Macro-/Microstructural Features and Temperature Distribution of Pure Aluminum 1070 in Novel Thin Slab Continuous Casting Process

Abstract

This study introduces a novel vertical bending continuous casting technology for producing thin slabs of pure aluminum and aluminum alloys, offering significant advancements over traditional vertical casting methods. The newly designed equipment and optimized process parameters enable the continuous casting of pure aluminum 1070 at a speed of 1.2 m/min, surpassing the casting speed of conventional direct-chill casting. Comprehensive investigations of the macrostructure and microstructure of the cast pure aluminum 1070 reveal a refined equiaxed crystal structure with an average grain size of 98 μm, significantly smaller than that obtained through conventional casting processes. This refinement in grain size is expected to enhance the material’s mechanical properties and processing capabilities. Additionally, simulations of the temperature distribution and solidification structure provide insights into the formation of a “U”-shaped sump and liquidus isotherms, which are crucial for understanding the solidification dynamics of the material. The refined grain structure and increased casting speed demonstrate substantial potential for advancing aluminum alloy production, especially in applications requiring superior performance and more efficient manufacturing processes.

1. Introduction

The production of thin slabs of pure aluminum and aluminum alloys is of critical importance to the aluminum industry due to their extensive applications across sectors such as aerospace, automotive, and construction [1,2,3,4,5]. Traditional direct-chill (DC) casting processes have been the cornerstone for producing casting billets [6,7]. However, this process is often limited by issues such as coarse grain structures [8,9,10,11], macrosegregation defects [12,13], internal stresses [14,15], and the need to remove the head and tail sections, which reduces the yield of finished products. In particular, coarse grains, uneven microstructures, and a low proportion of equiaxed grains are common problems that can significantly affect the mechanical properties and final applications of aluminum products. Additionally, the semi-continuous production mode can also negatively affect production efficiency.

In response to these challenges, this study explores the development of a novel vertical bending continuous casting (VC) technology for producing thin slabs of pure aluminum, such as the widely used pure aluminum 1070. Vertical bending continuous casting has been successfully applied in steel, where cracking issues are relatively easy to address due to low solidification shrinkage, high mushy zone strength and plasticity, and minimal cracking [16]. In contrast, aluminum and its alloys have higher thermal conductivity and solidification shrinkage coefficients than steel [17], leading to greater thermal stresses on the solidification shell. Their mushy zone strength and plasticity are also lower, making them more prone to severe cracks under mechanical stresses during bending. Cracking during casting is mainly caused by the combined effect of thermal stresses from solidification shrinkage [18,19] and mechanical stresses from casting equipment [20], which exceed the fracture stress of the mushy zone [21], with insufficient liquid supply to fill cracks [22]. Once cracks form, they are difficult to eliminate, increasing the risk of irreversible defects. These challenges have made the application of vertical bending continuous casting to aluminum and its alloys particularly complex. Building on previous experimental and simulation studies, we have successfully overcome these challenges, enabling the casting of aluminum and aluminum alloy thin slabs through VC technology. This innovative approach aims to address the limitations of conventional casting methods by introducing a vertical bending mechanism during the solidification process, which is expected to enhance heat transfer efficiency and structural uniformity. To achieve this, we have designed equipment and process parameters that are optimized for casting pure aluminum 1070 thin slabs. The research encompasses a comprehensive analysis of the macrostructure and microstructure of the pure aluminum 1070 slabs produced using this new VC method. The casting speed of 1.2 m/min was chosen for this study, and the resulting slabs were examined using both a camera and an optical microscope [23] to document the structural characteristics. Furthermore, to gain a deeper understanding of the solidification process and the effects of the VC technology, we conducted simulations of the temperature distribution and solidification structure. These simulations provide an intuitive visualization of the heat transfer and solidification dynamics, which are critical for evaluating the effectiveness of the new casting technique. To highlight the distinctive advantages of the VC process, a brief comparison with the traditional DC casting method was conducted, underscoring improvements in casting speed, structural uniformity, and grain refinement.

In summary, research on the novel vertical bending continuous casting technology for producing pure aluminum and aluminum alloy thin slabs remains limited. This study establishes a foundation for further development of the VC process and provides new insights into its unique features and advantages. As a result, it not only introduces a promising approach to the casting of pure aluminum and aluminum alloys but also contributes to the broader field of aluminum processing research.

2. Materials and Methods

Thin slabs with cross-sectional dimensions of 40 mm × 260 mm were produced using self-developed vertical bending continuous casting equipment. This equipment includes an intermediate frequency melting furnace, an insulation furnace, a pouring system, a vertical mold (380 mm high), a mold vibration device, a foot roller section (130 mm high), a vertical section (175 mm high), a bending section (625 mm high), an arc section (with a 2.5 m radius), a series of withdrawal straighteners, a dummy bar, and a set of horizontal roller tables. Currently, the maximum flow rates for the mold, foot roller section, vertical section, curved section, and arc section are 70.0 m³/h, 7.0 m³/h, 7.0 m³/h, 7.0 m³/h, and 7.0 m³/h, respectively. Process parameters, such as water flow and casting speed, are controlled and recorded using a computer equipped with data acquisition cards and software. The operation is semi-automatic, involving a manual start stage followed by a fully automatic continuous casting stage. One of the functions of this setup is to enable the adjustment of process parameters before each operation. The schematic representation and photographic documentation of the vertical bending continuous casting equipment for aluminum and aluminum alloy thin slabs is shown in Figure 1.

After equipment adjustments and process optimization, a successful vertical bending continuous (VC) casting process was developed for thin slabs of pure aluminum 1070. The chemical composition of pure aluminum 1070 [8], as shown in Table 1, was used as the experimental material. The continuous casting speed was set at 1.2 m/min, and the initial pouring temperature was maintained between 715 °C and 725 °C during the casting experiments. Refining was performed using the 6AB-E refining agent, which primarily consists of hexachloroethane, and 0.05% of Al-5Ti-1B grain refiner was added to improve the grain structure. Grain refiners, including Al-Ti-B [24,25,26], Al-5Ti-1B [27,28,29,30], Al-Ti-C [31,32], and Al-Ti-B-C [33] alloys, are employed to improve the process of grain refinement. This research chose the widely utilized Al-5Ti-1B alloy, enabling a comparison with previously published studies in the literature. The continuous casting thin slab within the working speed range was processed into samples suitable for macro-etching and metallographic observation. The cross-sectional half of the continuous casting thin slab sample was subsequently milled and sanded. After macro-etching with aqua regia, the sample was cleaned with alcohol and dried using a blow dryer. The macrostructure was then captured using a camera for documentation. The metallographic samples of pure aluminum 1070 were prepared using sandpaper, coarse polishing, and fine polishing. Subsequently, the samples were etched with a 0.5% hydrofluoric acid aqueous solution. After cleaning with alcohol and drying using a blow dryer, the microstructure was observed under an optical microscope.

Considering the symmetry of the system, one half of the casting slab was analyzed. The physical model’s cross-sectional dimensions are 130 mm × 40 mm. A preliminary model of the continuous casting process was developed in SolidWorks and subsequently imported into ProCAST. During the meshing process, ProCAST’s dedicated pre-processing module MeshCAST was used to discretize the computational domain with a hexahedral mesh, resulting in a mesh consisting of 11,756 2D elements and 96,321 3D elements. Boundary conditions, initial conditions, and material composition were defined accordingly. Surface temperature values at key locations were measured using an infrared temperature gun and used to adjust the heat transfer coefficients for accuracy. The unsteady process of the thin slab continuous casting of pure aluminum 1070 was simulated using the MiLE algorithm [34], yielding the temperature distribution throughout the casting process. In the numerical simulation of the solidification structure of pure aluminum 1070 thin slabs, the grain structure evolution was calculated and analyzed using the cellular automaton–finite element (CAFE) model [35,36,37,38]. This model is based on the coupling between the finite element (FE) [39] computation of solidification and cellular automaton (CA) technology [40]. The heat transfer model meshes for the vertical bending continuous casting of the pure aluminum 1070 thin slab and a flowchart outlining the calculation methodology applied in this study are shown in Figure 2.

3. Results

3.1. Continuous Casting Thin Slab of Pure Aluminum 1070: Macrostructure and Microstructure

Figure 3 illustrates the pure aluminum 1070 thin slab produced by the vertical bending continuous casting equipment, which was designed for aluminum/aluminum alloy thin slabs with a casting speed of 1.2 m/min. The length of the thin slab in this continuous casting process is 7.4 m, constrained by the capacity of the smelting furnace and the length of the horizontal roller table in the facility. The surface of the continuously cast pure aluminum 1070 slab is smooth and defect-free, with no visible cracks or surface imperfections, demonstrating the success and reliability of the continuous casting process.

Figure 4 presents the macrostructure of the pure aluminum 1070 thin slab produced by vertical bending continuous casting at a casting speed of 1.2 m/min. As shown in the figure, the cross-section of the continuous casting thin slab is characterized by a fine-grained macrostructure, where surface fine crystals gradually transition to coarse columnar dendrites as one moves inward and eventually to equiaxed dendrites. The ratio of the fine crystal region at the surface, the columnar crystal region, and the equiaxed crystal region is 3:8:14, with the equiaxed crystal region comprising 56% of the total volume.

The microstructure of the pure aluminum 1070 thin slab, produced at a casting speed of 1.2 m/min by vertical bending continuous casting, is shown in Figure 5. The microstructures in Figure 5a–c consist of small, nearly spherical grains, with representative morphologies highlighted by red circles. In Figure 5d,e,g, the grains exhibit a similar directional alignment, characterized by elongated axial growth and the formation of coarse columnar regions, with typical microstructures marked by yellow circles. In Figure 5f,h,i, the morphology predominantly shows a geometric spherical shape, with each grain exhibiting similar height and width when viewed from different angles. Typical equiaxed grain microstructures are indicated by blue circles. The equiaxed crystal microstructure exhibits an average grain size of 98 μm.

3.2. Simulation Study on the Temperature Distribution of Pure Aluminum 1070 Continuous Casting Thin Slab

The macroscopic diagrams of the temperature and solidification distribution of pure aluminum 1070 in vertical bending continuous casting are shown in Figure 6. Figure 6a illustrates the thickness center interface along the longitudinal cross-section and overall temperature distribution, revealing a gradual decrease in temperature as continuous casting progresses. The temperature distribution shows heat transfer from the high-temperature region at the center toward the cooler edge regions, with a cooling rate greater at the edges than at the center. Figure 6b provides the distribution of solid fraction along the longitudinal cross-section at the thickness and width center interfaces, clearly showing a liquid core length of 430 mm, which solidifies completely before the end of the foot roller section. Figure 6c presents the temperature distribution across the cross-section at the mold exit, showing temperatures of 616 °C at the center of the wide surface and 510 °C at the center of the narrow surface. The temperature differences between the core center and the centers of the wide and narrow surfaces are 43 °C and 149 °C, respectively. Figure 6d shows the solid fraction distribution at the mold exit cross-section, indicating a shell thickness of 12 mm on one side of the mold exit. In the vertical bending thin slab continuous casting process, cooling occurs from the periphery to the center and from the bottom to the top. Due to weak natural convection within the sump, the temperature distribution is relatively non-uniform. This results in the temperature at the center of the sump being higher than at the periphery and the temperature near the top being higher than at the bottom. Consequently, “U”-shaped liquidus isotherms are formed along the longitudinal section. The morphology of the liquid sump is influenced by the small temperature difference between the core and surface due to the dimensions of the thickness of the thin slab, the pouring gate arrangement at one-quarter and three-quarters of the slab width, and the characteristics of strong and uniform cooling during solidification.

The temperature distribution curves for the wide surface center, center point, narrow surface center, and corner vertex as a function of distance are shown in Figure 7. The experimentally measured temperatures were recorded at several positions, at the narrow surface center at the outlet of the bending section, the narrow surface center located at 500 mm and 1200 mm along the arc length from the inlet of the arc section, and the wide surface center at the outlet of the arc section. The corresponding temperatures were 455 °C, 446 °C, 420 °C, and 352 °C, respectively. The accuracy of the model was validated, with a difference of less than 5 °C compared to the simulation results.

The temperature distribution curves within a distance of 4500 mm from the meniscus are presented in Figure 7a. At a distance of 650 mm from the meniscus, the temperatures at the wide surface center, narrow surface center, and corner are 515 °C, 406 °C, and 383 °C, respectively. Beyond this point, a brief temperature rebound is observed, with the maximum rebound temperatures at the wide surface center, narrow surface center, and corner vertex reaching 531 °C, 460 °C, and 458 °C, respectively. The rebound temperature differences are 16 °C, 54 °C, and 75 °C. This rebound is attributed to the cessation of the secondary cooling water supply, after which only residual secondary cooling water dripping onto the thin slab provides limited cooling. The slab subsequently enters an air-cooling section, where heat dissipation primarily occurs through convective and radiative heat transfer from the surface, resulting in a gradual temperature reduction in the continuous casting slab.

The temperature distribution before complete solidification is shown in Figure 7b. It can be observed that during the solidification process of pure aluminum 1070, latent heat is gradually released as the liquid progressively transforms into a solid. As the latent heat of solidification is fully released and the liquid is completely transformed into a solid, the temperature at the center reaches below 650.9 °C. Notably, except for the continuous decrease in the temperature at the center point, the other three curves exhibit a temperature rebound near the mold exit. This phenomenon can be attributed to two factors. First, a gas gap forms between the thin slab and the mold near the mold exit due to the solidification shrinkage of the alloy. In this gas gap, heat transfer from the slab surface is dominated by radiation and convection, leading to a significant reduction in cooling intensity and the cooling rate. The convective heat transfer coefficient in this region is notably low. Second, as the thin slab enters the foot roll section, a sufficiently thick solidified shell has already formed. This shell, with its relatively low thermal conductivity, reduces the efficiency of heat transfer from the liquid core to the shell surface, thereby limiting the impact of external cooling water on the internal temperature. In the foot roll section, heat conduction becomes the dominant mode of heat transfer. However, as the shell thickness increases, the thermal resistance also rises, resulting in a significantly lower surface heat flux compared to the mold region. Consequently, cooling slows down, and the temperature decreases at a more gradual rate. As the casting process advances, the temperature of the thin slab continues to decrease, while its surface is directly exposed to secondary cooling water, which notably enhances the cooling rate.

3.3. Solidification Structure Evolution Simulation of Pure Aluminum 1070 Continuous Casting Thin Slab

Figure 8 presents the simulation results of the solidification structure evolution of pure aluminum 1070 in vertical bending continuous casting. Figure 8a–d displays the simulated solidification microstructure of the pure aluminum 1070 continuous casting thin slab at different positions, corresponding to distances of 47 mm, 142 mm, 356 mm, and 430 mm from the meniscus. Consistent with experimental observations, the cross-sectional microstructure of the thin slab consists of fine-grained regions at the edges, equiaxed grain regions at the center, and columnar crystal structures in the intermediate zone, with a similar distribution ratio. This analysis provides insight into the relevant nucleation parameters influencing the microstructure during the VC casting process. Key parameters, such as the nucleation rate (the formation of new crystals during solidification) and heat transfer (the movement of heat through the material), play a crucial role in determining the final outcome. A higher nucleation rate can lead to a finer grain structure, while more efficient heat transfer can accelerate the cooling process, influencing the material’s overall properties. In the continuous casting process of aluminum alloys, an increase in supercooling intensifies the heat transfer process, particularly during solidification. This is achieved by enhancing the heat flux density, influencing the temperature gradient, and accelerating the cooling rate, which drives the transfer of heat from the liquid aluminum alloy to the external environment, thereby affecting the solidification behavior and the final microstructure of the cast product. By adjusting these parameters, the simulation can provide more accurate predictions of the final product’s characteristics. The nucleation parameters for pure aluminum 1070, as used in the microstructure CAFE model of the VC casting process, are detailed in Table 2.

4. Discussion

As shown in Figure 6 and Figure 7, under strong cooling conditions (with a long mold and high cooling water flow rate), the temperature along the entire mold length rapidly and uniformly decreases, leading to high undercooling and a low temperature gradient. It is evident that the melt within the mold uniformly cools, which inherently increases both the nucleation rate and the number of nuclei. Thus, this uniform temperature distribution achieved by the VC casting process is one of the primary reasons for obtaining a refined equiaxed grain structure during solidification. According to Atsumi Ohno et al. [41], before the formation of the chill layer at the initial stages of pouring and solidification, the melt near areas such as the gating system and mold wall experiences undercooling due to localized chilling, which promotes heterogeneous nucleation and generates numerous free-floating chill dendrites within the melt. These small dendrites drift toward the central region of the mold with the flow of the liquid metal. If the pouring temperature is not excessively high, some of these small dendrites do not fully remelt and thus act as nuclei for equiaxed grains. This suggests that the internal equiaxed grain region results from non-spontaneous nucleation followed by growth and free-floating movement, especially when a substantial number of effective nucleation sites are present within the liquid metal, which increases the width of the equiaxed grain region and decreases equiaxed grain size. Therefore, intense chilling by strong cooling facilitates the formation of a refined equiaxed structure in the cast billet. Moreover, the incorporation of grain refiners plays a crucial role in the solidification process. By introducing heterogeneous nucleation sites, enhancing the nucleation rate, and inhibiting the growth of columnar grains, grain refiners facilitate the development of equiaxed grains. This process significantly improves the conditions for heterogeneous nucleation, making it a key strategy for achieving a fully equiaxed grain structure.

Conventional DC casting typically results in coarse dendritic structures, while semi-continuous casting, as reported in [8], produces a significantly higher number of refined equiaxed grains. This phenomenon is attributed to the application of a direct current pulsed magnetic field and inoculation. In contrast, the novel process presented in this study, which does not utilize an external magnetic field, results in a more uniform microstructure with finer equiaxed grain sizes compared to the structure produced by adding an equivalent amount of grain refiners and applying a 100A pulsed magnetic field, as shown in Figure 4d of reference [8]. This enhancement significantly improves the performance of the cast slab, an advantage that is expected to carry forward into subsequent processing stages. In the temperature distribution of the pure aluminum 1070 thin slab cast using the VC casting process, the morphology of the liquidus isotherm differs notably from that observed in conventional DC casting, both with and without intensive melt shearing. The formation of the “U”-shaped liquidus isotherms can be attributed to several key factors related to the unique thermal dynamics of the vertical bending continuous casting process. Unlike conventional DC casting, the VC casting process, with its specific pouring gate arrangement and strong uniform cooling, creates a more balanced heat transfer across the slab. The thin slab thickness and the pouring gate positioned at one-quarter and three-quarters of the slab width contribute to a more symmetrical temperature distribution. This arrangement minimizes the temperature difference between the core and surface, resulting in a flatter and more uniform liquidus isotherm profile. As the liquidus isotherm forms a “U”-shape, it reduces the temperature gradient across the slab, promoting more uniform solidification and cooling rates. This uniformity helps to prevent the formation of large temperature variations, which can cause defects such as macrosegregation or cracks. In addition, this more consistent temperature field plays a key role in forming a finer and more uniform microstructure, as evidenced by the increased number of equiaxed grains and the smaller equiaxed grain size observed in the cast slab. To provide a more intuitive comparison and analysis of the new process, Table 3 presents the key performance differences between the VC and DC casting processes. Beyond the differences in macrostructure, microstructure, and liquidus isotherm morphology, the newly developed process offers several distinct advantages. From an equipment perspective, the mold length is 380 mm, which extends the primary cooling length compared to the conventional DC process, allowing for more rapid cooling and supporting higher casting speeds. The novel process facilitates continuous production, reduces energy consumption, and increases the yield of aluminum. Additionally, the aluminum and aluminum alloy thin slabs cast by this process exhibit a near-net shape, reducing the need for subsequent processing steps and further decreasing energy consumption. This contributes to the green and sustainable advancement of the aluminum industry.

5. Conclusions

(1) This novel process enables the continuous production of aluminum, achieving faster casting speeds, reducing energy consumption, and increasing yield. In the case of pure aluminum 1070 thin slabs, the ratio of the fine crystal region at the surface, the columnar crystal region, and the equiaxed crystal region is 3:8:14, with the equiaxed crystal region comprising 56% of the total volume. The equiaxed crystal microstructure exhibits an average grain size of 98 μm.

(2) The liquid core length is 430 mm, and the shell thickness on one side at the mold outlet is 12 mm. Additionally, a “U”-shaped sump and “U”-shaped liquidus isotherms are formed along the longitudinal section.

(3) In this process, the maximum nucleation undercooling parameters for surface nucleation and volume nucleation in the pure aluminum 1070 thin slab solidification microstructure are 13 °C and 15 °C, respectively. The maximum numbers of nucleation for surface nucleation and volume nucleation are 1 × 10¹² and 5 × 10¹³, respectively.

Author Contributions

Q.M.: methodology, software, formal analysis, investigation, data curation, visualization, writing—original draft; F.H.: conceptualization, writing—review and editing, visualization, project administration; Z.W.: software, data curation; S.Z.: formal analysis; G.J.: data curation, supervision; G.Y. and G.W.: methodology, writing—review and editing, formal analysis, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The authors declare that the data supporting the findings of this study are available within the paper.

Conflicts of Interest

The authors declare no competing interests.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Schematic representation and photographic documentation of the vertical bending continuous casting equipment for aluminum and aluminum alloy thin slabs: (a) Schematic representation; (b) side-view photographic documentation; (c) front-view photographic documentation.

Figure 2. (a) Heat transfer model meshes for vertical bending continuous casting of pure aluminum 1070 thin slab; (b) flowchart outlining calculation methodology applied in this study.

Figure 3. The pure aluminum 1070 thin slab was produced by vertical bending continuous casting equipment (casting speed: 1.2 m/min).

Figure 4. The macrostructure of the pure aluminum 1070 thin slab produced by vertical bending continuous casting (casting speed: 1.2 m/min).

Figure 5. The microstructure of the pure aluminum 1070 thin slab produced by vertical bending continuous casting (casting speed: 1.2 m/min). (a,d,g) Samples taken from the edge in the width direction, at the surface, one-fifth of the thickness, and the center of the thickness, respectively; (b,e,h) samples taken from [Forumla omitted. See PDF.] in the width direction, at the surface, one-fifth of the thickness, and the center of the thickness, respectively; (c,f,i) samples taken from [Forumla omitted. See PDF.] in the width direction, at the surface, one-fifth of the thickness, and the center of the thickness, respectively.

Figure 6. Macroscopic diagram of the temperature and solidification distribution of pure aluminum 1070 in vertical bending continuous casting (casting speed: 1.2 m/min). (a) Thickness center interface along the longitudinal cross-section and overall temperature distribution; (b) the distribution of solid fraction along the longitudinal cross-section at the thickness and width center interfaces; (c) temperature distribution at the mold exit in the cross-section; (d) solid fraction distribution at the mold exit in the cross-section.

Figure 7. Temperature distribution curves for the wide surface center, center point, narrow surface center, and corner vertex as a function of distance in vertical bending continuous casting (casting speed: 1.2 m/min): (a) within 4500 mm from the meniscus; (b) within 450 mm from the meniscus.

Figure 8. Simulation results of the solidification structure evolution of pure aluminum 1070 in vertical bending continuous casting (casting speed: 1.2 m/min): (a) 47 mm from the meniscus; (b) 142 mm from the meniscus; (c) 356 mm from the meniscus; (d) 430 mm from the meniscus.

Table 1

Chemical composition of pure aluminum 1070 (wt. %).

Si	Fe	Cu	Mn	Mg	Zn	Cr	B	Ti	Al
0.058	0.17	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	Bal.

Table 2

Nucleation parameters for microstructure CAFE model of VC casting process.

Volume Nucleation			Surface Nucleation
$\Delta T_{v,\max }$(°C)	$\Delta T_{v,\sigma }$(°C)	$n_{v,\max }$(m−3)	$\Delta T_{s,\max }$(°C)	$\Delta T_{s,\sigma }$(°C)	$n_{s,\max }$(m−2)
15	1	5 × 1013	13	1	1 × 1012

Table 3

Key performance differences between the VC and DC casting processes.

Performance Indicator	VC Casting	DC Casting
Casting Speed	Higher (e.g., 1.2 m/min)	Lower (typically less than 1 m/min)
Cooling Rate	Faster (longer mold cooling zone and higher water flow rate)	Slower (shorter mold cooling zone and lower water flow rate)
Energy Efficiency	Improved (due to higher casting speed and faster cooling)	Lower (higher energy consumption with slower cooling)
Aluminum Alloy Quality	Refined equiaxed grain structure, smaller average grain size (e.g., 98 μm)	Larger grain size
Subsequent Processing Needs	Near-net shape, reducing the need for further processing	More downstream processing required
Production Cost	Potentially lower (reduced need for downstream processing and energy consumption)	Higher (due to more energy consumption and additional processing steps)
Product Performance	Better mechanical properties (finer grain structure)	Lower mechanical properties
Environmental Impact	Lower energy consumption, supports green and sustainable development	Higher energy consumption, greater environmental impact

References

1. Li, S.S.; Yue, X.; Li, Q.Y.; Peng, H.L.; Dong, B.X.; Liu, T.S.; Yang, H.Y.; Fan, J.; Shu, S.L.; Qiu, F. et al. Development and applications of aluminum alloys for aerospace industry. J. Mater. Res. Technol.; 2023; 27, pp. 944-983. [DOI: https://dx.doi.org/10.1016/j.jmrt.2023.09.274]

2. Banoth, S.; Babu, V.S.; Raghavendra, G.; Shankar, P.H. Study of various types of aluminium alloy grade series and their effect on mechanical performance. Mater. Today Proc.; 2023; pp. 1-6. [DOI: https://dx.doi.org/10.1016/j.matpr.2023.04.635]

3. Sun, Y. The use of aluminum alloys in structures: Review and outlook. Structures; 2023; 57, 105290. [DOI: https://dx.doi.org/10.1016/j.istruc.2023.105290]

4. Dursun, T.; Soutis, C. Recent developments in advanced aircraft aluminium alloys. Mater. Des.; 2014; 56, pp. 862-871. [DOI: https://dx.doi.org/10.1016/j.matdes.2013.12.002]

5. Warren, A.S. Developments and challenges for aluminum—A boeing perspective. Proceedings of the 9th International Conference on Aluminium Alloys; Brisbane, Australia, 2–5 August 2004.

6. Guo, G.H.; Yao, T.; Liu, W.S.; Tang, S.; Xiao, D.H.; Huang, L.P.; Wu, L.; Feng, Z.H.; Gao, X.B. Numerical simulation and machine learning prediction of the direct chill casting process of large-scale aluminum ingots. Materials; 2024; 17, 1409. [DOI: https://dx.doi.org/10.3390/ma17061409]

7. Zhang, L.; Li, X.Q.; Li, R.Q.; Jiang, R.P.; Zhang, L.H.; Guan, S.K.; Liu, B.B. Direct-chill casting of large-scale Al-Cu alloy ingot under ultrasound: Distribution of physical fields and analysis of microstructure. Adv. Eng. Mater.; 2021; 23, 2100432. [DOI: https://dx.doi.org/10.1002/adem.202100432]

8. Chen, H.; Jie, J.C.; Fu, Y.; Ma, H.J.; Li, T.J. Grain refinement of pure aluminum by direct current pulsed magnetic field and inoculation. Trans. Nonferrous Met. Soc. China; 2014; 24, pp. 1295-1300. [DOI: https://dx.doi.org/10.1016/S1003-6326(14)63191-5]

9. Zhu, C.; Zhao, Z.H.; Zhu, Q.F.; Wang, G.S.; Zuo, Y.B.; Li, Q.Q.; Qin, G.W. Hot-top direct chill casting assisted by a twin-cooling field: Improving the ingot quality of a large-size 2024 Al alloy. J. Mater. Sci. Technol.; 2022; 112, pp. 114-122. [DOI: https://dx.doi.org/10.1016/j.jmst.2021.09.053]

10. Wang, X.J.; Xu, Y.J.; Yang, L.F.; Chen, C.C.; Song, Z.X.; Cui, J.Z. Effect of lubricating oil and wiper on super-high strength 7055 aluminum alloy ingots. Crystals; 2023; 13, 88. [DOI: https://dx.doi.org/10.3390/cryst13010088]

11. Eskin, D.G.; Savran, V.I.; Katgerman, L. Effects of melt temperature and casting speed on the structure and defect formation during direct-chill casting of an Al-Cu alloy. Metall. Mater. Trans. A; 2005; 36, pp. 1965-1976. [DOI: https://dx.doi.org/10.1007/s11661-005-0059-6]

12. Glenn, A.M.; Russo, S.P.; Paterson, P.J.K. The effect of grain refining on macrosegregation and dendrite arm spacing of direct chill cast AA5182. Metall. Mater. Trans. A; 2003; 34, pp. 1513-1523. [DOI: https://dx.doi.org/10.1007/s11661-003-0263-1]

13. Nadella, R.; Eskin, D.G.; Du, Q.; Katgerman, L. Macrosegregation in direct-chill casting of aluminium alloys. Prog. Mater. Sci.; 2008; 53, pp. 421-480. [DOI: https://dx.doi.org/10.1016/j.pmatsci.2007.10.001]

14. Jamaly, N.; Phillion, A.B.; Drezet, J.-M. Stress—Strain predictions of semisolid Al-Mg-Mn alloys during direct chill casting: Effects of microstructure and process variables. Metall. Mater. Trans. B; 2013; 44, pp. 1287-1295. [DOI: https://dx.doi.org/10.1007/s11663-013-9902-0]

15. Eskin, D.G. Structure and casting defects of aluminum billets produced by direct-chill casting. Advances in the Science and Engineering of Casting Solidification; The Minerals, Metals & Materials Society: Pittsburgh, PA, USA, 2015; pp. 73-80.

16. Liu, Z.Q.; Li, B.K. Effect of vertical length on asymmetric flow and inclusion transport in vertical-bending continuous caster. Powder Technol.; 2018; 323, pp. 403-415. [DOI: https://dx.doi.org/10.1016/j.powtec.2017.10.034]

17. Sengupta, J.; Thomas, B.G.; Wells, M.A. The use of water cooling during the continuous casting of steel and aluminum alloys. Metall. Mater. Trans. A; 2005; 36, pp. 187-204. [DOI: https://dx.doi.org/10.1007/s11661-005-0151-y]

18. Ludwig, O.; Drezet, J.M.; Martin, C.L.; Suéry, M. Rheological behavior of Al-Cu alloys during solidification: Constitutive modeling, experimental identification, and numerical study. Metall. Mater. Trans. A; 2005; 36, pp. 1525-1535. [DOI: https://dx.doi.org/10.1007/s11661-005-0244-7]

19. Wang, R.; Zuo, Y.B.; Zhu, Q.F.; Liu, X.D.; Wang, J.C. Effect of temperature field on the porosity and mechanical properties of 2024 aluminum alloy prepared by direct chill casting with melt shearing. J. Mater. Process. Technol.; 2022; 307, 117687. [DOI: https://dx.doi.org/10.1016/j.jmatprotec.2022.117687]

20. Fabrègue, D.; Deschamps, A.; Suéry, M.; Poole, W.J. Rheological behavior of Al-Mg-Si-Cu alloys in the mushy state obtained by partial remelting and partial solidification at high cooling rate. Metall. Mater. Trans. A; 2006; 37, pp. 1459-1467. [DOI: https://dx.doi.org/10.1007/s11661-006-0090-2]

21. Davidson, C.; Viano, D.; Lu, L.; Stjohn, D. Observation of crack initiation during hot tearing. Int. J. Cast Met. Res.; 2006; 19, pp. 59-65. [DOI: https://dx.doi.org/10.1179/136404606225023291]

22. Suyitno,; Eskin, D.G.; Katgerman, L. Structure observations related to hot tearing of Al-Cu billets produced by direct-chill casting. Mater. Sci. Eng. A; 2006; 420, pp. 1-7. [DOI: https://dx.doi.org/10.1016/j.msea.2005.12.037]

23. Lloreda-Jurado, P.J.; Chicardi, E.; Paúl, A.; Sepúlveda, R. Effect of processing parameters on the properties of freeze-cast Ni wick with gradient porosity. Mater. Des.; 2021; 206, 109795. [DOI: https://dx.doi.org/10.1016/j.matdes.2021.109795]

24. Birol, Y. An improved practice to manufacture Al-Ti-B master alloys by reacting halide salts with molten aluminum. J. Alloys Compd.; 2006; 420, pp. 71-76. [DOI: https://dx.doi.org/10.1016/j.jallcom.2005.10.017]

25. Birol, Y. Effect of the salt addition practice on the grain refining efficiency of Al-Ti-B master alloys. J. Alloys Compd.; 2006; 420, pp. 207-212. [DOI: https://dx.doi.org/10.1016/j.jallcom.2005.11.010]

26. Birol, Y. The effect of holding conditions in the conventional halide salt process on the performance of Al-Ti-B grain refiner alloys. J. Alloys Compd.; 2007; 427, pp. 142-147. [DOI: https://dx.doi.org/10.1016/j.jallcom.2006.03.002]

27. Wang, L.W.; Wu, T.; Liu, A.P.; Yan, H.; Mao, Y.N.; Liu, Y.; Peng, Z.Z.; Wang, D.L.; Liang, Z.M.; Wang, Q. et al. Effect of Al-5Ti-1B grain reffner on microstructure and properties of arc-additive-manufactured Al-Mg alloy. Vacuum; 2022; 200, 111012. [DOI: https://dx.doi.org/10.1016/j.vacuum.2022.111012]

28. Wei, Y.S.; Wang, C.; You, J.; Qiu, D.; Gao, Y.P.; Xu, H.; Xu, J.; Wang, H.Y. Substantial grain refinement of Al-Mn-Si alloys mediate d by collaborative effect of Al-5Ti-1B refiner and sub-rapid solidification. J. Mater. Sci. Technol.; 2024; 187, pp. 230-239. [DOI: https://dx.doi.org/10.1016/j.jmst.2023.11.025]

29. Lee, J.Y.; Lee, J.M.; Son, K.S.; Jang, J.I.; Cho, Y.H. A study on the interaction between a Sr modiffer and an Al-5Ti-1B grain reffner in an Al-7Si-0.35Mg casting alloy. J. Alloys Compd.; 2023; 938, 168598. [DOI: https://dx.doi.org/10.1016/j.jallcom.2022.168598]

30. Pattnaik, A.B.; Das, S.; Jha, B.B.; Prasantha, N. Effect of Al-5Ti-1B grain refiner on the microstructure, mechanical properties and acoustic emission characteristics of Al5052 aluminium alloy. J. Mater. Res. Technol.; 2015; 4, pp. 171-179. [DOI: https://dx.doi.org/10.1016/j.jmrt.2014.10.017]

31. Vinod Kumar, G.S.; Murty, B.S.; Chakraborty, M. Development of Al-Ti-C grain refiners and study of their grain refining efficiency on Al and Al-7Si alloy. J. Alloys Compd.; 2005; 396, pp. 143-150. [DOI: https://dx.doi.org/10.1016/j.jallcom.2004.12.039]

32. Banerji, A.; Reif, W. Development of AI-Ti-C grain refiners containing TiC. Metall. Trans. A; 1986; 17, pp. 2127-2137. [DOI: https://dx.doi.org/10.1007/BF02645911]

33. Xu, C.; Xiao, W.L.; Wen, S.Z.; Zhen, Y.J.; Ma, C.L. Microstructures and grain refinement performance of Al-Ti-B-C grain refiner via powder metallurgy. Rare Met.; 2022; 41, pp. 2357-2362. [DOI: https://dx.doi.org/10.1007/s12598-015-0578-2]

34. Liu, Q.L.; Li, X.M.; Jiang, Y.H. Numerical simulation of EBCHM for the large-scale TC4 alloy slab ingot during the solidification process. Vacuum; 2017; 141, pp. 1-9. [DOI: https://dx.doi.org/10.1016/j.vacuum.2017.03.009]

35. Saucedo-Mora, L.; Marrow, T.J. Method for the explicit insertion of microstructure in Cellular Automata Finite Element (CAFE) models based on an irregular tetrahedral Finite Element mesh: Application in a multi-scale Finite Element Microstructure MEshfree framework (FEMME). Finite Elem. Anal. Des.; 2015; 105, pp. 56-62. [DOI: https://dx.doi.org/10.1016/j.finel.2015.07.001]

36. Ma, J.; Wang, B.; Zhao, S.L.; Wu, G.X.; Zhang, J.Y.; Yang, Z.L. Incorporating an extended dendritic growth model into the CAFE model for rapidly solidified non-dilute alloys. J. Alloys Compd.; 2016; 668, pp. 46-55. [DOI: https://dx.doi.org/10.1016/j.jallcom.2016.01.210]

37. Kim, M.S.; Lee, S.H.; Jung, J.G.; Eah, K. Prediction of grain structure in direct-chill cast Al-Zn-Mg-Cu billets using cellular automaton-finite element method. Prog. Nat. Sci-Mater.; 2021; 31, pp. 434-441. [DOI: https://dx.doi.org/10.1016/j.pnsc.2021.05.003]

38. Shterenlikht, A.; Howard, I.C. The CAFE model of fracture—Application to a TMCR steel. Fatigue Fract. Engng. Mater. Struct.; 2006; 29, pp. 770-787. [DOI: https://dx.doi.org/10.1111/j.1460-2695.2006.01031.x]

39. Wang, B.; Zhang, J.Y.; Li, X.M.; Qi, W.H. Simulation of solidification microstructure in twin-roll casting strip. Comp. Mater. Sci.; 2010; 49, pp. S135-S139. [DOI: https://dx.doi.org/10.1016/j.commatsci.2010.01.051]

40. Yamazaki, M.; Natsume, Y.; Harada, H.; Ohsasa, K. Numerical Simulation of Solidification Structure Formation during Continuous Casting in Fe-0.7mass%C Alloy Using Cellular Automaton Method. ISIJ Int.; 2006; 46, pp. 903-908. [DOI: https://dx.doi.org/10.2355/isijinternational.46.903]

41. Chen, Z.M.; Yu, W.Q. Formation and Control of Solidification Microstructures in Castings. Principles of Solidification in Casting Metals; 1st ed. Tong, J.X.; Huang, H.Z. Peking University Press: Beijing, China, 2014.