Patent Application
20130248134
The invention relates to a method of casting an alloy, and apparatus for the same. The invention relates in particular, but not exclusively, to casting aluminium alloys.
Known methods for casting alloys involve pouring molten alloys into a mould, placing the mould inside a chamber, and cooling the mould using a quenchant such as water, or the like. The molten alloy within the mould solidifies as the mould is cooled, and is cast into the general shape of the mould.
Known casting methods have a number of problems associated with them. One such problem is the formation of bubbles within a cast metal, known as a gas porosity defect. Gas porosity defects may reduce the strength of the metal, and potentially affect its appearance. Gas porosity defects occur because liquid metals, in general, hold a large amount of dissolved gas, whereas by comparison a solidified metal cannot. Therefore, as the metal cools during the casting process, bubbles of gas may form within the metal.
It is also a common problem for defects to occur, and for cast material to be wasted, where molten alloy has not filled the mould adequately. Molten alloy may not entirely fill the mould, particularly thin sections of mould. Furthermore, if the mould is cooled whilst the molten alloy is poured into it, portions of the material within the mould may solidify prematurely, before other portions of the mould have been filled.
The mechanical properties of metals formed using casting processes are affected by various aspects of the process used. Variation in the rate of cooling of the metal can alter the properties of the metal. By maintaining the quenchant in its liquid form for longer higher levels of heat removal may be sustained, yielding a metal with enhanced mechanical properties.
According to an aspect of the invention we provide a method of casting an alloy, comprising the steps of pouring molten alloy into a mould, moving the mould to a first position inside a chamber, the chamber including a volume of quenchant, increasing a pressure within the chamber to above atmospheric pressure, and moving the mould to a second position in which at least a portion of the mould is submersed in the quenchant, so as to reduce the temperature of the mould.
According to an aspect of the invention we provide a method of casting an alloy, comprising the steps of moving a mould to a position within a chamber, the chamber including a volume of quenchant, reducing a pressure within the chamber to below atmospheric pressure, and causing molten alloy to enter the chamber, and to enter the mould under the reduced pressure within the chamber.
According to another aspect of the invention we provide a casting apparatus, comprising a pressurisable chamber having a part for receiving a quenchant, a pressurising means for altering a pressure within the chamber, a sealable opening to provide access to an interior of the chamber, and a support part for supporting a mould, the support part being moveable between a first, non-quenching, position and a second, quenching, position wherein the second position is closer than the first position to a lowermost portion of the part for receiving a quenchant.
According to another aspect of the invention we provide a casting apparatus, comprising a pressurisable chamber having a part for receiving a quenchant, a sealable opening to provide access to an interior of the chamber, and a container for receiving molten alloy, the container being connected to an interior of the chamber by a passageway, and being closable by a meltable sacrificial seal to separate the container from the interior of the chamber.
Further features of the various aspects of the invention are set out in the claims appended hereto.
Embodiments of the invention will be described by way of example only with reference to the accompanying drawings.
With reference to the
A support arrangement is provided within the chamber, for supporting a mould 18. The mould may be a conventional investment cast mould, for example. The support arrangement comprises a generally flat support part 20 onto which a mould 18 may be placed. Preferably, the support part 22 is perforated to allow a fluid to pass through it. An embodiment of the support part 20 is shown in
The support part 20 is connected to a support arm 28 and one or more guide members 26. The support arm 28 is secured to the support part 20 at one end, and to an actuator 24 at its other end. The actuator 24 comprises a drive mechanism that is operable to move the support arm 28 axially in a first (downward) and a second (upward) direction, thereby causing the support part 20 to move in the first or second direction, respectively. In an embodiment, the actuator 24 includes a hydraulic drive mechanism. It should be understood that other types of drive mechanism may be used, such as a pneumatic drive mechanism. In an embodiment, a pair of guide members 26 is provided, the guide members 26 being spaced from on either side of the support arm 28. The support part 20 may comprise a pair of guide rings 27, each configured to surround a portion of a respective guide member 26, and each guide member 26 may be disposed generally upright within the chamber, and generally parallel to the support arm 28.
The actuator 24 may move the support arm 28 upwardly and downwardly, thereby causing upward and downward movement of the support part 20 within the chamber. The guide rings 27 engage with the guide members 26, to guide the support part 20 upwards and downwards, and prevent lateral rotation of the support about the support arm 28.
The lower portion 12 of the chamber is adapted to receive a volume of quenchant 16, which may be water (which may include additives), for example. The quenchant 16 should be fluid having a relatively high specific heat capacity, compared to that of air, for example, such that submersing the mould 18 within the quenchant 16 will result in increased heat transfer from the mould 18. The lower portion 12 of the chamber includes one or more conduits forming inlets and/or outlets 30, 32, which may be provided with valves, to allow quenchant to flow into or out of the chamber.
The support arrangement is operable to move the support part 20 between a first position in which at least a portion of a mould 18 supported on the support part 20 is not submersed in the quenchant 16, and a second position in which the mould 18 is at least partially submersed in the quenchant 16. The quenchant 16 is held within a lowermost portion of the lower portion 12, and therefore the first position of the support part is further that the second position from the lowermost part of the lower portion of the chamber.
When the mould 18 is lowered into the quenchant 16, the heat of the quenchant 16 rises as heat is transferred to the quenchant 16 from the mould 18. In the case in which the quenchant 16 is water, the water will vaporise and turn into steam once it reaches its vaporisation point, which at sea-level pressure is 100° C. It is preferable to maintain the water in its liquid state, since the specific heat capacity of water is much higher than the specific heat capacity of steam, and therefore the heat will be transferred from the mould 18 more effectively (and quickly) when submersed in water, than if it was submersed in steam.
In order to maintain the quenchant 16 in its liquid state as long as possible, and to as high a temperature as possible, the pressure within the chamber is increased. An increase from atmospheric pressure at sea-level, of 1 bar, yields a vaporisation point of approximately 120° C., and an increase of 3 bar yields a vaporisation point of approximately 144° C. Therefore, by increasing the pressure within the chamber, the quenchant 16 will stay in its liquid state until it reaches a higher temperature, allowing a greater amount of heat energy to be transferred from the mould 18 to the quenchant 16, prior to its vaporisation (at which point heat transfer becomes less efficient).
It should be understood that this aspect of the invention is not limited to any particular combination of pressure within the chamber and temperature of quenchant 16. The method is limited only by the limitations of chamber construction (i.e. it must remain sealed under pressure), and available compressed air pressure. However, it is envisaged that the pressure within the chamber is raised to a pressure significantly above atmospheric air pressure during the casting process.
The casting apparatus 10 is provided with a pressurising means, for increasing the pressure within the chamber. In this example, the chamber is provided with an inlet valve 36, that is connected to a source of compressed air. When the pressure within the chamber needs to be raised, the inlet valve 36 is opened and compressed air is introduced into the chamber. A venting valve 34 is provided for venting pressure from the chamber, to the atmosphere. By using the inlet valve 36 and venting valve 34, the pressure within the chamber may be controlled accordingly. The pressurising means may include a pump of a known type (such as a positive displacement pump, velocity pump, centrifugal pump, impulse pump, or any other suitable pump), or a compressor of a known type (such as a diaphragm compressor, a rotary screw or vain compressor, a scroll compressor, a reciprocating compressor, an axial flow compressor, a centrifugal compressor, or a mixed-flow compressor, for example).
Pressure and temperature sensors may be provided within the chamber (or within pipes connected to the chamber), so that the pressure and temperature within the chamber, and of the quenchant, may be monitored and/or controlled. A pressure-relief valve 44 is provided, through which pressure may be vented if the pressure within the chamber exceeds a maximum desired pressure. The pressure-relief valve 44 may be adjustable to a pre-determined level to be set by an operator.
A method of casting will now be described with reference to
Before, during or after the pressure has been increased within the chamber, the support arrangement is controlled so that the support part 20 and the mould 18, are lowered from the first position to the second position. This controlled lowering may take place at a pre-determined, constant rate, set by an operator.
As the heat transfers from the mould 18 to the quenchant 16, the metal within the mould 18 cools, and eventually solidifies. After a pre-determined time, or when initiated by an operator, the support arrangement moves the support part 20 from the second position back to the first position, lifting the mould 18 to be lifted out of the quenchant (out of the liquid quenchant, at least). At this point, the pressure within the chamber may be reduced back to atmospheric levels, by operating the venting valve 34. The upper portion 14 of the chamber may then be opened, providing access to the mould 18, which may then be removed from the chamber.
The temperature of the quenchant 16 may be monitored by temperature sensors, to allow an operator to assess the heat within the chamber. On completion of the casting operation (or at any other stage of the process), and prior to opening the sealed chamber to extract the mould 18, the temperature of the quenchant 16 may be regulated by removing heated quenchant 16 from the chamber via the one or more outlets 30, 32, and/or cooled quenchant may be introduced via the one or more inlets 30, 32.
With reference to
In addition to those features previously described, the venting valve 134 includes an additional exhaust valve 152, for switching between venting the pressure of the chamber to atmospheric pressure, and venting to a pressure-reduction means, which in this example is a vacuum pump 154. The use of the vacuum pump 154 enables the pressure within the chamber to be reduced to a level below that of atmospheric pressure. The pressure-reduction means may include any form of pump (or compressor-type apparatus) also suitable for use as a pressurising means, as described herein, configured to cause flow of gas from the chamber so as to reduce the pressure within the chamber.
The upper portion 114 of the chamber engages with an inlet arrangement, defining a passageway for providing molten alloy from a container 146 to a mould 18 whilst the mould 18 is positioned inside the chamber. The inlet arrangement comprises the container 146 for receiving molten alloy, the lower end of the container 146 forming a neck 158 that is engageable with the upper portion 114 of the chamber. In an embodiment, the neck 158 has a screw-threaded portion on an outer surface, for engagement with a corresponding screw-threaded portion 156 defined within an opening in the upper portion 114 of the chamber. When the neck 158 is engaged with the upper portion 114, the lowermost portion of the neck 158 lies adjacent, or abuts, a ridge defined within the opening.
A sacrificial seal 148 is provided, which preferably comprises a disc of material that will melt when exposed to molten alloy. Preferably, the disc is formed of aluminium, having properties similar or identical to the molten alloy to be cast. The ridge defined by the opening within the upper portion 114 of the chamber is configured to receive the sacrificial seal 148. The sacrificial seal 148 is held in position against the ridge by the lowermost portion of the neck 158, so that the sacrificial seal 148 is sandwiched between the two. In this manner, the passageway is sealable to separate the container 146 from the interior of the chamber.
A pouring valve 150 is provided in the upper portion 114 of the chamber, disposed below the inlet arrangement. The pouring valve 150 may be a ball valve, for example. When open, the pouring valve 150 provides communication between the interior of the chamber and the inlet arrangement.
In use, a mould 18 is placed inside the chamber on the support part 120, as shown in
Molten alloy is poured into the container 146 at its upper end 147, so as to contact the sacrificial seal 148 disposed at the lower end of the container 146, separating the container 146 and the interior of the chamber.
The mould 18 has an opening defined at its upper surface, for receiving a molten alloy. The opening of the mould 18 is disposed directly beneath the pouring valve 150, such that melting of the seal 148 permits molten alloy to flow from the container into the interior of the chamber, and into the mould 18.
The lowered pressure within the chamber means that there are reduced gas levels within the mould cavity. This enables the molten alloy to flow freely into the chamber and ensures that the molten alloy fills the lower portions of the mould 18. Furthermore, by assisting the mould-filling process in this way, the need to heat the molten alloy and the mould 18 to very high temperatures is reduced. This provides a more energy-efficient method of filling a mould 18.
As the molten alloy flows from the container 146, through the passageway and pouring valve 150, and into the interior of the chamber, the molten alloy blocks the opening between the interior of the chamber and atmospheric pressure outside the chamber. In this way, the lowered pressure within the chamber is preserved, until the metal has finished flowing into the chamber, and into the mould 18.
When molten alloy has filled the mould 18, the atmospheric pressure is immediately allowed to enter the chamber, through the passageway. At this point, the pouring valve 150 is closed, sealing the chamber once again from atmospheric pressure.
By reducing the mass of gas within the mould cavity, the pouring technique described above enhances the ability of the molten alloy to access thin and awkwardly-shaped parts of the mould cavity. A further advantage of this method is that the likelihood and/or extent of premature solidification of some portions of the metal within the mould 18 is reduced, resulting in an improved casting quality.
Once the sacrificial seal 148 has been used, it can be replaced for future operation of the casting apparatus by unscrewing the neck 158 from the upper portion 114 of the chamber.
The remainder of the method is identical to that of the first embodiment, wherein the pressure is increased within the chamber by operating the inlet valve 136, and the support part 120 is lowered into the quenchant 16, thereby accelerating the mould-cooling process.
The proposed method of enhanced solidification will offer mechanical properties superior to existing available levels, thus offering the possibility of weight and material reductions and/or improved performance of the cast product. Combining casting in an increased-pressure environment, with pouring the molten alloy into the mould under a reduced pressure, may result in reduced material costs due to reduced scrap being produced at the initial casting stage.
When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.
The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| GB1204884.9 | Mar 2012 | GB | national |
