2023 Volume 64 Issue 2 Pages 373-378
Shiny and un-shiny periodic patterns are formed on the cast strip surface of aluminum alloys fabricated by using vertical-type high-speed twin-roll casting. The periodic surface patterns reduce the ductility of the rolled sheets produced from the cast strip, and thus should be suppressed. The surface patterns are caused by the vibration of the molten metal in the gap at the contact point between the nozzle tip and roll surface. In the present study, three types of nozzles (normal, release agent, and bent tip nozzle) were prepared. The effect of the nozzle shape on the surface patterns of Al–3 mass% Si alloy containing 1 mass% Fe, 0.5 mass% Mg, and 0.8 mass% Cu cast strips was investigated. The normal nozzle produced a periodic surface pattern and Si segregation in the un-shiny region was observed. Cross-sectional observation showed a difference in cooling rate between the shiny and un-shiny regions caused by the vibration of the molten metal near the nozzle tip. The release agent nozzle suppressed the periodic surface pattern slightly and Si segregation remained. The bent tip nozzle produced a strip surface where the shiny and un-shiny regions were mixed instead of a periodic surface pattern, and Si segregation was not observed. Because the gap near the nozzle tip was small, the vibration of the molten metal was strongly suppressed.
The increasing demand for lightweight materials in the automotive industry has led to an enormous growth in the production of wrought aluminum alloys.1) Therefore, the amount of aluminum scrap is increasing and it is necessary to recycle aluminum alloys. However, aluminum alloy scrap contains a large amount of various impurity elements, some of which degrade the mechanical properties of the recycled alloys.1,2) Therefore, the secondary ingots made from aluminum alloy scrap cannot be used for ductile wrought aluminum alloys, and thus they have been mainly used for producing cast aluminum alloy products. The demand for cast aluminum alloys for engine blocks is decreasing due to the increase in electric vehicle production, and thus the current cascade recycling system will soon no longer be suitable due to the oversupply of recycled ingots.1) To solve this problem, it is necessary to develop “upgrade recycling”, in which secondary aluminum ingots produced from scrap can be used to fabricate wrought materials.
Cast aluminum alloys contain a large amount of Si to improve castability. For example, the JIS ADC12 alloy, which is widely used as a die casting alloy, contains about 11 mass% Si,2–4) and the JIS AC4CH alloy, which is a typical cast alloy, contains about 7 mass% Si.1) In these alloys, the amount of eutectic Si particles is large, and their size and morphology affect the mechanical properties of the materials. In the microstructure of Al–Si alloys, a low cooling rate forms eutectic Si particles that are coarse and plate-like,2) which degrades the mechanical properties of the cast materials. In addition, aluminum alloy scrap contains iron, which also degrades the mechanical properties. In particular, needle-shaped coarse β-AlFeSi phase particles formed in iron-containing aluminum alloys decrease the ductility, toughness, and corrosion resistance of the alloys.1,5,6) Therefore, we need to reduce the harmful effects of these impurity elements.
In this research, we focused on the excellent cooling ability of the vertical-type high-speed twin-roll casting (HSTRC).7,8) Figure 1 shows a schematic of the vertical-type high-speed twin-roll caster used in this study. This device consists of a pair of rotating water-cooled copper rolls, a movable spring for applying a roll separating force, a nozzle, and a side-dam for creating a molten metal pool. In this casting method, a cast strip is made directly from the molten metal by pouring the molten metal between a pair of rotating copper rolls. The casting speed of this method (30∼150 m/min)1) is faster than those of conventional casting methods, such as direct-chill casting (casting speed 0.09∼0.11 m/min) and horizontal-type twin-roll casting (casting speed 1∼5 m/min).7–11) No release agent is applied to the roll surface in order to exploit the high heat removal capacity of the copper rolls. The hydrostatic pressure of the molten metal pool is also used to maintain good contact between the molten metal and the roll surface and retain the heat removal capacity. One side of the roll is fixed to the device, whereas the other is fixed via a spring and is movable, so the roll follows the shrinkage of the cast strip, creating good contact between the roll surface and the cast strip during casting. Thus, this casting method has a high cooling rate of more than 103°C/s,12–14) allowing it to achieve supersaturation of impurity elements and refinement of secondary particles in the strips.14–17) The cast strips fabricated by this method can be used as a starting material for wrought products, such as cold-rolled sheets. This suggests that the present method can contribute to building the upgrade recycling system, in which low-grade aluminum scraps are used to fabricate high-grade wrought aluminum products.
Schematic of the vertical-type high-speed twin-roll caster used in this study. 1. Crucible, 2. molten metal, 3. nozzle, 4. side-dam, 5. melt head, 6. roll rotation speed, 7. solidification length, 8. copper roll, 9. spring load, 10. strip.
However, periodic shiny and un-shiny patterns containing many surface cracks can be formed on the HSTRC strip surface and have been observed in A356 and AC7A alloys.1,18) The surface cracks reduce the ductility of the cold-rolled sheets produced from the cast strip. To benefit from the advantages of HSTRC and obtain sound cast strips with a good surface, the surface pattern must be suppressed.
A pair of nozzles installed in the upper part of the HSTRC machine are in contact with both the molten metal and the rotating roll surfaces (Fig. 1). The way in which the nozzle tip sits on the roll surface affects the cooling and solidification of the molten alloy, and thus the surface condition and microstructure of the cast strip. We considered two possible reasons for the formation of the surface pattern observed in previous studies. The first was inverse segregation, in which the residual liquid phase is squeezed from the mid-thickness region toward the surface region18) and the second was vibration of the molten metal in the gap at the contact point between the nozzle tip and roll surface.1) In the present study, we focus on the vibration of the molten metal and the effect of the nozzle shape on the periodic surface patterns of Al–3 mass% Si alloy cast strips fabricated by HSTRC.
An Al–3 mass% Si alloy containing 1 mass% Fe, 0.5 mass% Mg, and 0.8 mass% Cu was used in this study. Table 1 shows the chemical composition of this alloy. This alloy composition simulated the secondary alloy produced by mixed scrap of JIS ADC12 (Al–Si–Cu) and A6022 (Al–Si–Mg) alloys, which are commonly used in automobiles. A schematic of the HSTRC used in this study is shown in Fig. 1. The pure copper rolls had a diameter of 300 mm and a width of 100 mm. The roll surface was polished with #120 and #400 waterproof abrasive papers. The pouring temperature of the melt was 650°C. The melt head was kept at 100 mm during casting. The solidification length, which was the contact length between the nozzle tip and the roll gap along the roll surface, was fixed as 100 mm. The initial roll gap was 1 mm. A 20 kN load was applied to one of the rolls by springs. The strip was fabricated at a speed of 40 m/min. About 2.5 kg molten metal was prepared for fabricating strips 3 m long and 100 mm wide. The cast strip was 3.0 mm thick in the constant-thickness region, which was about 2 m long in the strip.
Three types of nozzles were prepared. Figure 2 shows schematics of the (a) normal nozzle, (b) release agent nozzle, and (c) bent tip nozzle. The normal nozzle was a steel plate (thickness, t = 4.5 mm) covered with a heat-insulating ceramic fiber sheet (t = 8 mm). The release agent nozzle was the normal nozzle being sprayed with a release agent (Bunny Height Graphite Spray, Nippon Graphite Industry) to reduce the wettability of the molten metal on the nozzle tip. This makes it difficult for the molten metal to penetrate the gap between the nozzle tip and the roll surface, which suppresses the vibration of the molten metal and reduces the periodic surface pattern. The bent tip nozzle had a nozzle tip that extended along the roll rotation direction and the release agent was sprayed on the heat-insulating ceramic fiber sheet (t = 2 mm). The gap at the contact point between the nozzle tip and the roll surface was small because the nozzle tip extended along the roll-rotating direction and the sheet was thin. This may make it difficult for the molten metal to enter the gap. The bent tip nozzle was expected to suppress the vibration of the molten metal, producing a uniform strip surface without a periodic surface pattern.
Schematics of the nozzles. (a) Normal nozzle, (b) release agent nozzle, and (c) bent tip nozzle.
Microstructural observations were conducted on the constant thickness part of the cast strips. The cast strip surface was observed by scanning electron microscopy (SEM; VE9800, KEYENCE) and the composition analysis was conducted by field emission-scanning electron microscopy (FE-SEM; 7100, JEOL) with energy dispersive spectrometry (EDS). The cross-sectional microstructure of cast strips was observed with an optical microscope (BX51M, Olympus). The samples were mounted in resin and polished with abrasive paper from #120 to #2000. Afterwards, they were polished with diamond pastes with diameters of 6, 3, and 1 µm. A colloidal silica oxide polishing suspension solution was used for the final polishing. The polished samples were etched with Keller’s reagent (H2O: 95 mL; HNO3: 2.5 mL; HCl: 1.5 mL; HF: 1.0 mL; 40 s at room temperature).
Figure 3 shows the cast strip surface fabricated with the (a) normal nozzle, (b) release agent nozzle, and (c) bent tip nozzle. In Fig. 3(a), a periodic surface pattern was observed. This surface pattern was similar to the patterns observed on the surface of the A356 cast strip and the AC7A cast strip fabricated by HSTRC.1,18) The surface patterns consist of shiny regions and un-shiny regions. The shiny regions have a metallic luster, whereas the un-shiny regions appear as white bands. Many cracks were observed at the boundaries between regions. Figure 4 shows the SEM-secondary electron image (SEI) of a (a) shiny region and (b) an un-shiny region on the cast strip surface. The shiny region had a smooth surface, whereas the un-shiny region had a rough surface. When visible light is reflected well by a smooth surface, it is observed as a shiny region, whereas light is diffused by the rough surface of an un-shiny region. Figure 5 shows the cross-sectional images just below the surface in the (a) shiny region and (b) un-shiny region. The secondary particles were coarser in the un-shiny region than in the shiny region, suggesting that there was a difference in the cooling rate during solidification between the shiny and un-shiny regions. The rough surface of the strips in the un-shiny region also suggests that the un-shiny region was formed when there was poor contact between the molten metal and the roll surface. Periodic fluctuations in cooling rate have been reported in the A356 and AC7A alloy cast strips fabricated by HSTRC.1,18) This periodic cooling rate fluctuation is thought to be caused by changes in contact between the molten metal and the roll surface due to the vibration of the molten metal near the nozzle tip. A similar periodic fluctuation in cooling rate was also observed in this study, suggesting that molten metal vibration occurred near the nozzle tip, as in A356 and AC7A alloy cast strips fabricated by HSTRC.
Surface appearance of cast strips fabricated with the (a) normal nozzle, (b) release agent nozzle, and (c) bent tip nozzle.
SEM-SEI of cast strip surface. (a) Shiny region and (b) un-shiny region.
Cross-sectional images of cast strip corresponding to the (a) shiny region and (b) un-shiny region.
Figure 6 shows a schematic of the vibration of the molten metal near the nozzle tip. When the molten metal comes into contact with the roll, a solidification shell is formed. The molten metal enters the gap formed at the contact point between the nozzle tip and the roll surface due to the hydrostatic pressure of the molten metal (Fig. 6(a)). The solidification shell is moved down by the roll rotation when the molten metal is dragged out of the gap (Fig. 6(b)). The vibration of the molten metal occurs at the contact point between the nozzle tip and roll surface. Haga and Suzuki also reported that during the horizontal-type twin-roll casting, the bouncing of the molten metal meniscus occurred at the contact point between the nozzle tip and roll surface.19) Figure 7 shows the results of EDS composition analysis on the cast strip surface fabricated with the (a) normal nozzle, (b) release agent nozzle, and (c) bent tip nozzle. Si was segregated in the un-shiny region in the cast strip fabricated with the normal nozzle (Fig. 7(a)). In the EDS composition analysis for the periodic surface pattern of the A356 cast strip fabricated by HSTRC, Si segregation was also detected in the un-shiny region.1) Si segregation occurred in the present alloy for the same reason as in the A356 alloy. Figure 8 shows a schematic of the mechanism of Si segregation on the strip surface near the nozzle tip. As shown in Fig. 8(a), when the molten metal contacts the roll surface, the solidification shells form on the roll surface. This causes solute segregation in ahead of the solid growth front by according to the normal segregation manner. As shown in Fig. 8(b), (c), when the molten metal momentary enters the gap by the hydrostatic pressure of the molten metal, and solute-enriched part of the molten metal and the other part with no solute segregation contact the roll surface and they solidify. After that as shown in Fig. 8(d), when the solidification shell is dragged out by roll-rotating, the solute-enriched molten metal is also considered to be dragged out. This may result in the formation of periodical Si segregation on the strip surface. Thus, the Si segregation with a rough surface in the un-shiny region observed in this study and in the A356 alloy strips occurred due to the vibration of the molten metal at the contact point between the nozzle tip and roll surface.1)
Schematic showing the vibration of molten metal near the nozzle tip. (a) Molten metal enters the gap. (b) Molten metal is dragged out of the gap.
Composition analysis of the cast strip surface fabricated with the (a) normal nozzle, (b) release agent nozzle, and (c) bent tip nozzle. Red circles indicate the region measured.
Schematic of the mechanism of the Si segregation on the strip surface near the nozzle tip.
Solidification crackings were frequently observed at the boundary between the shiny and un-shiny region. Figure 9(a) shows a cross-sectional image of a crack. Figure 9(b) shows the magnified image of the circled part in (a). Dendrite branches were observed in the interior of the crack,20) which indicated that these cracks were connected shrinkage cavities. The difference in cooling rate between the shiny and un-shiny region caused the crack formation at the boundaries.
(a) SEM-SEI of surface crack on the cast strip and (b) magnified image of the region indicated by the circle in (a).
The periodic surface pattern was slightly suppressed on the surface of the cast strip fabricated with the release agent nozzle (Fig. 3(b)). Si segregation was observed in the un-shiny region, similar to the strip fabricated by using the normal nozzle (Fig. 7(b)). The release agent may decrease the wettability of the molten metal on the nozzle tip, which prevents the molten metal penetrating the gap at the contact point between the nozzle tip and roll surface, suppressing the vibration of the molten metal.1) However, the periodic surface patterns and Si segregation remained. This means that the vibration of the molten metal did not completely disappear.
The bent tip nozzle produced a strip with a surface on which the shiny and un-shiny regions were mixed (Fig. 3(c)) instead of in a periodic pattern (Figs. 3(a) and 3(b)). Si segregation was no longer observed in the un-shiny region (Fig. 7(c)). Because the heat-insulating sheet of the nozzle tip was thin (t = 2 mm) and the nozzle tip was extended along the roll rotation direction, the gap at the contact point between the nozzle tip and roll surface was small. This may prevent the molten metal from entering the gap. Therefore, the vibration of the molten metal was strongly suppressed, and the molten metal solidified almost uniformly on the rotating roll surfaces at a relatively constant cooling rate, and no Si segregation was observed. However, the strip surface was not uniform and shiny, and although there was no periodic pattern, the surface contained a mixture of shiny and un-shiny regions. The vibration of the molten metal was not completely suppressed and surface cracks were still observed in the cast strip. However, Fig. 10 shows the size of the surface crack is smaller than that on the cast strip fabricated by using normal nozzle. In this study, a bent nozzle tip reduced the periodic surface pattern of the cast strip produced by HSTRC substantially. However, further nozzle improvement is needed to produce a cast strip with a perfectly uniform shiny surface.
OM observations of surface crack on the cast strip fabricated by using (a) normal nozzle and (b) bent tip nozzle.
In this study, the effect of nozzle shape on periodic surface patterns of Al–3 mass% Si alloy containing 1 mass% Fe, 0.5 mass% Mg, and 0.8 mass% Cu alloy cast strips fabricated by HSTRC was investigated. Our conclusions are as follows.
This study was supported by New Energy and Industrial Technology Development Organization (NEDO: project number JPNP21003). We would like to express our deep gratitude here.
