薄带连铸Twin roll casting of aluminium alloys

Materials Science and Engineering A280(2000)116–123Twin roll casting of aluminium alloysM.Yun,S.Lokyer,J.D.Hunt *Department of Materials ,Uni 6ersity of Oxford ,Oxford OX 13PH ,UKAbstractTwin roll casting can be used to produce aluminium sheet from :10to 0.5mm thick directly from the melt.Although sheet can be produced,various defects arise which limit the range of operating conditions suitable for commercial exploitation.Over the past 10years the twin casting process has been modelled,and extensive experimental work carried out in Oxford.This has enabled many of the microscopic and macroscopic defects that occur to be described and explained.©2000Elsevier Science S.A.All rights reserved.Keywords :Twin roll casting;Aluminium sheet;Microscopic defects;Macroscopic defects /locate /msea1.IntroductionTwin roll casters have been used for almost 50years in the aluminium industry.Generally,commercial twin roll cast aluminium alloys have narrow freezing ranges and are cast with a sheet thickness of about 6mm.Although high quality sheet is produced,productivity is relatively low and the range of alloys which can be cast is small.Over the last 10years research has been carried out at Oxford in collaboration with Kvaerner Metals (formerly Davy International)on understand-ing,modelling and improving the roll-casting process.A series of numerical models have been developed [1–4]which have modelled the process and have led to a better understanding of the complex combined so-lidification and rolling which occurs.Early numerical and experimental work showed that productivity could be improved by casting thinner,and it led to extensive commercial interest in thin sheet casting.Work in Oxford has cast sheet down to 0.5mm thick produced at 50m min −1.The defects that occur during roll casting have been characterised,and models have been developed to ex-plain their occurrence.Most recently,the effect of defects on downstream processing have been investi-gated.Work is currently being carried out on a wide range of other metals.In twin roll casting,molten metal is fed onto water-cooled rolls,where it solidifies and is then rolled.The work in Oxford was carried out with either a horizontal or a vertical caster designed and built in collaboration with Kvaerner Metals.Horizontal or vertical refers to the plane of the metal strip.The roll diameter of the horizontal caster is 400mm and that of the vertical caster is 600mm and they are thus comparable with those used on commercial casters.The face width of the rolls is about 250mm.The metal is fed into the roll gap typically with a tip set-back of 45–55mm.The tip is narrower than the roll face width and soft side dams are used to contain the metal.Typically,the strip is cast 150mm wide,but wider and narrower strip can be produced.Loads of width up to :1tonne mm −1can be produced with the narrow strip.The strip speed can be varied from about 1m s −1to about 0.7m min −1and casts can be either 20or 60kg.Strip has been produced with a thickness of 0.5–6.5mm.A very precise application of lubricant is needed for thin strip casting.The two twin roll casters have com-puter-controlled sprays where the rate of application depends on roll speed,strip thickness and alloy.Over an extended period,a very wide range of alloys has been cast.These include examples from the AA lxxx,2xxx,3xxx,4xxx,5xxx,6xxx and 8xxx series.Trials have been carried out at different gauges and loads to investigate defect formation,and to investigate the suitability of the cast material for downstream application.*Corresponding author.Tel.:+44-1865-273712;fax:+44-1865-273789.E -mail address :john.hunt@ (J.D.Hunt)0921-5093/00/$-see front matter ©2000Elsevier Science S.A.All rights reserved.PII:S 0921-5093(99)00676-0M.Yun et al./Materials Science and Engineering A280(2000)116–123117In this paper recent numerical modelling will be described,and the results will be reported regarding the the investigation into sticking and lubrication,various macroscopic defects such as buckling,and microscopic defects such as surface bleeds,channel and deformation segregates.2.Numerical modellingThe most recent numerical model produced in Ox-ford by Bradbury[4,5]treats the solidification,heat flow,fluidflow in the liquid and deformation within the solid and semisolid.The model uses a control-volume method with non-orthogonal curvilinear linear coordi-nates.Typical control volumes in the roll are shown in Fig.1a.The deformation in the solid and semi-solid is treated as a modified viscosity which is a function of temperature;that is the metal is assumed to be a non-work-hardening isotropic material.Sticking fric-tion is assumed along the strip/roll interface.The strip exit speed(amount of forward slip)is calculated by the model.The heat-transfer coefficient between the metal and the roll is taken to be determined[5]by the local pressure exerted on the roll and the fraction solid to be given by the Scheil equation.Typical results for fraction solid,effective strain rate and pressure are shown in Fig.1b,c and d.A compari-son between predicted and experimentally measured properties as a function of roll speed for two strip gauges are shown in Fig. 2.Fig.2a and b show predicted strip and roll temperatures;Fig.2c shows separating force and Fig.2d the power used assuming a 10%loss in the gear train.The agreement between theory and experiment is very good.One feature predicted by the model is that the growth velocity at the base of the sump is smaller than the roll speed because solid and semi-solid is pushed backwards by the deformation process.Most of the deformation occurs in the hottest material towards the centre of the strip.Under some conditions,this backward deforma-tion reduces the velocity so much that melting rather than freezing occurs.In a real process it seems likely that instability across the width of the strip will occur before uniform melting(or recirculation)takes place in the centre of the strip.The model showed that a large tip set-back and thin gauge favours recirculation.It is likely that the largest practical tip set-back is deter-mined by the onset of recirculation.3.Macroscopic defectsTwo macroscopic twin roll-casting defects will be discussed;sticking and buckling.3.1.Sticking/lubricationFor thick sheet,:6mm,the sticking of the metal strip to the rolls is not a very serious problem.During commercial use,a layer of metal/metal oxide builds up on the roll and only intermittent application of lubri-cant is needed.As the thickness of the strip is reduced, sticking becomes much more likely.When the strip thickness becomes less than about2mm a uniform, metered quantity of lubricant must be applied for most aluminium alloys.Sticking of the metal to the roll can be so severe that the aluminium has to be ground off the steel roll,and if strip gets attached to both rolls at different points across the width,the strip can be torn into two parts.Partial sticking can lead to the coating on the roll being removed and this then leads to complete sticking after one revolution of the roll.Expe-rience indicates that sticking can always be prevented by the application of a uniform layer of lubricant of sufficient thickness.Too thick a layer leads to a poorFig.1.(a)Finite volume grid.(b)Fraction solid.(c)Effective strain rate.(d)Pressure isobars plotted on actual modelled shape.M.Yun et al./Materials Science and Engineering A280(2000)116–123118Fig.2.Experimental points and predicted lines for(a)strip exit temperature;(b)roll temperature;(c)separating force;(d)energy used;plotted against roll speed.heat-transfer coefficient and the lubricant is picked in significant quantities on the strip.Typically the strip exits the caster4–10%faster than the roll speed be-cause of solid-state rolling.A number of different lubricants and lubrication methods have been investigated.Early trials showed that the uniformity of application was critically impor-tant,as was a metered supply rate.It is not very clear whether a parting layer or a lubricant is required. Materials which might be expected to act as a solid lubricant seem to act more efficiently.Most success has been achieved with a water-based graphite emulsion which is sprayed onto the hot rolls.The water must be completely dried off before the roll returns to the tip.A metered computer-controlled pump is used to pump the graphite emulsion.The pump rate is set to vary with strip speed,gauge and alloy type.Alloys which contain Mg are much less likely to stick,and there is practically no sticking problem when more than about2wt%Mg is present in the alloy.This observation can possibly be correlated with a mixed spinel forming at low Mg concentrations and magnesia above2wt%Mg[6].A high-magnesium alloy is usually used to condition newly ground rolls or ones where bad sticking has occurred.3.2.Macroscopic bucklingTrials have been carried out on the Oxford casters to investigate buckling in a range of alloys.An example of a buckle is shown in Fig.3.In this example one side of the sheet is tight and the other is long so that the additional material appears as a buckle.The loose material can appear on either side or in the centre of the strip.On the Oxford casters the buckle usually appears on one side because the strip is relatively narrow;under severe conditions,the length of the long edge can be up to twice the length of the short edge. Before the strip is picked up by the coiler the strip often moves to the right or left.On the Oxford caster,the buckle is the result of restricting the strip’s sideways motion.On occasions the direction of motion changes before the strip reaches the coiler,indicating that the effect was not the result of caster setup.In the experimental trials the usual procedure was to start with a gauge of about1mm then increase the thickness or load until buckling ceased.Buckling was then reintroduced by decreasing the load or decreasing the gauge.A summary of some of the results is shown in Fig.4.It should be stressed that the transition did not occur on a precise line but occurs over some band of values.In most cases,when the load was decreased the amount of buckling was reduced,but eventually heat lines or sticking ually one side of the strip was tight the other loose but occasionally sheet was produced with a loose centre.For the purer alloys when buckling occurred heat lines[7–9]tended to be formed on the tight edge.For wider freezing range alloys the sheet appeared to be externally of reasonable quality but metallographic examination shows that the long edge had been heavily deformed whereas the short edge appeared to have had little rolling.A number of experiments have been carried out to characterise the defect.A larger tip set-back appears to increase the tendency to buckle.Certain alloys are more prone to buckling;AA1200is much worse than AA1100,despite having a relatively similar composi-tion.Perhaps the most significant observation is the effect of coiler tension;buckling could be reduced or eliminated by increasing the coiler tension;steady state results in AA1200are shown in Fig.5.It is concluded that buckling can be understood in terms of different amounts of forward slip produced during the rolling part of the process.Any non-unifor-M .Yun et al ./Materials Science and Engineering A 280(2000)116–123119Fig.3.An example of buckling in AA1200.mity such as metal feed temperature,amount of lubri-cation,camber on the rolls and tip position will lead to more metal solidifying in one region of the sheet.The increased amount of rolling in this region of the strip leads to more forward slip and thus to a loose pocket.For thick sheet the stiffness of the sheet and the tension of the coiler will tend to prevent the sheet buckling.As the sheet thickness is reduced the stiffness of the sheet will be reduced and the sensitivity of the solidification process to non-uniformities will increase.Eventually buckling will start;once started,the application of lubricant will tend to stabilise the buckle.The heavily rolled region will scour the surface film from the roll,thus increasing the heat-transfer coefficient and further increasing the quantity of solid.Similar buckling occurs during hot and cold rolling when more material is rolled in one region.In twin roll casting it will always be difficult to ensure that a uniform quantity of solid having uniform properties is produced across the width of the caster.For this rea-son,at first sight,it is surprising that good quality sheet can ever be produced without buckling on a caster.A number of features make twin roll casting,as opposed to rolling,more forgiving.One of these is that the amount of backward slip can differ at different posi-tions across the width because in casting the incoming material is liquid.Similarly,material can be dragged through the roll bite by the surrounding material with-out being constrained by an incoming solid sheet as it would in a rolling process.Another is that sideways distribution of material can occur over larger distances because the initial semi-solid is very soft.It is concluded that the onset of buckling is a mea-sure of the non-uniformity of the solidification processresulting in more forward slip over parts of the strip.It is suggested that buckling will always occur at a thin enough gauge and that the transition will tend to be machine-and running-condition specific.Fig.4.Experimental plots of strip thickness versus specific load in twin roll cast aluminium strip.Solid circles,no buckling;open circles,buckling.M .Yun et al ./Materials Science and Engineering A 280(2000)116–123120Fig.5.Buckling as a function of coiler tension and specific load.Solid circles,no buckling;open circles no buckling.Fig.6.Surface bleeds in a twin roll cast Al-0.3wt%Fe alloy;(a)type-I surface bleed;(b)type II surface bleed.4.Surface defectsA number of surface defects exist;one of the more important is a surface bleed.4.1.Surface bleedSurface bleeds are pockets of solute-rich material which form on the strip surface and contain a higher concentration of intermetallic particles.These pockets are hard and difficult to deform during downstream cold rolling.The size of the surface bleed varies from :0.05mm long and 0.01mm deep to 1.5mm long and 0.1mm deep.An example is shown in Fig.6.The amount and size of the bleeds were measured by taking a section right across the strip width and mea-suring the area of bleed per unit width.Surface bleeds are more frequent and more severe at low specific loads and fine gauges.Eventually,at high enough loads,surface bleeds become much smaller and less frequent.The amount of surface bleeds was found to depend critically on alloy.For example AA 1100is very suscep-tible to surface bleeds,whereas bleeds do not appear to be formed in AA 3003.It is proposed [10,11]that surface bleeds are the result of a gap (or small buckle;not the macroscopic buckling discussed previously)opening up between the roll and the semi-solid sheet.The space is then partially filled with solute-rich liquid.Initially near the tip,semi-solid material is formed on the roll and moves towards the roll gap at the roll speed.Near the sump the speed of the semi-solid will be slightly less than the roll speed because of backward slip resulting from the deforma-tion of the solid and semi-solid.The differences in speed will lead to compressive forces in regions A of Fig.7and could lead to the formation of a small buckle.The compressive forces will mean that liquid can be squeezed into the gap that is formed.It is suggested that the susceptibility to bleeding for differ-ent alloys is determined by the freezing range of the alloy after the liquid fraction has reached a few percent.When the strip is rolled in the caster any remaining gapbetween the roll and strip is closed up and the surface bleed appears to remain relatively undeformed.5.Internal defectsChannel segregates,deformation segregates and the banded structure will be discussed.Channel segregates are cylindrical low melting point regions oriented in the casting direction (see Fig.8).Deformation segregates are equiaxed regions of low-melting-point material dis-tributed in a central band.(see Fig.9).When a banded structure forms,the outer surfaces of the sheet have very different grain structure and secondary arm spac-ing when compared to that within a central band (see Fig.10).Fig.7.Schematic diagram showing interaction between the strip and the roll surfaces.M .Yun et al ./Materials Science and Engineering A 280(2000)116–123121Fig.8.Channel segregates in AA6111.(a)lengthwise section;(b)transverse section.5.1.Deformation segregatesThese segregates occur when the deformation process is so rapid that the solid and liquid deforms together rather than the liquid being squeezed out of the solid.During the deformation process small liquid regions are formed between the solid grains.Careful metallo-graphic examination shows that these are equiaxed and not elongated in the casting direction [11].An example is shown in Fig.9.5.2.Banded structuresAt high loads and low speed,before the banded structure is formed,the secondary arm spacing is small on the surface and in the centre of the sheet.The secondary arm spacing is small in the centre of the sheet despite the fact that heat must be transported further.This occurs because the heat transfer coefficient between the sheet and the roll becomes dramatically larger when sufficient solid forms near the base of the sump.When solid exists throughout the strip pressure builds up between the rolls thus increasing the heat transfer coefficient.The larger heat-transfer coefficientFig.9.Deformation segregates in a twin roll-cast AA6111alloy:(a)longitudinal and (b)transverse section.All the different morphologies are thought to be the result of the combined solidification and rolling process.The different structures can be discussed in terms of decreasing the specific load at fixed sheet thickness.This is equivalent to increasing the casting rate.For thick sheet under high loads,defect-free struc-tures are formed.As the casting velocity is increased the depth of the sump increases.Deformation of the semi-solid leads to liquid being squeezed back towards the sump.When liquid flows from a cold to a hot region in a casting,the liquid must change its composi-tion and this melts solid.Flow in one region leads to melting,to further flow and,thus,to channel formation [12,13].Channels are formed in a number of casting situations where liquid metal flows between the den-drites [12–14]from a cold to a hot ually in twin roll casting the channels are formed in the central plane of the sheet and have an almost constant spacing.As the casting rate is increased the channels are shorter and occur over a central band.As the casting rate increases further deformation segregates are formed.M.Yun et al./Materials Science and Engineering A280(2000)116–123 122Fig.10.Examples of banded structure(inhomogeneity in grain structure)in twin roll cast AA3003.(a)fine/coarse central band;(b) coarse/fine band.thixotropic nature of the semi-solid metal.Once de-formed,the semi-solid material in the central band remains soft until it reaches a much lower temperature compared to grains in the outer band.It is not clear why some grains have a much coarser structure but this may be associated an increased local rate of back deformation.It has been suggested that for large tip set-backs,the central region can be pushed back so fast that melting or recirculation occurs[4,5].5.3.Defect limit diagramThe experimental results are summarised in a defect-limit diagram for AA1100,AA3003and AA6111(see Fig.11).It is not really possible to draw definite lines, but trends can be shown.Defect-free material is found at the high-load thick-sheet corner.Moving towards the thin-sheet low-load corner,firstly channel segregates are formed and then deformation segregates.Eventually, banded structures are formed.The central banded re-gions are labelledfine and coarse.As indicated earlier, atfirst the central band is mainlyfine;then more coarse grains form,and eventually most of the band has a coarse structure.The presence of surface bleeds has not been included on the plot because there appears to be a gradual increase in severity and quantity going from the thick-sheet high-load corner.Buckling has not been included on the plots since it is felt that the onset of buckling will depend too critically on caster set-up. 6.SummaryTheoretical and experimental work has helped in the understanding of some of the more important defectsmeans that the cooling rate is much more rapid and thus the secondary arm spacing is smaller in the central regions.The process is modified at relatively low loads.In-stead of a continuously varying secondary arm spacing, there is a sudden change midway through the sample and the sheet can be divided into inner and outer bands.Generally,the inner band has a veryfine sec-ondary arm spacing although small regions have a much coarser structure(see Fig.10a).The amount of coarse structure increases with increasing the coating rate(see Fig.10b).The whole central region of the strip appears to be pushed back towards the liquid as a result of the deformation process.Our numerical calculations[4] indicate that the base of the sump becomesflatter (more’U’-shaped)and the temperature gradient in the casting direction increases when extensive deformation occurs.Although the rate of motion of an equiaxed grain in the central region is less than the strip velocity, the equiaxed grains are cooled more rapidly because of the higher temperature gradient,thus explaining thefiner structure.It is suggested that the sharp division between the outer and central band is a result of the Fig.11.Plots showing experimental points and rough microstructural regions.Defect limit diagram.M.Yun et al./Materials Science and Engineering A280(2000)116–123123which occur in twin roll cast aluminium alloys.We conclude that the more important defects produced in twin roll cast sheet can be divided into three main categories:surface defects or bleeds,internal defects and macroscopic bucking.Experimental results are re-ported and mechanisms are suggested to explain the defect formation.References[1]M.J.Bagshaw,J.D.Hunt,R.M.Jordan,A steady state modelfor roll 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