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THE FEEDING OF LIGHT ALLOY CASTINGS
CHAPTER 17
THE F E E D I N G OF L I G H T ALLOY CASTINGS T. A. BURNS
The light alloys normally include the whole range of aluminium base alloys together with the magnesium base range. Neglecting the fact that special pre cautions with respect to exposure to atmosphere have to be taken with the magnesium base range both can, for the purposes of considering feeding, be conveniently dealt with together. Although the magnesium range of alloys normally require pouring tempera tures of the order of 100°C or more above those for aluminium both groups can be considered as falling within a medium pouring temperature range and the requirements for aluminium, which is the more common group and will be discussed in detail, will also apply to magnesium.
FREEZING RANGE
Both short and long freezing range alloys occur within the group. The majority of commercial castings are made from the aluminium/silicon/copper alloys when the freezing ranges are variable but are, in general, long. Only pure aluminium, which is little used except for slush casting and some pressure diecastings, together with the eutectic aluminium/silicon alloys possess a short freezing range. Theoretically the extent of the freezing range can be an important consideration since it will affect the way a casting solidifies. It may simplify some problems but enhance others. When a pure metal or short freezing range alloy is cast against a mould there is a strong initial chilling effect from the mould that will give a thin layer of fine equi-axially orientated crystals but thereafter solidification will proceed against the thermal gradient (heat is being lost at a more or less constant rate to and through the mould material) by means of growth of columnar crystals. In other words a freezing front will advance in fairly regular fashion from the mould wall and virtually parallel to it. Unless adequate feeding arrangements are made for an alloy freezing in this manner gross unfed shrinkage defects are likely to result at the centre of the section or at hot spots. On the other hand a long freezing range alloy, after the initial skin chilling effect, will form random crystal nucleii dispersed throughout the body of the casting, these "seeds" then growing as the mass slowly cools, at a rate proportional to the rate of heat extraction, until they touch each other and form a coherent network or skeleton, further growth being restricted by availability of liquid feed metal. 499
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APPLIED SCIENCE IN THE CASTING OF METALS
D = kV - C where D = thickness of solidified metal, k = a constant depending on casting size and mould conductivity, c = a function of degree of superheat. SOLIDIFICATION BY SKIN FORMATION (narrow freezing range alloys) iLiq.
crystals
advancement; of continuous;. solidification^ front
coarse columnar grains 40% Si
Jl1-7%Si
FIG. 1. Solidification by skin-formation (narrow freezing range alloys). SOLIDIFICATION OF WIDE FREEZING RANGE ALLOYS (i.e."Pastv"solidification) Pasty band Mg^Alg ♦ Liq.
advancement ofdiscontinuous solidification front 'equiaxed crystals
100% Al
'lO%Mg
35% Mg
FIG. 2. Solidification of wide freezing range alloy.
At any intermediate stage, therefore, these alloys will consist of a mushy mass of solidified crystals surrounded by still liquid metal. This type of solidification pattern is most likely to give unfed shrinkage defects in the form of small dispersed interdendritic cavitation with, however, some tendency again to concentrate at heat centres. The two forms of solidification are neatly summed up by the descriptions "skin formers" and "pasty solidifiers".
SOLIDIFICATION PATTERN
Bearing the foregoing in mind it will be appreciated that it should only be necessary to ensure that metal in those parts of the mould remote from the feeders should begin to solidify first and that solidification should then proceed continuously back to the riser to ensure completely sound castings. This will apply equally to long and short freezing range alloys provided the thermal gradient is sufficiently steep. The term "directional solidification" is used to
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describe this pattern and provided certain requirements are met then a sound casting must result. These requirements are principally that the desired axis of solidification must proceed at a faster rate than a lateral advance from side walls otherwise bridging will occur to hinder the flow of feeding metal. It also assumes that in a long freezing range alloy the "pasty" band between fully solidified and fully liquid metal is relatively narrow, which condition is seldom approached in a practical casting due to interference from casting shape, moulding, running and feeding factors, etc. It is probable that under practical conditions of casting light alloys and particularly where the rate of solidification is relatively slow, e.g. in sand and gravity (permanent mould) die castings almost the whole mass of solidifying metal consists of a mixture of solid and liquid irrespective of the classification of the alloy in terms of freezing range. This is because of the necessity for bottom gating and consequent temperature inversion together with the effect of turbulence. For the purposes of running and risering a casting therefore it is not usual to take undue account of the charac teristics of the particular alloy but to apply certain general considerations covering the light alloy group as a whole. Nevertheless, these will be aimed at satisfying the needs of good general directional solidification and may be summarized as follows: 1. 2. 3. 4. 5.
Correct gating and feeding. Correct pouring temperature. Use of chills. Nucleation. Correct mould dressing (mainly gravity diecasting).
1. Correct Gating and Feeding (a) Sand Castings Because of their ready oxide-forming properties the light alloys must be cast via bottom gates to eliminate or control oxide formation within the mould cavity and possible subsequent entrainment within the casting proper. Since also, to take advantage of gravity and for convenience, risers are situated on the top of a casting it is apparent that the first metal into the mould, and therefore the coolest, is most likely to be displaced into the feeder heads. An inverted undesirable temperature gradient could therefore be set up that would quite likely persist throughout solidification unless the risers were particularly massive. Adverse temperature differences of the order of 20°C between the top and bottom of an uphill run sand cast test bar 1 in. diameter x 11 in. long in aluminium and magnesium alloys have been observed. With most light alloy sand castings moulding and/or coring considerations will dictate which way up the casting will be made and it follows that sites for ingates and risers will probably be restricted to "obvious" areas to conform to the general scheme of bottom gating and top feeding. Some correction of the consequent unfavourable temperature gradient can often be obtained by pouring
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A P P L I E D SCIENCE IN THE CASTING OF METALS
the casting through the running system until the metal just enters the base of the risers and then topping up with hotter metal from the crucible direct into the risers. This method is often useful with medium or even some heavy section castings but is unlikely to succeed with light castings since the rate of solidifica tion is generally too rapid.
©
®
® F I G . 3. Semi-Durville casting process.
Some complicated castings may even require a form of semi-Durville type running system in which a pronounced tilt is given to the mould at the commence ment of pouring, the mould being returned to horizontal as pouring proceeds. In addition to its principal aim of reducing turbulence the system can also assist feeding in that the first metal in is relatively localized and return to horizontal can, depending of course on casting shape, ensure that this cool metal is displaced gently to a remote mould region leaving the hotter follow-up metal in a more favourable position from the temperature gradient point of view. If this mould tilting method is used, the ingate/downsprue should always be at the lowest point in the tilted mould. The method can also be used in conjunction with subsequent topping up of risers as discussed previously. It has sometimes been proposed that a secondary gate or bridge from the downsprue into the base of the riser, if conveniently located, can ensure hot metal into the riser and so provide available feed potential. The principal objections to this system are essentially that the flow of molten metal down the
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sprue, once established, is difficult to change and the bridge seldom performs as it is supposed to, the metal continuing to enter the casting via the normal gate. Additionally the bridge is idle during the early part of the pour, interferes with the geometry of the running system and usually only provides an ideal configura tion for the aspiration of air into the metal stream by virtue of a venturi effect. Such bridging may also introduce mechanical stressing of the casting due to differing rates of solidification, so much so that some casting may hot tear or "spring" during fettling. The objections are considered to outweigh the ad^ vantages by a wide margin and any proposed use of the method should be carefully considered as a more acceptable and more efficient system can most probably be substituted.
FIG. 4. Use of branch gate or bridge.
So far running and risering have been considered only as two distinct systems performing separate functions but this need not necessarily always be so. In addition to introducing metal in as quiet and clean a condition as possible to the mould cavity the running system, suitably sized and laid out, can also perform a useful function as a source of feed metal. To prevent pressure build up and avoid squirting into the mould, also to exercise a skimming effect, running systems for light alloys are often sized around 1:2:1 or thereabouts in terms of ratios of effective cross-sectional areas of downsprue/runner bar/ ingate(s). Control of the metal velocity and volume depends only on the downsprue and ingate(s) cross-section and it is possible to increase cross-sectional area and mass of the runner bar without detriment to the mechanics of the system as a means of controlling metal flow. The law of continuity states that in a system of filled channels the flow rate is the same at all points and where the channel cross-section is smaller the velocity will be correspondingly higher whereas where the cross-section is larger the velocity will have to be corres pondingly lower to maintain the system filled. A suitably enlarged runner bar R
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APPLIED SCIENCE IN THE CASTING OF METALS
can therefore function as a reservoir of liquid feed metal and since the running system as a whole must pass the entire volume of metal to make the casting it will pick up sufficient superheat to enable it to function effectively as a feeder. The weak point is of course the ingate area since despite the preheat due to the passage of metal it may still freeze off prematurely. The system as outlined is often extremely useful and further variations are possible extending to the use of shrinkbobs or even side risers or full box height risers located on the running system rather than on the casting proper. Very seldom, however, can the method be used as the sole means of feeding a light alloy casting and it is usually used supplementary to conventional risering. It should be remembered that by definition the light alloys have a low specific gravity and the displacement factor with respect to mould air is therefore low. Top risers provide an escape route for displaced air and so help prevent air locking. This is a further con sideration that should be borne in mind when methoding a casting with respect to siting of risers and it should not be confused with mis-running due to low pouring temperatures. The difficulties, both practical and theoretical, of ensuring favourable thermal gradients in a solidifying light alloy are, therefore, well recognized bearing in mind that the liquid-to-solid volume contraction, which will require feeding, is around 7% theoretical but, as has been indicated, about 3 times this figure is required practically, it is understandable that relatively heavy risers, etc., must be used. The average aluminium alloy sand casting will carry virtually its own weight of risers and running system, i.e. casting yield will be around 50 % assuming an all green sand mould without additional feeding aids (which will be discussed later). Riser Shape The function of a riser is to retain a supply of available feed metal for as long as there is a demand from the casting. It is, therefore, important that it should lose its heat at as low a rate as possible and thermodynamic considerations indicate a sphere as having the lowest surface/volume ratio and hence the ability to retain heat longest. Except possibly in the case of disappearing" expanded polystyrene or polyurethane premoulded forms a spheroidal riser is a practical impossibility and the next thermally most efficient shape—a cylinder— should be used. Again, however, efficiency will decrease as the height: diameter ratio increases and as far as possible this should be kept fairly close to 3:1, i.e. a 2 in. diameter riser will be 6 in. high. The area of contact with the casting is also important since this is the route for feed metal and necking down must be restricted, to prevent premature freezing, only to allow access for fettling tools. Sometimes the casting shape cannot be conveniently fed from cylindrical risers and kidney shapes or wedges must be used. The isotherms in an open riser, assuming steady-state conditions, indicate that the heat centre, i.e. the last part to solidify, will be at or slightly above the geometric centre due to apid heat loss from the surface to atmosphere. The geometric centre of a
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V section wedge will be nearer the open end of the V and thus relatively distant from the casting. Such a riser is likely to bridge across in the early stages of solidification and its full potential will not be realized. A much better section from this point of view is a chunky U that brings the heat centre down much nearer the casting. Heat loss to atmosphere from an open riser can be an important factor affecting temperature gradients in a cast mould; obviously its effect will be greater the larger the surface area of the riser. The major proportion of a casting/ riser complex will be in contact with the sand and while the mould will have a strong initial chilling effect—a cold damp surface—once the immediate surface layers have heated up and moisture has been driven deeper into the sand, further temperature loss will be at a slow steady rate. On the other hand both radiation and convection will continuously assist rapid loss from a riser surface. Full riser potential will, therefore, be better obtained if a layer of insulating, or better still, a suitably sensitized exothermic material, is applied immediately on completion of the pour. An insulating layer will equate heat loss all round the riser surface but some exothermic materials, if correctly chosen, release sufficient heat both to have a powerful self-heating effect and also increase riser temperature. Burnt out exothermic materials should subsequently provide good insulation and in both cases, exothermics and insulators, further heat loss to atmosphere is severely restricted and the riser is that much more effective. Sleeved Risers—Insulating and Exothermic Sleeves Pursuing the above idea a stage further there would obviously be considerable advantage in lining the whole riser with heat conserving materials. While heat losses from the riser to the sand of the mould are not excessive any improvement would be welcome since it would increase efficiency and yield and also provide a safety factor to compensate for other minor variations that might affect casting soundness. Materials having better insulating values than green sand and which have been used for fabricating riser sleeves are numerous and include resin bonded sand, asbestos paper and board, plaster of Paris, oil bonded sand, etc. For a long period the most popular insulating sleeve material was an aerated or foamed form of plaster of Paris. A particular refractory type of plaster is mixed with water in the usual way and a small quantity of surface active agent then added before transferring to a high speed mixer whose principal purpose is to entrap thousands of small air bubbles in the slurry. These bubbles are stable and are trapped in the solidified plaster thus further improving the insulating properties. The external surfaces of such sleeves are continuous and show no evidence of the internal honeycomb effect. After proper drying the best insulating charac teristics of such sleeves are markedly superior to green sand. Recently even further improvements have been obtained from the use of a bonded fibre type of sleeve where in addition to improved heat conservation other advantages include resistance to damping back from the green sand mould in which they are rammed and a toughness and resilience notably absent in the extremely
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APPLIED SCIENCE IN THE CASTING OF METALS
brittle plaster sleeves. Better technical performance, better handling charac teristics and a faster production rate (plaster sleeve production is extremely slow due to mixing and pouring requirements and core box limitations) have meant these felt-like fibre sleeves have practically superseded all other forms of insulation and use has been extended to launders, furnace plugs, semicontinuous cast die heads, etc., where their particular properties have proved attractive.
900\ 800]
700-
1
600]
500' 400
RISER MATERIALS 1) Exothermic (FEEDEX A) ""**** (a)Riser top uncovered. ,,_„.„_„,. (b) „ .. covered with 1"of exothermic(FEEDEX*) 2) Insulating foamed plaster (a) No top cover. (b) 1" cover of plaster. N.B. Plaster contained 50% 3) Green sand 4) ■Felt-like" fibre sleeve.
sleeve
***
2a
voids.
15 20 10 TIME (minutes) ADDITIONAL DATA:- Pouring temperature of metal - 700 °C. Mould material -green sand. Riser sleeve dimension = height 6"t outside dia. 4" wall thickness 1/2". 5
FIG. 5. Cooling curves of commercially pure aluminium showing the effect of riser material on solidification of the metal.
A further gain in thermal efficiency of a riser, as can be seen from the cooling curve diagram, can be obtained from an exothermic sleeve. A range of exo thermic materials is available to suit all commonly cast metals and riser diameters and from this range can be selected a type suitable for the light alloys. Generally speaking, since they need to be rapidly sensitive to medium range metal tem peratures the sleeve sensitivity will fall into the high sensitivity grouping, i.e. they will fire quickly when the molten metal touches them, burn hot and provide good subsequent insulation. Internal temperatures of a burning high sensitivity exothermic material can go as high as 1700-1800°C so even allowing for an approximation that some of the heat will escape to the mould material, a considerable input to the riser itself is possible. This dramatic restoration of favourable temperature gradient often means that appreciable economy in head size, and therefore improvement in yield, is possible.
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In both cases, of course, i.e. insulating and exothermic sleeved risers, the full effect must be maintained by application of a layer, of approximately the same thickness as the riser wall, of a similar material on to the riser/air interface. In the case of insulating sleeves it is permissible and advantageous to apply an exothermic carbonaceous type powder as the anti-piping compound or hot topping material.
FIG. 6. Casting in "Alufont 3". A railway carriage bearing plate with sand heads and exothermic sleeved risers showing the considerable saving of metal.
Riser Size As previously discussed a continuous struggle exists in light alloy founding between the necessity for bottom running and the high feed requirements of a casting against a usually unfavourable temperature gradient. For plain, unsleeved risers in green sand this means that the effective diameter (in the case of cylindrical risers) must be appreciably greater than the section it is intended to feed. The theoretical aspects of riser dimensioning in respect of a casting/riser system in thermal equilibrium are dealt with elsewhere. Almost no fundamental work on heat flow or even thermal characteristics of the light alloys at or near the solidification point is available to put green sand riser dimensioning on a scientific basis and as a result the subject has been treated essentially to an empirical approach. In practice this usually means that a large safety factor is allowed and any subsequent adjustments reluctantly made, if the casting is
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APPLIED SCIENCE IN THE CASTING OF METALS
successful, on a "hit or miss" basis. This view is understandable in the light of the complexity of the problem where the interplay of such factors as casting shape, pouring temperature, metal/sand mass, absence or presence of coring and metal/core relationship, mould material, composition of the alloy and its freezing range, etc., can all affect results to varying degrees. Most castings can be visually sectioned into regular geometric shapes and the heat centres (thicker sections that will need feeding or chilling) will suggest themselves. Arrangements can then be made to supply feed metal to these areas from suitably placed and sized risers or from a modified running system as previously discussed. To isolated bosses and thicker sections it is usual to allow 100% feed metal, i.e. the riser will be the same mass as the visually isolated section to be fed, and riser/casting contact area must be carefully considered to prevent premature freezing and bridging over. Surface/volume ratios and probable temperature gradients indicate that the riser should be somewhat thicker in section than the section to be fed and factors of 1-3 to 1-5 times the casting thickness are usual. Similar calculations must be applied to any blind risering (side risers that do not reach the top of the mould— unusual in light alloy practice) or runner bar feeding. The general feed requirements of a given casting can, therefore, be built up into a composite picture and the optimum running and risering systems to satisfy these demands worked out. (b) Gravity Die (Permanent Mould) Castings Although the overall rate of solidification in a metal mould is very much faster than in sand, similar arrangements in respect of ensuring a preferred solidification pattern must be made to obtain good castings consistently. Because of the speed of solidification, temperature gradients are steep and feed requirements short and sharp. It is, therefore, usual to apply very generous risering (especially runner/risering) to gravity die castings, the lower yield being accepted in view of the increased production rate possible with the process combined with the quicker return of clean metal for remelting. Similar considerations with regard to bottom running, risering, etc., will apply as described for sand castings and a proposed casting may be visually assessed in the same way to determine location of gate(s), riser(s), etc. Here, however, more regard must be paid to die construction since it may not be possible to incorporate all the desired features in a rigid mould. Gravity dies must obviously be kept as simple as possible with the minimum of coring, loose pieces, draw backs, etc. Die operating temperature is obviously an important factor in promoting or opposing a good solidification pattern and running and risering is often overgenerous in terms of actual requirements in order to maintain die temperature alone. For further details on these points and on some of the expedients and coating arts commonly used to produce acceptable castings see under the appropriate chapter.
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2. Correct Pouring Temperature To an important extent the pouring temperature of an alloy can have an effect on grain size, shape and distribution in the solidifying casting and this factor can sometimes be used to advantage, e.g. with some copper base alloys in ensuring a columnar pressure resistant skin. With aluminium alloys, however, because the solidification rate, whatever the method of moulding, is quite rapid, the effect is not so useful and in general only a pouring temperature just sufficient to ensure a completely run casting is employed. Long solidification periods in the mould must be avoided because of possible gas pick up, high volume contraction and the normal pasty method of crystallization. The correct pouring temperature must therefore be decided upon—and adhered to—in the light of casting section, alloy used and running method employed. High pouring temperatures—say above 760°C/780°C—and consequent longer time at high temperature in the furnace must be avoided from the points of view of enhanced possibility of gas absorption (see chapter on Aluminium) and grain coarsening due to aggregation of nucleii. Where a large surface area-thin section casting is involved it is preferred that, rather than increase the pouring temperature to run the thin sections, the running/ingating system be enlarged and the casting made more quickly using multiple ingates if necessary.
3. Use of Chills The desirability of directional, controlled solidification has been stressed in earlier sections. Chills in the form of metallic inserts in the mould (sand castings) obviously remove heat from the casting at a more rapid rate, at least during the early part of solidification, than the relatively insulating sand and they can, therefore, often be used to advantage to propagate a preferred gradient. Localized surface sinks at isolated bosses and section junctions can often be overcome with suitably sized and located chills, since they will have the effect of evening out small section variations. Chills can be of steel, cast iron, copper or even aluminium. They should be at least as thick as the section requiring chilling and massive individual contact areas with the casting should be avoided: instead use multiple smaller or well drilled chills to allow escape of gas. Chills should always be kept in good condition and coated with a suitable dressing to reduce damping back and to decrease the effect of any hard initial chilling that could leave a shut on the casting. Blowing from chills is usually the result of sand moisture condensation on the chill face so they should be preheated and the moulds closed and cast without delay. A proper dressing will also prevent welding—especially important in the case of copper or aluminium chills. Run ning/ingating systems should be arranged so that incoming metal does not continuously flow over any chills thus strongly heating them and destroying the effect required.
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APPLIED SCIENCE IN THE CASTING OF METALS
4. Nucleation The chapter on Aluminium (Chapter 7) has described how aluminium and its alloys can be grain refined by means of the addition, in salts or metallic hardener form, of suitable foreign nucleii of similar lattice structure. It follows that during solidification, particularly with the wide freezing range alloys, better mass feeding will be obtained if the solids content of the pasty mass is in a finely divided, i.e. nucleated, form. Mechanical bridging will be less likely to occur and effective mass movement and hence better feeding, more likely to persist to a later stage in the solidification process. Provided, therefore, that the methoding of a particular casting is correct, nucleation of the metal can provide some assistance in the feeding process. If mould design and layout is unsatisfactory, however, the reverse may apply and grain refinement will succeed only in collecting a dispersed shrinkage distribution into a gross cavitation.
5. Correct Mould Dressing In a sand mould and similarly in a pressure die casting, the nature and applied thickness of any dressing can have no significant effect on the mode or speed of solidification. On a gravity die, on the other hand, the die dressing exists as a layer of (usually) insulating material between two masses of metal. Its charac teristics and thickness can therefore play an important part in governing heat flow from the cast metal to the die. It follows that the type of die dressing and the way it is applied can have a controlling effect on the mechanism and direction of solidification and therefore on the quality of the casting. These aspects are discussed in greater detail in Chapter 12 on gravity die casting.
THE F E E D I N G OF L I G H T ALLOY CASTINGS T. A. BURNS
The light alloys normally include the whole range of aluminium base alloys together with the magnesium base range. Neglecting the fact that special pre cautions with respect to exposure to atmosphere have to be taken with the magnesium base range both can, for the purposes of considering feeding, be conveniently dealt with together. Although the magnesium range of alloys normally require pouring tempera tures of the order of 100°C or more above those for aluminium both groups can be considered as falling within a medium pouring temperature range and the requirements for aluminium, which is the more common group and will be discussed in detail, will also apply to magnesium.
FREEZING RANGE
Both short and long freezing range alloys occur within the group. The majority of commercial castings are made from the aluminium/silicon/copper alloys when the freezing ranges are variable but are, in general, long. Only pure aluminium, which is little used except for slush casting and some pressure diecastings, together with the eutectic aluminium/silicon alloys possess a short freezing range. Theoretically the extent of the freezing range can be an important consideration since it will affect the way a casting solidifies. It may simplify some problems but enhance others. When a pure metal or short freezing range alloy is cast against a mould there is a strong initial chilling effect from the mould that will give a thin layer of fine equi-axially orientated crystals but thereafter solidification will proceed against the thermal gradient (heat is being lost at a more or less constant rate to and through the mould material) by means of growth of columnar crystals. In other words a freezing front will advance in fairly regular fashion from the mould wall and virtually parallel to it. Unless adequate feeding arrangements are made for an alloy freezing in this manner gross unfed shrinkage defects are likely to result at the centre of the section or at hot spots. On the other hand a long freezing range alloy, after the initial skin chilling effect, will form random crystal nucleii dispersed throughout the body of the casting, these "seeds" then growing as the mass slowly cools, at a rate proportional to the rate of heat extraction, until they touch each other and form a coherent network or skeleton, further growth being restricted by availability of liquid feed metal. 499
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APPLIED SCIENCE IN THE CASTING OF METALS
D = kV - C where D = thickness of solidified metal, k = a constant depending on casting size and mould conductivity, c = a function of degree of superheat. SOLIDIFICATION BY SKIN FORMATION (narrow freezing range alloys) iLiq.
crystals
advancement; of continuous;. solidification^ front
coarse columnar grains 40% Si
Jl1-7%Si
FIG. 1. Solidification by skin-formation (narrow freezing range alloys). SOLIDIFICATION OF WIDE FREEZING RANGE ALLOYS (i.e."Pastv"solidification) Pasty band Mg^Alg ♦ Liq.
advancement ofdiscontinuous solidification front 'equiaxed crystals
100% Al
'lO%Mg
35% Mg
FIG. 2. Solidification of wide freezing range alloy.
At any intermediate stage, therefore, these alloys will consist of a mushy mass of solidified crystals surrounded by still liquid metal. This type of solidification pattern is most likely to give unfed shrinkage defects in the form of small dispersed interdendritic cavitation with, however, some tendency again to concentrate at heat centres. The two forms of solidification are neatly summed up by the descriptions "skin formers" and "pasty solidifiers".
SOLIDIFICATION PATTERN
Bearing the foregoing in mind it will be appreciated that it should only be necessary to ensure that metal in those parts of the mould remote from the feeders should begin to solidify first and that solidification should then proceed continuously back to the riser to ensure completely sound castings. This will apply equally to long and short freezing range alloys provided the thermal gradient is sufficiently steep. The term "directional solidification" is used to
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describe this pattern and provided certain requirements are met then a sound casting must result. These requirements are principally that the desired axis of solidification must proceed at a faster rate than a lateral advance from side walls otherwise bridging will occur to hinder the flow of feeding metal. It also assumes that in a long freezing range alloy the "pasty" band between fully solidified and fully liquid metal is relatively narrow, which condition is seldom approached in a practical casting due to interference from casting shape, moulding, running and feeding factors, etc. It is probable that under practical conditions of casting light alloys and particularly where the rate of solidification is relatively slow, e.g. in sand and gravity (permanent mould) die castings almost the whole mass of solidifying metal consists of a mixture of solid and liquid irrespective of the classification of the alloy in terms of freezing range. This is because of the necessity for bottom gating and consequent temperature inversion together with the effect of turbulence. For the purposes of running and risering a casting therefore it is not usual to take undue account of the charac teristics of the particular alloy but to apply certain general considerations covering the light alloy group as a whole. Nevertheless, these will be aimed at satisfying the needs of good general directional solidification and may be summarized as follows: 1. 2. 3. 4. 5.
Correct gating and feeding. Correct pouring temperature. Use of chills. Nucleation. Correct mould dressing (mainly gravity diecasting).
1. Correct Gating and Feeding (a) Sand Castings Because of their ready oxide-forming properties the light alloys must be cast via bottom gates to eliminate or control oxide formation within the mould cavity and possible subsequent entrainment within the casting proper. Since also, to take advantage of gravity and for convenience, risers are situated on the top of a casting it is apparent that the first metal into the mould, and therefore the coolest, is most likely to be displaced into the feeder heads. An inverted undesirable temperature gradient could therefore be set up that would quite likely persist throughout solidification unless the risers were particularly massive. Adverse temperature differences of the order of 20°C between the top and bottom of an uphill run sand cast test bar 1 in. diameter x 11 in. long in aluminium and magnesium alloys have been observed. With most light alloy sand castings moulding and/or coring considerations will dictate which way up the casting will be made and it follows that sites for ingates and risers will probably be restricted to "obvious" areas to conform to the general scheme of bottom gating and top feeding. Some correction of the consequent unfavourable temperature gradient can often be obtained by pouring
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A P P L I E D SCIENCE IN THE CASTING OF METALS
the casting through the running system until the metal just enters the base of the risers and then topping up with hotter metal from the crucible direct into the risers. This method is often useful with medium or even some heavy section castings but is unlikely to succeed with light castings since the rate of solidifica tion is generally too rapid.
©
®
® F I G . 3. Semi-Durville casting process.
Some complicated castings may even require a form of semi-Durville type running system in which a pronounced tilt is given to the mould at the commence ment of pouring, the mould being returned to horizontal as pouring proceeds. In addition to its principal aim of reducing turbulence the system can also assist feeding in that the first metal in is relatively localized and return to horizontal can, depending of course on casting shape, ensure that this cool metal is displaced gently to a remote mould region leaving the hotter follow-up metal in a more favourable position from the temperature gradient point of view. If this mould tilting method is used, the ingate/downsprue should always be at the lowest point in the tilted mould. The method can also be used in conjunction with subsequent topping up of risers as discussed previously. It has sometimes been proposed that a secondary gate or bridge from the downsprue into the base of the riser, if conveniently located, can ensure hot metal into the riser and so provide available feed potential. The principal objections to this system are essentially that the flow of molten metal down the
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sprue, once established, is difficult to change and the bridge seldom performs as it is supposed to, the metal continuing to enter the casting via the normal gate. Additionally the bridge is idle during the early part of the pour, interferes with the geometry of the running system and usually only provides an ideal configura tion for the aspiration of air into the metal stream by virtue of a venturi effect. Such bridging may also introduce mechanical stressing of the casting due to differing rates of solidification, so much so that some casting may hot tear or "spring" during fettling. The objections are considered to outweigh the ad^ vantages by a wide margin and any proposed use of the method should be carefully considered as a more acceptable and more efficient system can most probably be substituted.
FIG. 4. Use of branch gate or bridge.
So far running and risering have been considered only as two distinct systems performing separate functions but this need not necessarily always be so. In addition to introducing metal in as quiet and clean a condition as possible to the mould cavity the running system, suitably sized and laid out, can also perform a useful function as a source of feed metal. To prevent pressure build up and avoid squirting into the mould, also to exercise a skimming effect, running systems for light alloys are often sized around 1:2:1 or thereabouts in terms of ratios of effective cross-sectional areas of downsprue/runner bar/ ingate(s). Control of the metal velocity and volume depends only on the downsprue and ingate(s) cross-section and it is possible to increase cross-sectional area and mass of the runner bar without detriment to the mechanics of the system as a means of controlling metal flow. The law of continuity states that in a system of filled channels the flow rate is the same at all points and where the channel cross-section is smaller the velocity will be correspondingly higher whereas where the cross-section is larger the velocity will have to be corres pondingly lower to maintain the system filled. A suitably enlarged runner bar R
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can therefore function as a reservoir of liquid feed metal and since the running system as a whole must pass the entire volume of metal to make the casting it will pick up sufficient superheat to enable it to function effectively as a feeder. The weak point is of course the ingate area since despite the preheat due to the passage of metal it may still freeze off prematurely. The system as outlined is often extremely useful and further variations are possible extending to the use of shrinkbobs or even side risers or full box height risers located on the running system rather than on the casting proper. Very seldom, however, can the method be used as the sole means of feeding a light alloy casting and it is usually used supplementary to conventional risering. It should be remembered that by definition the light alloys have a low specific gravity and the displacement factor with respect to mould air is therefore low. Top risers provide an escape route for displaced air and so help prevent air locking. This is a further con sideration that should be borne in mind when methoding a casting with respect to siting of risers and it should not be confused with mis-running due to low pouring temperatures. The difficulties, both practical and theoretical, of ensuring favourable thermal gradients in a solidifying light alloy are, therefore, well recognized bearing in mind that the liquid-to-solid volume contraction, which will require feeding, is around 7% theoretical but, as has been indicated, about 3 times this figure is required practically, it is understandable that relatively heavy risers, etc., must be used. The average aluminium alloy sand casting will carry virtually its own weight of risers and running system, i.e. casting yield will be around 50 % assuming an all green sand mould without additional feeding aids (which will be discussed later). Riser Shape The function of a riser is to retain a supply of available feed metal for as long as there is a demand from the casting. It is, therefore, important that it should lose its heat at as low a rate as possible and thermodynamic considerations indicate a sphere as having the lowest surface/volume ratio and hence the ability to retain heat longest. Except possibly in the case of disappearing" expanded polystyrene or polyurethane premoulded forms a spheroidal riser is a practical impossibility and the next thermally most efficient shape—a cylinder— should be used. Again, however, efficiency will decrease as the height: diameter ratio increases and as far as possible this should be kept fairly close to 3:1, i.e. a 2 in. diameter riser will be 6 in. high. The area of contact with the casting is also important since this is the route for feed metal and necking down must be restricted, to prevent premature freezing, only to allow access for fettling tools. Sometimes the casting shape cannot be conveniently fed from cylindrical risers and kidney shapes or wedges must be used. The isotherms in an open riser, assuming steady-state conditions, indicate that the heat centre, i.e. the last part to solidify, will be at or slightly above the geometric centre due to apid heat loss from the surface to atmosphere. The geometric centre of a
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V section wedge will be nearer the open end of the V and thus relatively distant from the casting. Such a riser is likely to bridge across in the early stages of solidification and its full potential will not be realized. A much better section from this point of view is a chunky U that brings the heat centre down much nearer the casting. Heat loss to atmosphere from an open riser can be an important factor affecting temperature gradients in a cast mould; obviously its effect will be greater the larger the surface area of the riser. The major proportion of a casting/ riser complex will be in contact with the sand and while the mould will have a strong initial chilling effect—a cold damp surface—once the immediate surface layers have heated up and moisture has been driven deeper into the sand, further temperature loss will be at a slow steady rate. On the other hand both radiation and convection will continuously assist rapid loss from a riser surface. Full riser potential will, therefore, be better obtained if a layer of insulating, or better still, a suitably sensitized exothermic material, is applied immediately on completion of the pour. An insulating layer will equate heat loss all round the riser surface but some exothermic materials, if correctly chosen, release sufficient heat both to have a powerful self-heating effect and also increase riser temperature. Burnt out exothermic materials should subsequently provide good insulation and in both cases, exothermics and insulators, further heat loss to atmosphere is severely restricted and the riser is that much more effective. Sleeved Risers—Insulating and Exothermic Sleeves Pursuing the above idea a stage further there would obviously be considerable advantage in lining the whole riser with heat conserving materials. While heat losses from the riser to the sand of the mould are not excessive any improvement would be welcome since it would increase efficiency and yield and also provide a safety factor to compensate for other minor variations that might affect casting soundness. Materials having better insulating values than green sand and which have been used for fabricating riser sleeves are numerous and include resin bonded sand, asbestos paper and board, plaster of Paris, oil bonded sand, etc. For a long period the most popular insulating sleeve material was an aerated or foamed form of plaster of Paris. A particular refractory type of plaster is mixed with water in the usual way and a small quantity of surface active agent then added before transferring to a high speed mixer whose principal purpose is to entrap thousands of small air bubbles in the slurry. These bubbles are stable and are trapped in the solidified plaster thus further improving the insulating properties. The external surfaces of such sleeves are continuous and show no evidence of the internal honeycomb effect. After proper drying the best insulating charac teristics of such sleeves are markedly superior to green sand. Recently even further improvements have been obtained from the use of a bonded fibre type of sleeve where in addition to improved heat conservation other advantages include resistance to damping back from the green sand mould in which they are rammed and a toughness and resilience notably absent in the extremely
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brittle plaster sleeves. Better technical performance, better handling charac teristics and a faster production rate (plaster sleeve production is extremely slow due to mixing and pouring requirements and core box limitations) have meant these felt-like fibre sleeves have practically superseded all other forms of insulation and use has been extended to launders, furnace plugs, semicontinuous cast die heads, etc., where their particular properties have proved attractive.
900\ 800]
700-
1
600]
500' 400
RISER MATERIALS 1) Exothermic (FEEDEX A) ""**** (a)Riser top uncovered. ,,_„.„_„,. (b) „ .. covered with 1"of exothermic(FEEDEX*) 2) Insulating foamed plaster (a) No top cover. (b) 1" cover of plaster. N.B. Plaster contained 50% 3) Green sand 4) ■Felt-like" fibre sleeve.
sleeve
***
2a
voids.
15 20 10 TIME (minutes) ADDITIONAL DATA:- Pouring temperature of metal - 700 °C. Mould material -green sand. Riser sleeve dimension = height 6"t outside dia. 4" wall thickness 1/2". 5
FIG. 5. Cooling curves of commercially pure aluminium showing the effect of riser material on solidification of the metal.
A further gain in thermal efficiency of a riser, as can be seen from the cooling curve diagram, can be obtained from an exothermic sleeve. A range of exo thermic materials is available to suit all commonly cast metals and riser diameters and from this range can be selected a type suitable for the light alloys. Generally speaking, since they need to be rapidly sensitive to medium range metal tem peratures the sleeve sensitivity will fall into the high sensitivity grouping, i.e. they will fire quickly when the molten metal touches them, burn hot and provide good subsequent insulation. Internal temperatures of a burning high sensitivity exothermic material can go as high as 1700-1800°C so even allowing for an approximation that some of the heat will escape to the mould material, a considerable input to the riser itself is possible. This dramatic restoration of favourable temperature gradient often means that appreciable economy in head size, and therefore improvement in yield, is possible.
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In both cases, of course, i.e. insulating and exothermic sleeved risers, the full effect must be maintained by application of a layer, of approximately the same thickness as the riser wall, of a similar material on to the riser/air interface. In the case of insulating sleeves it is permissible and advantageous to apply an exothermic carbonaceous type powder as the anti-piping compound or hot topping material.
FIG. 6. Casting in "Alufont 3". A railway carriage bearing plate with sand heads and exothermic sleeved risers showing the considerable saving of metal.
Riser Size As previously discussed a continuous struggle exists in light alloy founding between the necessity for bottom running and the high feed requirements of a casting against a usually unfavourable temperature gradient. For plain, unsleeved risers in green sand this means that the effective diameter (in the case of cylindrical risers) must be appreciably greater than the section it is intended to feed. The theoretical aspects of riser dimensioning in respect of a casting/riser system in thermal equilibrium are dealt with elsewhere. Almost no fundamental work on heat flow or even thermal characteristics of the light alloys at or near the solidification point is available to put green sand riser dimensioning on a scientific basis and as a result the subject has been treated essentially to an empirical approach. In practice this usually means that a large safety factor is allowed and any subsequent adjustments reluctantly made, if the casting is
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successful, on a "hit or miss" basis. This view is understandable in the light of the complexity of the problem where the interplay of such factors as casting shape, pouring temperature, metal/sand mass, absence or presence of coring and metal/core relationship, mould material, composition of the alloy and its freezing range, etc., can all affect results to varying degrees. Most castings can be visually sectioned into regular geometric shapes and the heat centres (thicker sections that will need feeding or chilling) will suggest themselves. Arrangements can then be made to supply feed metal to these areas from suitably placed and sized risers or from a modified running system as previously discussed. To isolated bosses and thicker sections it is usual to allow 100% feed metal, i.e. the riser will be the same mass as the visually isolated section to be fed, and riser/casting contact area must be carefully considered to prevent premature freezing and bridging over. Surface/volume ratios and probable temperature gradients indicate that the riser should be somewhat thicker in section than the section to be fed and factors of 1-3 to 1-5 times the casting thickness are usual. Similar calculations must be applied to any blind risering (side risers that do not reach the top of the mould— unusual in light alloy practice) or runner bar feeding. The general feed requirements of a given casting can, therefore, be built up into a composite picture and the optimum running and risering systems to satisfy these demands worked out. (b) Gravity Die (Permanent Mould) Castings Although the overall rate of solidification in a metal mould is very much faster than in sand, similar arrangements in respect of ensuring a preferred solidification pattern must be made to obtain good castings consistently. Because of the speed of solidification, temperature gradients are steep and feed requirements short and sharp. It is, therefore, usual to apply very generous risering (especially runner/risering) to gravity die castings, the lower yield being accepted in view of the increased production rate possible with the process combined with the quicker return of clean metal for remelting. Similar considerations with regard to bottom running, risering, etc., will apply as described for sand castings and a proposed casting may be visually assessed in the same way to determine location of gate(s), riser(s), etc. Here, however, more regard must be paid to die construction since it may not be possible to incorporate all the desired features in a rigid mould. Gravity dies must obviously be kept as simple as possible with the minimum of coring, loose pieces, draw backs, etc. Die operating temperature is obviously an important factor in promoting or opposing a good solidification pattern and running and risering is often overgenerous in terms of actual requirements in order to maintain die temperature alone. For further details on these points and on some of the expedients and coating arts commonly used to produce acceptable castings see under the appropriate chapter.
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2. Correct Pouring Temperature To an important extent the pouring temperature of an alloy can have an effect on grain size, shape and distribution in the solidifying casting and this factor can sometimes be used to advantage, e.g. with some copper base alloys in ensuring a columnar pressure resistant skin. With aluminium alloys, however, because the solidification rate, whatever the method of moulding, is quite rapid, the effect is not so useful and in general only a pouring temperature just sufficient to ensure a completely run casting is employed. Long solidification periods in the mould must be avoided because of possible gas pick up, high volume contraction and the normal pasty method of crystallization. The correct pouring temperature must therefore be decided upon—and adhered to—in the light of casting section, alloy used and running method employed. High pouring temperatures—say above 760°C/780°C—and consequent longer time at high temperature in the furnace must be avoided from the points of view of enhanced possibility of gas absorption (see chapter on Aluminium) and grain coarsening due to aggregation of nucleii. Where a large surface area-thin section casting is involved it is preferred that, rather than increase the pouring temperature to run the thin sections, the running/ingating system be enlarged and the casting made more quickly using multiple ingates if necessary.
3. Use of Chills The desirability of directional, controlled solidification has been stressed in earlier sections. Chills in the form of metallic inserts in the mould (sand castings) obviously remove heat from the casting at a more rapid rate, at least during the early part of solidification, than the relatively insulating sand and they can, therefore, often be used to advantage to propagate a preferred gradient. Localized surface sinks at isolated bosses and section junctions can often be overcome with suitably sized and located chills, since they will have the effect of evening out small section variations. Chills can be of steel, cast iron, copper or even aluminium. They should be at least as thick as the section requiring chilling and massive individual contact areas with the casting should be avoided: instead use multiple smaller or well drilled chills to allow escape of gas. Chills should always be kept in good condition and coated with a suitable dressing to reduce damping back and to decrease the effect of any hard initial chilling that could leave a shut on the casting. Blowing from chills is usually the result of sand moisture condensation on the chill face so they should be preheated and the moulds closed and cast without delay. A proper dressing will also prevent welding—especially important in the case of copper or aluminium chills. Run ning/ingating systems should be arranged so that incoming metal does not continuously flow over any chills thus strongly heating them and destroying the effect required.
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APPLIED SCIENCE IN THE CASTING OF METALS
4. Nucleation The chapter on Aluminium (Chapter 7) has described how aluminium and its alloys can be grain refined by means of the addition, in salts or metallic hardener form, of suitable foreign nucleii of similar lattice structure. It follows that during solidification, particularly with the wide freezing range alloys, better mass feeding will be obtained if the solids content of the pasty mass is in a finely divided, i.e. nucleated, form. Mechanical bridging will be less likely to occur and effective mass movement and hence better feeding, more likely to persist to a later stage in the solidification process. Provided, therefore, that the methoding of a particular casting is correct, nucleation of the metal can provide some assistance in the feeding process. If mould design and layout is unsatisfactory, however, the reverse may apply and grain refinement will succeed only in collecting a dispersed shrinkage distribution into a gross cavitation.
5. Correct Mould Dressing In a sand mould and similarly in a pressure die casting, the nature and applied thickness of any dressing can have no significant effect on the mode or speed of solidification. On a gravity die, on the other hand, the die dressing exists as a layer of (usually) insulating material between two masses of metal. Its charac teristics and thickness can therefore play an important part in governing heat flow from the cast metal to the die. It follows that the type of die dressing and the way it is applied can have a controlling effect on the mechanism and direction of solidification and therefore on the quality of the casting. These aspects are discussed in greater detail in Chapter 12 on gravity die casting.