Effect of Casting Speed on Floating Grains in Direct-Chill Casting of Aluminum Alloys

Abstract

The effects of casting speed on macrosegregation and grain structure during the direct-chill casting of an Al–Mg–Si alloy are experimentally investigated. Results show that increasing the casting speed yields a more pronounced negative centerline segregation. Regard this mechanism, apart from the increased contribution of solidification shrinkage, the variation in floating grains is newly observed. The increased casting speed significantly increases the size and fraction of floating grains at the central portion of the billet, thus resulting in a more severe negative segregation. To clarify the experimentally observed phenomena, the solidification and distribution of floating grains in billets cast at different casting speeds are numerically simulated. The increased casting speed deepens the slurry zone significantly, thus providing more time for the floating grains to freely float and develop before settling into the mushy zone. Additionally, the greater slope of the coherency isotherm resulting from the increased casting speed promotes the movement and sedimentation of the floating grains toward the central portion of the billet. Consequently, more floating grains are detected at the billet center.

1. Introduction

Direct-chill (DC) casting is the main technology used to produce wrought aluminum billets/ingots for subsequent deformation. However, macrosegregation and non-uniform grain structures often occur during this process because of the inhomogeneity of solidification across the billet section. These constitute irreversible defects that cannot be eliminated during the subsequent heat treatment. The presence of casting defects can severely deteriorate the formability of billets/ingots and the mechanical properties of the final products. Therefore, the importance of macrosegregation and grain structure control during DC casting cannot be overemphasized.

In addition to the billet dimension and type, casting parameters such as casting speed, casting temperature, and water flow rate determine the occurrence of casting defects. Among them, casting speed exerts a dominant effect on the microstructural features and macrosegregation extent. Increasing the casting speed can refine the grain structure and non-equilibrium eutectics because of increased solidification rates.^1–5) The improvement in the as-cast microstructure can optimize the formability of the billets and reduce the risk of hot cracking. However, by increasing the casting speed, negative centerline segregation becomes more severe.^6–8) Eskin et al. concluded that the aggravation in negative segregation was due to the greater contribution of shrinkage-induced flow in the mushy zone because the increased casting speed significantly deepened the liquid sump during DC casting.^7,9) In addition to solidification shrinkage, the movement of floating grains contributes to negative centerline segregation.^10–13) Moreover, the appearance of floating grains deteriorates the evenness of the grain structure.^12,14) Although several studies have investigated the effect of casting speed on the as-cast structure and macrosegregation, the effect on floating grains is yet to be clarified. Investigations pertaining to the grain size, morphology, and volume fraction of floating grains with respect to different casting speeds is crucial not only for understanding macrosegregation variation, but also for improving the overall quality in terms of subsequent processing.

In this study, the effect of casting speed on the macrosegregation of DC cast billets is investigated. Microstructural analysis is performed to reveal the variation in the grain structure resulting from different casting speeds. Subsequently, the solidification patterns and distribution of floating grains during DC casting at different casting speeds are numerically investigated. Based on the results, the effect of the casting speed on the floating grains is clarified.

2. Experimental

Segregation and structure analyses were performed on the Al–Mg–Si DC cast billets with a diameter of 106 mm. The casting temperature was 970 K, the water flow rate was 60 L/min, the water temperature was 28°C, and three different casting speeds (200, 250, and 300 mm/min) were applied. The compositional variation across the billet cross section was analyzed using a spark spectrum analyzer. To examine the macrostructure, the billet cross section was etched with 10% NaOH solution and a nital cleaning agent. Several metallographic samples obtained at different distances from the billet center were electrolytically etched with Barker’s reagent at 20 V direct current for microstructural analysis; the reagent comprised 5 mL of HBF₄ (48%) in 200 mL of H₂O. To investigate the microsegregation, area-scanning analysis was performed on the floating grains using an electron probe microanalyzer (EPMA, Shimadzu EPMA-1720, Japan) operating at an acceleration voltage of 15 kV, an electron beam current of 80 nA, and a sampling time of 100 ms. To clarify the experimentally obtained effect of the casting speed on the floating grains, a coupled model was developed to simulate the solidification and motion of the floating grains during DC casting. Details regarding the mathematical model and simulations are available in earlier work.¹⁵⁾

3. Results and Discussion

3.1 Grain structure and segregation

Figures 1(a)–(c) show macrographs of the as-prepared billets produced at different casting speeds, i.e., (a) 200 mm/min, (b) 250 mm/min, and (c) 300 mm/min. The results clearly show inhomogeneity in the grain structure. At 200 mm/min, structural inhomogeneity occurred in the entire billet cross-section, which was characterized by the dispersive distribution of black spot-like anomalous structure. When the casting speed was increased to 250 mm/min, the unevenness of the structure improved, and the black spot-like anomalous structure reduced significantly. When the casting speed was further increased to 300 mm/min, the structural inhomogeneity improved further, and the black spot anomalies disappeared. However, upon closer examination, the result shown in Fig. 1(c) revealed another type of structure inhomogeneity, which appeared mainly in the central region of the billet resulting from the floating grains.¹³⁾ Correspondingly, a certain extent of macrosegregation was observed in all three billets, as shown in Figs. 1(d)–(f). The compositional deviation was used to assess the macrosegregation extent, i.e., ΔC = (C_i − C_i,0)/C_i,0, where C_i and C_i,0 are the experimentally measured and nominal compositions of alloy element i, respectively. The macrosegregation profiles illustrated the same trend in segregation patterns among the different casting speeds, i.e., negative segregation at the billet center and positive segregation in the quarter region. However, the increase in casting speed resulted in more pronounced segregation patterns, particularly negative centerline segregation. The compositional deviation of Si in the center of the billet decreased from −0.0208 at 200 mm/min to −0.0367 at 300 mm/min. In addition to the higher contribution of solidification shrinkage resulting from the deepening of the sump, variations in the characteristics of floating grains in the billets among the different casting speeds exerted a certain effect on negative segregation.

Fig. 1

Macrographs and macrosegregation profiles of billet produced at different casting speeds: (a), (d) 200 mm/min; (b), (e) 250 mm/min; (c), (f) 300 mm/min.

Figure 2 illustrates the variation in the grain structure across the billet cross-section with respect to the casting speed. As shown, the occurrence of a duplex grain structure, i.e., a mixture of coarse- and fine-dendrite grain structures was observed in the billet at all the casting speeds investigated. The grains exhibiting coarse dendrite features were recognized as floating grains, based on microsegregation analysis.^11,15) Although the structural analysis indicated the appearance of floating grains at all three casting speeds and that most of them appeared in the central portion of the billets, the distribution and morphology of the floating grains were different. At casting speeds of 200 and 250 mm/min, some floating grains were clearly observed at the quarter and periphery of the billets, although the amount was significantly less than that at the center of the billets. When the casting speed was increased to 300 mm/min, floating grain was invisible at the periphery of the billet. In addition, when the casting speed was low, the floating grains showed a clear clustering distribution at the billet center. The identification of floating grains in the central portion of the billet at 300 mm/min was difficult owing to the absence of coarse dendrite clustering. However, the result of the structural analysis shows that the area fraction of the floating grains increased from ∼50.45% at 200 mm/min to ∼58.1% at 300 mm/min, as illustrated in Fig. 4(a).

Fig. 2

Grain structure along billet diameter based on different casting speeds.

Figure 3 shows the microsegregation features of typical floating grains (along with the surrounding normal grains) at the center of the billets at different casting speeds. Three solute elements, including the eutectic elements Mg and Si and the peritectic element Cr, were selected for analysis using the EPMA. Results from the analysis show more severe microsegregation of the floating grains than the surrounding grains, with the concentrations of eutectic and peritectic elements being significantly lower and higher than those in the surrounding grains, respectively. As reported in previous studies,^7,13) such microsegregation characteristics of floating grains are a key feature being the innate contribution to negative centerline segregation. However, the floating grains in the billet with a higher casting speed did not exhibit more pronounced microsegregation, which can be explained by the formation mechanism of the floating grains. The solute-depleted coarse dendrites inside the floating grains nucleated and grew in the slurry region, where the solute concentration was similar to that of molten aluminum. Changes in the casting speed did not cause significant changes in the solute concentration in this region. Therefore, the floating grains growing in this region only demonstrated morphological differences, and no clear distinction was observed in terms of the microscopic solute concentration. However, the macrosegregation profiles shown in Fig. 1 clearly indicate aggravated negative centerline segregation as the casting speed increased. Therefore, further investigations into the variation in the distribution of floating grains at different casting speeds was performed.

Fig. 3

Microsegregation of typical floating grains at billets center at different casting speeds.

Figure 4(a) shows the average size and area ratio of the floating grains at the center of the billets obtained at different casting speeds, and Fig. 4(b) shows the average grain size across the entire billet cross-section. The statistical results indicate the refinement effect of the grain structure as the casting speed increased due to the increased solidification rate during casting, which is consistent with the results reported previously.^1–3) However, evident coarsening was observed in the floating grains at the center of the billets after the casting speed increased, with the average size increasing from ∼357.7 µm at 200 mm/min to ∼423 µm at 300 mm/min. Moreover, as mentioned above, the fraction of floating grains in the central portion of the billet increased with the casting speed. As previously reported, one of the main formation mechanisms for negative centerline segregation is the accumulation of solute-depleted floating grains at the billet center. Therefore, more severe negative centerline segregation with increased casting speed appear reasonable. The observed distribution of floating grains can be attributed to the variation in the solidification patterns of the DC casting billet at different casting speeds. This will be discussed further based on the results of the numerical simulation.

Fig. 4

(a) Average size and area ratio of floating grains at the center of billets, (b) average size of grains with different distances from billet center.

3.2 Solidification and distribution of floating grains

The effect of casting speed on the size and distribution of the floating grains is related to the variation in the solidification patterns with respect to the different casting speeds. To clarify this, numerical simulations of the solidification and movement of floating grains during DC casting were performed. Figure 5 presents the liquid fraction contour in the longitudinal section of the billets predicted at four casting speeds, i.e., (a) 100 mm/min, (b) 200 mm/min, (c) 250 mm/min, and (d) 300 mm/min. The transition region between the liquidus and solidus isotherms can be further categorized into slurry and mushy zones, bounded by a coherency isotherm. For the DC casting of wrought aluminum alloys, the solid fraction at which coherency transition typically occurs is between 0.2 and 0.33;¹⁶⁾ hence, a solid fraction of 0.3 (i.e., liquid fraction of 0.7, as shown in Fig. 5) was adopted in this study. The results indicate that casting speed significantly affected the depth and shape of the sump. The increased casting speed caused a significant deepening of the transition region, particularly in the slurry zone, which width at the billet center increased from 0.036 m at 100 mm/min to 0.112 m at 300 mm/min, whereas the resultant change was not noticeable in the mushy zone. In terms of the macrosegregation formation, different mechanisms acted in different zones, e.g., the movement of floating grains mainly occurred in the slurry zone, whereas solidification shrinkage occurred in the mushy zone. Therefore, the change in the depth and shape of the slurry zone significantly affected the size and distribution of the floating grains in the DC casting billet. The resulting variation in the distribution of the floating grains further affected centerline segregation.

Fig. 5

Predicted liquid fraction contour at different casting speeds: (a) 100 mm/min; (b) 200 mm/min; (c) 250 mm/min; (d) 300 mm/min.

The simulated distribution of floating grains (represented by the particles; different colors were set for better distinction) in the DC casting billet is shown in Fig. 6 for different casting speeds, i.e., (a) 100 mm/min, (b) 200 mm/min, (c) 250 mm/min, and (d) 300 mm/min. Although a similar dispersive distribution was obtained for the grains at all four casting speeds, a certain difference was observed. The grains gradually accumulated toward the billet center as the casting speed increased. This difference can be observed more clearly in the statistical results illustrated in Fig. 7. The number fraction of the floating grains increased from 33.86% at 100 mm/min to 50.74% at 300 mm/min in the central portion of billet but reduced significantly at the billet periphery. The simulated results agreed well with the experimental statistical data shown in Fig. 4(a). Owing to the increased number fraction of floating grains at the billet center, the negative segregation became more severe after the casting speed was increased.

Fig. 6

Distribution of floating grains predicted at different casting speeds: (a) 100 mm/min; (b) 200 mm/min; (c) 250 mm/min; (d) 300 mm/min.

Fig. 7

Statistic number fraction of floating grains along billet diameter at different casting speeds.

As mentioned above, the variation in the distribution of the floating grains contributed to the aggravated negative centerline segregation at high casting speeds. In addition, the floating grains became larger as the casting speed increased, as shown in Fig. 4(a). The effects of the casting speed on the size and distribution of the floating grains can be clarified based on the formation mechanism of the floating grains, as schematically illustrated in Fig. 8. As previously reported,¹³⁾ the unique duplex dendrite structure, which comprises a mixture of internal coarse and peripheral fine dendrites, exhibited a certain solidification sequence of floating grains within the slurry and mushy zones. Therefore, the floating grains were identified as grains that originated from heterogeneous nucleation or detached dendrites that first freely floated and grew in the slurry zone, and then became entrapped by the mushy zone during DC casting. For the case with a low casting speed, as illustrated in Fig. 8(a), the movement of the floating grains in the slurry zone was relatively restricted by the narrow and flat slurry zone. This may have caused the clustering of floating grains in the billet at a low casting speed, as shown in Fig. 2. The increased casting speed significantly deepened the slurry zone, which provided more time to the floating grains to freely float and develop before settling into the mushy zone. Therefore, the size of the floating grains increased when the casting speed increased. Additionally, the greater slope of the coherency isotherm resulting from a higher casting speed promoted the movement and sedimentation of the floating grains toward the central portion of the billet. Consequently, more floating grains were detected at the billet center. Notably, the discussion above is based on a theoretical analysis of the experimental and simulated results. The current mathematical model cannot describe the coupling process of microstructure evolution and macroscale movement during casting, which will be investigated in future research.

Fig. 8

Schematic diagram illustrating effects of casting speed on floating grains.

4. Conclusion

The effects of casting speed on the segregation and grain structure of DC-cast Al–Mg–Si alloy billets were experimentally investigated. Additionally, the solidification and distribution of floating grains in billets cast at different casting speeds were numerically simulated. The results obtained are as follows:

1. (1)    Increasing the casting speed resulted in severe negative centerline segregation, with the compositional deviation of Si decreasing from −0.0208 at 200 mm/min to −0.0367 at 300 mm/min.
2. (2)    The distribution of floating grains differed significantly at different casting speeds. At 200 mm/min, a portion of the floating grains appeared at the billet periphery, whereas the floating grains only appeared in the central region of the billet when the casting speed was increased to 300 mm/min.
3. (3)    The area fraction of floating grains in the billet center increased from ∼50.45% at 200 mm/min to ∼58.1% at 300 mm/min, along with significant coarsening despite the refinement effect of regular grains. In addition, the microsegregation features of the floating grains owing to the different casting speed did not differ significantly.
4. (4)    The simulated results showed that the slurry zone deepened significantly as the casting speed increased, which resulted in a longer floating process of the floating grains before they settled into the mushy zone. Additionally, the greater slope of the coherency isotherm promoted the movement of the floating grains toward the central portion of the billet.

Acknowledgments

This work was supported by the National Natural Science Foundation of China [grant numbers 52204395, 52150710544] and the Natural Science Foundation of Jiangsu Province [grant number BK20210723].

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