~500 mm above the start of the casting zone, from which samples for chemical composition analysis and microstructural analysis were subsequently taken.
2.6 Methods for microstructural and chemical composition analysis
For Microstructural Analysis, we used optical microscopy to characterize the microstructure of the 7075 alloy. Samples were prepared by grinding, polishing, and etching using a 0.5% hydrofluoric acid solution in water.
Grain size was measured using the linear intercept method, and orientation contrast images (IPF maps) were obtained using the EBSD (Electron Backscatter Diffraction) method. Grain size determination was conducted using the Feret method. The chemical composition of the slab samples, as well as the phases identified, was determined using Energy Dispersive X-ray Spectroscopy (EDS) [28,29]. Additionally, for chemical analysis of the samples, an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used (model: iCAP PRO XP DUO Manufacturer: Thermo Fisher Scientific).
3. RESULTS AND DISCUSSION
3.1 Slab casting
Under DC casting conditions, the process proceeded stably until a height of 1500 mm was reached. At this point, a sound of initial cracking was heard in the lower part, around the starter head, resulting in the formation of a small crack up to 150 mm length.
Upon reaching a slab height of 1800 mm, casting was completed and the slab was moved to the primary cooling zone. However, during 20~30 minutes of natural cooling in the air, starting from the lower part, the DC slab instantly cracked with the formation of a wide central longitudinal crack, as shown in Fig 5a, due to accumulated post-casting thermal stress.
Meanwhile, the EMS slab showed no cracks and did not form any cracks upon casting, cooling up to the ambient temperature, further cutting, indicating the complete removal of thermal stress (Fig 5b).
Additionally, an evaluation of the temperature difference between the edge and the center of the slab was conducted by direct measurement at 10 mm immersion into the melt on the meniscus zone. It was found that under EMS conditions, the temperature difference at 100 mm from the center to the long slab edge was 4.3~5.5°C, while for DC conditions, it was between 30~65°C. This explains the significant reduction in post-casting thermal stress and confirms the effectiveness of the EMS application.
The application of a unidirectional pulsating traveling magnetic field (U-PTMF) with the proposed method during casting proved to be highly effective in improving the quality of high-strength aluminum alloy slabs. This method provided significant defect reduction, and was particularly successful in preventing central cracking and reducing thermal stress.
3.2 Microstructural analysis of the slab crosssection
The microstructure of the cast DC slab (Fig 6a, 7a) was severely damaged by micropores on the surface, in the middle and in the central part. The surface of the DC slab (Fig 6a) in the range of 0~10mm from the surface had a pronounced dendritic heterogeneous hybrid type, dendriticglobular microstructure with an average size of 160 μm and up to 90~150 μm.
In the case of U-PTMF 5 Hz (Fig 6b), at a distance of 2 to 5 mm from the surface it also had a hybrid uniform microstructure with a grain size of 90 to 125 μm, but at a distance of 8~10 mm from the surface, a sharp change in morphology, homogenization and reduction in micro grain size, to a compact homogeneous globular morphology with a grain size of up to 90 μm, was observed.
Under DC conditions the middle and central axial parts of the slab (Fig 7a, c) were distinguished by the presence of micro inclusions and microporosity, with a grain size heterogeneity in the range of 90~125 μm. For U-PTMF (Fig 7b, d) the microstructure was homogeneous and compact without significant micropores and with a grain size of 80~90 μm.
The effect of structural refinement for EMS can be explained by several key physical effects induced by the alternating pulsed electromagnetic field:
1. Thermal Stabilization via Eddy Currents: When a pulsed magnetic field interacts with an electrically conductive molten metal, it induces an internal variable electric field, generating eddy currents. These eddy currents contribute to a more homogeneous heat distribution and thermal stabilization of the solidification front at the interface between the solid and liquid phases. This effect occurs due to the significant difference in electrical conductivity between the solid and liquid aluminum. The result is a reduction in heat transfer rates and a decrease in grain growth speed, promoting the formation of a globular microstructure instead of a coarse dendritic structure [30,31].
2. Electromagnetic Stirring and Shear Flow Effects: The induced electromagnetic body forces generate forced convection inside the molten metal, leading to the formation of shear flows, micro-vortices, and enhanced turbulence. These effects promote the homogenization of the temperature gradient at the solidification front and within the entire cross-section of the slab, thereby equalizing the solidification front across the entire slab cross-section. This leads to thermal stabilization of the crystallization process and reduces segregation [32,33].
3. Fragmentation of Dendritic Arms: The application of pulsed electromagnetic forces at the dendrite growth front generates intermittent shear flows and electromagnetically induced stresses, which contribute to the detachment of primary and secondary dendrite arms. These detached dendritic fragments are subsequently dispersed into the melt, where they act as new nucleation sites, promoting the refinement of microstructural grains and improving overall grain homogeneity [34].
4. Magnetohydrodynamic (MHD) Mechanical Vibration Effect: Pulsed electromagnetic forces, particularly near the solidification front, induce a dual-frequency mechanical electromagnetic vibration (with frequencies of 22.5 Hz and 5 Hz). This mechanical vibration is transmitted to both the solidification front and the growing dendritic structures from the mold toward the slab center. This results in the mechanical detachment of primary and secondary dendritic arms, further contributing to the generation of additional crystallization nuclei and promoting the formation of a fine and uniform microstructure [35,36].
The combination of thermal stabilization, electromagnetic stirring, dendrite fragmentation, and MHD-induced mechanical vibrations leads to the formation of a finer and more homogeneous microstructure in aluminum alloy slabs processed with EMS. These effects not only improve grain refinement but also reduce porosity and macro segregation, making EMS an effective method for controlling solidification structures in industrial casting applications.
Given the observed effect of reducing microporosity (as shown in the microstructures in Figs 6 and 7), the mechanism can be explained as follows. EMS generates shear forces and turbulence, which promote bubble coalescence and floatation, and larger bubbles rise to the surface more efficiently, reducing gas entrapment in the melt [37]. During conventional solidification, dissolved gases form micropores due to insufficient escape time, but the EMS enhances melt flow, disrupting the formation of gas pockets at the solidification front [38]. Also, the stirring effect of EMS allows for better diffusion and transport of dissolved hydrogen toward the melt surface, improving hydrogen removal [39].
3.3 Analysis of distribution and homogeneity of chemical composition across the slab cross-section
As presented in Table 1, in the central region of the slab (75~55 mm), under DC casting the chemical composition is relatively close to nominal values, but EMS ensures a more uniform distribution across the entire slab thickness. In the middle region (35~45 mm), DC casting resulted in a significant depletion of Zn, Mg, and Cu, whereas EMS maintained a stable composition. At the surface (5~15 mm), DC casting leads to severe macro segregation, with a relative deviation (RD%) by location in the slab in the range of Zn: 5.27~44%; Mg: 7,2~33,6%, Cu: 18~51%), while EMS resulted in a composition very close to the specification: 0.18~2.73% for Zn, 0.4~2.8% for Mg and 0.67~8.7% for Cu as well.
The coefficient of variation (CV%) for Zn listed in Table 1 was reduced from 21.4% (DC) to 1.2% (EMS), for Mg it decreased from 16.6% (DC) to 0.9% (EMS) and for Cu from 19.1% (DC) to 3.5% (EMS), indicating a significant improvement in chemical homogeneity.
During DC casting (without EMS), the following effects are observed: Gravitational segregation, where heavier elements (Zn, Cu) tend to settle in the lower regions of the ingot; differential cooling of the meniscus, leading to the formation of supercooled semi-solid regions at the periphery, where low-melting-point elements accumulate; insufficient mixing of the molten metal, causing interdendritic enrichment of alloying elements [40].
This effect results from the electromagnetic convection, as Lorentz forces generate strong flows within the molten metal. Enhanced mixing of the chemical elements reduces compositional gradients, and gravity-driven segregation is minimized due to a more uniform distribution of heavy and light elements [41].
Also, as was confirmed, the EMS promotes dendrite fragmentation, accelerating the columnar-to-equiaxed transition (CET). The equiaxed grain structure enhances the uniformity of the alloying elements [42,43]. Additionally, the reduction in solutal buoyancy effects, where the EMS disrupts convective cells that could otherwise transport alloying elements to specific zones, ensures a more uniform chemical composition across the entire cross-section [44].
3.4 Analysis SEM microstructure
With DC casting (Fig 8 a), the uneven contours of grains along the edges and inconsistent thickness indicate that uneven cooling and crystallization occurred. This leads to the formation of inclusions in the intergranular space, ranging from 0.93 μm to 6.4 μm in thickness. Such conditions are less favorable to a homogeneous microstructure, and can negatively affect the mechanical properties of the material. Grain size was determined to be in the range of 90~145 μm.
As shown in Fig 8 b, in samples processed using the electromagnetic method (EMS) with currents of 275~315 Amperes, more rounded grain shapes and a tendency toward uniform intergranular zones thickness up to 3 μm are observed. This indicates more stable crystallization conditions and improved heat distribution, ensuring a homogeneous microstructure. Grain size was in the range of 80~90 μm.
3.5 Mechanical Properties and Energy Efficiency and Industrial Feasibility Discussion
The refined microstructure obtained with EMS is expected to enhance mechanical performance, as observed in similar studies on aluminum alloy casting. According to Davis (1993) [1], finer equiaxed grains contribute to higher yield strength and fatigue resistance by reducing stress concentration at grain boundaries. Further, the improved solute distribution minimizes localized softening zones, which can otherwise lead to premature failure under cyclic loading. Future work will focus on direct tensile and hardness testing to quantify these expected improvements.
Another factor to consider when implementing EMS at an industrial scale is energy consumption. The power input for the U-PTMF system used in this study was estimated at 1.8 kW per ton of metal processed, which is 20~30% lower than conventional AC EMS methods [45]. This suggests that U-PTMF provides an energy-efficient solution for improving cast quality while maintaining operational cost-effectiveness. Further research is needed to evaluate long-term cost benefits in large-scale production settings.
4. CONCLUSIONS
A novel electromagnetic influence method (U-PTMF) for microstructural control during the vertical semi-continuous casting of 7075 alloy slabs was proposed and experimentally validated. A comparative analysis with DC casting demonstrated significant advantages in structure, degassing, stabilization of post-casting thermal stresses, and overall metallurgical quality .
The application of U-PTMF effectively eliminated the formation of central cracks, a critical defect in semi-continuous casting. The improved crystallization conditions enhance metallurgical quality and reduce machining costs.
Pulsating stirring at 5 Hz promoted a transition from a dendritic to a more uniform globular structure. With DC casting, a coarse dendritic morphology was observed, with grain sizes of 160 μm (surface) and 90~125 μm (center), while EMS produced a more uniform grain structure (80~90 μm in the center) and eliminated gas porosity .
EMS significantly minimized compositional deviations across the slab cross-section, particularly for Zn (from 5.3~44% RD to 0.18~2.7%), Mg (from 7.2~33.6% RD to 0.4~2.8%), and Cu (from 18~51% RD to 0.7~8.7%).
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