Patent ID: 12202034

DETAILED DESCRIPTION OF THE INVENTION

Referring firstly toFIGS.1aandbthere is illustrated a casting system in accordance with one embodiment of the invention. In this embodiment the system is a counter gravity casting system.

The system includes a heating assembly2in the form of a dry hearth tower which includes a flue4which allows gases to pass up through the descending charge, pre-heating the charge efficiently as a counter-flow heat exchanger. The use of the dry hearth6melting provides an advantage over crucible or bath melting because the primary oxide skins on the charge materials can be separated completely from the metal. The oxide skins remain on the sloping hearth6and are scraped off periodically. The molten metal and/or metal alloy, free from the primary oxides which would normally have been present, enters the refining system. Typically the dry hearth melter can maintain the melt level8constant to within a few millimetres and feed-back from the monitor of the metal level can be used to match the melting rate to the casting rate.

The transfer assembly26includes a launder or channel to allow the flow of the liquid metal and/or metal alloy from the heating assembly2to the mould filling assembly12in the direction of arrow14. Along the launder there is provided a portion18which acts as an oxide sediment pump (OSP)16to allow sedimentation of oxide bifilms from the liquid metal and/or metal alloy in a controlled manner, together with extraction via a pump18.

As the liquid metal and/or metal alloy flows, traversing over the top of a sump or vessel20, the rate of descent of bifilms ensures that the larger, heavier bifilms have sufficient time to sink through the overlying flow and be captured by entering the sump or vessel20. Their rate of descent can be controlled by the rate of addition of heavy alloy addition. The vacuum assisted pump18periodically lifts the sediment (the concentrated oxide and liquid aluminium mixture) out of the sump or vessel20and into a pigging ingot so that the cast sediment, is provided in a uniquely convenient ingot form to facilitate the recovery of the entrained aluminium.

In one embodiment the OSP channel or launder can be split into two channels or launders so reducing the metal speed by half and doubling the time for sedimentation. This also facilitates the blocking off of one channel for cleaning or maintenance without stopping production.

The relatively shallow depth of the channel or channels10will allow for passive degassing by the counter-directional flow of dry nitrogen above the melt.

Furthermore, as the system has no moving parts, requires zero human intervention, and all the processes are ‘passive’ i.e. occur naturally without intervention the efficiency and implementation of the apparatus is significantly improved with regard to conventional apparatus.

Typically the liquid metal and/or metal alloy is kept molten by overhead electrical elements24which extend along the length of the channel or channels10. The temperature is designed to be low from the heating assembly2and through the transfer assembly26to maximise sedimentation and degassing efficiencies, reduce energy consumption, and extend refractory life.

Filters22can be introduced as required to filter materials from the metal and/or metal alloy prior to the same reaching the mould filling assembly12.

The mould filling assembly12includes a pump28and the liquid metal and/or metal alloy is introduced via the opening of a valve in the base9ppof the pump body. The body is then pressurised to raise the metal up the riser tube30and into the mould. A non-return system keeps the melt at the top of the riser tube30ready for the next mould (avoiding falling back to avoid the generation of oxides in the riser tube).

In one embodiment the volume of the body32of the pump28is approximately only 1 percent of the volume of a low pressure furnace, so that very little pressurising gas is required to operate the pump. Oxidation inside the pump can therefore be avoided relatively economically by the use of the completely inert argon gas. The casting pressures are typically only in the region of 0.2-0.4 bar providing a further economy in the use of argon. Fill time can be programmed to suit.

The small pump chamber or cavity34also means that the pump is responsive so a ‘fill profile’ can be generated for castings if necessary. Combined with modelling flow software it would be possible to know when to speed up or to slow down the rate of fill to reduce the generation of oxides for castings with complex internal geometry which is a significant improvement.

Typically, in this embodiment, a pump of 20-30 kg capacity is suspended in a crucible furnace36of a required capacity, such as in the range 300-500 kg aluminium. Electrical resistance heating elements surrounding the crucible permit the raising of the metal temperature to an appropriate casting temperature just prior to casting.

The ability to slide the filled mould casting as will be subsequently described, off the riser tube30in, for example the direction of arrow38and along the casting platen40onto an adjacent support surface has major advantages over current systems and the sliding action allows the cast metal in the mould cavity to start the cooling process ‘off-line’ and frees up the riser tube30within only a few seconds for the next casting mould to be placed over the same.

The casting platen40is typically formed of steel and is the surface on which moulds are cast and is flush with the top of the riser tube and the pump28is suspended from the platen. Below the casting platen40there is provided a production system which can be arranged to provide ultra-clean liquid metal and/or metal alloy and deliver it into moulds in a controlled and repeatable flow avoiding damage to the liquid.

Above the casting platen40the user is free to adapt the type of mould and mould handling facilities as they require. One such process employs a vacuum assisted investment block moulding technique for very thin wall precision castings that has been adapted to a counter gravity filling process.

Alternatively, a design involving permanent steel dies is possible for the manufacture of automotive wheels and when compared to current systems, increased quality, reduced production costs and higher production rates seem achievable. The current practice is roughly a wheel every 3-4 minutes with a scrap rate varying between 10-18%. With good mechanical handling for the dies above the platen, a reasonable estimation is that a rate of casting a wheel every 1-2 minutes is achievable with practically zero scrap. The use of a carousel for the steel dies could produce a defect and scrap free 25 kg casting every 30-40 seconds.

Furthermore the same apparatus can be used for the filling of sand moulds.

Embodiments of the respective assemblies of the system are now described and it should be appreciated that the assemblies below the casting platen can be used to advantage in combination or independently and in conjunction with further types of assemblies above the casting platen which may be conventionally available.

With respect to the heating assembly2then, in operation, when the charge materials which are to be used to form the liquid metal alloy are placed in the heating assembly heating means in the melter heat the charge materials and heating gases which are created pass in the direction of arrow100upwardly through the shaft and thereby heat the charge materials as they pass downwardly through the shaft in the direction of arrow101to reach the dry hearth6which transforms the charge materials into a liquid molten form. The molten material then runs down the slope103and into a holding bath8with the skin of the charging material which remains after the liquification of the same remaining on the hearth6. After a period of time, such as for example, an hour or more, the accumulating pile of oxide skins are removed from the hearth6and into a dross bin. This is in contrast to common melting procedures which use baths or crucibles which necessarily mix the highly deleterious oxide skins of the charge into the liquid metal. The dry hearth melting technique effectively segregates the major oxide content, the primary oxide skins on the charge from the melt and with the skins moving to dross bin and the molten material progressing in a different direction.

Turning now to theFIGS.1a-band5a-ethe liquid metal and/or metal alloy transfer assembly26is illustrated in greater detail. The molten metal alloy moves from the furnace heating assembly2into the transfer assembly26and moves along channels10and preferably at a uniform height so as to avoid any fall of the material and allow the distribution of the molten material to the required locations in the foundry.

The launder transfer system also includes the overhead electrical resistance element heater24to maintain the temperature of the molten material and maintain the same in the liquid condition whilst, at the same time, ensuring that the temperature of the same is as low as can be practically allowed, whilst preventing solidification. The reason for keeping the temperature as low as possible is to provide several benefits which are: the improved degassing of the molten material from hydrogen gas, particularly if a dry gas is flowed counter current over its surface and secondly, the precipitation of second phases onto bi-films occurs more efficiently at lower additions of sedimenting elements, thirdly, the extension of refractory life, and fourthly, the saving of energy.

At the oxide sedimentation pump16location, a proportion of the bi-films and their suspension in the molten material will detrain and settle out and can be effectively caught by the system at the base vessel20. However, it is found that the natural rate of settling is relatively low due to the near neutral buoyancy of the bi-films in the molten material. Thus, in order to enhance the rate at which bi-films settle, prior to entering the detrainment station, the molten material is dosed by a continuous wire feed105of elements which will promote sedimentation. The wire feed is, in this embodiment, an aluminium based alloy containing one or more elements which form heavy intermetallic compounds and which deposit and grow on bi-films when, as in this embodiment the molten material is a liquid aluminium alloy. The wire can be fed from, for example a similar apparatus as is used to feed welding wire and the wire feed promotes the controlled sedimentation to occur depending on the flow rate of the molten material.

In one embodiment titanium (Ti) can be added to enhance the rate of sedimentation of bifilms from the liquid metal. The elimination of oxide bifilms down to perhaps a size of 10 um is dependent on a number of factors including (i) the cleanness of the charge material: (ii) the melting rate: (iii) the level of alloy additions: (iv) the temperature.

As the dosed molten material enters the oxide sedimentation pump16of the launder, the tendency is for the molten material to continue its horizontal flow as indicated by arrow107and to traverse the upper level of the sedimentation volume because of the slightly favourable temperature gradient resulting from the overhead heating causing the upper layers of liquid in the launder to be slightly lower density. The horizontal flow distance across the top of the sedimentation volume is targeted to give adequate time for bi-films above a maximum size to settle out of the flow. For instance, a flow time of one minute across the unit might result in the descent of bi-films by at least 100 mm thereby allowing them to escape the horizontal flow stream and reach the largely stagnant liquid below at the vessel20. This eliminates bi-films of a certain size, corresponding for instance to the elimination of all bi-films down to a 5 micrometres diameter, whereas a 10 minute traverse time would eliminate bi-films down to perhaps 1 micrometre diameter. These elimination dimensions will depend on the concentration of the dosing alloy, the speed of the molten metal across vessel20, and the temperature of the liquid.

In a continuous flow regime, such as is required for DC casting, a continuous flow through the sedimentation unit could also be subject to the same logic requiring length of time, lowness of temperature and concentration of dopant to effect a useful cleaning action down to an acceptable maximum size of oxide bi-film defect.

Typically, the sedimentation vessel20is shaped to concentrate the sediment at a point of maximum concentration and at that point, the sediment is sucked out by a bubble lift or vacuum pump (OSP)18shown in more detail inFIGS.5a-e. The molten material enters the OSP assembly18at the entry137and at which typically the wire105is located. The molten material passes into the sedimentation vessel20at which tubes138conveying inert gas are located to provide gas down into the sedimentation volume at a sufficient rate so as to create intermittent bubbles and preferably forms bubbles at a conveniently high point in a riser tube140because bubbles created low down in the riser tube expand as they rise, finally filling the whole tube and continuing to accelerate, lifting the flocculent by means of a vacuum. The result is an exit of sediment in an accelerated manner which is effectively uncontrolled. To better control the lifting action, the bubble tubes138are gradually lowered into place so that controlled, trauma free movement of the sediment from the molten material in the sedimentation volume is achieved.

The bubbles from the exit139of the tubes138then rise up the respective coaxial outer riser tubes140and as they do so the bubbles act as a piston so as to move the contents of the riser tubes140with the flow of the bubbles in the direction of arrows141to the exit142and into dross bins144. Once primed, each bubble will lift up what is in the tube above it and suck in more oxides and sediment from the bottom of the sediment vessel20. Typically the shape of the exit142will be chamfered and has the effect of pushing the oxides and sediment clear from the exit and therefore allows the same to be clear for the next bubble and entrained sediment.

Typically the larger the gap between the respective inner tube138and outer riser tube140, then the greater amount of sediment which can be removed by each bubble from the molten material.

The bubble generation rate and/or frequency and the provision of specific types of wire feed can be electronically linked to the required flow rate of the molten material as dictated by, for example, the furnace and/or required casting rate. Typically the higher the flow rate then the more additions of wire material and removal of sedimentation is required.

Also, the inner diameter of the riser tube may be increased to counter the effect of a bubble, or agglomeration or bubbles, ejecting liquid and sediment from the exit142. The lifting action of the bubbles to provide a discreet pulsing action therefore assists the flow of the flocculant volume of sediment. The gas involved is required to be an inert gas such as argon.

Alternatively, instead of a tube with an exit142from which bubbles are released with the sediment, a porous plug is provided in the base of the sedimentation vessel20and the riser tube140is adjustable so as catch the bubbles emerging from the porous plug.

Alternatively to the provision of dross bins, it may be advantageous to cast the sediment into ingots near the top of the riser tube and so that instead of dross bins, ingot moulds are provided to be filled, and then the ingots can be moved away and stacked and this can, in one embodiment be performed in an automated manner. In this embodiment, then in place of an overflowing chute to carry away the sediment from the top of the riser tube, the riser tube connects directly with an ingot mould, causing sediment to fill the mould in a counter gravity manner and when the mould is filled, the mould and its solidifying sediment is moved away to be replaced by a new mould and so on. In one embodiment, the mould can be handled and emptied by a robot which may also perform the stacking operation and therefore not require any human intervention.

In an alternative example of the apparatus in accordance with the invention instead of the use of a bubble lift to extract sediment, a vacuum lift is used which has the advantage of increased control, avoiding the possible instability caused by the uncontrolled expansion of the bubble during its rise. The vacuum lift once again has the benefit of few moving parts. It can also be connected to an ingot casting station, in which newly formed ingots of sediment are cast, and can be transported away and stacked by robot.

In one embodiment, the sedimentation collection process can be used in series to further perform a cleaning effect on the molten material in stages. Although this embodiment might suggest that the same benefit could be gained by enlarging the sedimentation pump, effectively lengthening the distance which the liquid metal flows over the top of the sedimentation volume and so lengthening the time available for sedimentation, such an embodiment is not recommended in at least certain instances. This can be because the sediment requires walls at some angle of repose in the region of 45 degrees, thereby necessarily forming a sedimentation volume deeper than approximately 0.5 metre. Most furnaces containing liquid aluminium are limited to approximately this depth because of the danger of leakage. With increasing depth beyond 0.5 metre, it is widely accepted that the danger of leakage becomes unacceptable.

After the molten material has traversed the sedimentation stage16on the transfer assembly it is necessary to ensure that the metal does not suffer any further entrainment affect whereby fresh bi-films would be created.

Thus it is ensured that after the liquid metal has traversed the sedimentation pump18, the metal is never poured, but continues its horizontal flow along channels10, to the mould filling assembly and then finally transported counter-gravity into the moulds as will be described to manufacture cast products.

Referring now toFIGS.2a-eand3, there is illustrated in a schematic manner a mould filling assembly12in accordance with one embodiment of the invention and the apparatus includes a supply of liquid metal104, such as a holding furnace36which retains the metal at a sufficient temperature so as to be held in a liquid state. A pressurised means such as a pump28allows the liquid metal to be supplied to a riser tube30in the liquid state and to pass upwardly in the direction of arrow110through the riser tube. The riser tube has an outlet or orifice interface112which is provided for the selective location with an ingate114of a mould116which has a cavity118which is to be filled with the liquid metal and/or metal alloy.

The apparatus further includes a support surface in the form of the casting platen40and the surface may be formed of a relatively “cool” material such as steel which is sufficiently hardwearing to withstand the sliding action of the moulds116thereacross. In one embodiment, the surface of the casting platen40are acted upon by a cooling medium applied to its underside which is supplied from a cooling medium source, such as cool water or air so as to assist in maintaining the said support surfaces40in a relatively cool low temperature condition.

FIG.2aillustrates the mould116having been placed in position on the casting platen with its ingate114aligned with the riser tube30and in connection with a gasket seal which can be formed of a compressible ceramic fibre and held in place, if necessary, by being partially recessed into the mould so that the ingate114of the mould116is connected to the orifice interface112of the riser tube30as shown. The liquid metal104is pressurised to a pressure P1 by the pump28which is just enough to provide the liquid metal104at a level L1 which is substantially level with the support surface40or a few millimetres below the same.

The condition of the pump28is then changed to increase the pressure of the liquid metal104to P2 which causes the level of the liquid metal to rise upwardly as indicated by arrow128to level L2 as shown inFIG.2bso that the liquid metal104fills the cavity118of the mould. When the mould cavity is filled a valve130which until this point has been open, is closed so retaining the liquid metal104at height L2 and isolating the mould116from the pump28.

As the mould116cavity118is now full of liquid metal104and is now isolated from the pump28and is in the condition shown inFIG.2c, the mould116can then be slid sideways as indicated by the arrow132inFIG.2d, so as to remove the ingate114from the orifice interface112of the riser tube30and the ingate114is maintained in contact with the support surface40) as the sliding movement occurs. Any imperfections in the base of the mould can be accommodated and mitigated by the provision of the ceramic fibre gasket around the mould ingate114, which acts to seal the sliding interface. Also, no liquid metal spurts from the open orifice of the riser tube30as the level of the liquid metal104in the riser tube30remains at L1 and cannot rise while the valve130remains closed. During the closure of the valve130the pump28may be recharged with liquid metal104from the holding furnace36.

The mould116is then continued to be slid in the direction of arrow132until the mould116reaches a position which is sufficiently remote or removed from the orifice112so that the orifice is then free and can be subsequently reused by the placement of a new mould116′ which is slid into position as indicated by arrow138as shown inFIG.2eso as to bring the ingate114′ into connection with the riser tube30and the cycle of steps is repeated to fill that mould116′ and so on with successive moulds.

FIG.4illustrates a plan view of the casting platen40which has located thereon the mould116which has been filled as described with regard toFIGS.2a-eso that this first mould116can be retained in position on the surface40for as long a period of time as required so that the cooling effect of the casting platen40solidifies the liquid metal104in the cavity118to a sufficient extent so that the first mould116can subsequently be removed from the casting platen40without any risk of loss of metal which may cause damage to the casting therein or harm to personnel. While this mould116is cooling, the ingates114′ of subsequent mould116′ and then ingate of mould116″ have been connected, in sequence, to the orifice interface112of the riser tube and the liquid metal104introduced into the same using the method as described and without delay as the orifice112and surrounding casting platen40is quickly cleared for use. As the process steps are repeated for successive moulds, the moulds116′, and116″ are shown as having also been slid as indicated by arrows134and136respectively to locations on the casting platen40and a fourth mould116′″ is shown in connection with the orifice interface112and is in the process of being filled.

It will therefore be appreciated that at any given time, there may be one, two or further moulds all cooling on the casting platen40while subsequent moulds are being filled. As a result, the productivity time using this process, is significantly improved so that for example, with minimal moving parts, and minimal loss of liquid metal and costs, a typical cycle time for a 1-2 kg mould casting is estimated to comprise:5 seconds for the siting of the mould over the riser tube5 seconds for the filling of the mould5 seconds for the actuating slide and pressure reduction and opening of valve V

So that a total of 15 seconds is the estimated cycle time. For castings which are larger, typically up to approximately 200 kg, the filling time would be increased but only extending the cycle time to perhaps 30-40 seconds.

Turning now toFIGS.6a-8fthere are illustrated components utilised in the mould filling assembly12which are now described in detail.

In the pump28illustrated inFIGS.6a-hthere is shown a pump body32which may be of height which is selected to suit the desired capacity of the particular pump. In addition, the length of the riser tube30and valves44,46need to be provided of a length to match that of the selected pump body32and all of the other components of the pump28can be common and regardless of the particular capacity of the pump. Filter48is bonded to an insert50of the pump and the filter48prevents ingress of oxides into the pump body32and the filter52protects the valves44,46from oxide debris and prevent the same from settling on the valve seats. A baffle box55is located in the body32by pins from the interior of the pump body in order to minimise any leaks to externally of the pump and the working melt level of the liquid metal and/or metal alloy is 10 mm lower than face56of the baffle.

When the pump is out of use or requires cleaning the insert50can be removed and if necessary replaced by moving the same as indicated by arrow54inFIG.6cinto the main cavity34of the pump body, typically using a hydraulic press. The insert includes a tunnel58therein which, in this embodiment is toroidal in shape, and allows the flow of the liquid metal from the main cavity34, through the tunnel and into the riser tube30from where the liquid metal and/or metal alloy move upwardly towards the casting platen40and the mould located thereon to thereby flow into the mould cavity to fill the same.

The tunnel and the components of the insert are shown in greater detail inFIGS.7a-gand it will be seen that the tunnel includes a tapered post60to allow the liquid metal and/or metal alloy to enter the tunnel with reduced turbulence.

The tunnel58floor can have upon it a textured surface, this textured surface is designed to snag and hold any bi-films that have entered the pump.

The riser tube30lower end has a filter directly below it, such that any metal that wants to pass into the riser tube30from the tunnel58, has to go through the filter. This is to trap and snag any bi-films that have made it this far into the pump.

The upper face62of the insert includes the interface64with the riser tube30lower end and the interfaces66,68with the respective in valve46and out valve44of the pump.

InFIGS.8a-fthere is illustrated a component for use with the furnace36of the mould filling assembly, and into which furnace the liquid metal or metal enters from the channel10of the transfer assembly26in the direction of arrow72. In accordance with this embodiment the component70is elongate and is formed of a refractory material and is attached to the inner wall74of the furnace36adjacent the entrance76. The component has a lip or wall78which is located above the height of the base of the entrance76and hence prevents overflow of the liquid metal and/or metal alloy into the crucible36before the level of the liquid metal and/or metal alloy in the crucible reaches the height of the base of the channel10and hence reduces the possibility of the generation of oxides in the liquid metal and/or metal alloy in the crucible. This therefore encourages the priming of the liquid metal and/or metal alloy in the crucible from the bottom of the furnace36. The component can also be provided with a pour formation80) passing to the bottom of the furnace to further direct the flow of the liquid metal and/or metal alloy. Furthermore, the component can be left in place, as even once the priming from the bottom of the furnace has been achieved and the furnace is full and overflows the melt level would be the same height and so there would be no drop-off of the liquid metal and/or metal alloy on the other side. It is also envisaged that the melt rate of the dry hearth melter assembly2can be controlled to match the hole in the component70.

It should also be appreciated that the size and capacity of the pump28which is used can be altered to suit the size of castings required. For example, if the castings are typically 3 kg, the melt rate 200 kg/h, a 20 kg pump can be used which has the potential to deliver about 1000 kg/h. However alternatively a larger pump, for example, with a 40 kg capacity can be used which would yield for instance 20 kg castings filled in approximately 10 seconds, giving approximately 100 castings/h, requiring a melt rate of 2000 kg/h. The size of the pump can be increased to 200 kg shot size. However, of course, for such large castings, the numbers of such castings per day is usually limited.

The apparatus and method as herein described provides an optimum casting package for most mould types, providing superior casting quality and the reliability of a system with few moving parts, low energy consumption, efficient use of metal and low-cost replaceable consumables.

The system requires a minimal labour input beyond an operator in charge of melting and periodic primary oxide removal, and an operator to monitor the oxide pump and casting station whilst at the same time reducing the conventional scrap rate of between 10-18% to near zero with oxide-depleted metal and controlled, non-turbulent filling, for any foundry using this technology.

Furthermore there is a significant potential for improved properties of the cast alloy. A sufficiently high ductility will allow, for the first time, an acceptance of the necessary accompanying loss of some of the ductility by the addition of higher levels of alloys to increase strength. A high and reliable ductility (elongation to failure) will allow users of the technology to enter into markets that have traditionally been a no-go area for castings. The cast material which is created as a result of use of the system, is expected to have a high thermal and electrical conductivity as a result of the absence of bifilms. The bifilm populations generally present in cast Al alloys, acting as a population of cracks, cause the heat to travel via circuitous, lengthy routes, because it cannot cross the ‘air gap’ of the cracks. In contrast, of course, in ultra-clean metal, heat or electrons will flow efficiently in straight lines, offering minimal resistance to flow.