Solidification Structure and Casting Defects in High-Speed Twin-Roll CastAl­2mass% Si Alloy Strip

Min-Seok Kim+ and Shinji Kumai

Department of Materials Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan

Al­2mass% Si alloy, which has wide freezing temperature range, was cast using a laboratory-scale vertical-type high-speed twin-rollcaster. The solidification structure and several kinds of casting defects were examined using OM, SEM and SEM-EDS. The results showed thatthe unstable melt pool condition could cause the surface defects such as the ripple mark, “un-shiny” zone and the inverse segregation. In thepresent nozzle type, constructing a stable high melt pool level was seemed to be essential to obtain a sound strip with no defect on the stripsurface. A large-scale internal cracking along the casting direction was also observed in the high-speed twin-roll casting of aluminum alloy. Thepresent results revealed that the cracking was related to distribution of the residual liquid in the central band region, which could be controlled bythe roll separating force near the roll nip. It is considered that the roll separating force caused the shear localization in the central band region andpromoted a formation of continuous liquid film in the shear localized region. The liquid film was a cause of the internal cracking when the strippassed through the roll nip. [doi:10.2320/matertrans.L-M2013824]

(Received December 19, 2012; Accepted June 20, 2013; Published September 6, 2013)

Keywords: twin-roll casting, aluminum­silicon alloy, surface defect, ripple mark, inverse segregation, internal cracking, grain structure, shearlocalization

1. Introduction

Ever since a twin-roll continuous caster was firstly devisedby Bessemer in the middle of nineteenth century,1) manyresearchers and manufacturers have tried to apply the casterto the thin strip production because it was a quite attractiveprocess through which the production steps could be greatlyreduced.2,3) Generally, twin-roll casters can be roughlydivided into three types depending on the casting rollarrangements: vertical, horizontal and inclined casters.Several melt feeding systems are devised to achieve stablecasting conditions.4)

In steel industry, since the first commercial applicationswere developed in the middle of twentieth century2)

considerable progress has been attained in developing atwin-roll strip casting process with high casting speed.5,6) Inaluminum industry, the twin-roll casting operation startedaround the same time by Hunter and Lauener.2) However,until recently, the twin-roll casting for aluminum sheetproduction was limited mostly to horizontal-type.4,7) Thehorizontal-type casters are generally operated at very lowcasting speed in comparison with the vertical-type caster. Inaddition, the process is available only for alloys with narrowfreezing temperature range.7)

About a decade ago, Haga et al. developed a vertical-type caster in aluminum sheet fabrication with remarkablyincreased casting speed.8,9) They used pure copper rolls withno lubricant on the roll surface to remove heat effectively.A feeding nozzle was installed over the rolls to introduce alarge hydrostatic pressure. Various kinds of aluminum alloystrips such as Al­Mg­Si based,10­13) and Al­Si alloys14) werefabricated by using the caster. The cast strip with refinedsolidification structure due to the high cooling rate improvedtoughness and formability.15­18)

However, a severe problem arose in the high-speed twin-roll cast alloy with a wide freezing temperature range.11,12,14)

Large-scale cracking occurred in the mid-thickness regionduring the strip casting. Fractography of the crack surfacerevealed that a shrinkage cavity was the primary reason forthe cracking.11,14) This suggested that distribution of residualliquid in the mid-thickness region when the strip goesthrough the roll nip was a dominant factor controlling theinternal cracking.

In order to investigate the influence of the freezingtemperature range on the internal cracking, the thicknessand microstructure of Al­Mg­Si based 6022 alloy strip witha wide freezing temperature range was compared to thoseof the pure aluminum strip with no freezing temperaturerange.11) The thickness of the 6022 strip was much largerthan that of pure aluminum strip. Moreover, a thick centralband consisting of fine globular grains was formed onlyin the 6022 alloy strip, and a large internal cracking wasobserved in the central band region. These differences wererelated to change in solidification manner: cellular growthwith planar growth-front in pure Al and dendritic growth withmushy-type growth-front in the 6022 alloy. The formation ofrelatively thick solidifying shell in the mushy-type growthcontributes to the large strip thickness. It was also thoughtthat twiggy dendrite branches in the mushy layer of thegrowth-front could be fragmented when two solidifyingshells encountered near the roll nip. As a result, thefragmented grains and some floating grains in the melt wereconcentrated in the mid-thickness region of the cast strip andformed a band structure. Consequently, solidification of theremained liquid in the central band region immediately afterthe strip went through the roll nip was considered to be amain reason of the internal cracking.

More precise investigation was made to study the influenceof the freezing temperature range on the strip thickness andthe internal cracking using Al­Si binary alloys.14) For thispurpose, hypo-eutectic binary Al­Si alloys with various Sicontents were cast under the same casting conditions. Thestrip thickness changed distinctly with changing the Sicompositions, and the largest strip thickness was obtained in+Graduate Student, Tokyo Institute of Technology

Materials Transactions, Vol. 54, No. 10 (2013) pp. 1930 to 1937©2013 The Japan Institute of Light Metals

Al­2mass% Si strip. Microstructure observation found thatwhole strips exhibited band structures, which were macro-scopically distinguishable from the outer shells in the mid-thickness region of the cast strip. The internal cracking wasobserved in the strip with 1­2mass% Si compositions. Thiscomposition range (1­2mass% Si) roughly corresponded tothe composition of the widest freezing temperature range inequilibrium Al­Si phase diagram. In these strips, the thickcentral band consisted of fine globular grains and very smallamount of Al­Si eutectic phase at the inter-grain region. Theamount of the eutectic phase corresponds to that of residualliquid when the strip went through the roll nip. These resultssuggested that not only the formation of the central band, butalso the distribution and the amount of residual liquid in thecentral band region should be paid attention to reveal thecause of internal cracking.

In the present study, to investigate the formation manner ofstrip with a wide freezing temperature range more precisely,Al­2mass% Si alloy was cast using the high-speed twin-rollcaster. An extensive investigation was conducted on theoverall strip to examine not only the internal cracking, butalso the formation of various casting defects that could beinfluenced by the mushy-type solidification. Based on thechange in strip thickness along the casting direction, theirformation mechanisms are discussed from a view point ofmelt pool condition in the nozzle that can affect the contactcondition between the solidifying shells and the roll surface.Since internal cracking always occurs in the central bandregion, the relationship between the roll separating force andthe central band formation is also studied, and the internalcracking mechanism is proposed.

2. Experimental Procedures

2.1 Alloy preparationAl­2mass% Si alloys were produced by mixing the ingots

of commercially pure aluminum (99.88mass%) and Al­25mass% Si alloy. The chemical composition of the ingotswas displayed in Table 1. The ingots were melted in thealumina-coated carbon crucible in the electric furnace, andtotal weight of molten metal was around 2.5 kg. The moltenmetal was degassed by Ar gas bubbling for 20min (gas flux³0.3 L/min).

2.2 High-speed twin-roll castingFigure 1 shows a schematic illustration of the laboratory-

scale vertical-type twin-roll caster used in the present study.In the caster, a large feeding nozzle, which is not available forthe conventional vertical-type caster, was used to maintain ahigh melt pool level during the strip casting. The feedingnozzle consisting of steel plates covered with a heat-insulating material was mounted on the rolls so that thebottom-end of the nozzle contacted directly with the rollsurface. A pair of side dams made of steel plates covered withreplaceable heat insulating sheets was set on the side-edge ofthe nozzle and the casting roll. The caster was equipped witha pair of pure copper rolls (made of tough-pitch copper) with300mm in diameter and 100mm in width. No lubricant wasapplied on the roll surface. The casting rolls were internallycooled with running water during the casting. One of the rolls

was loosely fixed by being supported by a series of springsto control the roll separating force, while the other one wasfixed firmly. The roll separating force was controlled by usingsix springs connecting in parallel. Each spring had a freelength of 150mm. The initial roll separating force wasobtained by setting their length to the pre-determined values.For example, for a single spring, the spring constant is0.47 kN/mm. The initial roll separating force of 0.47 kN wasobtained by spring length reduction of 1mm from the initiallength. Since six springs supported the roll, total force was2.82 kN. The value obtained in this way was regarded asinitial roll separating force. During strip casting, the pre-setforce was applied on the strip near the roll nip by enlargingthe roll gap. In the present study, the initial roll separatingforce was controlled in a range from 3 to 60 kN.

The molten metal of 2.5 kg in the crucible was poured intothe nozzle from the fixed roll side toward the movable rollside when the superheat of the melt reached approximately5°C. The pouring was conducted manually as fast as possible,but very carefully so that the maximum height of the meltpool in the nozzle was kept at 100mm. Roll rotating speedwas 60m/min. The initial roll gap was 1mm. This procedureallowed to obtain a 3­4m long and 100mm wide strip.

2.3 Microstructure observationsThe strip surface was observed by using a scanning

electron microscope (SEM). The solidification structureobservation was conducted on the longitudinal cross sectionof the mid-width of the cast strip. For cross-sectionalmicrostructure observation, the longitudinally sectionedsamples were mounted in an epoxy resin and polished. Thepolished cross-section was etched with 2% HF and distilled

Table 1 Chemical compositions (mass%) of Al­25mass% Si and com-mercially pure aluminum ingots.

Si Fe Cu Mn Ti Al

Al­Si 25.3 0.15 <0.01 bal.

Pure Al 0.04 0.1 <0.01 <0.01 0.01 99.88

Melt

Crucible

Feeding nozzle

A series of springPure Cu rolls

(diameter : 300 mm)

Side dam

Strip

Melt pool height in

feeding nozzle~ 100 mm

Fig. 1 A schematic illustration of the vertical-type high-speed twin-rollcaster.

Solidification Structure and Casting Defects in HSTRC Al­2mass% Si Alloy Strip 1931

H2O solution, and observed using an optical microscope(OM). As-polished cross-sections were also anodized at40V voltages in a 3.3% HBF4 and distilled H2O solution toreveal the grain structure. Compositional change by inversesegregation at the surface region of the cast strip wasexamined by using a scanning electron microscopy withenergy-dispersive X-ray spectroscopy (SEM-EDS).

3. Results

3.1 Characterization of the cast strip3.1.1 Change in strip thickness along the casting

directionA strip about 3m long with 100mm in width was

fabricated under the initial roll separating force of 20 kN.Figure 2 shows the change in strip thickness along the striplength. The x-axis of the figure indicates the length from thestrip head. The origin of the x-axis corresponds to the leadingedge of the strip. The thickness of the cast strip was measuredfor two positions: mid-central line of the strip and 20mmapart from the central line. No thickness difference wasobserved in the width direction. The thickness change alongthe strip length can be divided into three zones. In the zone Iin Fig. 2, the strip thickness increased continuously. This isthe stage when the molten metal poured from the crucible isfilling the nozzle and a thin solid skin starts growing on theroll surfaces. The zone I is followed by zone II in Fig. 2,where the strip thickness was almost constant. The constantthickness in the zone II is attributed to the constant meltheight level in the nozzle. Gradually running out of the melt,the melt pool level in the nozzle decreases and thus the stripthickness decreases continuously. This stage corresponds tothe zone III in Fig. 2.3.1.2 Macro- and microscopic appearance of the strip

surfaceFigure 3 shows the appearance of strip surface of each

zone shown in Fig. 2. At the initial part (zone I in Fig. 2),where the strip thickness gradually increases, discontinuous“un-shiny” regions were observed on the strip surface, asindicated by arrows in the Fig. 3(a). In addition, periodicaloscillation patterns, which are formed normal to the castingdirection, were observed on the surface. In the zone II,where the strip thickness is constant, the surface is flat andshiny and un-shiny region and oscillation patterns were not

observed, as shown in Fig. 3(b). In the zone III, where thestrip thickness continuously decreases, two distinct featuresare exhibited. In the zone III-(C), the periodical ripple markswere clearly observed on the strip surface (Fig. 3(c)). Sucha ripple mark is frequently observed on the strip surfaceproduced by the horizontal-type twin-roll casting.19) In thezone III-(D), no ripple marks were observed, but the un-shinyregion appeared again on the strip surface, as indicated byarrows in Fig. 3(d).

Figure 4 shows the SEM-image of the strip surface. Finedendrite cells were clearly observed on the shiny stripsurface. These polygonal dendrite cells are probably thechilled crystals on the roll surface. As shown in the Fig. 3,the strip surface in the zone I and III consists of two regions:relatively shiny, smooth region and un-shiny region.Figures 4(c) and 4(d) display the surface morphology ofthe relatively shiny and smooth region. In this region, a largeamount of fine eutectic Si particles and plate-like Al­Fe­Sibased intermetallic compound particles are dispersed on thesurface. Figures 4(e) and 4(f ) exhibit the microstructure ofthe “un-shiny region”, as indicated by the arrows in theFig. 3. In this region, the strip surface is not flat, but, asshown in the Fig. 4(f ), the surface is covered with manyclumps. In addition, cracks were observed in this region,as shown in Fig. 5. The micrograph with high magnificationin Fig. 5(b) reveals that the surface of the crack is a smoothdendrite cell surface.3.1.3 Grain structure

Figure 6 shows the grain structure for each part of the caststrip shown in Fig. 2. It is noted that the left-hand side of

00

1

2

3

4

5

Center

20 mm from center

Strip length, l / m

Stri

p th

ickn

ess,

t / m

m

(A) (B) (C) (D)

Leading edge of strip

1 2 3

Fig. 2 Change in the strip thickness along the strip length. (The result wasdivided into three zone, I, II and III depending on the macroscopic trendof the thickness change).

(a) (b)

(c) (d)

Fig. 3 Appearance of the strip surface: (a) zone I, (b) zone II, (c) zone III-(C) and (d) zone III-(D) in Fig. 2.

M.-S. Kim and S. Kumai1932

each picture corresponds to the strip surface of the movableroll side in Fig. 1. The microstructure of the very initial part(Fig. 6(a)) is characterized by columnar grains grown fromboth roll surfaces, and equiaxed grains in the mid-thicknessregion. In the zone I-(B), in which the strip thicknessincreases, well-developed columnar grains are observed inthe inner region of the movable roll side (Fig. 6(b)). Incontrast, equiaxed grains are observed in the fixed roll side(Fig. 6(b)). In the zone II, with a constant strip thickness, themicrostructure exhibits small columnar grains in chill zone atthe strip surface, equiaxed grains in the sub-surface regionand a large amount of fine globular grains in the mid-thickness region (Fig. 6(c)). The globular grains form a bandstructure which is clearly distinguishable from the outershells consisting of the fine columnar and equiaxed grains.In the final part of the strip (zone III), the microstructure issimilar to that of the zone II, but there is no fine columnargrains in the surface region, i.e., no chill zone formed at thestrip surface (Fig. 6(d)).

A large-scale internal cracking formed continuously isobserved in the mid-thickness region, as indicated by anarrow in the Fig. 6(c). The cracking always appears in thecentral band region consisting of fine globular grains, mostlyat a boundary between the central band and the outer shell.3.1.4 Central band thickness and internal cracking

A change of the strip thickness and the central bandthickness are shown in Fig. 7. The central band thickness wasobtained as follows. An area of the central band region wasmeasured for the pre-determined length in the longitudinalcross section, and the area was divided by the length to obtainthe band thickness. Six parts were measured to obtain anaveraged value. In the constant strip thickness zone, thecentral band thickness showed nearly constant value, about400­500 µm. In the final part of the strip, the central bandbecomes large, about 700 µm in thickness, which is abouthalf of the strip thickness.

A zone, in which internal cracking is observed, is alsoindicated in Fig. 7. The area roughly corresponds to the zoneII. In contrast, no crack was observed at the final part of thestrip.

( a )

( c )

( b )

( e )

( d )

( f )

20 µm

20 µm

20 µm

( )

( )

5 µm

5 µm

5 µm

Fig. 4 SEM-image of strip surface. Microstructure of (a), (b) the constantthickness zone (zone II in Fig. 2), (c), (d) relatively shiny and smoothsurface region in zone I and III, (e), (f ) the “un-shiny region” in zone Iand III in Fig. 2; (i) and (ii) indicate Si and Al­Si­Fe intermetalliccompound particles, respectively.

( b )

( a )

Crack

5 µm

50 µm

Fig. 5 Microstructure of the “un-shiny” region revealing the “hot tearing”:(a) low-magnification, (b) magnified microstructure of the “hot tearing”region in (a).

1 mmInternal cracking

(a) (b)

(c) (d)

Fig. 6 Grain structure: (a) zone I-(A), (b) zone I-(B), (c) zone II and (d)zone III in Fig. 2; Note that the left-hand side of each picture correspondsto the strip surface fabricated at the movable roll side in Fig. 1.

Solidification Structure and Casting Defects in HSTRC Al­2mass% Si Alloy Strip 1933

3.1.5 Inverse segregationFigure 8 shows OM-image of the cross-section near the

surface and the results of SEM-EDS compositional mappingfor Al and Si elements in a part of these areas. For the stripsurface in zone II, no segregation was detected. On the otherhand, for the strip surface in zone I and III, a thin layer rich inSi was detected by EDS composition analysis.

3.2 Influence of the roll separating forceIn order to examine the effect of roll separating force on

the internal cracking, Al­2mass% Si alloy strips were alsocast under two different initial roll separating forces of 3 and60 kN, which are smaller and higher than 20 kN, respectively.Figure 9 shows the comparison of the strip thickness and thecentral band thickness under the two conditions. In the zoneII showing constant strip thickness, about 0.5mm thicknessreduction took place by the increase of the roll separatingforce from 3 to 60 kN, while no prominent change inthickness was observed in zone I and III. It is shown that theamount of strip thickness reduction corresponds to that of thecentral band region. Figure 10 shows the microstructuralchange of the central regions of the cast strips corresponding

to the increase of the initial roll separating force from 3 to60 kN. For 3 kN, the thick central band consists of fineglobular grains, as shown in the Fig. 10(a). A large volumeof porosity was also observed. In this case, the large-scale

0 1 2 30

1

2

3

4

5

Strip length, l / m

Thi

ckne

ss, d

/ m

m Strip thickness Central band thickness

Internal cracking zone

Fig. 7 Change in the strip thickness and the central band thickness alongthe casting direction; Initial roll separating force: 20 kN.

Zone Zone and

OMimage

ED

S c

ompo

sitio

n m

ap

Al

Si

50 µm 50 µm

Fig. 8 OM-image in the surface region and the result of EDS compositionmapping for Al and Si elements; I, II and III indicate each zone in Fig. 2.

0 1 20

0.5

1

1.5

Strip length, l / m

Cen

tral

ban

dth

ickn

ess,

t / m

m 3 kN 60 kN

0 1 2 30

1

2

3

4

5

Strip length, l / m

Stri

p th

ickn

ess,

t / m

m

3 kN 60 kN

Fig. 9 Changes in the strip thickness and the central band thickness alongthe casting direction for two initial roll separating force of 3 and 60 kN.

: Central band region 250 µm

(a)

(b)

Fig. 10 Microstructure of the mid-thickness region of the cast strips forinitial roll separating force of (a) 3 and (b) 60 kN.

M.-S. Kim and S. Kumai1934

internal cracking, which appeared under the condition of20 kN, did not occur. On the other hand, in the case of 60 kN,a semi-continuous thin band structure was observed in themid-thickness region, as shown in Fig. 10(b). No internalcracking was observed in this case, either.

4. Discussion

4.1 The effect of melt pool level on the casting defectsand the solidification structure

4.1.1 The role of the melt pool level in strip fabricationThe melt feeding system, which controls the fluid flow

of the molten metal, is very critical for twin-roll castingoperation because it affects the strip quality: such as surfacedefects, strip thickness deviation and solidification structure,etc.4,20­24) The stabilized melt supply is closely related toa stable and uniform heat transfer condition between thegrowing solid shell and the moving substrate (i.e., the rotatingroll surface). In order to achieve a stable feeding condition,several trials have been made so far. Among them, modifyingthe casting rolls arrangement (roughly divided into threetypes: vertical, horizontal and inclined-type) and optimizationof the design and the position of the feeding nozzle4) werethe most important trials. Hwang et al. performed bothexperimental and simulated works21) using an inclined twin-roll caster. They reported that the combined effect of meltpool level and nozzle position brought about the change inrecirculation pattern of the melt, which directly affect fluc-tuation of the meniscus where solid shell, melt and air contact.A high melt pool level resulted in a relatively stable flowcondition, yet, a violent fluctuation at the meniscus causedthe formation of dross-trapped and wrinkled strip surface aswell as strip thickness deviation. The computational resultsby Bae et al.22) indicated that the flow pattern affects thecooling rates of the solidifying shells on the roll surface. Theyalso mentioned that a high level of the melt pool is desiredfor the stable growth of shells on the rotating roll surface.

Keeping a stable contact condition between the solidifyingshell and the roll surface is very important to fabricate thesound strip with no surface defect and thickness deviation.In order to achieve this by the present high-speed twin-rollcasting method, a hydrostatic pressure is applied on thesolidifying shell traveling on the roll surface using a melthead in the nozzle. The hydrostatic pressure is indispensableto compensate for the thermal contraction and shrinkage ofthe solidifying shell, which can lead to air gap formationbetween the solidifying shell and the roll surface. In thepresent study, the melt feeding to the nozzle was conductedmanually by tilting the crucible. Therefore, precise controlof the melt flow is difficult in the beginning of feeding.However, once the feeding nozzle is filled with the melt andthe high melt pool level is attained, the turbulent flowinduced by the successive pouring does not affect thesolidification condition on the roll surface. The largehydrostatic pressure induced by high melt pool level in thenozzle also improves the contact condition between thesolidifying shell and the roll surface. The formation of well-developed chill zone (Fig. 8) reflects the improved contactcondition at the initial stage of the solidification where thesolid shell began to form on the roll surface. The good

surface quality and constant strip thickness in the zone II inFig. 3 are considered to result from this effect.

In the present study, a single shot of the melt from thecrucible filled the nozzle and formed a single strip. Therefore,the change in melt pool height directly affected the stripthickness. In Fig. 2(a), the strip thickness increases withincrease of the melt pool level at the initial stage, and thenreaches the relatively constant thickness under the constantmelt pool level. At the final stage, the thickness decreases asthe melt pool level decreases. The change in the melt poollevel causes the change in hydrostatic pressure, and affectsheat transfer between the solidifying shell and the rollsurface. Therefore, these suggest that constructing high meltpool level using the present nozzle type is effective toimprove the contact condition, which can control a formationof strip thickness, solidification structure and surface defects.4.1.2 The formation of the ripple mark and the

“un-shiny region” on the strip surfaceThe surface defect is one of the most troublesome

problems in the twin-roll casting because the strip surfaceis not removed by machining and is supplied to successivehot rolling or cold rolling. For the surface of the strip, whichwas fabricated under the relatively low melt pool levelcondition, two kinds of macroscopic defects were observed:ripple mark and “un-shiny” region. The ripple mark, shownin the zone III-(C) in Fig. 3, is a typical defect in horizontal-type twin-roll casting process.19) This can be formed whenunstable solidification takes place in the section where a thinsolid shell starts to grow on the roll surface. At this stage, ifthe hydrostatic pressure is scarce around the thin shell, thedetachment of the shell from the roll surface can take placedue to the force of the solidification shrinkage. This allowsthe following melt to permeate to the gap created, resultingin an overlaid structure. This step occurs continuously, andconsequently ripple marks are formed periodically on the caststrip (zone III-(C) in Fig. 3).

The “un-shiny” region, observed in Fig. 3, also seems tobe related to poor contact condition. A large number ofclumps result in uneven surface condition in the “un-shiny”region, as shown in Figs. 4(e) and 4(f ). Since the distancebetween the clumps roughly corresponds to the dendritearm spacing in the sub-surface region of zone I and III,as displayed in the upper-right side in Fig. 8, the clumpformation is considered to result from inter-dendritic leakageof liquid during the strip casting. Furthermore, the well-projected clumps on the surface indicate poor contactcondition of this region until the strip passed through theroll nip. Therefore, due to locally degraded heat extractioncondition in this region, the liquid at the dendrite cellboundary is more likely to be preserved, even near the surfaceregion. As a result, surface cracking induced by the “hottearing” can occur in the un-shiny region, as shown in Fig. 5.4.1.3 Formation of inverse segregation

Inverse segregation is a typical defect which deterioratesthe surface quality in horizontal-type twin-roll cast aluminumalloy strips.25­27) A similar defect was also observed in thevertical-type high-speed twin-roll cast strip in the presentstudy (Figs. 4(c)­4(f ) and 8). It has been reported that theinverse segregation is caused mainly by the solidificationshrinkage, which promotes the liquid flow in the opposite

Solidification Structure and Casting Defects in HSTRC Al­2mass% Si Alloy Strip 1935

direction to the shell growth direction.28) A liquid flow that isdriven by the pressure drop due to the air gap formation onthe casting surface is also an important phenomenon of theinverse segregation.29) Forbord et al.27) used AlStrip-modelto reproduce the condition for the inverse segregation inhorizontal-type twin-roll casting. In their model, it wasshown that the low pressure zone was constructed near thesurface region of solidifying shell, and the zone correspondedto the area of the inverse segregation. In the present casting,in the same context, due to the solidification shrinkage andthe formation of the large air gap (poor contact condition),the liquid at the inter-dendritic region can be exuded intothe surface region. Consequently, an undesired layer with ahigh solute content appeared on the strip surface.4.1.4 Formation of grain structure

Change in solidification structure along the castingdirection (Fig. 6) indicates that the structure formation isalso affected by the melt pool level. Basically, roll temper-ature increase is expected to some extent in one revolutionof the roll. Actually, we produced a 15m long strip usingthe present caster without thickness change along the castingdirection in the 12m long stage II region. Therefore, thetemperature increased after one revolution is considered to bemaintained somewhat constant even though it can show aslight increase if the casting proceeds for much longer time.The diameter of the roll in the present study was 300mm, andso circumferential length was about 1m. Therefore, the initialpart of the strip, at least 1m long, should be regarded as theunstable part of the strip. However, this unstable initial partprovided us important information concerning the solid-ification manner of the cast strip. Therefore, we focused onthis stage.

In the very initial part of the strip (Fig. 6(a)), a steepthermal gradient, originating from the low roll surfacetemperature, promotes the growth of the columnar grainson the roll surface. As the casting proceeds, this zone isfollowed by the transition zone, in which columnar andequiaxed grains co-exist, as shown in Fig. 6(b). Since thepart in the zone I-(B) was fabricated during the stage whenthe poured melt did not fill the nozzle completely yet, theresultant solidification structure is related to the unstable meltpool condition. In general solidification, it is known thatsolidification structure is mainly determined by the ratio ofthermal gradient, G and growth rate, R, (G/R).30) In thepresent casting, it is possible that the unstable contactcondition between the growing shell and the roll surfacefluctuates G/R value, and the fluctuation triggers a structuretransition from columnar to equiaxed grain.31) There areseveral possibilities to produce the unstable contact con-dition. The solidification of the transition zone takes placebefore constructing the stable melt pool level in the nozzle.As the strip thickness increases continuously, an overallcontraction of the shell can lead to the large air gapformation. Another possibility is related to the turbulentflow induced by the melt pouring. In the transition zone, localpressure change, induced by the turbulent fluid flow, can begenerated. As a result, the contact condition can be fluctuatedlocally. Since the melt pouring is conducted from the fixedroll side toward the movable roll side in the present study, theturbulence of the melt flow seemed to affect the solidifying

condition on the movable roll side. At the constant thicknesszone (Fig. 6(c)), the constant melt pool level in the nozzlefacilitated a stable contact between the solidifying shell andthe roll surface, resulting in quite homogeneous grainstructure. At this stage, after one revolution, the rolltemperature seemed to increase to some extent. An effectof the increased roll temperature and the low melt superheat(about 5°C in this study) is considered to decrease thethermal gradient, G, at the growth front and promote theformation of the equiaxed grains,30) as shown in Fig. 6(c).

4.2 Formation mechanism of internal crackingA large-scale internal cracking is a serious problem in

high-speed twin-roll cast aluminum alloy strips.11) It wasalready shown in the previous study14) that a thick bandstructure consisting of fine globular grains formed in thealloys with wide freezing temperature range, and the internalcracking always occurred in the central band region. Thefractography14) found that the crack surface was covered withsmooth round tips of dendrite branches and globular grains.This indicates that the internal cracking is not a post-solidification fracture, but results from the continuouslyconnected shrinkage cavities.

In Fig. 7, the internal cracking is observed in the zone II.In contrast, no crack was observed at the final part of thestrip, in spite of the thick central band. It is assumed thatthe final part of the strip is formed under a very small rollseparating force due to a thin strip thickness (i.e., smallenlargement of the roll gap near the roll nip). At the final partof strip in Fig. 9, no prominent change in the central bandthickness also supports this. These finding suggest that theapplied roll separating force is a controlling factor of theinternal cracking.

In addition to this, according to the results described in 3.2,following two conditions are considered to cause the internalcracking: (1) formation of a thick central band consisting ofthe fine globular grains and (2) applying a certain extent ofexternal stress (i.e., the roll separating force) to the centralband region in the semi-solid state.

The amount of residual liquid in the central band region isgreatly influenced by the roll separating force.11) As the rollseparating force increases, the cooling rates near the roll nipincreases due to improved contact condition between theroll surface and the solidifying shell.11) In addition, the liquidin the central band region can be reduced through “squeezingout”, by applying a high roll separating force.11) Conse-quently, the amount of residual liquid decreased with theincrease of the roll separating force. This explains why either athin or no central band are formed, and is also the reason whyno internal cracking in the strip at 60 kN exists (Fig. 10(b)).

In Fig. 6(c), the internal cracking occurs continuously atthe boundary between the central band and the outer shell.For developing such a continuous cracking, therefore,continuous liquid film must exist in the central band region.The present study shows that the cracking rather takes placeat 20 kN than at 3 kN. This suggests that the distribution ofthe residual liquid is affected by the external stress induced bythe roll separating force. In this case, the “shear localization”is considered to be an important mechanism in controllingthe distribution of liquid in the central band region.

M.-S. Kim and S. Kumai1936

With respect to the “shear localization”, a similar phe-nomenon can be found in a simple compression experiment.Tzimas and Zavaliangos32) performed the compression testof semi-solid Al­4mass% Cu alloy consisting of equiaxedgrains. They reported that the shear localization was observedat a high solid fraction, especially above 0.85 and the shearlocalized zone appeared as a porosity band in the compressedspecimens. Such shear localization occurs to accommodatethe compressive force introduced when the overall grainrearrangement is restricted by the increase of grain boundarycohesion at a high solid fraction. In this case, the specimenwas easily split into two parts even under a small force intension when the crosshead was reversed after the end of thecompression. The fracture surface exhibited smooth grainsurfaces, which indicates that the grain boundaries weresurrounded by liquid.

In a similar context, the roll separating force is consideredto cause the shear localization in the central band region insemi-solid state. Once the shear localization takes place, sincethe external stress disturbs the creation of a bridge betweengrains in the shear localized zone, the rapid shearingfacilitates the formation of continuous liquid film at thisregion. If the liquid film was maintained until the strip passesthrough the roll nip, it would eventually appear as the internalcracking.

5. Conclusions

A strip of Al­2mass% Si alloy, which has a wide freezingtemperature range, was fabricated using a high-speed twin-roll caster. In this study, a large feeding nozzle was usedto obtain a high melt pool level during the strip casting.Formation of solidification structure and several castingdefects such as inverse segregation and internal crackingwere studied, and the following conclusions were drawn.(1) Un-stable contact condition between the solidifying

shell and the roll surface was considered to result inseveral kinds of surface defects such as the ripplemark, “un-shiny” region and the compositional inversesegregation as well as in-homogeneous grain structure.As the melt pool level in the nozzle increased, the stripthickness increased by improved contact conditionbetween the solidifying shell and the roll surfaceand the overall contact condition became stable. Byconstructing and keeping the high melt pool level in thefeeding nozzle, the formation of the surface defects wasprevented effectively.

(2) A large-scale internal cracking was observed in thecentral band region consisting of fine globular grains.The cracking was assumed to relate with the residualliquid in the central band and the roll separating force.It was considered that the roll separating force causedthe shear localization in the central band region andpromoted a formation of continuous liquid film in the

shear localized region, and consequently, the connectedshrinkage cavity in the liquid film resulted in thecontinuous internal cracking when the strip wentthrough the roll nip.

REFERENCES

1) H. Bessemer: U. S. Patent No. 49, 053, July 25 (1865).2) N. Zapuskalov: ISIJ Int. 43 (2003) 1115­1127.3) R. Cook, P. G. Grocock, P. M. Thomas, D. V. Edmonds and J. D. Hunt:

J. Mater. Process. Technol. 55 (1995) 76­84.4) B. Q. Li: JOM 47-5 (1995) 29­33.5) R. Wechsler: Scand. J. Metall. 32 (2003) 58­63.6) K. Schwerdtfeger: ISIJ Int. 38 (1998) 852­861.7) M. Yun, S. Lokyer and J. D. Hunt: Mater. Sci. Eng. A 280 (2000) 116­

123.8) T. Haga and S. Suzuki: J. Mater. Process. Technol. 143­144 (2003)

895­900.9) T. Haga, K. Takahashi, M. Ikawa and H. Watari: J. Mater. Process.

Technol. 140 (2003) 610­615.10) K. Suzuki, S. Kumai, Y. Saito, A. Sato and T. Haga: Mater. Trans. 45

(2004) 403­406.11) M. S. Kim, Y. Arai, Y. Hori and S. Kumai: Mater. Trans. 51 (2010)

1854­1860.12) M. S. Kim and S. Kumai: Proc. 12th Int. Conf. on Aluminum Alloys,

September 5­9, Yokohama, Japan, (2010) pp. 693­697.13) K. Suzuki, S. Kumai, K. Tokuda, T. Miyazaki, A. Ishihara, Y. Nagata

and T. Haga: Mater. Sci. Forum 519­521 (2006) 1821­1826.14) M. S. Kim and S. Kumai: Mater. Trans. 52 (2011) 856­861.15) D. Shimosaka, S. Kumai, F. Casarotto and S. Watanabe: Mater. Trans.

52 (2011) 920­927.16) O. Umezawa, M. Nakamoto, Y. Osawa, K. Suzuki and S. Kumai:

Mater. Trans. 46 (2005) 2609­2615.17) K. Suzuki, S. Kumai, Y. Saito and T. Haga: Mater. Trans. 46 (2005)

2602­2608.18) T. Haga, M. Ikawa, H. Watari, K. Suzuki and S. Kumai: Mater. Trans.

46 (2005) 2596­2601.19) H. Esaki, Y. Watanabe, K. Ueda, H. Uto and K. Shibue: J. Japan Inst.

Light Met. 56 (2006) 266­270.20) N. Zapuskalov and M. Vereschagin: ISIJ Int. 38 (1998) 1107­1113.21) J. D. Hwang, H. J. Lin, J. S. C. Jang, W. S. Hwang and C. T. Hu: ISIJ

Int. 36 (1996) 690­699.22) J. W. Bae, C. G. Kang and S. B. Kang: J. Mater. Process. Technol. 191

(2007) 251­255.23) K. Shibuya and M. Ozawa: ISIJ Int. 31 (1991) 661­668.24) H. Yasunaka, K. Taniguchi, M. Kokita and T. Inoue: ISIJ Int. 35 (1995)

784­789.25) Ch. Gras, M. Meredith and J. D. Hunt: J. Mater. Process. Technol. 167

(2005) 62­72.26) D. Monaghan, M. B. Henderson, J. D. Hunt and D. V. Edmonds: Mater.

Sci. Eng. A 173 (1993) 251­254.27) B. Forbord, B. Andersson, F. Ingvaldsen, O. Austevik, J. A. Horst and

I. Skauvik: Mater. Sci. Eng. A 415 (2006) 12­20.28) M. C. Flemings: Solidification Processing, (McGrow-Hill, USA, 1974)

pp. 234­262.29) E. Haug, A. Mo and H. J. Thevik: Int. J. Heat Mass Transfer 38 (1995)

1553­1563.30) W. Kurz and D. J. Fisher: Fundamentals of Solidification, (Trans Tech

Publications Ltd., Switzerland, 1998) pp. 69­74.31) C. A. Siqueira, N. Cheung and A. Garcia: Metall. Mater. Trans. A 33

(2002) 2107­2118.32) E. Tzimas and A. Zavaliangos: Acta Mater. 47 (1999) 517­528.

Solidification Structure and Casting Defects in HSTRC Al­2mass% Si Alloy Strip 1937