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Surface quality and microstructure of Al-Mg alloy strips fabricated by vertical-type high-speed twin-roll casting
Journal of Manufacturing Processes 37 (2019) 332–338
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical paper
Surface quality and microstructure of Al-Mg alloy strips fabricated by vertical-type high-speed twin-roll casting
T
⁎
Daisuke Kikuchia, Yohei Haradab, , Shinji Kumaib a b
Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama Meguro-ku, Tokyo, 152-8552, Japan Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama Meguro-ku, Tokyo, 152-8552, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Vertical-type high-speed twin-roll casting Al-Mg alloy strip Periodic patterns
Al-Mg alloy strips were fabricated by the vertical-type high-speed twin-roll casting. Periodic patterns on the strip surface were observed in the casting direction consisting of the shiny region and un-shiny region with cracks. The un-shiny region and cracks remain even after successive cold rolling. Therefore, formation of the periodic patterns during casting should be suppressed. In this study, the formation mechanism of the periodic patterns of the JIS-AC7A strips was examined by microstructural observation and compositional analysis. The cross-sectional observation in the un-shiny region revealed the inverse segregation of Mg and Fe from the mid-thickness area of the strip toward the surface along the grain boundaries. It is considered that the residual liquid is squeezed out from the mid-thickness area of the strip toward the surface through the grain boundaries, and results in the formation of the un-shiny region. Temperature measurement was performed at the gap formed between the roll surface and the nozzle tip by using thermocouples. The strips were also fabricated by using the nozzles with various tip shapes in order to change the contact conditions between the melt and roll surface. The oscillation of the molten metal meniscus in the gap between the roll surface and the nozzle tip is considered to be one of the possible reasons of the formation of the periodic patterns on the strip surface.
1. Introduction Al-Mg series alloys have good corrosion resistance, formability and weldability. The alloys are applied to body panels of the motor vehicle [1]. However, hot tearing or solidification cracking has been a crucial problem due to the large solidification range in the Al-Mg alloys [2]. Especially in the DC casting, residual Mg-rich liquid inside an ingot or a billet penetrated to the surface, and surface defects called bleed bands are often formed [3]. These surface defects are also anticipated in production of Al-Mg alloy strips with the twin-roll casting. Vertical-type high-speed Twin-roll casting is a cost-effective method to fabricate thin aluminum alloy strips directly from the molten metal. The fabrication speed is faster than other twin-roll castings, such as horizontal-type and melt-drag-type twin-roll casting [4]. This method can omit several hot rolling processes in the conventional strip manufacturing process, realize lower energy consumption and production cost. Although this method is beneficial for producing thin strips economically, the rolling reduction in the cold rolling process is smaller as compared with that of the conventional method. It is anticipated that the surface quality of the as-cast strips directly affect the surface quality and characteristics of the final products.
⁎
The surface quality has been investigated for the Al-Mg alloy strips fabricated by the horizontal-type twin-roll casting and the melt-dragtype twin-roll casting. Haga et al. reported that periodic defects along the casting direction were formed on the bottom surface of the 5182 aluminum alloy strips fabricated by the melt-drag-type twin-roll casting. They mentioned that the possible reason was air gaps formed between the melt and the roll surface at the nozzle tip [5]. Forbord et al. investigated periodic surface segregation on the 5052 aluminum alloy strip produced by the horizontal-type twin-roll casting. Microstructural observation and chemical analysis were conducted for the sample obtained by the interrupted test. Numerical simulation was also performed to investigate the formation of segregation. The periodic surface segregation was considered to result from the reduced contact pressure between the melt and the roll surface near the meniscus at the nozzle tip [6]. Harada et al. reported periodic patterns on the surface of the A356 (Al-Mg-Si) alloy strips fabricated by the vertical-type high-speed twin-roll casting. The periodic patterns were observed in the casting direction and they consisted of the shiny region and un-shiny region. Compositional analysis of the strip surface revealed that Si segregated in the un-shiny region. It was also reported that the cooling rate near the strip surface periodically changes in the casting direction [7].
Corresponding author. E-mail address: [email protected] (Y. Harada).
https://doi.org/10.1016/j.jmapro.2018.12.007 Received 31 August 2018; Accepted 3 December 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 37 (2019) 332–338
D. Kikuchi et al.
kept at about 100 mm during casting. Solidification length, the contact length between a tip of the nozzle and the roll gap along the roll surface, was fixed to 100 mm. Initial roll gap was 1 mm. 11 kN load was applied to one of the rolls by springs in advance before the casting. The strip was fabricated at the speed of 60 m/min. About 2.7 kg molten alloy was prepared for fabricating about a 3 m-long and 100 mm-wide strip. When the melt head was stable at about 100 mm, thickness of the strip was constant at about 2.3 mm. The microstructural analysis was conducted in the range of about 1.5 m-length with the constant thickness of the strip. The cross-section for normal or transverse to the surface along casting direction was observed with an optical microscope (OM) after etched by Keller’s reagent (H2O: 95 mL, HNO3: 2.5 mL, HCl: 1.5 mL, HF: 1.0 mL, 40 s at room temperature) or Weck’s reagent (H2O: 200 mL, NaOH: 2.0 g, KMnO4: 8.0 g, 12 s at room temperature). The strip surface was also observed by using a scanning electron microscope (SEM) and the compositional distribution was analyzed with an electron probe micro analyzer (EPMA).
Table 1 Chemical composition of JIS-AC7A alloy used in this study (mass%). Cu
Si
Fe
Mg
Mn
Ni
Ti
Al
0.01
0.11
0.08
4.9
0.5
0.01
0.02
Bal.
Fig. 1. Schematic diagram of 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. Spring load, 9. Strip.
3. Results and discussion
However, very few studies have investigated the surface quality of the strips fabricated by the vertical-type high-speed twin-roll casting. In this study, we investigated the effect of the casting condition and nozzle tip shape on the surface quality and microstructure of the strips fabricated by this casting method.
Fig. 2 shows a strip surface fabricated by the vertical-type highspeed twin-roll casting. Periodic patterns were observed on the strip surface in the range of the constant thickness. They consist of “shiny region”, the shiny band showing a metallic luster and “un-shiny region”, the white band including many surface cracks. The surface cracks in the un-shiny region remained after the successive cold rolling, because the strips were cold-rolled without the surface grinding. Therefore, the periodic patterns should be suppressed from the cast strip surface. Fig. 3(a) and (b) shows secondary electron images (SEI) of the surface structure in the shiny region and the un-shiny region, respectively. The shiny region was relatively smooth, and the grain boundary of chilled crystals solidified on the roll surface was clearly observed (Fig. 3(a)). On the other hand, the un-shiny region was rough and some protrusions with the surface cracks were observed (Fig. 3(b)). The rough surface in the un-shiny region could be seen as a white band due to diffused reflection of the visible light. Fig. 4 shows compositional distributions of Al, Mg and Fe with a SEM-SEI on the strip surface including both the shiny region and the un-shiny region. Concentrations of Mg and Fe were higher on the uneven surface with some protrusions in the un-shiny region. Fig. 5 shows compositional distributions of Al, Mg and Fe with a compositional image in the cross-section near the
3.1. Surface segregation in the un-shiny region
2. Experimental procedures The Al-Mg alloy used in this study was JIS-AC7A. The chemical composition of the alloy is listed in Table 1. A schematic diagram of the vertical-type high-speed twin-roll caster used in this study is shown in Fig. 1. The diameter and the width of pure copper rolls are 300 mm and 100 mm, respectively. Roll surface was polished with #120 and #400 waterproof abrasive papers in sequence before casting. One roll is firmly fixed to the pedestal, whereas the other roll is attached to the pedestal with springs, and it is possible to apply the load to the strips. The initial roll separating force, namely initial spring load, applied before the casting was controlled by the contraction of spring length. The spring load was used so that a strong contact between the roll surface and the solidification shells can be kept to achieve an excellent heat extraction and high cooling rates. Two steel plate nozzles (thickness, t = 4 mm) and two side-dams covered with heat-insulating ceramic fiber (t = 8 mm) were attached on the roll surfaces and on the roll sides, respectively. The molten metal poured from the top flowed down and formed the melt pool in the space surrounded with nozzles, side-dams, and the rolls. Solidification begins when the molten metal contacts the roll surface. Rapid cooling can be realized by the applied hydrostatic pressure to the solidification shells on the water-cooled rolls. Solidification shells growing from the roll surfaces encountered before they were introduced to the roll gap. The strip widens the gap under the roll separating force. Table 2 describes casting conditions. The melt temperature just before casting was 665 °C. The melt head was Table 2 Casting conditions. Melt temperature
Melt head
Solidification length
665 °C
100 mm
100 mm
Initial roll gap
Roll rotation speed
Initial spring load
1 mm
60 m/min
11 kN
Fig. 2. Strip surface fabricated by vertical-type high-speed twin-roll casting. 333
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present strip surface fabricated by the vertical-type high-speed twin-roll casting is considered to be comparable to this. As the dendrites in solidification shells were growing, Mg or Fe elements were concentrated in the residual liquid along the inter-dendrites or the grain boundaries. The residual liquid was squeezed toward the surface by solidification shrinkage, resulting in the uneven surface with Mg and Fe segregation in the un-shiny region, namely inverse segregation. Fig. 6(a) shows SEM-SEI of the crack formed in the un-shiny region. Magnified image of the framed part in (a) was shown in (b). Some dendrite tips and grain boundaries were observed. If the healing does not occur at the interdendrites or the grain boundaries after the residual liquid was squeezed, the surface cracks were formed eventually. Therefore, the crack formation mechanism is considered to be close to the hot tearing during the conventional DC casting. This is why most of the surface cracks existed in the un-shiny region.
Fig. 3. Secondary electron images (SEI) of surface structure on (a) shiny region and (b) un-shiny region.
surface under the un-shiny region. The columnar grains were observed. The surface cracks propagating along the highly Mg concentrated grain boundaries are also shown. Forbord et al. investigated the periodic surface segregation of AA5052 strips fabricated by horizontal-type twin-roll casting. They reported that Al-Fe-Si particles and protrusions with high Mg content were formed on the strip surface. They considered that the residual liquid located in the inter-dendrites was squeezed toward lower-contact-pressure region between the roll and solidification shell [6]. Formation mechanism of the un-shiny region on the
3.2. Difference in cooling rates between the shiny region and the un-shiny region Fig. 7(a) and (b) shows the cross-section of the strip corresponding to the shiny region and the un-shiny region, respectively. The midthickness area of the strip showed the band structure consisting of
Fig. 4. Compositional distributions of Al, Mg and Fe with SEM-SEI on strip surface including both shiny region and un-shiny region. 334
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Fig. 5. Compositional distributions of Al, Mg and Fe with compositional image in cross-section near surface under un-shiny region.
coarse globular grains with many micro-voids and secondary particles. This region is considered to be formed by trapping the floating α-Al crystals between the solidifying shells growing from the roll surfaces. During casting, separation of the nucleated crystals from the roll surface and fragmentation of dendrite arms of solidification shells growing from the roll surface can be occurred [8]. Afterwards, they can grow into the globular grain under the upward convection produced by highspeed roll rotation and encounter of each solidification shell [9]. The outer region corresponds to the solidification shell that grew on the roll surface. The thickness of solidification shell located under the un-shiny region is thinner than that under the shiny region, as shown by arrows in Fig. 7. This means that the local cooling rate under the un-shiny region was lower than that of the shiny region. This also suggests that periodical change of cooling rate possibly occurred at the formation of
Fig. 6. a) SEM-SEI of crack formed in un-shiny region. (b) Magnified image of framed part in (a).
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Fig. 7. Cross-section of strip corresponding to (a) shiny region and (b) un-shiny region.
Fig. 8. Schematic diagram showing oscillation of molten metal meniscus.
the oscillation of the molten metal meniscus, the nozzle shape is improved to eliminate the gap between the roll surface and the nozzle tip, and the release agent is coated to the nozzle tip so that the molten metal does not enter the gap. By using these nozzles, the periodic patterns on the strip surface are reduced. Even in the Al-Mg alloy, the oscillation of the molten metal meniscus is thought to greatly influence the formation of the periodic patterns. Fig. 8 is a schematic diagram showing the oscillation of the molten metal meniscus. A gap is formed at the contact point between the roll surface and the nozzle tip. During casting, molten metal enters this gap by the hydrostatic pressure. However, it is considered that the molten metal is dragged from the gap by the roll rotation. The meniscus oscillation is thought to occur intermittently as the molten metal exits from the gap. In order to confirm the oscillation of the molten metal
the solidification shell on the rotating copper roll surface. In the part where the cooling rate is low and the columnar grains are formed instead of the equiaxed grains, the inverse segregation occurs in which the residual liquid is squeezed out from the mid-thickness area of the strip through the grain boundaries toward the surface, and this is considered to result in the formation of the un-shiny region.
3.3. Melt motion at the gap between the nozzle tip and the roll surface The periodic patterns consisted of shiny region and un-shiny region are also observed on the surface of the A356 alloy strips fabricated by the vertical-type high-speed twin-roll casting [7]. The patterns are thought to be formed by oscillation of the molten metal meniscus in the gap formed by the roll surface and the nozzle tip. In order to suppress 336
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Fig. 9. Temperature measurement by using thermocouples for periodic oscillation of molten metal meniscus at gap formed by roll surface and nozzle tip.
Fig. 10. Temperature change at thermocouples No. 1 and No. 2.
meniscus, the tip of the thermocouple (No. 1) was set in the gap formed by the roll surface and the nozzle tip as shown in Fig. 9, and the temperature was measured. If the oscillation of the molten metal meniscus occurs, the temperature change corresponding to the oscillation should be detected. For comparison, the temperature in the molten metal pool, which is free from the oscillation was also measured (No. 2). Fig. 10 shows the results of temperature measurement. In both No. 1 and No. 2, temperature decrease of about 5 °C was observed immediately after casting. The important finding in this figure is that the No. 1 thermocouple shows fine temperature fluctuation (about 1 °C amplitude). Such a feature is not observed in the No. 2 thermocouple. The result confirmed the oscillation of the molten metal meniscus in the gap formed between the roll surface and the nozzle tip. If the oscillation of the molten metal meniscus results in the periodic
Fig. 12. Frequencies of periodic patterns on strip surface at each nozzle.
patterns on the strip surface, the feature of patterns should be changed depending on the nozzle tip shape. In order to examine this, three kinds of nozzles as shown in Fig. 11 were prepared. Nozzle A is a conventional-type. A heat insulator with a thickness of 8 mm is wrapped around a steel plate. Nozzle B is a modified one. In this case, a thin (2 mm-thick) heat insulator is wrapped on the steel plate. The gap formed by the roll surface and the nozzle tip is smaller than the Nozzle A. By using this nozzle, the entry and exit of the molten metal to and from the gap easily occurs, and the frequency of the periodic patterns on the strip surface is considered to increase. Nozzle C is the type, which a release agent is coated on the surface of the Nozzle A. The release agent hinders the introduction of the molten metal into the gap. The exit of the molten
Fig. 11. Schematic diagrams of each nozzle tip. 337
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(2) The cross-sectional observation in the un-shiny region found that Mg and Fe segregated along the grain boundaries from the midthickness area of the strip toward the surface. The thickness of the solidified shell corresponding to the un-shiny region of the surface was thinner than that of the shiny region. The results suggest that the local cooling rate of the strip periodically changed. In the part where the cooling rate is low, the inverse segregation occurs in which the residual liquid with high concentration of Mg and Fe is squeezed out from the mid-thickness area of the strip through the grain boundaries toward the surface, and this is considered to result in the formation of the un-shiny region. (3) Temperature was measured at the gap formed between the roll surface and the nozzle tip by using thermocouples. The observed temperature-time curves showed a possibility that oscillation of the molten metal meniscus was occurring. The strips were also fabricated by using nozzles with various tip shapes to examine the change of the periodic patterns. It was cleared that the oscillation of the molten metal meniscus in the gap formed between the roll surface and the nozzle tip has a large influence on the formation of the periodic patterns on the strip surface through the periodic change of the local cooling rate.
Fig. 13. Relationship between roll rotation speed and frequency of periodic patterns of Nozzle A.
metal easily occurs and the frequency of the periodic pattern on the strip surface is considered to increase. Fig. 12 shows the frequencies of the periodic patterns on the strip surface obtained by using different types of nozzles. The frequency f (Hz) is the result of dividing the roll rotation speed (m / s) by the pattern interval (m). The frequencies of Nozzle B and Nozzle C were larger than that of Nozzle A. Fig. 13 shows the relationship between the roll rotation speed and the frequency of the periodic patterns of the Nozzle A. As the roll rotation speed increased, the frequency of the periodic pattern increased. The possible reason is that the molten metal is more likely to exit from the gap formed by the roll surface and the nozzle tip, as the roll rotation speed increases. From these results, it was cleared that the oscillation of the molten metal meniscus in the gap formed between the roll surface and the nozzle tip has a large influence on the formation of the periodic patterns on the strip surface.
References [1] Romhanji E, Popović M, Glišić D, Stefanović M, Milovanović M. On the Al-Mg alloy sheets for automotive application: problems and solutions. Metalurgija-J Metall 2004;10:205–16. [2] Phillion AB, Cockcroft SL, Lee PD. X-ray micro-tomographic observations of hot tear damage in an Al-Mg commercial alloy. Scr Mater 2006;55(5):489–92. [3] Bayat N, Carlberg T. Surface structure formation in direct chill (DC) casting of Al alloys. JOM 2014;66(5):700–10. [4] Haga T, Suzuki S. Study on high-speed twin-roll caster for aluminum alloys. J Mater Process Technol 2003;143-144(1):895–900. [5] Haga T, Nishiyama T, Suzuki S. Strip casting of A5182 alloy using a melt drag twinroll caster. J Mater Process Technol 2003;133:103–7. [6] Forbord B, Andersson B, Ingvaldsen F, Austevik O, Horst JA, Skauvik I. The formation of surface segregates during twin roll casting of aluminum alloys. Mater Sci Eng A 2006;415:12–20. [7] Harada Y, Yamamoto H, Nagano S, Kim MS, Kumai S. Fabrication of high-quality wrought product-grade cast aluminum alloys by high-speed twin-roll casting. J Japan Found Eng Soc 2015;87(11):772–81. [8] Kim MS, Kumai S. Effect of Si content on strip thickness and solidified structure in high-speed twin-roll cast Al-Si alloy strips. Mater Trans 2011;52(5):856–61. [9] Xu M, Zhu M. Numerical simulation of the fluid flow, heat transfer, and solidification during the twin-roll continuous casting of steel and aluminum. Metall Mater Trans B 2016;47(1):740–8.
4. Conclusion The surface quality of the Al-Mg alloy strips fabricated by the vertical-type high-speed twin-roll casting method was examined by the microstructural observation and compositional analysis. (1) Periodic patterns consisting of the shiny region and the un-shiny region with cracks were observed on the strip surface along the casting direction. The surface of the un-shiny region was rough showing some protrusions segregated with Mg and Fe.
338
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical paper
Surface quality and microstructure of Al-Mg alloy strips fabricated by vertical-type high-speed twin-roll casting
T
⁎
Daisuke Kikuchia, Yohei Haradab, , Shinji Kumaib a b
Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama Meguro-ku, Tokyo, 152-8552, Japan Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama Meguro-ku, Tokyo, 152-8552, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Vertical-type high-speed twin-roll casting Al-Mg alloy strip Periodic patterns
Al-Mg alloy strips were fabricated by the vertical-type high-speed twin-roll casting. Periodic patterns on the strip surface were observed in the casting direction consisting of the shiny region and un-shiny region with cracks. The un-shiny region and cracks remain even after successive cold rolling. Therefore, formation of the periodic patterns during casting should be suppressed. In this study, the formation mechanism of the periodic patterns of the JIS-AC7A strips was examined by microstructural observation and compositional analysis. The cross-sectional observation in the un-shiny region revealed the inverse segregation of Mg and Fe from the mid-thickness area of the strip toward the surface along the grain boundaries. It is considered that the residual liquid is squeezed out from the mid-thickness area of the strip toward the surface through the grain boundaries, and results in the formation of the un-shiny region. Temperature measurement was performed at the gap formed between the roll surface and the nozzle tip by using thermocouples. The strips were also fabricated by using the nozzles with various tip shapes in order to change the contact conditions between the melt and roll surface. The oscillation of the molten metal meniscus in the gap between the roll surface and the nozzle tip is considered to be one of the possible reasons of the formation of the periodic patterns on the strip surface.
1. Introduction Al-Mg series alloys have good corrosion resistance, formability and weldability. The alloys are applied to body panels of the motor vehicle [1]. However, hot tearing or solidification cracking has been a crucial problem due to the large solidification range in the Al-Mg alloys [2]. Especially in the DC casting, residual Mg-rich liquid inside an ingot or a billet penetrated to the surface, and surface defects called bleed bands are often formed [3]. These surface defects are also anticipated in production of Al-Mg alloy strips with the twin-roll casting. Vertical-type high-speed Twin-roll casting is a cost-effective method to fabricate thin aluminum alloy strips directly from the molten metal. The fabrication speed is faster than other twin-roll castings, such as horizontal-type and melt-drag-type twin-roll casting [4]. This method can omit several hot rolling processes in the conventional strip manufacturing process, realize lower energy consumption and production cost. Although this method is beneficial for producing thin strips economically, the rolling reduction in the cold rolling process is smaller as compared with that of the conventional method. It is anticipated that the surface quality of the as-cast strips directly affect the surface quality and characteristics of the final products.
⁎
The surface quality has been investigated for the Al-Mg alloy strips fabricated by the horizontal-type twin-roll casting and the melt-dragtype twin-roll casting. Haga et al. reported that periodic defects along the casting direction were formed on the bottom surface of the 5182 aluminum alloy strips fabricated by the melt-drag-type twin-roll casting. They mentioned that the possible reason was air gaps formed between the melt and the roll surface at the nozzle tip [5]. Forbord et al. investigated periodic surface segregation on the 5052 aluminum alloy strip produced by the horizontal-type twin-roll casting. Microstructural observation and chemical analysis were conducted for the sample obtained by the interrupted test. Numerical simulation was also performed to investigate the formation of segregation. The periodic surface segregation was considered to result from the reduced contact pressure between the melt and the roll surface near the meniscus at the nozzle tip [6]. Harada et al. reported periodic patterns on the surface of the A356 (Al-Mg-Si) alloy strips fabricated by the vertical-type high-speed twin-roll casting. The periodic patterns were observed in the casting direction and they consisted of the shiny region and un-shiny region. Compositional analysis of the strip surface revealed that Si segregated in the un-shiny region. It was also reported that the cooling rate near the strip surface periodically changes in the casting direction [7].
Corresponding author. E-mail address: [email protected] (Y. Harada).
https://doi.org/10.1016/j.jmapro.2018.12.007 Received 31 August 2018; Accepted 3 December 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 37 (2019) 332–338
D. Kikuchi et al.
kept at about 100 mm during casting. Solidification length, the contact length between a tip of the nozzle and the roll gap along the roll surface, was fixed to 100 mm. Initial roll gap was 1 mm. 11 kN load was applied to one of the rolls by springs in advance before the casting. The strip was fabricated at the speed of 60 m/min. About 2.7 kg molten alloy was prepared for fabricating about a 3 m-long and 100 mm-wide strip. When the melt head was stable at about 100 mm, thickness of the strip was constant at about 2.3 mm. The microstructural analysis was conducted in the range of about 1.5 m-length with the constant thickness of the strip. The cross-section for normal or transverse to the surface along casting direction was observed with an optical microscope (OM) after etched by Keller’s reagent (H2O: 95 mL, HNO3: 2.5 mL, HCl: 1.5 mL, HF: 1.0 mL, 40 s at room temperature) or Weck’s reagent (H2O: 200 mL, NaOH: 2.0 g, KMnO4: 8.0 g, 12 s at room temperature). The strip surface was also observed by using a scanning electron microscope (SEM) and the compositional distribution was analyzed with an electron probe micro analyzer (EPMA).
Table 1 Chemical composition of JIS-AC7A alloy used in this study (mass%). Cu
Si
Fe
Mg
Mn
Ni
Ti
Al
0.01
0.11
0.08
4.9
0.5
0.01
0.02
Bal.
Fig. 1. Schematic diagram of 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. Spring load, 9. Strip.
3. Results and discussion
However, very few studies have investigated the surface quality of the strips fabricated by the vertical-type high-speed twin-roll casting. In this study, we investigated the effect of the casting condition and nozzle tip shape on the surface quality and microstructure of the strips fabricated by this casting method.
Fig. 2 shows a strip surface fabricated by the vertical-type highspeed twin-roll casting. Periodic patterns were observed on the strip surface in the range of the constant thickness. They consist of “shiny region”, the shiny band showing a metallic luster and “un-shiny region”, the white band including many surface cracks. The surface cracks in the un-shiny region remained after the successive cold rolling, because the strips were cold-rolled without the surface grinding. Therefore, the periodic patterns should be suppressed from the cast strip surface. Fig. 3(a) and (b) shows secondary electron images (SEI) of the surface structure in the shiny region and the un-shiny region, respectively. The shiny region was relatively smooth, and the grain boundary of chilled crystals solidified on the roll surface was clearly observed (Fig. 3(a)). On the other hand, the un-shiny region was rough and some protrusions with the surface cracks were observed (Fig. 3(b)). The rough surface in the un-shiny region could be seen as a white band due to diffused reflection of the visible light. Fig. 4 shows compositional distributions of Al, Mg and Fe with a SEM-SEI on the strip surface including both the shiny region and the un-shiny region. Concentrations of Mg and Fe were higher on the uneven surface with some protrusions in the un-shiny region. Fig. 5 shows compositional distributions of Al, Mg and Fe with a compositional image in the cross-section near the
3.1. Surface segregation in the un-shiny region
2. Experimental procedures The Al-Mg alloy used in this study was JIS-AC7A. The chemical composition of the alloy is listed in Table 1. A schematic diagram of the vertical-type high-speed twin-roll caster used in this study is shown in Fig. 1. The diameter and the width of pure copper rolls are 300 mm and 100 mm, respectively. Roll surface was polished with #120 and #400 waterproof abrasive papers in sequence before casting. One roll is firmly fixed to the pedestal, whereas the other roll is attached to the pedestal with springs, and it is possible to apply the load to the strips. The initial roll separating force, namely initial spring load, applied before the casting was controlled by the contraction of spring length. The spring load was used so that a strong contact between the roll surface and the solidification shells can be kept to achieve an excellent heat extraction and high cooling rates. Two steel plate nozzles (thickness, t = 4 mm) and two side-dams covered with heat-insulating ceramic fiber (t = 8 mm) were attached on the roll surfaces and on the roll sides, respectively. The molten metal poured from the top flowed down and formed the melt pool in the space surrounded with nozzles, side-dams, and the rolls. Solidification begins when the molten metal contacts the roll surface. Rapid cooling can be realized by the applied hydrostatic pressure to the solidification shells on the water-cooled rolls. Solidification shells growing from the roll surfaces encountered before they were introduced to the roll gap. The strip widens the gap under the roll separating force. Table 2 describes casting conditions. The melt temperature just before casting was 665 °C. The melt head was Table 2 Casting conditions. Melt temperature
Melt head
Solidification length
665 °C
100 mm
100 mm
Initial roll gap
Roll rotation speed
Initial spring load
1 mm
60 m/min
11 kN
Fig. 2. Strip surface fabricated by vertical-type high-speed twin-roll casting. 333
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present strip surface fabricated by the vertical-type high-speed twin-roll casting is considered to be comparable to this. As the dendrites in solidification shells were growing, Mg or Fe elements were concentrated in the residual liquid along the inter-dendrites or the grain boundaries. The residual liquid was squeezed toward the surface by solidification shrinkage, resulting in the uneven surface with Mg and Fe segregation in the un-shiny region, namely inverse segregation. Fig. 6(a) shows SEM-SEI of the crack formed in the un-shiny region. Magnified image of the framed part in (a) was shown in (b). Some dendrite tips and grain boundaries were observed. If the healing does not occur at the interdendrites or the grain boundaries after the residual liquid was squeezed, the surface cracks were formed eventually. Therefore, the crack formation mechanism is considered to be close to the hot tearing during the conventional DC casting. This is why most of the surface cracks existed in the un-shiny region.
Fig. 3. Secondary electron images (SEI) of surface structure on (a) shiny region and (b) un-shiny region.
surface under the un-shiny region. The columnar grains were observed. The surface cracks propagating along the highly Mg concentrated grain boundaries are also shown. Forbord et al. investigated the periodic surface segregation of AA5052 strips fabricated by horizontal-type twin-roll casting. They reported that Al-Fe-Si particles and protrusions with high Mg content were formed on the strip surface. They considered that the residual liquid located in the inter-dendrites was squeezed toward lower-contact-pressure region between the roll and solidification shell [6]. Formation mechanism of the un-shiny region on the
3.2. Difference in cooling rates between the shiny region and the un-shiny region Fig. 7(a) and (b) shows the cross-section of the strip corresponding to the shiny region and the un-shiny region, respectively. The midthickness area of the strip showed the band structure consisting of
Fig. 4. Compositional distributions of Al, Mg and Fe with SEM-SEI on strip surface including both shiny region and un-shiny region. 334
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Fig. 5. Compositional distributions of Al, Mg and Fe with compositional image in cross-section near surface under un-shiny region.
coarse globular grains with many micro-voids and secondary particles. This region is considered to be formed by trapping the floating α-Al crystals between the solidifying shells growing from the roll surfaces. During casting, separation of the nucleated crystals from the roll surface and fragmentation of dendrite arms of solidification shells growing from the roll surface can be occurred [8]. Afterwards, they can grow into the globular grain under the upward convection produced by highspeed roll rotation and encounter of each solidification shell [9]. The outer region corresponds to the solidification shell that grew on the roll surface. The thickness of solidification shell located under the un-shiny region is thinner than that under the shiny region, as shown by arrows in Fig. 7. This means that the local cooling rate under the un-shiny region was lower than that of the shiny region. This also suggests that periodical change of cooling rate possibly occurred at the formation of
Fig. 6. a) SEM-SEI of crack formed in un-shiny region. (b) Magnified image of framed part in (a).
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Fig. 7. Cross-section of strip corresponding to (a) shiny region and (b) un-shiny region.
Fig. 8. Schematic diagram showing oscillation of molten metal meniscus.
the oscillation of the molten metal meniscus, the nozzle shape is improved to eliminate the gap between the roll surface and the nozzle tip, and the release agent is coated to the nozzle tip so that the molten metal does not enter the gap. By using these nozzles, the periodic patterns on the strip surface are reduced. Even in the Al-Mg alloy, the oscillation of the molten metal meniscus is thought to greatly influence the formation of the periodic patterns. Fig. 8 is a schematic diagram showing the oscillation of the molten metal meniscus. A gap is formed at the contact point between the roll surface and the nozzle tip. During casting, molten metal enters this gap by the hydrostatic pressure. However, it is considered that the molten metal is dragged from the gap by the roll rotation. The meniscus oscillation is thought to occur intermittently as the molten metal exits from the gap. In order to confirm the oscillation of the molten metal
the solidification shell on the rotating copper roll surface. In the part where the cooling rate is low and the columnar grains are formed instead of the equiaxed grains, the inverse segregation occurs in which the residual liquid is squeezed out from the mid-thickness area of the strip through the grain boundaries toward the surface, and this is considered to result in the formation of the un-shiny region.
3.3. Melt motion at the gap between the nozzle tip and the roll surface The periodic patterns consisted of shiny region and un-shiny region are also observed on the surface of the A356 alloy strips fabricated by the vertical-type high-speed twin-roll casting [7]. The patterns are thought to be formed by oscillation of the molten metal meniscus in the gap formed by the roll surface and the nozzle tip. In order to suppress 336
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Fig. 9. Temperature measurement by using thermocouples for periodic oscillation of molten metal meniscus at gap formed by roll surface and nozzle tip.
Fig. 10. Temperature change at thermocouples No. 1 and No. 2.
meniscus, the tip of the thermocouple (No. 1) was set in the gap formed by the roll surface and the nozzle tip as shown in Fig. 9, and the temperature was measured. If the oscillation of the molten metal meniscus occurs, the temperature change corresponding to the oscillation should be detected. For comparison, the temperature in the molten metal pool, which is free from the oscillation was also measured (No. 2). Fig. 10 shows the results of temperature measurement. In both No. 1 and No. 2, temperature decrease of about 5 °C was observed immediately after casting. The important finding in this figure is that the No. 1 thermocouple shows fine temperature fluctuation (about 1 °C amplitude). Such a feature is not observed in the No. 2 thermocouple. The result confirmed the oscillation of the molten metal meniscus in the gap formed between the roll surface and the nozzle tip. If the oscillation of the molten metal meniscus results in the periodic
Fig. 12. Frequencies of periodic patterns on strip surface at each nozzle.
patterns on the strip surface, the feature of patterns should be changed depending on the nozzle tip shape. In order to examine this, three kinds of nozzles as shown in Fig. 11 were prepared. Nozzle A is a conventional-type. A heat insulator with a thickness of 8 mm is wrapped around a steel plate. Nozzle B is a modified one. In this case, a thin (2 mm-thick) heat insulator is wrapped on the steel plate. The gap formed by the roll surface and the nozzle tip is smaller than the Nozzle A. By using this nozzle, the entry and exit of the molten metal to and from the gap easily occurs, and the frequency of the periodic patterns on the strip surface is considered to increase. Nozzle C is the type, which a release agent is coated on the surface of the Nozzle A. The release agent hinders the introduction of the molten metal into the gap. The exit of the molten
Fig. 11. Schematic diagrams of each nozzle tip. 337
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(2) The cross-sectional observation in the un-shiny region found that Mg and Fe segregated along the grain boundaries from the midthickness area of the strip toward the surface. The thickness of the solidified shell corresponding to the un-shiny region of the surface was thinner than that of the shiny region. The results suggest that the local cooling rate of the strip periodically changed. In the part where the cooling rate is low, the inverse segregation occurs in which the residual liquid with high concentration of Mg and Fe is squeezed out from the mid-thickness area of the strip through the grain boundaries toward the surface, and this is considered to result in the formation of the un-shiny region. (3) Temperature was measured at the gap formed between the roll surface and the nozzle tip by using thermocouples. The observed temperature-time curves showed a possibility that oscillation of the molten metal meniscus was occurring. The strips were also fabricated by using nozzles with various tip shapes to examine the change of the periodic patterns. It was cleared that the oscillation of the molten metal meniscus in the gap formed between the roll surface and the nozzle tip has a large influence on the formation of the periodic patterns on the strip surface through the periodic change of the local cooling rate.
Fig. 13. Relationship between roll rotation speed and frequency of periodic patterns of Nozzle A.
metal easily occurs and the frequency of the periodic pattern on the strip surface is considered to increase. Fig. 12 shows the frequencies of the periodic patterns on the strip surface obtained by using different types of nozzles. The frequency f (Hz) is the result of dividing the roll rotation speed (m / s) by the pattern interval (m). The frequencies of Nozzle B and Nozzle C were larger than that of Nozzle A. Fig. 13 shows the relationship between the roll rotation speed and the frequency of the periodic patterns of the Nozzle A. As the roll rotation speed increased, the frequency of the periodic pattern increased. The possible reason is that the molten metal is more likely to exit from the gap formed by the roll surface and the nozzle tip, as the roll rotation speed increases. From these results, it was cleared that the oscillation of the molten metal meniscus in the gap formed between the roll surface and the nozzle tip has a large influence on the formation of the periodic patterns on the strip surface.
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4. Conclusion The surface quality of the Al-Mg alloy strips fabricated by the vertical-type high-speed twin-roll casting method was examined by the microstructural observation and compositional analysis. (1) Periodic patterns consisting of the shiny region and the un-shiny region with cracks were observed on the strip surface along the casting direction. The surface of the un-shiny region was rough showing some protrusions segregated with Mg and Fe.
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