Patent Publication Number: US-2013248134-A1

Title: Method and Apparatus for Casting

Description:
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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a casting apparatus of the present invention, containing a mould; 
         FIG. 2  is a plan view of a support part; 
         FIG. 3  is a side view of the casting apparatus of  FIG. 1 , showing the mould being positioned relative to the support part; 
         FIG. 4  is a side view of the casting apparatus of  FIG. 1 , showing the support part in a lowered position, wherein the mould is partially submersed in quenchant; 
         FIG. 5  is a side view of the casting apparatus of  FIG. 1 , showing the mould being removed from the chamber; 
         FIG. 6  is a side view of another embodiment of a casting apparatus according to the invention, containing a mould, in which the pressure within the body of the chamber is lowered; 
         FIG. 7  is a side view of the casting apparatus of  FIG. 6 , showing the mould being positioned relative to the support part; 
         FIG. 8  is a side view of the casting apparatus of  FIG. 6 , wherein the pressure within the chamber is raised; 
         FIG. 9  is a side view of the casting apparatus of  FIG. 6 , showing the support part in a lowered position, wherein the mould is partially submersed in quenchant; 
         FIG. 10  is a side view of the casting apparatus of  FIG. 6 , showing the mould being removed from the casting apparatus; and 
         FIG. 11  is a side view of the passageway of the casting apparatus of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the  FIGS. 1 to 5  of the drawings, a casting apparatus  10  is provided, including a chamber having an upper portion  14  and a lower portion  12 . The upper portion  14  forms a ‘lid’ which is connected by a hinge  40  to the lower portion  12 , forming a sealable opening to provide access to the interior of the chamber. A clamp  42  is provided to ensure a tight seal is maintained between the upper portion  14  and lower portion  12  when the lid is sealed in a closed position. 
     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  FIG. 2 , in which the support part  20  comprises a plurality of bars  22  disposed in a grid. In this manner, the support part  20  provides a perforated, and generally flat, surface for supporting the base of a mould  18 . It will be apparent that while the support part  20  has a general cross-hatch pattern in this embodiment, other forms of support part  20  suitable for supporting a mould  18  may be used. 
     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.  FIG. 1  shows the support part  20  in its first position, in which the mould  18  is supported in a position clear of the quenchant  16 .  FIG. 4  shows the support part  20  in its second position, in which the mould  18  is largely submersed in the quenchant  16 . In this second position, a large surface area of the mould  18  is in contact with quenchant  16 , and therefore the heat from the mould  18  will transfer to the quenchant  16 , cooling the mould  18  and its contents. The perforations in the support part  20  allow the support part  20  to be raised and lowered through the quenchant  16  without meeting significant resistance. 
     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  FIGS. 1 , and  3  to  5 .  FIG. 3  shows the chamber lid (upper portion  14 ) in its open position, and a mould  18  being placed within the chamber, on the support part  20  which is in its first (i.e. raised) position. At this stage, the chamber is at atmospheric pressure.  FIG. 1  shows the chamber in a closed configuration, with the upper portion  14  being sealed against the lower portion  12 , and held closed by the clamp  42 . Once the chamber has been closed, and sealed, the pressure within the chamber is increased. Compressed air is introduced into the chamber through the inlet valve  36  until a pre-determined increase over atmospheric pressure is achieved. Readings from one or more pressure sensors  38  within the chamber, or within pipes connected to the chamber, may be used to detect whether the desired pressure level has been achieved. Once the pressure within the chamber has reached the desired level, the inlet valve  36  is closed to seal the pressure within the chamber. 
     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  FIGS. 6 to 11  of the drawings, a further embodiment of the invention is described. Many of the features of this embodiment are equivalent, or identical to, respective features of the first embodiment described, in which case the integers of the second embodiment are numbered with corresponding reference numerals with a prefix of ‘1’ (for example, lower portion  12  of the first embodiment, is labelled  112  in the second embodiment). 
       FIG. 6  shows casting apparatus  110  including all of the features of the previously described embodiment. While in the embodiment described, the apparatus and method include the features of the previous embodiment, it should be understood that the additional features described below, that are not included in the previous embodiment, may be applied to alternative casting apparatus and methods. It is not envisaged that the use of the additional features should be limited to use in conjunction with the features of the previous embodiment. 
     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  FIG. 7 . Following that, the chamber is closed and sealed, using the clamp  142 . The vacuum pump  154  is then operated, to reduce the pressure within the chamber, to a pre-determined level below atmospheric pressure. 
     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.