MELTING ALUMINIUM• Molten aluminium has a very high affinity for oxygen • but the oxide skin which forms on the surface of the melt effectively protects it from

further oxidation. • The tendency is to use melting furnaces in which the minimum of turbulence is

produced at the surface of the molten metal so that the oxide skin remains unbroken. • crucible furnaces were widely used for melting aluminium alloys• but in recent years the tendency has been to use such furnaces for holding molten

metal rather than for melting it, bearing in mind the low thermal efficiency of the crucible.

• These holding furnaces may be bale-out or tilter furnaces fired by gas or oil.• Reverberator furnaces are now more widely used for melting purposes and these are

generally designed so that instead of the flame impinging on the surface of the charge it is directed at the roof of the furnace, thus reducing turbulence of the molten metal.

• A modern unit used for melting aluminium alloys and working either on the shaft or shaft/reverberatory principle is the 'tower' furnace

• The recovery of swarf, baled foil, decorated foil scrap and tube off-cuts is often carried out in the coreless induction furnace where the stirring action assists in absorbing thin scrap before it is oxidised.

• The principal disadvantage in channel type induction furnace use is that the channel tends to become clogged with small particles of aluminium oxide.

• Since these particles are not affected by the magnetic field they tend to be 'squeezed' to the outside of the metal stream and on to the refractory walls of the channel itself.

• Hence the Tama-type channel furnace is generally used for melting aluminium. • It has rectangular channels which can be cleaned periodically using suitable

broaches.• In recent years the bulk melting of aluminium and its alloys has become

popular. • Thus, in the Birmingham area, delivery of the molten metal from melter to user

sometimes involves a road journey of up to 16 km.• Hydrogen, which may arise from various sources, dissolves readily in molten

aluminium alloys, but is almost insoluble in solid alloys. • As solidification takes place - the gas is liberated in the partly solid casting,

producing blow-holes• Reverberatory melting increases the danger of hydrogen absorption, since the

gas may exist in considerable quantities in the furnace atmosphere. • Hence the reverberatory furnace is used mainly for melting alloys for die-

casting, where, due to the rapid solidification which takes place gas porosity is less able to manifest itself.

• Both oxidation and gas solution increase with temperature, and it is therefore important that accurate pyrometric control is available to avoid overheating the melt.

• It is particularly important that those alloys containing zinc or magnesium should not be overheated.

• Much of the former would be lost by volatilization, whilst the latter oxidises readily at high temperatures.

• The necessity for super-heating molten aluminium alloys should be avoided where possible by preheating stirrers, plungers and ladles so that their chilling effect on the molten metal is reduced.

• Whilst overheating will accelerate the formation of dross, it is by no means the only cause.

• As already mentioned, unnecessary turbulence or stirring of the melt will repeatedly expose fresh metal surfaces which immediately oxidise and form additional scum.

• use of small scrap, with its relatively large area of oxide skin, and corroded scrap will increase the amount of dross produced.

• most frequent cause for the rejection of aluminium alloy castings is porosity• due to the liberation, during solidification, of hydrogen which has been dissolved

by the metal at some stage in the melting process

• hydrogen does not arise from the negligible amount of the free gas present in the atmosphere

• generally produced as a result of a chemical reaction between the molten aluminium and water vapour from some other source2Al + 3H2O = Al2O3 + 6H

• Whilst this hydrogen is still in its atomic state it is readily absorbed by the molten aluminium.

• It is interesting to note that 1 m3 of air may contain as much as 10 g of water vapour. • If this water vapour reacts with molten aluminium according to the above equation

more than 1 g of hydrogen will be liberated, and this would be sufficient to cause such unsoundness in about 1 tonne of cuttings as to make necessary their rejection

• solubility of hydrogen in solid aluminium is very low, but as the metal melts the solubility increases very rapidly

• Conversely as the molten aluminium solidifies the solubility of hydrogen falls again so that it is rejected from solution,

• Since dendrites of metal have already begun to form, the tiny bubbles of gas are trapped between the growing arms, forming blow holes in the resultant casting.

• In practice, molten aluminium with a gas content of less than 100 mm3/kg will produce castings virtually free from porosity.

Common sources of moisture which give rise to the production of hydrogen include:(a) products of combustion in the furnace atmosphere;(b) water vapour in the atmosphere of the foundry itself;(c) corrosion products on the surface of the scrap;(d) condensed moisture on the surface of the scrap;(e) badly dried melting pots and foundry tools;(f) sand adhering to scrap, such as runners and risers.

• The oxide skin which forms on the surface of the melt prevents to some extent contact between the metal and either the foundry atmosphere or the products of combustion.

• The skin must therefore be disturbed as little as possible when further additions of the charge are being made.

• When aluminium alloys are melted in pit furnaces the crucible should be covered with a lid or, better still, the molten charge should be protected by a layer of flux.

• The use of flux is essential with alloys containing more than 2% magnesium, since these alloys are very prone to the solution of hydrogen

• In order to expel moisture from corrosion products present on the surface of ingots or scrap, the material should be preheated on the side of the melting furnace before being charged to the crucible.

• Because of their greater heat capacity, ingots require heating for a longer period than thin scrap.

• This prolonged preheating is necessary to evaporate moisture, which condenses initially on the ingot surfaces from the products of combustion emitted from the furnace.

• Preheating of ingots should be common practice with all metals, since the presence of moisture on the ingot surface will cause molten metal to be thrown out of the furnace with explosive violence when such ingots are charged to the crucible

• Hydrogen may also be produced as a result of reactions between the molten charge and various lubricants adhering to process scrap, whilst ingot metal which was originally melted under unfavourable conditions may contain blow-holes filled with hydrogen.

• This hydrogen may be dissolved again by the metal as it melts.DE-GASSING ALUMINIUM ALLOYS• If an aluminium alloy has been melted under conditions which have allowed the

absorption of hydrogen to take place, then a de-gassing treatment must be applied to the molten metal just before casting it.

• hydrogen can be removed in one of the following methods• (A) by bubbling another gas through the molten alloy• (b) by treatment of the molten alloy with a suitable liquid flux in the presence of a hydrogen-free atmosphere;• (c) by pre-solidification• If the hydrogen in solution in the molten alloy is in equilibrium with an atmosphere

containing a certain amount of hydrogen, then, obviously, removal of all the hydrogen from the atmosphere in contact with the melt will lead to the evolution by the metal of its hydrogen content

• As this evolved hydrogen is continually swept away, more will diffuse from the metal into the atmosphere until equilibrium is reached at zero hydrogen content of both metal and atmosphere.

• The removal of hydrogen from the melt by diffusion into a hydrogen-free atmosphere is not quite so simple as it sounds, because not only must the atmosphere be free from hydrogen and hydrogen-containing gases, such as water vapour, but the metal surface must also be clean and free from oxide-film barriers, to enable the gas to diffuse into the atmosphere.

• Thus methods (a) and (b) mentioned above are similar in principle. • In each case intimate contact is effected between a dry hydrogen-free gas and

the molten metal; on the one hand, by passing bubbles of the gas through the melt and, on the other, by fluxing the oxide skin at the surface of the melt and exposing the metal itself to dry air.

• the gas method is the most frequently used, and involves passing either nitrogen or chlorine through the melt.

• The function of this process as indicated above is to 'flush' hydrogen from the molten alloy.

• The gas, whether nitrogen or chlorine, is supplied in cylinders, and it goes it must be as free as possible from moisture, since the latter would tend to introduce hydrogen into the melt

• A graphite or refractory tube is used to bubble the gas through the molten alloy, and it is connected to the gas cylinder by rubber tubing which carries an asbestos-wool filter.

• The refractory tube, which should reach to the bottom of the melt, is best closed at the lower end and drilled with a number of 3 mm diameter holes rather in the manner of a domestic gas poker.

• Before de-gassing is carried out the fuel supply should be turned off.• Treatment is usually carried out with the melt at a temperature of 730° C if

chlorine is used, but de-gassing with nitrogen is not very effective below 740° C. • The flow of gas should not be such that excessive• Turbulence is caused at the surface of the melt; a steady stream of bubbles is all

that is required. • It is advisable to remove most of the dross from the surface by means of a

suitable liquid flux before de-gassing is attempted. • Fluxes commonly employed for this purpose contain mixtures of the fluorides

and chlorides of magnesium and potassium for use with magnesium-bearing alloys,

• mixtures of sodium chloride and sodium silicofluoride for use with magnesium-free alloys

• When the dross has been removed along with the layer of flux the surface of the melt is protected by the addition of a further layer of flux.

• Effective de-gassing takes from 10 to 20 minutes, depending largely upon the size of the melt, which is then allowed to stand for a further 10 minutes before removing the flux layer with a perforated ladle. The melt is then cast.

• The main advantage in the use of nitrogen for de-gassing is that unlike chlorine it is not poisonous.

• In other respects, however, nitrogen is less satisfactory than chlorine.• Not only does cylinder nitrogen contain some moisture and oxygen but, being

lighter than chlorine, it also disperses more quickly into the atmosphere. • Chlorine, being a heavy gas, forms a protective layer on the surface of the melt,

and this further assists in the removal of hydrogen. • some aluminium chloride forms as a heavy vapour along with the chlorine and

makes its own contribution towards de-gasification and protection.• Despite its toxicity therefore, chlorine is much more widely used than nitrogen as a

de-gassing agent. Provided that efficient fume extraction is employed,• chlorine is quite safe to use, and has the further advantage that it has a cleansing

action on the melt. • The use of chlorine tends to produce coarse grain in some alloys, but this can be

overcome by adding a grain refiner after degassing is complete.

• All the advantages attendant on the use of chlorine as a de-gasser can be obtained by treating the melt with the compound hexachlorethane,

• This is a solid which when plunged below the surface of the melt decomposes with the liberation of chlorine.

• Obviously this provides a convenient method of de-gassing, since gas cylinders and ancilliary apparatus are not required.

• Suppliers of foundry equipment sell hexachlorethane—generally under a trade name—as either powder or tablets in weighed units for the treatment of a specific amount of molten alloy

• When de-gassing with hexachlorethane, the furnace is first shut off before the melt has reached its casting temperature.

• The temperature which the melt should have reached when the fuel supply is cut off will be determined largely by experience, and will be related to the size of the charge and the heat capacity of the furnace.

• The surface of the melt should be treated with flux to remove most of the dross and covered with another thin layer of flux.

• The temperature of the melt should be between 700° and 750° C. • The required amount of hexachlorethane is plunged below the surface with a special

bell-shaped plunger and held near to the bottom of the crucible until the evolution of chlorine has ceased.

• This should take about 3 minutes.

• C2 Cl6 2C + 3Cl2

• The plunger is then removed and the melt allowed to stand for about 10 minutes during which time it should attain its pouring temperature (the fuel supply is, of course, still turned off).

• Finally, the flux is stirred into the surface of the melt in order that it shall dissolve dross and other suspended matter.

• The flux is then skimmed off with a perforated ladle and the melt is ready for casting.

Treatment with a liquid Flux• It was mentioned earlier that the oxide scum which forms on the surface of

molten aluminium is a fairly effective protection against the absorption of hydrogen and that, conversely, it will also prevent any dissolved gas from escaping again, as would happen if the surface of the molten alloy were exposed to dry air after the melting process was complete.

• Reasonably effective de-gassing is therefore obtained by covering the surface of the melt with about 1% by weight of a dry mixture consisting of two parts sodium chloride to one part sodium fluoride

• As soon as it melts this flux is rabble into the surface of the melt for about 5 minutes.

• It is then thickened by the addition of some higher-melting-point flux so that it can be removed with a perforated ladle.

• This treatment is most effective if the crucible is removed from the furnace before the flux is added, since water vapour from products of combustion may still be present in the furnace atmosphere after the fuel has been cut off.

• However, removal from the furnace may only be possible in the case of large crucibles with a big heat capacity, and small crucibles which cool quickly may have to remain in the furnace whilst treatment is carried out.

• One disadvantage of treatment with flux of this nature is that the liquid flux attacks refractory crucibles.

• This can be overcome to a large extent by confining the flux in an iron ring which fits into the top of the crucible

• Although the flux does not come into contact with the sides of the crucible, it cleans a sufficient area of the surface of the melt for its action to be effective.

• Pre-solidification is sometimes applied to reduce the gas content of very gassy melts.

• The molten metal is allowed to cool and solidify very slowly in the crucible, care being taken to prevent the surface from freezing over until the rest of the melt is solid.

• During cooling and subsequent solidification the solubility of the hydrogen falls almost to zero, and if the surface is prevented from freeezing over, most of the gas will be liberated.

• The charge can then be remelted under satisfactory conditions and should be relatively gas-free.

• Treatment of this type is best carried out in iron or steel pots, since refractory crucibles would be liable to crack when attempts were made to remelt the solid charge.