PRESENT TRENDS IN METHODS OF PRODUCTION

OF ALUMINIUM ALLOY SLABS AND

BILLETS FOR WORKING

M. R. Reeve

P

RODUCTION of aluminium in all parts of theworld is increasing rapidly and continuously atthe present time. For example, in Europe,

400,000 metric tons of primary aluminium wereproduced in 1950, but by 1956 this output was morethan doubled. Apart from two minor recessions in1953 and 1957, an increase has occurred each year.

Increase in output of the aluminium industry as awhole is very evident in the wrought field, and is relatedincreasingly to various applications in industrial anddomestic markets. Examples of comparatively newwrought aluminium alloy applications are evident in thebuilding industry, vehicle bodies, shipbuilding andkitchen ware. Greater demand in the wroughtindustry, more than in other fields of aluminiumproduction, has had to be met by marked changes inproduction techniques. Such changes have had toenable outputs to be increased whilst maintaining andimproving metal quality. The purpose of this paper isto comment on changes in techniques now evident, andto describe briefly some of the newer productionmethods now being adopted.

The stages in production of slabs and billets arenormally as follows :

(1) Melting : Virgin ingot and/or scrap aremelted and alloying additions made to obtainthe desired chemical composition.

(2) Conditioning : The molten alloy is treated

(3)

for removal of gas and oxide , and grain re-fining additions are made, if necessary.Transfer to casting station : The requiredquantity of clean , gas-free metal of correctchemical composition is transferred from thefurnace to the casting machine.

(4) Casting : Production of slabs or billets forrolling, forging or extrusion . and semi -finishedrod or strip.

These stages are illustrated in diagrammatic formsin Fig . 1, and the direction of present trends inproduction methods is shown from left to right acrossthe diagram.

Mr. M. R. Reeve, B.Sc., Senior Research Metallurgist, FosecoInternational Limited, Birmingham , England.

Melting

In older wrought plants, melting is carried out infuel fired single hearth reverberatory furnaces, in whichalloying and conditioning also take place, and fromwhich the metal is cast. The tendency in modernplants is to use two furnaces in series, or a single largefurnace divided into two parts. Melting takes place inthe first unit. and metal is then transferred to the secondunit, known frequently as a holding bath, for alloying,conditioning and casting. The main advantage of thetwin bath arrangement is that greater production ratesare possible, because melting in the first unit can conti-nue whilst conditioning and casting are in progress inthe second.

In addition, conditioning treatments are more easilycontrolled, as a major degree of oxide removal fromthe bath takes place by the very fact of melting sepa-rately. By melting under a suitable liquid flux coverand by charging solid scrap into the pool of metalbelow that cover, oxidation of materials during meltingis kept to a minimum and surface oxides on solid scrapare more easily removed. The partially purified metalis then in a better condition for final treatment whentransferred to the holding unit. Typical twin batharrangements are oil fired reverberatory furnaces, inwhich the melting bath of 12 tons capacity is separatedfrom the holding bath of 6 tons capacity by a refractorybridge as illustrated in Fig. 2. Metal is transferredthrough a curved external transfer launder. On thecontinent of Europe, electrical melting is much favouredand a typical installation may consist of a low frequencyinduction melting furnace and resistance-elementholding furnace, each of about 3-4 tons capacity.

Furnace charging arrangements have improved asproduction rates have increased. Whereas older andsmaller furnaces are charged by hand with some assis-tance from a crane when dealing with the heavier piecesof scrap, mechanical chargers are now in use for thelargest furnaces which may be of up to 30 tons capacity.The furnace roof can be made removable to permitcharging of large quantities of scrap.

Future designs of furnaces for melting large quantitiesof aluminium are likely to change considerably,

139

OLDER BATCH METHODS

RENERBERATONY SINGLE HEARTH

CONTINUOUS

CONDITIONING

LADLE

SMALL SEMI%WNIINVDUS

(BILLETS AND SI ABS)

TRANSFER TO CASTINGSTATION

CASTING

FULLY CONTINUOUS

(sues)

NEWER CONTINUOUS METHODS

LAUNDER

LARGE SEMI.CONTIN000S FULLY CONTINUOUS WN(f.

(BILLETS AND SLABS) (STRIP AND BOO)

Fig. I.

Older batch methods-Newer continuous methods.

especially in countries where electric power is relativelyexpensive and fuel firing must be retained for economicproduction. The major disadvantage of fuel-firing isthat the products of combustion are introduced into thefurnace and aluminium is maintained in an oxidisingatmosphere, resulting in considerable melting loss. Theadvent of radiant heat tubes for Fuel-fired furnaces,

however, means that the products of combustion canbe excluded from contact with the metal, and use ofsuch tubes will no doubt be made in the furnace ofthe future. Tubes could even he submerged in themetal bath itself to obtain optimum heat transfer, butconsiderable refractory problems would have to besolved before this became a practicable possibility.

140

DUPLEX HEARTH FURNACE FOR RE -MELTING ALUMINIUM ALLOYS

Charging Door Trans Charging Door(light and haavystrap, ingots,etc ter Launder

--3 (fludng,degassing)

Melting Both Holding BothSECTIONAL PLAN

Flue

ELEVATION

Fig. 2.

Duplex hearth furnace for re-melting aluminium alloys.

Conditioning

Methods for gas and oxide removal and grain refiningcome under this heading.

Gas removal: As is well known, dissolved hydrogenmay be removed from aluminium alloys by treatmentwith gaseous chlorine, or chlorine-nitrogen mixtures, orby plunging tabletted hexachlorethane into the melt.I n older plants, such treatment is typically carried outin the ladle used to transfer metal to the casting station.A ladle contains a relatively deep bath of a relativelysmall quantity of metal and is therefore a very suitableunit for degassing. The degassing is usually carriedout in a fume chamber containing one or more refrac-tory chlorination tubes or facilities for plungingdegasser tablets.

In more modern plants, metal is transferred from theholding furnace to the casting station by a launder, andan effort has therefore been made to degas metal in thebath prior to casting. Such degassing is essentially abatch operation, and is relatively inefficient because themetal bath in the furnace is usually shallow with alarge surface area. Reabsorption of gas from thefurnace atmosphere, particularly in fuel-fired furnaces,is therefore a problem, and further reabsorption froma moist ambient atmosphere by metal flowing in thelaunder during casting may take place.

Methods of continuously treating the flowing metalbetween furnace and casting station have thereforebeen devised, using gaseous chlorine or chlorine,' nitro-gen mixtures. In one of these illustrated in Fig. 3, flow-ing metal is directed through a deep, externally heatedchamber in which degassing is carried out. In another,metal flows through an enclosed refractory laundercontaining baffles to the downstream, of which thedegassing agent is introduced and made to flow counter-current to the metal. These methods are more effectivethan batch treatment in the furnace and problems ofgas reabsorption are reduced. They are therefore com-ing into more general use in wrought plants.

Continuous degassing with chlorine is thus effectivein reducing the hydrogen content of molten aluminiumand its alloys to very low levels and some removal ofoxide may also result from the considerable scavengingwhich takes place during treatment. However, manychlorine users are aware that residual oxide may yet bea problem in chlorine treated aluminium alloys, andthat further treatment is necessary.

Oxide removal: Methods for removal of oxide in-clusions suspended in an aluminium melt depend on theuse of a washing flux consisting of a mixture of saltsfluid at the operating temperature and able to "wet"the solid particles. Inclusions, once "wetted" by theflux, remain in the flux layer, and are removed when thelayer is subsequently skimmed off the surface of thebath. It will be seen that very thorough contact betweenflux and metal is essential for complete removal ofoxides suspended in the bath.

The conventional method of using a washing flux is torabble the fused powder into the metal bath. The fluxliquefies rapidly and being less dense than metal, rises

Fig. 3.Apparatus for chlorination of flowing stream of

molten aluminium (Kaiser Patent).

141

to the bath surface taking with it the oxides. For sucha treatment to he wholly successful, a subsequent"holding" treatment is necessary during which metal isallowed to stand with falling temperature under thewashing flux cover. During this holding period, furtheroxides settle out into the flux layer and a degree of out-gassing takes place. The process is particularly suitedto electric resistance holding furnace in which holdingtimes of as long as 5 to 8 hours may be used. In fuel-fired furnaces, a holding treatment is fully effectiveonly in the absence of flame from the burners and it istherefore the practice to use the fuel to preheat the bathprior to treatment with the fuel supply off. Rate oftemperature loss would, in this instance, limit holdingtime. It is evident that use of a washing flux with aholding treatment is time consuming and inefficient andtends to curtail production rates.

A process of continuous fluxing, analogous to conti-nuous degassing and called flux washing, has thereforebeen developed. The object of the process is to treata flowing stream of metal and obtain rapid and inti-mate flux-metal contact to remove suspended oxideseffectively. The treatment unit consists of an exter-nally heated refractory washing chamber containinga deep layer of molten flux retained in the chamberby a refractory baffle which does not reach the hearth.Metal for treatment flows continuously through thewashing chamber and washed metal free from fluxflows under the baffle and out. In the unit illustratedin Fig. 4. a horizontal plate is introduced into theupper portion of the flux layer to spread the metalstream and improve flux-metal contact. The unit hasto be maintained continuously at service temperatureand may be based for convenience on a modified com-mercially available foundry crucible or bale-out basinplaced in a small furnace.

Composition of a flux for washing is of prime im-portance and it is possible to modify the latter to obtaina degree of gas removal and, if desired, grain refining,in addition to oxide removal. Comprehensive metalconditioning may thus be achieved in a single compacttreatment unit without the need for any holding timein the furnace other than that needed to bring themetal to the correct temperature for treatment,normally below 720,C. Alternatively, if desired, theprocess can be made complementary to a continuousdegassing unit. Flux washing therefore offers a meansof considerably increasing production rates.

Grain refining: The grain size of an aluminium alloycasting depends on the alloy composition and castingconditions, and can be varied, within limits, bycontrol of these factors. The grain size of wroughtmetal depends partly on initial as-cast grain size and

partly on subsequent thermal and mechanical treatment.Fine grain is desirable in a finished or semi-finishedproduct to obtain the optimum combination of mecha-nical properties and when control of production condi-tions is unable to produce sufficient artificial meansof reducing grain size are resorted to.

The grain of an aluminium alloy casting can berefined by the addition of a small quantity of suitablenucleating elements, of which the most important

0®®®C®l

I Ent ry Launder 7 Plumbago Baffle2 Tundish 8 Fluid Flux Layer

3 Downs pout 9 Exit Launder

4 Circulkr5 readerPlate 10 Refracto ry Furnace Lining

S Plumbs oCrucible I I Sheet Steel Retainer6 Lid 12 Refractory Stool

Fig. 4.

Flux washing tank (Foseco Patent).

examp:es are titanium and boron. Titanium alone iswidely used in the wrought industry, actual nucleationbeing effected by titanium carbide formed by actionof carbon in the melt. The proportion of titaniumrequired to obtain such refinement is quite high, beingof the order of 0' 15%, and the process suffers fromthe disadvantage that titanium carbide nuclei tend toaggregate and lose their effectiveness during prolongedholding or after remelting of the alloy. The set ofphotographs in Fig. 5 shows the progressively increas-ing grain size of aluminium treated with 0'15%titanium after remelting four times. As will be seen,the sample P.4 shows a coarse grain structure (1-2 mm)despite the fact that the material contains a fairlyhigh titanium content at 0'15°c. It is thereforenecessary to introduce titanium for grain refining to

the bath immediately prior to casting. Where scrapis recirculated for remelting, such grain refinementmay result in a build-up of titanium of the alloy to anundesirable extent.

By comparison, titanium and boron together astitanium boride are able to effect nucleation at themuch lower levels of 0'03°,) Ti and 0'003";, B and theeffect largely persists throughout holding or remeltingtreatments because the nuclei remain discrete particles.

142

6. P. 1. P. 2. P. 3. P. 4.

'FIT AN] UM

CONTENT ... 0'15°0 0'15°'; 0.15 °'0 0' 15 /o 0'15°,0(;RAIN'IZE '5 mm '7 m m '8 nim '9 mm P2 nun

Fig. S.

The progressive grain growth on remelting samples -Grain refined with titanium hardener.

The effect is clearly illustrated in the set of photographsof macro-etched sections through a rolling slab inFig. 6. Consequently, the treatment is markedlysuperior to that with titanium alone and proprietaryproducts are marketed containing reducible titaniumand boron salts together with a quantity of hexachlore-thane, the vigorous dissociation of which aids dispersalof the nucleating elements. Such grain refiners areplunged, in tabletted form, into the holding bath ata convenient stage prior to casting.

An alternative means of grain refining is, as hasalready been mentioned, to use a washing flux contain-ing grain relining salts.

Transfer to casting station

In older plants, metal is transferred from the meltingfurnace or holding bath to the casting machine in aladle. Ladle sizes have tended to be restricted byplant layout and other considerations and have notkept pace with the quantity of metal required forproduction of the larger slabs and billets. In theseinstances, where two or more ladles are required tocomplete a casting operation, a considerable degreeof work study is required to ensure that the rightquantity of metal at the right temperature reaches themachine at the right time. In addition, use of ladleshas the disadvantage that metal must be tappedfrom the furnace at a relatively high temperature toallow for the temperature loss in the ladle duringtransfer and casting. The temperature variation duringcasting entailed in use of ladles is also undesirable.

In modern plants, metal flows direct from the holding

bath via a launder. Control of the metal flow rate isobtained by the degree of tilt of a tilting furnace or byone or more control valves placed in the launder systemwhen a fixed tap-hole is used. Where several slabs orbillets are cast simultaneously, metal distribution maybe obtained by dividing the launder as in Figs. 7 and 8or by the use of a tundish. The arrangement has theadvantage that the tapping temperature can be relativelylow, and the casting temperature maintained relativelyconstant so that the amount of metal which can be castat any one time is limited only by the capacity ofthe holding bath. On the other hand, correct launderdesign is of prime importance to avoid gas or oxidepick-up or the generation of dross, hence contact ofmetal with the ambient atmosphere and generation ofturbulence of the stream must be minimised. If the dropin metal level is sufficiently great to warrant the use ofvertical downspouts, these must not admit air with themetal. If flow control valves are used, these should beseated below the lowest metal surface to avoid induct-ance of air as shown in Fig. 9. Tap-holes of a fixedfurnace are best operated submerged beneath the metalsurface in the launder to avoid dross generation. Thematerial of construction of a launder is important ;certain refractories absorb moisture at ambient tempera-tures which may contaminate the metal stream. Ingeneral, the shorter the launder and the less the leveldrop, the less is the liability of the metal to reabsorp-tion of gas or oxide. Many Continental Europeanplants use launders only 30 in long with a level drop oftypically 3 in.

Complete enclosure of launders is gaining favourin many plants to reduce contact with the ambient

143

Photograph A.

Before grain refinement.

Photograph B.

After grain refinement with 0.03 titanium and0-003 boron . Very small equi .axed crystalsand complete freedom from columnar growth.

Photograph G.

Structure after a time lag of I hour. Almostidentical to Photograph B.

Photograph D.

After pouring 5 tons metal from the 10-toncharge and replacing bath with a further5 tons without additional grain refinement.Time lag of 2T hours. Grain refined structurealmost wholly retained.

Fig. 6.

A set of macro- photographs showing thegrain refinement at various stages.

144

Poker for 4low control

60-AM

Four Slab t.".ulds

Fig. 7.

Plan of typical launder system feeding four slabs moulds.

I

Fig. 8.

Semi-continuous casting of commercial purity aluminiuminto four slabs , 8 in. 28 i n.' ; 88 in. long.

(By courtesy at Alcon Industry Ltd.)

3

Fig. 9.

Vertical section through needle valvefor controlling metal flow between

two launders at different levels.

atmosphere. The atmosphere inside the launder may be Castingfurther controlled by admission under positive pressureof nitrogen or argon or, as has already been mentioned, Chill casting in static moulds of slabs and billets forchlorine gases, working has long been superseded in all but it very few

145

Fig. 10.

Section through mould of semi-continuous casting machine.

Fig. II.

Ejection of four circular-section billets from small semi-continuouscasting machine . (By courtesy ofJames Bocth Aluminium Ltd.)

wrought aluminium plants by semi-continuous methods.The principles of semi-continuous casting are too wellknown to require more than a brief description. One ormore horizontal water-cooled open-ended metal mouldsare used, each of which at the commencement of cast-ing is closed by a "stool". All stools are mounted on acommon platen which is withdrawn as casting proceedsby mechanical or hydraulic means, as shown in Fig. 10.The closed cavity formed by stool and die is filled withmolten metal and as solidification commences, the ma-chine is started and stool and embryo casting withdrawnbelow the mould. Cooling is effected by water spraysimpinging on the outer surface of the mould wall and onthe solid skin of the casting itself as it is withdrawn be-low the skirt of the mould. The process continues until aslab or billet of the required length has been cast, whenthe supply of metal is stopped and the solid castingejected from the pit by raising the platen (Fig. 11).Removal from the machinc is effected by crane. Thesize of the casting is defined by the mould cross-section and the depth of the pit into which the platen islowered. A typical large slab may have a cross-sectionof 40 :8 in and a length of 120 in and weigh rathermore than 11 tons.

Slabs and billets particularly of the higher strengthalloys cast by these methods are subject to internalstresses produced by the rapid rates of cooling and themagnitude of the stresses increases with. and tends toimpose a limit on, the cross-section of the casting.Control and limitation of the amount and distributionof cooling water has been suggested to reduce the rateof chill and hence the liability to stress.

Such control may be achieved, for example, by remov-ing the fluid coolant at a predetermined distance belowthe mould skirt by air jets or a mechanical syntheticrubber wiper.

Another feature of semi-continuous casting is theirliability to segregation of alloying elements and parti-cularly inverse segregation and exudation of low meltingpoint constituents through the cast skin. Exudationis aided by contraction of the skin away from themould wall which occurs immediately on solidificationleaving an air gap which tends to insulate the metal fromthe cooling water and retard freezing. Low meltingpoint constituents from the partially solidifying zonesof the casting are forced through the skin and reach thesurface in the air gap. Hard beads on the slab surfaceare so formed which arc normally removed by millingprior to preheating for working. Milling is an expens-ive operation requiring costly equipment and, ofcourse, entails the cooling of cast material to roomtemperature. Means of reducing or eliminating segre-gation have been suggested to overcome this. One ofthese, for example, is to use a porous sintered metalskirt to the mould through which water sprays can beforced into the air gap so overcoming the insulatingeffect. There is no doubt that reduction of segregationto the extent that the need for milling could be obviatedwould represent a considerable step forward in slabproduction. Castings could be transferred from themachine to a soaking pit for preheating for workingwithout intermediate cooling to ronni temperature as

146

Refractory distributorunit with twin nozzlessubme ged below metal

\ surface

Correct position ofvalve seat to avoid

/ Induction, of air,

Refroc ory float

etaslevel

Fig. 12.A method of introducing metal to slab mould of

semi-continuous casting machine.

Lever mechanism

in wrought steel production, and costly and thermo-dynamically inefficient cooling and reheating of slabswould be eliminated.

Considering the casting machines themselves, theolder semi-continuous type is comparatively small,mechanically or hydraulically operated, and fed withmetal from a ladle. The size and number of slabs orbillets are restricted. Both this type and the few fullycontinuous machines in the wrought aluminium indus-try are being superseded on economic grounds bylarge hydraulically operated semi-continuous machinesfed by a launder from the holding bath and these cancast 6 tons or more of metal in one "drop" (Fig. 8).This quantity of metal may comprise typically fourrolling slabs or four to twelve extrusion billets depend-ing on size.

Metal is normally fed to moulds of smaller machinesfrom an open launder, but larger moulds of moremodern machines usually employ some form of distri-butor. For example, metal in a slab mould may bedistributed via two refractory nozzles submerged in themolten pool as illustrated in Fig. 12.

The level of the pool in the mould should be main-tained constant, because minor variations can have amarked effect on cast structure and the incidence ofsegregation. Some form of automatic flow regulatingdevice in which a refractory valve operated by a re-fractory float on the metal surface makes correctivechange to rate of flow is now normally employed.

Casting wheels for continuous productionof strip and rod

Aluminium alloy strip and rod, in common with the

majority of semi-finished products, are produced con-ventionally by respectively rolling or extruding semi-continuously cast slab or billet. The sequence ofoperations is comparatively lengthy, and involvesthe milling of the cast slab, preheating for working anda multiplicity of working operations. Hence trans-formation of molten metal into a semi-finished productin one operation is obviously an economically desirableaim. Since the War, a number of fully continuousmoving-mould machines for casting strip or rod havebeen developed and these have proved successful andcompetitive with conventional methods for manufactureof such products. Examples of such machines are thefollowing:

The Properzi process: This was the first fullycontinuous casting wheel, and design changes sincethe War have greatly improved the efficiency of opera-tion, so that the process is simple and economic.Present-day machines produce a rod of about 1'5 sq. in.cross-section on a water-cooled casting wheel with acopper rim and iron side plates. The groove is closedby a water-cooled steel belt. Molten metal passesthrough a cast-iron holding pot from which it is meteredto the wheel cavity by a flow control device. Duringpassage of the metal from the inlet to the outlet, anangle of 180' is described as illustrated in Fig. 13 andcomplete freezing of the cross-section is obtained. Theusual casting speed is 24-36 r.p.m. The rod at 480'C

Fig. 13.Properzi casting process.

147

Fig. 14.

Aluminium laboratories casting process.

Fig. 15,The Hunter engineering process casting

and rolling principle.

Fig. 16.

Basic principles of the Hazelett machine.

is delivered direct to a multi-stand rolling mill whichreduces the rod cross-section to h" in diameter.

Alurnirriuru laboratories rotary strip casting process:This machine which is illustrated in diagrammatic formin Fig. 14 was designed to overcome the main disadvan-tage of the Properzi process, namely, the limitation incast width, and enable production of aluminium strip.Metal is fed to the top of the water-cooled forged steelmould wheel and completely solid strip is withdrawnfrom the bottom at a speed of up to 35 r.p.m. Recentpatent literature has disclosed a novel form of flowcontrol device for metering metal to this machine.The device consists of a level detector projecting intothe pool of metal at the top of the wheel which causesan air pressure device to make corrective changes inmetal flow rate.

Hunter engineering process : This is a combinedcontinuous strip casting and rolling machine. Moltenmetal is delivered through a metal spreader device tothe bottom of the gap between two water-cooled rolls(Fig. 15). The rolls serve as it mould in which themetal freezes, and as they rotate, the cast strip receivesa hot rolling pass. The machine can produce a 38 instrip at 3/4 r.p.m., which is delivered via a leveller andan edge miller to an up-coiling machine.

Haze lets process : The process produces strip up to36 in wide on a machine by introducing molten metalbetween two endless fl:xihle mild steel belts (Fig. 16).Strip thickness is controlled by adjusting the distancebetween the parallel lengths of the belt, whilst width iscontrolled by provision of side dams attached to thelower belt. Cooling water sprays are applied to bothbelts to obtain rapid chilling of metal. Control of heattransfer conditions is added by treating belt surfaceswith an insulating dressing and this also improvessurface finish of the strip.

Summary of conclusions

The need for increased production rates of aluminiumalloy semi-finished products has resulted in a progress-ive improvement in production methods. Batch meltingand methods of metal treatment in older plants havetended to give way to continuous methods in modernplants, and such methods, especially in the field of metaltreatment, are becoming increasingly important. Specialattention is drawn to continuous methods of degassing,using chlorine, and of oxide removal using the fluxwashing process.

Looking further ahead, considerable alterations inmelting practice are anticipated in the future as new andeven revolutionary melting furnaces are developed. Ex-clusion of molten aluminium from contact with the am-bient atmosphere at all stages prior to casting is expectedto become common practice. Minimising or eliminat-ing inverse segregation during semi-continuous castingwould obviate the need for surface milling prior to pro-cessing and result in major economy and acceleration ofproduction. Further developments in fully continuousmethods of casting are anticipated.

148

Acknowledgement

The author gratefully acknowledges the assistance andencouragement of his colleagues and the Directors ofFoseco International Limited, by whose permission thispaper has been prepared.

References

1 Non-ferrous Metals Statistics -- October 1959 , Organisationfor European Economic Co - operation.Brit . Patent 745, 769- Kaiser Aluminium and ChemicalCorporation.A. Cibula, The Grain Refining of Aluminium Alloy Castingsby Addition of Titanium and Boron-J. Inst. Met., Vol. 80Sept., 1951.

4 D. M. Lewis and J. Savage, The Principlzs of ContinuousCasting of Metals -Inst. or Metals Metallurgical Reviews,Vol. 1, 1956.

s Brit . Patent 757, 569- Kaiser Aluminium and ChemicalCorporation.

6 G. C. Fear, Continuous Casting of Aluminium -J. of Metals,Jan., 1960.

DISCUSSIONS

Mr. N. Majuindar. Indian Aluminium Co. Ltd., Calcutta:I must thank the author for his very interesting paperand I would like to add the following :

The author mentions that holding baths are used foralloying , conditioning and casting in conjunction withmelters. In our view, alloying is best done in themelting furnace as this requires stirring of the bath foruniformity of composition. One important reasonwhy holding baths are used is to enable holding ofthe metal undisturbed for a period before casting soas to effect separation of the oxides. Alloying in theholding bath would defeat this purpose. Secondly,holding baths are normally designed for accuratecontrol of temperature and have relatively low heatinput and melting of alloying elements and hardenersin the holding bath would require considerably moretime.

The author mentions "flux washing" for removalof oxides. There is another notable development sofar as removal of oxides is concerned and that is metalfiltration using fibre glass cloth. The advantage of metalfiltration over flux washing is that it may be appliedat the mould and oxide contamination during launder-ing from the flux washing tank to the mould isprevented . Secondly, the method is simple and requiresno capital investment.

Syphon transfer from melting furnace to the holdingbath as well as from holding bath to the casting unitis being increasingly adopted in large installations.The basic advantage of syphon transfer is the rapiditywith which a large quantity of metal may be trans-ferred in a relatively short time. Metal flow througha large tapping hole is difficult to control and the rate

of transfer as such is limited to about 300 Ibimt/tappinghole whereas, by syphoning as much as 10,000-15,000 lbof metal may be transferred per minute.

Mr. Reeve mentions about continuous degassingusing nitrogen or chlorine. In this field there is anotable development, the so-called "gas lift", wherebythe level of hydrogen in strong alloys of aluminiummay be reduced to a level hitherto unattainable byconventional degassing methods. The principle issimple and is illustrated below

MOLTEN METAL LIFTED AS FROTHY MASS

Mr. M. R. Reeve (Author): I am very grateful toMr. N. Majumdar for his comments and would agreethat alloying in the melting furnaces is common practicein most plants. However, in the layout of some modernplants, increased flexibility is achieved by operatingone or more holding furnaces in conjunction with onelarge capacity melter. Alloying in the holding fur-naces is perfectly feasible, provided the weight ofalloying addition is not too great and one or moredifferent alloys can then be produced from metalmelted in a single furnace.

Siphoning devices and also molten metal pumps are,of course, being developed for emptying metal baths.It is understood that difficulties are currently beingencountered in devising a refractory with suitable pro-perties for these applications, even the best gradesof silicon carbide available being not entirely adequate.

Regarding glass cloth treatment of molten aluminium"filtering", in my opinion, is really a misnomer and'istraining" would be a more correct term as the finestcloth normally used is 110 mesh. This cloth willremove only the coarser non-metallic particles and evensome of these get through if they are needle shaped,together with finer particles which are a cause of manydefects in wrought products. A "chemical filter" orflux washing treatment will remove these finer particles,and is therefore incomparably more efficient.

149