QUIESCENT TRANSFER OF MELTS

[0001] This disclosure relates to the melting, holding, degassing and casting of liquid metals, particularly light metals such as aluminum and magnesium, by a totally quiescent means. This encourages the detrainment of non-metallic inclusions from the melt, and avoids the re-entrainment of surface oxides into the metal during degassing and the transfer to the point of casting and during the casting process itself.

Background of the Disclosure

[0002] Conventional methods of transferring and casting liquid metals by pouring effectively in free fall under gravity practically always introduce defects into the liquid because of the turbulent folding over of the liquid surface. Such transfer folds in the surface oxide into the bulk of the liquid metal. These folded films possess no bonding between the opposed oxide surfaces, and so act as cracks in the liquid. The freezing-in of the doubled-over films (known as 'bifilms') into castings results in poor and erratic mechanical properties and low fatigue resistance of the cast component. [0003] Once introduced, bifilms are not easily eliminated from liquid metals, particularly oxide bifilms in liquid aluminum and its alloys. The reason for this is that the bifilm initially ravels into a compact ball, allowing it to pass through most filters. Later, in the casting, it can unravel, becoming a serious crack-like defect that reduces the properties of the casting. Bifilms in Al and its alloys will also not sink or float in any reasonable time. The reason for this is that the aluminum oxide is slightly denser than liquid Al and so should sink. However, when folded in by surface turbulence (for instance during a pouring action) the air entrained between the folded-over film (giving it

its name 'bifilm' to denote its doubled-over nature) causes it to float. This nearly neutral buoyancy together with its film-like aspect ratio confers an extremely low Stokes velocity. Thus any slight convection in the melt will cause the suspended oxide defects to circulate for hours or days.

[0004] It is beginning to be widely understood therefore that quiescent transfers of the melt, avoiding the "bucket technology" approach that is so common in the industry, are necessary for good results. However, the quest for totally quiescent melting, and totally quiescent transfers to effect casting, has not been easy, and has thus far been elusive.

[0005] Prior to casting, the cleaning of the metal is usually nowadays carried out by the production of thousands or millions of minute bubbles of an inert gas introduced beneath the surface of the melt. This can be achieved via porous plugs inserted into the walls or base of the holding vessel, or by rotary degassing, in which an immersed rotor is caused to spin in the liquid, releasing clouds of minute bubbles. Such techniques use inert gas to flush out dissolved hydrogen and usually operate with impressive efficiency. Furthermore, it is thought that the bubbles attach to suspended oxides and carry them to the surface of the melt, from where they can be skimmed off. Unfortunately, however, this technique also re-introduces millions of minute double oxide films because the inert gas cannot be truly inert; it will always contain sufficient contaminating gases to create a thin oxide layer on the surface of every bubble. In addition, of course, the bursting of the bubbles at the surface of the melt necessarily reveals the interior of the bubble to the air, so increasing the possibilities for the re-introduction of oxide defects.

[0006] Thus the so-called cleaning process is one in which the few large double oxides are probably replaced by millions of small double oxides. For this reason, the total cleaning action is less than optimum. Even worse, it is not uncommon for rotary degassers to operate in such a way that a vortex is formed around the rotor shaft which carries air down into the melt, re-introducing oxides as fast as they can be removed. In addition to these problems, if the rotor assembly is not completely dry (the refractories are likely to have absorbed up to ten per cent water vapor over a weekend for instance) the first several minutes of operation of the rotary degasser will cause an increase in the gas content of the melt. Thus the melt will get worse before it gets better. For those systems working on an automatically-controlled (i.e. rather short) degassing time, the final result is likely to be a melt with increased rather than decreased hydrogen content. [0007] Also, of course, the general stirring actions in the liquid that any kind of bubble degassing introduces to the melt, eliminates any chance of inclusions separating via a sink or float process.

[0008] Moving on to the problems of casting, the transferring of the molten metal into the mold cavity, there has been considerable interest in the use of pneumatic dosing systems. However, the embodiment of such systems has so far involved the use of furnaces that are large pressurized vessels. These are not easily controlled because of their large volume of compressible gas, and large amount of heavy liquid that needs to be accelerated into the mold cavity. Even more seriously, such units have to be filled with liquid metal, and the filling is usually carried out by pouring under gravity, often from a considerable height, thus introducing the very defects that the process seeks to avoid.

[0009] A significant advance was made by the Cosworth Casting Process, in which the melt is made by an unspecified melting process, held for some time in a large holding furnace of conventional design, but casting is carried out by the use of an electromagnetic pump. In this case, the holding furnace is of such a shape, with its large depth, that the cooler walls encourage downward flow of cooler metal, setting up a convective stirring regime that prevents the efficient settling of inclusions in suspension. The existing electro-magnetic pumps are also somewhat counter-productive because of the huge power dissipation in the working volume of the pumps, resulting in very high redundant forces which cause intense high velocity stirring; only a minute fraction of the electrical power is used in the useful propulsion and pressurization of the metal. At the present time, despite the useful and praiseworthy historical advance that this system represents, neither the melting, nor holding, nor casting technology is in accordance with the recommendations of the present disclosure. The current application seeks to improve greatly on this previous top world-class casting process. [0010] Other previous attempts at improved melting and casting processes can be mentioned that also generally fall short of the advantages noted in the current application. It is noteworthy that the geometry of the holding vessels is not described in any of these descriptions of prior art. Thus the central issue of quiescent holding to the degree that suppression of convection is achieved, encouraging inclusions of nearly neutral buoyancy to settle under gravity, is not considered at any point. Additional features of these attempts are considered briefly below.

[0011] US Patent 3,809,379 describes a succession of stations in which the metal is melted, held, degassed by vacuum, and the transfers between stations effected

by electromagnetic pump. The use of electromagnetic pumps is considered less than ideal because of the problems cited above. More fundamentally, the active transfer of the melt is seen to be counter-productive in achieving a quiescent transfer system with minimal moving parts so as to avoid disturbance to the melt, and the consequent stirring-in of bifilm defects. The different levels of the melt in the various sections is again counter-productive because it is not easily achieved without the action of powerful forces, and the probable consequential eventual fall under gravity at some later stage. [0012] US Patent 3,890,139 discloses a succession of treatment stations for the production of blister copper. The stations are not required to be at the same height, but the process necessarily runs continuously, requiring the melts to run downhill between the various vessels. The locally high velocities that will be generated by this downhill motion of the liquid are highly injurious to the aims discussed herein. The proposed heating by arcs will also, of course, create Lorentz forces in the liquid, causing localized intense stirring, again to the severe detriment of the melt as envisioned herein. [0013] US Patent 4,741 ,514 is typical of many modern systems for the holding and dosing of liquid aluminum. The furnace body has to be filled by pouring in metal under gravity. This is a highly damaging action, creating and entraining much dross (i.e. macroscopic oxide defects in addition to copious amounts of microscopic oxide bifilms). Whilst filling a dispensing chamber, a stopper then allows the melt to fall a second time, thus further causing degradation of the quality of the melt. Finally, the melt is dosed by gravity into the mold, again damaging the melt.

[0014] US Patent 4,881 ,670 describes a connected succession of vessels of unspecified geometry as usual. However, in addition, degassing by bubbles is

disadvantageously introduced, together with a plunger displacement mechanism that transfers the melt out of the final vessel in some unspecified manner. One assumes that the molten metal falls into a vessel or channel for use in one or more casting machines located at some distance away. This fall, and the use of traditional casting units, all of which are known to contain elements of turbulent motion of the liquid, ensures that any benefit conferred by the melting and holding system is likely to be undone at the casting stage.

[0015] US Patent 4,974,817 describes a holding vessel of unspecified geometry as usual, but heated by gas burner, and so both disturbing the surface and providing products of combustion so that the hydrogen content of the melt is not easily reduced. Subsequent efforts to reduce hydrogen are carried out by immersed porous lances to create bubbles, introducing more defects as we have discussed, and preventing settling of suspended debris under gravity by the creation of circulation currents in the liquid. Finally, the system is designed merely to provide a baling well, from which metal is baled out via a small ladle. This technique introduces oxides back into the melt at this point of delivery, mainly as a result of oxide adhesion to the walls of the ladle, and because of the usual pouring action that is a consequence of the ladle adjusting the level of its contents prior to lifting clear of the well.

[0016] US Patent 5,662,859 is a complicated system involving the use of many moving parts, all of which are known to introduce severe practical problems for the reliable operation of systems for the handling of molten Al and its alloys. It includes no means of degassing the melt. Level control in the main body of the unit is achieved by a displacement plunger. The varying level of this body will create oxides by the wash of

metal up and down its sides. Similar oxide creation on the walls of the containment vessel will damage the melt when the unit is used in the gas-pressurized delivery mode. When used with the electromagnetic pump (as a kind of linear motor) the strong internal turbulence expected in this device is again counter-productive. The movement of the whole unit on rails to effect contact with the casting machine is also seen to be disadvantageous because of the danger of slopping and surging of the melt, again raising sediment, and creating oxide wash on the internal walls of the various chambers. [0017] US Patent 5,725,043 describes a variant of a conventional low pressure casting system. As such, it has the standard drawbacks of large, expensive pressure vessel, and large volumes of air or nitrogen gas to pressurize the melt to effect casting. Control is rather 'spongy' as a result of the large compressible volume of gas that is used to accelerate the large volume of heavy molten liquid (the system is equivalent to operating a heavy battering ram by holding it only by weak elastic bands). The valve immersed in the melt is a source of concern regarding its reliability in production. Also, the filtering system comprising the deep bed effectively prevents any rapid change of alloy, and is a major task when the bed requires to be changed. There is effectively no holding furnace in this arrangement, and no opportunity to degass the melt. Furthermore, between castings, or at least from time to time as the pressurized vessel is re-filled, the pressure in the vessel will have to be released. This will cause the metal in the riser tube to fall, equalizing levels with the melt. This washing of melt up and down the delivery tube will generate oxides at the very point where they are most in danger of entering the mold and thus degrading the casting.

[0018] (i) In summary, there are many disturbances that affect the operation of a conventional holding furnace. These arise from such events as (i) the filling of the furnace (usually by pouring from a considerable height), (ii) emptying from the furnace (again, often by pouring from a significant height) into the mold or device that transfers to the mold, or (iii) from such treatments as rotary degassing, (iv) from the mechanical stirring-in of alloy additions, or (v) from significant changes in level of the melt that would create washing effects up and down the walls of the holder so as to create oxides. [0019] Accordingly, it has been considered desirable to develop a new and improved quiescent transfer process for molten metals which would overcome the foregoing difficulties and others while producing better and more advantageous results. In particular, no prior work targets the whole melting and casting procedure to ensure that oxides are not only (i) reduced at the melting stage, but (ii) are eliminated, or at least greatly reduced (detrained and/or filtered) in a holding stage, and (iii) cast counter- gravity so as to avoid re-entrainment during the final mold filling stage. Only a holistic approach tackling all these three issues simultaneously has a chance of solving the problem of the manufacture of castings with low oxide defect populations. This is the target of the present disclosure.

[0020] The principle involved is that of quiescence. The novel principle is the design for conditions of quiescence of both the surface of the melt, and the bulk liquid beneath the surface.

[0021] Once melted (and the melting process advantageously includes a dry hearth technique for the separation of the heavily oxidized skins of the charge materials), the surface of the melt is maintained at a substantially unvarying and

horizontal level and is undisturbed, and the melt beneath the surface enjoys quiescent, non-convecting conditions. The imposition of these conditions requires the elimination of conventional degassing and cleaning of the melt by techniques involving the passage of bubbles of inert gas through the melt. Novel non-disturbing techniques are employed such as hydrogen diffusion out of the melt and its removal from the system by counter- current flow of a dry, hydrogen-free gas. Cleaning of the melt from inclusions is accomplished efficiently for the first time by sedimentation.

Brief Summary of the Disclosure

[0022] According to one aspect of the present disclosure, a quiescent melt handling system is provided. More particularly in accordance with this aspect of the disclosure, the quiescent melt handling system comprises a holding furnace for a molten metal, wherein the holding furnace comprises a relatively large surface area and a relatively shallow depth, having a width to depth ratio in the range of at least approximately 4 to 1 , but which may be as high as 100 to 1 , and a means for separating inclusions from the melt in the holding furnace.

[0023] In accordance with another aspect of the present disclosure, a quiescent transfer casting method is provided. More particularly in accordance with this aspect of the disclosure, the method comprises the steps of melting a metal to form a melt. The melt is then transferred into a holding furnace where the hydrogen in the melt is passively equilibrated with a dry atmosphere maintained in the holding furnace. The melt is subsequently withdrawn from the holding furnace. [0024] According to still another aspect of the disclosure, a quiescent melt handling system is provided. More particularly in accordance with this aspect of the

disclosure, the quiescent melt handling system comprises a melting furnace, which transforms solid metal into a melt, and a holding furnace for the melt, wherein the holding furnace comprises a shallow bath having a relatively large surface area with a heater located above the surface of the melt. The holding furnace has a width to depth ratio on the order of at least approximately 4-100 to 1 and a length to width ratio on the order of at least approximately 1 -100 to 1. A first channel communicates the holding furnace with the melting furnace ensuring that a melt level within both furnaces is substantially equal. A means is provided for withdrawing the melt from the holding furnace.

Brief Description of the Drawings

[0025] The disclosure may take physical form in certain parts and arrangements of parts, preferred embodiments of which will be described in detail in this specification and illustrated in the accompanying drawings in which: [0026] Figure 1 is a schematic side elevational view in cross-section of a quiescent transfer casting furnace system according to a first embodiment of the present disclosure;

[0027] Figure 2 is a schematic side elevational view in cross-section of a quiescent transfer casting furnace system according to a second embodiment of the present disclosure;

[0028] Figure 3 is a schematic side elevational view in cross-section of a quiescent transfer casting furnace according to a third embodiment of the present disclosure;

[0029] Figure 4 is a schematic side elevational view in cross-section of a holding furnace according to a fourth embodiment of the present disclosure;

[0030] Figure 5 is a schematic side elevational view in cross-section of a holding furnace according to a fifth embodiment of the present disclosure;

[0031] Figure 6 is a schematic plan view of another embodiment of a holding furnace according to the present disclosure; and,

[0032] Figure 7 is a schematic plan view of a holding furnace according to still another embodiment of the present disclosure.

Detailed Description of the Preferred Embodiments

[0033] It is necessary for the holding furnace to operate at a substantially constant level of liquid metal. This is achieved by ensuring that the rate of supply to the unit and rate of delivery from it are, on average, sufficiently well matched so that the change of level on additions to or deliveries from the unit cause the surface level to change by only relatively few millimeters. This is easily achieved by monitoring the height of metal in the holding furnace, and adjusting the rate of melting from the melting furnace accordingly. The task of maintaining the constant height is facilitated if the area of the melt in the holder compared to the volume changes is sufficiently large. [0034] It is desirable that this furnace is electrically heated by elements in the roof

(although fuel-firing inside heater tubes is of course possible). This has the benefit of creating a positive vertical temperature gradient in the liquid bath that will assist to stabilize the melt against convection. In addition, the provision of a dry atmosphere (free from the water vapor characterizing fuel-fired furnaces) allows the unit to take

advantage of passive degassing as will be described below, so avoiding the necessity to disturb the melt using conventional bubble-inducing degassing systems. [0035] Because such electrical heating elements (or gas-heated tubes) are limited in length the width of most holding furnaces of this type are limited to perhaps one or, at the most, two meters. Thus if extra area is needed because of the proposed manufacture of castings of large volume, it will generally be necessary to increase the length of the furnace.

[0036] The key aspect of the furnace that is central to this patent is the aspect ratio of the width (or length) of the bath compared to its depth. [0037] For instance many holding furnace baths have a width to depth ratio of approximately 1.0. Sometimes, the bath might be only half the depth compared to the width, i.e. a ratio of 2. Such ratios are the norm. However, the relatively long vertical sidewalls lose a significant amount of heat, causing them to be slightly cool. The melt in contact with the side walls therefore cools, becoming denser, and so setting up a flow downwards, turning at the base of the furnace, and meeting the opposing flow from the far walls, rises in the center. Thus the whole of the contents of the furnace are subject to this double torroidal circulation pattern. The result is that even without other disturbances, inclusions of near-neutral buoyancy are allowed no chance to settle. [0038] This application proposes that the holder width to depth ratio should be in the range 4 to 40 times as wide as it is deep, or preferably 5 to 50 or more preferably 10 to 100. In other words, the holding furnace should be at least four times as wide as it is deep, or possibly up to one hundred times as wide as it is deep (It is assumed that in most cases the length of the furnace will exceed its width, otherwise, the ratio would

logically apply to the furnace length). Thus, the length to width ratio can be in the range of 1-100, or more, to 1.

[0039] The reason for the small relative depth is that the effect of the side walls is now very much reduced. Not only is the convection effect not accumulated over a large height, but the temperature difference between top and bottom is now much less, plus any residual circulation is confined to the vicinity of the side walls. Thus the main body of the melt remains static.

[0040] The static nature of the main body of liquid now allows inclusions to sink and float even though their Stokes velocity (their natural sedimentation speed) is extremely low. Naturally, the longer the melt is allowed to dwell in the furnace, the smaller the inclusion that will be encouraged to separate out. The dwell time is, of course, related to the weight of castings produced and the number cast per unit time. For instance considering a relatively large Al alloy automotive cylinder block casting weighing 25 kg, it is easily shown that a furnace with a bath only 1 m wide, 100 mm deep and 3 m long would experience an average forward movement of the melt by 100 mm per casting, leading to a drop in surface level of 3 mm per casting. A casting every minute would give a dwell time of only 30 minutes. This is rather minimal, and is easily extended by a wider or longer bath. Less advantageously, the depth of the bath might be increased to an extent limited by the allowed ratios for effective operation. [0041] Because a crankcase of this weight might expect to be filled in approximately 30 seconds, during which time the melt travels on average 100 mm, the actual average velocity of the melt during the period of casting will be of the order of only 3 mm/s, after which the melt comes to a stop once more. Thus even the maximum

velocity of the melt is extremely modest. Such gentle movements are not expected to result in any significant turbulence, even in this example where the holder is perhaps somewhat undersize for its task.

[0042] Interestingly, from these rough figures, the inventory of liquid metal in the proposed system can be seen to be quite small, making alloy changes, or cleaning out of the system, relatively easy and quick. For instance for a depth of 100 mm and width of 1 m, each meter length of the furnace holds 250kg. Thus a 3 m long unit will hold only 750kg. This value contrasts with conventional holders for processes such as Cosworth Process in which a holder for such a casting duty would be expected to have a dwell time of at least 4 hours and hold 10,000 to 20,000kg, but whose considerable depth would provide convection currents to swamp most of the benefits of the 4 hour dwell period.

[0043] In this disclosure, the degassing of the melt is achieved without disturbance of the liquid metal. The limited depth of the melt and its large area ensures that gas can diffuse out of the metal within the dwell time, provided the atmosphere above the melt is maintained free from hydrogen, or sources of hydrogen such as water vapor or hydrocarbon gases. Thus dry nitrogen, or possibly even dry air, can be introduced above the melt. The volume of air or other gas present in the holding furnace above the melt is the same as in conventional holding furnaces of a given size. [0044] The dry gas can be introduced near the exit from the furnace. The dry atmosphere therefore would travel beneficially in a counter-current direction to the flow of the liquid metal, and would be allowed to exit via the numerous natural leaks at the far end of the furnace. (Even so, of course, the additional natural leaks from the sides of

the furnace will reduce the overall efficiency somewhat, but the effect is insignificant provided the lid is sitting down on the walls of the body of the furnace with a reasonable sealing face; the joint possibly additionally sealed with lengths of refractory blanket or rope. Similarly, the holes for the heating elements will also benefit from sealing in a 'rough and ready' manner with refractory wool or similar sealing material.) [0045] The counter-current system is advantageous because the gas which enters at the end of the furnace where the melt is about to exit is the purest. Thus, despite leakage and contamination of the gas, as it flows toward the melt entrance in the holding furnace, the melt exiting the holding furnace is expected to be excellent. [0046] The rate of flow of the gas in the holding furnace varies with the size of the holding furnace. For example, the rate of gas flow can be between about 2 to 6 liters per minute for a furnace which is 1 m wide 3 m long and 0.5 m high on the inside, above the melt.

[0047] Delivery of molten metal into the holder can be accomplished by a dry hearth type melting furnace. Such a furnace is conveniently fuel-fired, and attains a high melting efficiency by either (i) the use of twin recuperating burners so that charging can be carried out at floor level; or (ii) the furnace is a tower variety in which high thermal efficiency is achieved by conveying the spent gases from the melting burners up the stack, counter-current to the charge which descends as melting takes place. [0048] Such furnaces deliver a high quality metal because the main oxide skins on the charge material (and any iron inserts such as cylinder liners etc) remain on the sloping dry hearth, and can be removed at intervals as necessary via a side door. (This benefit contrasts with induction and other crucible type furnaces, where the oxides on

the surface of the charge materials and all remaining inserts are necessarily incorporated into the melt).

[0049] The melt from such furnaces is, however, at rather a low superheat. Thus an additional function of the holding furnace is to bring the melt temperature up to a casting temperature, and stabilize the melt at this temperature. This is easily achieved because of the large area and small depth of the melt.

[0050] The fact that the melt is at a low temperature for much of the area of the holder ensures that degassing is carried out with maximum effectiveness, since degassing efficiency is higher at lower metal temperatures.

[0051] The rate of delivery of metal can be adjusted to maintain the melt level in the furnace between close limits by controlling the rate of burning of fuel by the burners.

The rate can therefore be turned up or down at will. Thus the system is a 'just-in-time' or an 'on-tap' melt delivery system.

[0052] Metal delivery from the holding furnace can be envisioned to occur in a number of ways in which disturbance is minimized to the main body of the melt.

[0053] Referring now to the drawings wherein the showings are for purposes of illustrating preferred embodiments of the disclosure only and not for purposes of limiting same, Figure 1 is an example of a 3-Element interpretation of the current disclosure. A melting furnace, element 1 , preferably of a dry hearth type, is connected to a second element, a holding furnace of shallow depth and relatively large surface area 2, preferably of a radiant roof type, that is in turn connected to the third element, constituting a pump well furnace 3. In the pump well furnace, can be sited a pneumatic pump 4 of the type described in US Patent No. 6,103,182 dated 15 August 2000. The

simple channel connections 5 and 6 between the three elements (which may or may not be three separate furnaces) are arranged to convey the liquid metal 7 at constant surface level 8.

[0054] The dry hearth furnace may be of a conventional tower design requiring a lifting device, often a type of skip hoist design, to raise and load the charge materials into the top of the furnace. Alternatively, as shown for purposes of example only, the furnace may be of a horizontal design, employing twin regenerative burners, using air for combustion heated by the spent gases, to raise melting efficiency. Charging is then carried out conveniently at floor level by fork lift truck via a door 11 , opened to admit the metal charge 12. The charge is then melted by heat from fuel burners or electrical elements (not shown) under a radiant roof 13. The melted metal runs down the sloping hearth 14 into the well 15 of the dry hearth furnace, or, possibly via the connecting channel 5, directly into the holding furnace.

[0055] The significance of the choice of melting unit as a dry hearth furnace is that the oxide skins of the ingots or foundry returns (runners, risers and castings etc) are designed to be left on the dry hearth. Only the melted metal runs down the hearth and into the furnace well. The skins, dross, or even steel insert parts from mixed scrap, can be separated by raking off from the hearth at intervals via a side door. Thus only relatively clean metal is provided to the start of the melt supply. The dry hearth furnace has the further benefit that it provides liquid metal on demand, can operate continuously, and the rate of delivery can be adjusted by control of the burners. Moreover, a dry hearth furnace has practically no moving parts and requires relatively little attention or maintenance.

[0056] The dry hearth furnace may have a well, forming a reservoir in which gentle mixing under a tranquil surface is encouraged to provide a smoothing of chemical changes as charges to the furnace are changed, or as the melt rate changes. The well region of the furnace allows a second burner or other heater to be sited to raise the temperature of the melt, since the metal drains from the dry hearth practically at its melting point. Usually at least 50 to 150 C superheat (the temperature above the melting point) would be required to make most castings. Thus most of the superheat will require to be made up by the second burner or other heater in the melting furnace (the final adjustments to temperature can, of course, be made later in the holding furnace, or even in the pump well itself).

[0057] The quiescent transfer of molten metal from the melting furnace to the holding furnace is achieved if the two units are in good hydrostatic communication and so have substantially identical liquid levels. In such a situation the melt travels with extreme gentleness from one to the other.

[0058] Of course, it should be appreciated that any other conventional type of furnace could be used instead, such as the recently developed electrical resistance immersion heated furnaces, or even a crucible melter. In fact, the liquid metal need not be melted in the foundry. It could be supplied direct from a smelter possibly many miles away and transported to the foundry by road or rail in a specially insulated container, as is well known in the industry. The liquid metal can be off-loaded and stored at the foundry in the special container, or transferred into a second holding furnace. From such a container or second holding furnace, the molten metal could be subsequently transferred at a controlled rate to the holding furnace so as to substantially retain

constant melt level, such as by a snorkel ladle. Such an embodiment will be described hereinbelow. (It is important that such supplies of liquid metal are not introduced suddenly into the present system as commonly happens with liquid metal supplies. This would lead to a violation of the principles of this disclosure; greatly changing the melt level, disturbing both the surface oxide and bottom sediments, so that the melt would become contaminated with re-introduced inclusions.)

[0059] The melt 15 may exit the melting furnace via a filter 16 which may be of any suitable type such as the ceramic foam or bonded particle types. Alternatively, if used at all, a filter may be placed in the entrance to the holding furnace 2.

[0060] The dry hearth furnace well and the holding furnace are connected via a simple ceramic channel 5 (although other connections such as a ceramic tube can be envisaged to work equally well). Such connections necessarily require the normal attention to detail including good thermal insulation, and reasonable exclusion of air by suitable heated or insulated cover. Alternatively, of course, the furnaces may be effectively integrated to avoid any necessity for a separate link.

[0061] The melt then enters the holding furnace. That furnace is an important element of the melt handling concept. This furnace comprises a base part 21 that contains a shallow bath of liquid metal, and a roof 22 that contains the heating elements. The source of heating can be electrical resistance elements or tubular gas radiant elements and the mechanism of heat transfer to the melt is radiation, as is normal for a reverbatory type furnace.

[0062] The large area and shallow depth of the melt in this element acts beneficially in a number of ways:

[0063] i. the large area acts as a reservoir and buffer to smooth temporary imbalances between the melt supply and demand, maintaining a substantially constant level;

[0064] ii. sufficient area is provided to allow the casting temperature to be adjusted rapidly if necessary, when changing cast products, or when assisting recovery from power failures or other breakdown, and the shallow depth allows equilibration of temperature throughout the melt by conduction;

[0065] iii. inclusions are given the time to encourage separation by a sink or float process because the aspect ratio (width/depth) of the furnace effectively suppresses convective stirring driven mainly by the cooling from sidewalls since the effect of sidewalls is reduced to insignificance in this design of furnace; [0066] iv. hydrogen is allowed to equilibrate with the dry atmosphere above the melt that is constantly replenished while the hydrogen-contaminated spent gas is constantly removed. This degassing technique avoids the turbulent techniques commonly employed throughout the industry such as purging with bubbles from immersed lances or rotary degassing machines. Such turbulent degassing techniques can cause damage to the melt, leading to variable melt quality. The shallow depth of the furnace is calculated to allow the majority of hydrogen time to diffuse to the surface, and so escape into the atmosphere that is being continuously replenished with a hydrogen- free and moisture-free atmosphere.

[0067] v. The shallow bath is easy to clean, and contrasts with deep baths that sometimes are found to be impossible to clean because of oxide material which can remain out of sight for long periods and becomes fused to the furnace lining.

[0068] vi. The liquid metal inventory is unusually low for a high production casting system. Liquid metal inventories are often in the region of 10 to 50 tonnes of liquid metal, so that an alloy change can rarely if ever be contemplated. Liquid metal inventories according to the current disclosure are expected to be in the region of 0.5 tonnes making maintenance or alloy changes easier.

[0069] The liquid metal in the holding furnace 2 can connect to a pump well in a separate casting unit via a launder channel 6. The pump well requires sufficient depth to accommodate the pump 4, plus any box type filter unit 32 that may surround or be inserted under the pump. Furthermore, it is an advantage to be able to provide additional heat input at this final element of the melt line. (This is because many such lines have pump wells which are part of the holding furnace, and have no separate heating facility. Thus, such wells often run too cool to allow the line to produce some difficult thin-walled products. Worse still, a failure of the refractory lining of this integrated holder/pump well furnace causing a leak of metal through to the steel casing of the furnace is then a death blow to such designs. The ensuing heat losses and fall in temperature of the metal in the pump well make almost all casting impossible, forcing a shut down of the casting operation and a furnace re-line. The provision of a crucible furnace avoids the critical problem of low temperatures in the well with a low cost, simple, 'off-the-shelf solution.)

[0070] These considerations for the pump well lead naturally to that particular design provided by a conventional crucible furnace. This is because the crucible has the required depth, and is surrounded by a heating source. The furnace usually includes a

steel shell 33 with an insulating lining 34, which, in turn, contains a conventional silicon carbide or graphite based crucible 31.

[0071] The heating source surrounding the crucible (not shown) may be gas or electric. However, electrical resistance heating is normally to be preferred at this point in the line because waste gas ducts above the furnace are thereby avoided, and would otherwise complicate the siting and operation of a casting station. Also, the permeability of the crucible to hydrogen means that the hydrogen in solution in the melt is probably kept lower by avoiding gas heating, with its accompanying load of water vapor as products of combustion. The crucible is held in place on its ceramic stool 37 with a cover assembly 35 and the whole covered by a lid 36 to retain heat and exclude moist air.

[0072] The pump itself 4 can be of the type described in U.S. Patent No.

6,103,182 which is incorporated by reference hereinto in its entirety. This design of liquid metal pump has the capability of totally quiescent operation: it has an intake (not shown) for liquid metal beneath the surface of the liquid, transfers the liquid into the mold via a riser tube 41 , and at a rate which can be precisely controlled by the application of gas pressure into the body of the pump. The small volumes of gas and liquid involved ensure that the precision of control is excellent. [0073] Such pumps, whether pneumatic or electromagnetic, work with better repeatability if the melt level is constant, so that in this case such devices have optimum working conditions. In addition, the crucible furnace has its own source of heat, so that last minute adjustments to the casting temperature can be made. The generous (more

than adequate) heating facility is far preferable to the usual geometry of a charge well as an extension to a normal electrically heated reverbatory furnace as outlined above. [0074] Furthermore, the liquid can be held close to the open top of the riser tube

41. This is an important feature because the uncontrolled fall of the liquid down the riser tube on the completion of a casting is a feature of many less well controlled delivery systems. The fall of liquid acts to stir the metal inside the pressure vessel, thus stirring up once again inclusions that have taken time to settle to the bottom of the vessel. In addition, the fall of the liquid uncovers the inner surface of the riser tube, allowing the formation of a skin of oxide that can be incorporated into the melt during the next time the melt ascends the tube for the filling of the next mold. Thus, maintaining the riser tube continuously full of liquid metal not only saves a second or so off the filling time, but more importantly retains the excellent defect-free quality of the melt for each casting.

[0075] The connections between the elements of the melt line are made by simple known techniques, for instance by the use of proprietary preformed ceramic launder (channel) sections or ceramic tubes as has been mentioned above. Good hydrostatic communication between the melting furnace, holding furnace, and casting station ensures substantially similar levels between the melter, holder and casting station, with the result that the transfer of the melt between the units occurs only with extreme gentleness, without turbulence. Some difference in level is expected to build up as filters become blocked, but the essential feature of the gentle transfer of the liquid is not thereby threatened, as will be evident to any who have worked with in-line filtration as part of a melt supply system.

[0076] A filter, possibly of a ceramic foam type, may be placed at the exit from the melting unit to keep back some of the larger inclusions. The filter will need to be replaced from time to time. Similarly, a box filter of well-known form may be placed around the pump, or a ceramic foam filter may be sited underneath, to provide a final filtering operation.

[0077] A second example of the novel melting and casting system that is characterized by very little molten metal inventory in the whole system is described in this patent application with reference to Figure 2. As before, a melting furnace, element 110, preferably of a dry hearth type, is connected to a second element, a holding furnace of shallow depth and relatively large surface area 120, preferably of a radiant roof type. The simple channel connections 150 between the two elements (which may or may not be separate furnaces) are arranged to convey the liquid metal 170 at constant surface level 180.

[0078] A door 111 is opened to admit a metal charge 112 that is then melted by heat from fuel burners or electrical heating elements (not shown) under a radiant roof 1 13. The melted metal runs down the sloping hearth 114. No well at the exit of the dry hearth is proposed for this system. This reduces the total inventory of liquid metal in the system, so facilitating alloy changes. The dry hearth melting furnace therefore delivers metal direct, via the connecting launder or tube 150 if any, into the holding furnace 120. [0079] The melt enters the holding furnace. As previously, this furnace comprises a base part 121 that contains a shallow bath of liquid metal, and a roof 122 which contains the heating elements. Preferably the source of heating is some dry system

such as electrical resistance and the mechanism of heat transfer to the melt is radiation, as is normal for a reverbatory type furnace.

[0080] The large area of the melt in this element acts beneficially as before. In contrast to the embodiment of Figure 1 , however, no pump and pump well furnace is required, further reducing the inventory of liquid metal in the whole system.

[0081] Close to the delivery end of the holding furnace the melt is extracted via a tip 218 of a conventional snorkel ladle 220. Very little, if any, extra depth is required for the melt at the point at which the tip of the ladle samples the melt. Such a ladle is dry and preheated. It sucks up the liquid metal from the furnace, filling a reservoir 222 constituted by the upper part of the ladle. When full, a stopper (not visible) of the snorkel ladle is closed.

[0082] The ladle is withdrawn from the holding furnace and is transferred to a casting station 230 via a conventional mechanism 240 operating on an overhead rail

242. Here, the snorkel ladle's delivery tube is lowered into the mold, the snorkel stopper is raised, and the contents of the snorkel ladle are transferred into the mold. By careful design of the system of channels for the filling of the mold, using techniques well known to those skilled in the art of the design of casting filling systems, the transfer into the mold can be carried out without substantial surface turbulence, maintaining the integrity of the liquid metal.

[0083] A load cell 244 may be provided on a support cable 246 of the ladle 220 to gauge the weight of molten metal held by the ladle. In the embodiment illustrated, two apertures 252 and 256 are shown as extending through the roof 122 of the holding

furnace 120. The aperture 256, spaced further from the melting furnace 110, is used to charge the snorkel ladle 220.

[0084] In contrast, the aperture 252 near the melting furnace 110 is used simply to return back into the holding furnace 120 any molten metal remaining in the snorkel ladle 220 after it has discharged the molten metal at the casting station 230. This is illustrated with the snorkel ladle in dashed outline in Figure 2. It should be appreciated that more than one snorkel may be needed for the embodiment of Figure 2.

[0085] Also, it may be necessary to employ a conventional turnover and air blast station (not illustrated) to clean the snorkel ladle 220 between fillings. This may be needed to provide an otherwise simple filling system that can successfully avoid the use of a pump and still fill castings without surface turbulence. Such a system as described in this second example is useful, for instance, in foundries that are required to make frequent changes of alloy. The metal delivery technique via snorkel ladle as described here, as an example only, is especially appropriate for certain castings, where access to the mold filling channels is appropriate and the casting has limited vertical height. The system is also economical in terms of capital plant, since only the dry hearth furnace and the holding furnace, described above, are required.

[0086] The long snout of the snorkel can be lowered deeply into a mold, engaging with the base of the running system for the casting, so that the casting fills from the bottom upwards thus avoiding the falling of the metal with the danger of the entrainment of oxide defects inside the mold.

[0087] With reference now to Figure 3, a snorkel 340 can be envisioned to be a permanent fixture to a delivery end of a holding furnace 320 connected to a melting

furnace 310. In this way molds can be brought to the holding furnace and offered up to the snorkel. Again, the snorkel can operate so as to deliver clean melt directly into the base of molds, so that the mold can fill in a counter-gravity mode. [0088] Figure 4 shows a further embodiment of the disclosure. In this embodiment, a snorkel ladle 450 is used to transport liquid metal from some outside source, and fill a quiescent transfer furnace 420 in a controlled manner. As previously mentioned, the outside source could be at a location miles away from the holding furnace. Another snorkel 440 can be a permanent fixture at a delivery end of the holding furnace. It can deliver clean melt directly into a base of a mold 470. The transport snorkel 450 is advantageous for adding new melUinder the liquid surface and at a controlled rate so as to create minimal disturbance to the surface of the melt. [0089] It should be appreciated that in the example illustrated in Figure 4 the delivery to the holder, or holding furnace, is, as in other examples, quiescent, occurring under the surface of the liquid metal, so as to disturb the surface and disturb the body of the melt in the holder only minimally. The beneficial actions of the holder in reducing inclusions, reducing dissolved gas, and adjusting temperature are all carried out without undue disturbance to the surface or the bulk of the melt in the holder. In fact, even gentle stirring by convection is also suppressed by the aspect ratio of the bath together with the positive, stable density gradient provided by the radiant roof heating. Also, the delivery of metal from the holder is again carried out in such as way as to reduce disturbance to the surface and to the bulk of the melt in the holder. [0090] Figure 5 illustrates an embodiment of the invention. Delivery from a holding furnace 520 is again via a snorkel 540 fixed in the base of the furnace.

However, a heated refractory U tube arrangement 550 allows the counter-gravity filling of molds, similar to a known arrangement previously described in 1974 in US Patent 3,828,974. This patent provides a description of details of safety aspects of design that would be desirably included. The disclosure of this patent is incorporated herein, in its entirety

[0091] A known device, a slide gate (not shown), affixed to the base of a mold

570, operates at the junction between the orifice from the U tube and the mold. A snorkel stopper 544 is opened to fill the mold. When the mold is full the slide gate is closed, and the snorkel stopper 544 is closed. The mold can then be lifted clear and a new mold with an open slide gate can be planted on the melt delivery point. [0092] Figure 6 shows a schematic plan view of a holding furnace 620 according to another embodiment of the present disclosure. In this embodiment, molten metal enters near the point marked with an arrow 630. This could be from a sloping dry hearth melting furnace, or transferred via another snorkel. The furnace is shown with a site 636 for a fixed snorkel.

[0093] Filters 650, in the form of ceramic filter blocks, are shown placed in the bath. Their purpose is not primarily filtration. In operation the holder will gradually fill with settled-out debris of nearly neutral buoyancy. Thus this effectively floatable material will travel with the melt rather than sitting firmly on the base of the furnace. It will build up to become a slurry of sundry debris. This porridge-like sludge will, if not restrained, travel as a body along the furnace. The purpose of the semi-circle of ceramic filter blocks 650 is to restrain this movement, holding the debris in place. Clearly, this role could easily be undertaken by a sintered ceramic particle box filter or other suitable

device (or groups of such filters or devices) for holding back this slurry. In addition, of course, it may be found advantageous for the attainment of certain qualities of melt in certain alloys to introduce multiple such barriers.

[0094] For example, in Figure 4, two such spaced barriers, or sets of barriers,

660 and 670 are shown. In Figure 5, one such barrier or set of barriers 680 is shown. Interestingly, it is not the intent that these barriers actively provide any significant filtering action. The main inclusion separation mechanism is sedimentation. However, as sedimentation proceeds, the debris itself will build up its own filtration efficiency as it accumulates in depth, and only become too efficient when the melt cannot pass through it sufficiently quickly to re-fill the region of the furnace above the snorkel outlet prior to the next casting. At this point the rate of production will have to be slowed, forcing a cleaning operation for the holder.

[0095] It is to be expected that complete cleaning out of the furnace will be required, perhaps, once a week, or at some other regular interval depending on the quality of the incoming charge material.

[0096] Depending on the nature of the material charged to the melting unit and transferred into the holding furnace, the holder may benefit from additional barriers to the flow of debris. Thus additional arrays of heavy filter blocks (not shown) or other barriers to hold back the general flow of settled-out debris can be envisioned. [0097] Alternatively, it can be envisioned that some barriers could be simple upward steps in the floor of the holder furnace, proceeding towards the exit. With reference now to Figure 7, another holding furnace 720 according to the present disclosure is there illustrated. In this embodiment, the holding furnace is provided with

an inlet opening 722 on a first end thereof and an outlet opening 724 on a second end thereof. The holding furnace includes a floor 730 and a set of upward steps 732-738 therein. The melt 740 is at a level to cover all the upward steps. The space in the holding furnace 720 above the melt is indicated by numeral 750. Each upward step, perhaps only 10 mm high, would help to restrain the general movement of the whole mass. The bifilms will have time to open in this quiescent environment, thus impinging and interlocking to form, macroscopically, a kind of fairly solid cake or gel. Thus relatively isolated points for pinning movement would tend to act on the whole mass because of its degree of integrity. It is believed that the up-steps would not interfere unduly with the raking out of the debris from the metal entrance end when the furnace will require its routine cleaning.

[0098] With continued reference to Figure 7, the holding furnace 720 is provided with a gas inlet 752 through which a dry gas atmosphere flows into the holding furnace, as illustrated by arrow 754. The dry atmosphere or gas allows hydrogen in the melt to equilibrate with the dry atmosphere being replenished into the holding furnace via inlet 752. An outlet for the dry atmosphere is shown at outlet port 756 through which the gas, which has now picked up hydrogen, flows outwardly as shown by arrow 758. In this way, a counter current flow of the dry atmosphere takes place such that the cleanest gas, i.e., a gas which has not yet picked up any hydrogen, flows into the holding furnace via the inlet port for gas 754 adjacent the outlet port 724 for liquid metal. This allows the melt exiting the holding furnace to be of the best quality. Gas which has picked up hydrogen from the melt flows away from the outlet port 724 for metal, in the holding furnace, and towards the outlet port for gas 758 adjacent to the inlet port for

metal 722 thereof. Also provided in the space 750 above the melt 740 are an array of heaters 770. As mentioned previously, the heaters can be radiant electric heaters of a conventional design. Of course, any type of dry heating means (i.e., any method of heating which does not involve burning a flame of combustible gas above the surface of the melt so that the products of combustion are exposed to the melt) would be acceptable.

[0099] Other geometries of the holding furnace can be envisioned in which two or three or more counter-gravity casting stations are set around a furnace of considerable area, emerging as a row at one end, or from all available sides etc.

[0100] According to the disclosure, the manufacturing technique and system combines a number of components in a novel manner to achieve the quiescent transfer of melts, leading to unique advantages in the properties of the cast product.

[0101] More particularly, the method and apparatus employs a holding furnace from which all disturbances are designed to be suppressed, encouraging the sedimentation of inclusions, and achieving the simultaneous degassing of the melt, thus achieving liquid metal of high quality. This high quality is maintained into the cast product only if the melting and holding system described above is linked to a suitably quiescent system for transferring melt out of the furnace directly into a mold or into a transfer system that delivers to a mold.

[0102] In summary, the usual manner of filling a holder is simply by pouring in fresh liquid metal. Such trauma completely disturbs the whole melt, stirring sedimented inclusions back into suspension, and entraining new inclusions since all pouring actions involve much folding-in of the surface oxide. In contrast, the operation of the melt

system proposed in the current application is with minimal stirring or upsetting of the molten material at any stage of the progress of the melt, during its melting, temperature adjustment, cleaning, degassing and casting.

[0103] In those examples of this disclosure where more than one furnace is employed, it is important that all are linked so that the melt level in all furnaces is substantially the same. This condition naturally confers extremely gentle transfer of the melt between the furnaces. It contrasts with many prior examples in the industry where the melt is transferred down a ramp, or is actually poured freely through the air, causing considerable oxidation and damage to the quality of the melt by the entrainment of oxides.

[0104] A quiescent transfer casting line can include a melting furnace 1 , 110 which transforms solid metal into a melt. The melt is then transferred to a holding furnace 2, 120, 320 - 720. A first channel 5, 150 can communicate the holding furnace with the melting furnace. The holding furnace can be connected to a pump furnace 3. A second channel 6 can communicate the holding furnace with the pump furnace. Alternatively, a snorkel ladle 220 can be used both to withdraw and to dispense melt from the holding furnace. The holding furnace 2, 120, 320 - 720 has a relatively large surface area and a relatively shallow depth, having a width to depth ratio in the range of 4 - 100 to 1. A method employing such a casting line encourages the cleaning of the melt by the detrainment of non-metallic inclusions and avoids the re-entrainment of surface oxides into the body of the melt, in addition to achieving a degassing action and adjusting the temperature of the casting as required.

[0105] Thus, the technique proposed herein is designed to achieve filling, transfers out or dosing to a casting unit or mold, degassing, and effective filtration. All this occurs without moving parts, except for one or more stoppers that move only relatively slowly through a relatively few millimeters vertically through the local melt surface, so as to disturb the surface only insignificantly.

[0106] All this is seen to achieve various solutions to the difficulties of melting, and degassing liquid metal, and transferring into a mold without the introduction of deleterious bifilms.

[0107] The disclosure has been described with reference to several preferred embodiments. Obviously, modifications and alterations will occur to others upon the reading and understanding of the preceding specification. It is intended that the invention by construed as including all such alterations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.