Casting A Metal Object

Cross Reference to Related Applications

This application claims priority from United Kingdom Patent Application No. 07 21 063.6, filed 26 October 2007, the whole contents of which are incorporated herein by reference in their entirety

Technical Field

The present invention relates to apparatus for casting metal in a mould, and a method of casting a metal object in a mould.

Background of the Invention Apparatus and methods are known for casting metal objects in which a vacuum induction melting unit consists of two water-cooled chambers. The primary chamber contains a furnace in which the metal is melted under vacuum. When the melt is ready for pouring, a ceramic shell mould, preheated to 1000°C is placed in the second chamber that is then also evacuated. As soon as the pressure levels in the two chambers are equal, a valve between the chambers is opened. The mould is transferred into the primary chamber and filled with the molten metal. The mould is then retracted into the secondary chamber, the valve closed and the secondary chamber allowed back up to air pressure. This method therefore requires relatively expensive equipment including: a vacuum induction melting unit having a pair of chambers separated by a valve of sufficient size to allow the passage of the mould; and heating apparatus for preheating the ceramic shell. In addition, the vacuum induction melting unit must have vacuum pumping equipment that is capable of evacuating the second chamber very quickly to minimise loss of temperature of the ceramic shell. Due to the high expense of the equipment and process, the twin chamber system serves a relatively narrow market

when the global potential for special steel and nickel based casting manufacture is considered.

There are few such vacuum induction melting units in the world that have a melting capacity greater than 100kg. Because of the capital and operational costs, and the difficulty of producing large ceramic shells that have to be poured at 1000 ⁰ C, their application can only be justified for castings where cost is less important than safety and weight, such as in the aerospace sector.

The above described method also has the problem of poor energy efficiency, firstly due to the need to pre-heat the mould, and secondly due to heat loss during transfer of the mould between the pre-heat furnace and casting chamber.

It should also be understood that transferring the ceramic shell mould, when at a temperature of 1000 ⁰ C, between pre-heating apparatus and the vacuum induction melting apparatus also produces handling problems and requires suitable handling equipment.

Maintaining the temperature of the mould between the preheat furnace and the casting chamber can also prove to be difficult, particularly for large moulds. Moreover, as the size of the mould increases the size of chambers required to receive the mould also increases. For a given pumping capacity, a larger second chamber generally takes a longer time to evacuate than a smaller one. Consequently the time taken to transfer a mould between the pre-heat furnace and primary (casting) chamber is generally increased for larger chambers. This further contributes to the difficulty in maintaining mould temperature between the preheat furnace and the casting chamber. Of course, pumping capacity may be increased to keep the speed of evacuation of the second chamber and transfer of the mould between pre-heat furnace and casting chamber at practical levels, but this leads to increased capital and running costs.

Brief Summary of the Invention

According to a first aspect of the present invention, there is provided a method of casting a metal object in a mould, comprising the steps of: (a) establishing a mould within a mould box containing particulate material; (b) positioning said mould box within a casting chamber; (c) charging a crucible with metal; (d) while said mould box and said crucible are located within said chamber, evacuating air from said chamber to reduce pressure in said chamber; (e) heating the metal within the crucible to melt said metal; (f1) after the evacuation of air from said chamber is terminated, introducing a non- oxidising gas into said casting chamber; (f2) applying a vacuum directly to said mould box for assisting a flow of molten metal into said mould; and (g) pouring molten metal from said crucible into said mould, while continuing to pump gases from said mould box.

As the mould box is located within the casting chamber, along with the crucible, before the metal within the crucible is heated and melted, this process allows for the use of a single-chamber vacuum induction-melting unit to be used. Furthermore, because the transfer of the mould into the chamber takes place before the chamber is evacuated of air, the equipment required and method of transferring is simplified. Also, as the whole process takes place in a single chamber, it is not necessary to transfer a hot mould (at around 1000 ⁰ C) through a secondary chamber, to the casting chamber. Consequently, large castings can be produced by this method. For the purposes of this specification, a "large casting" is one that has a finished mass of 100 kg or more. Preferably, the mould is positioned within the casting chamber while it is cold. For the purposes of this specification, "cold" is herein defined as a temperature of between ambient air temperature and 300°C. As the mould is not required to be hot (around 1000°C), heating equipment and handling equipment may be simplified, particularly for large castings. In addition, energy costs and heat losses are reduced.

Preferably the mould is not preheated prior to step (g). Consequently, handling of the mould is further simplified.

According to a second aspect of the present invention, there is provided a method of casting a metal object in a mould, comprising the steps of: (a) establishing a mould within a mould box containing particulate material; (b)positioning said mould box within a casting chamber; (c) charging a crucible with metal; (d) while said mould box and said crucible are located within said chamber, evacuating air from said chamber to reduce pressure in said chamber; (e) heating the metal within the crucible to melt said metal; (f) after the evacuation of air from said chamber is terminated, introducing a non-oxidising gas into said casting chamber; and (g) pouring molten metal from said crucible into said mould.

According to a third aspect of the present invention, there is provided apparatus for casting metal in a mould, comprising: a casting chamber having (i) a first port configured to be connected to a gas supply for introducing a non-oxidising gas into the casting chamber, (ii) a second port configured to be connected to pumping equipment for evacuating gases from the casting chamber, and (iii) a duct having means for connecting to pumping equipment located outside of the casting chamber and means for connecting to a low- pressure chamber located within mould apparatus located in the casting chamber; and electrical heating means for melting metal located within a crucible within said casting chamber.

Brief Description of the Several Views of the Drawings

Figure 1 shows a schematic cross-sectional diagram of apparatus 101 for casting a metal object;

Figure 2 shows mould apparatus 201 for use within the casting apparatus 101;

Figure 3 shows flowchart outlining a method of casting a metal object, using the apparatus 101 of Figure 1;

Figure 4 shows the apparatus 101 being used in accordance with the method outlined in Figure 3;

Figure 5 shows a flowchart providing further detail of the step 301 of preparing mould; Figure 6 shows sand mould apparatus 601 being used within the apparatus 101 for casting a metal object; and

Figure 7 shows sand mould apparatus 701 being used within the apparatus 101 of Figure 1.

Description of the Best Mode for Carrying out the Invention Figure 1

Apparatus 101 for casting a metal object is shown in the schematic cross-sectional diagram of Figure 1. The apparatus 101 comprises a housing 102 defining a casting chamber 103. The housing 102 is shown in Figure 1 positioned on a foundry floor 151. Access into the chamber 103 is provided by a main door 104 and crucible door 105 in the walls of the housing 102.

Thus a worker, such foundry worker 152 is able to access the chamber 103 via the main door 104. Both the main door 104 and the crucible door 105 are provided with seals 106 such that when the doors are closed, as shown in Figure 1, the chamber is essentially airtight. The chamber 103 has an inlet port 107 configured to be connected to a gas supply for introducing a non-oxidising gas into the casting chamber 103. Thus, in the present example, the port 107 has a flange 108 connected to pipework 109 providing a supply of argon gas from an argon gas cylinder 110. A valve 111 is provided within the pipework 109 to control the flow of gas from the cylinder into the chamber 103.

In the present case an inert gas is used, but it is anticipated that this gas may be replaced by another gas, such as nitrogen, depending upon the type of metal being cast.

It should be noted that the word "metal" as used herein refers to pure

metal and metal alloys. Thus the metal may be an alloy such as a steel, stainless steel or a nickel based alloy.

The chamber 103 has an outlet port 112 configured to be connected to pumping equipment for evacuating air from the casting chamber. Thus, the port 112 has a flange 113 connected to a corresponding flange on vacuum line 114. The vacuum line 114 provides communication between the outlet port 112 and pumping equipment 115. In the present case the pumping equipment comprises a pair of roots blowers connected in series and backed by a rotary piston pump. The vacuum line 114 is provided with a solenoid actuated valve 116 to control the pumping of air from the chamber 103. It should be understood that during use of the apparatus 101, the door seals 106 provide a vacuum seal between the chamber 103 and the surrounding space occupied by workers, such as worker 152.

In addition to ports 107 and 112, the chamber 103 has a separate duct 117 having means for connecting to pumping equipment located outside of the casting chamber and means for connecting to a low-pressure chamber located within the casting chamber. Thus, in the present case, the duct 117 is a pipe having a flange 118 at an end outside the chamber 103 and a second flange 119 at the opposite end located within the chamber. The flange 118 in the present example is connected to pumping equipment 125 via a second vacuum line 120. The vacuum line is also provided with a solenoid actuated valve 121 to control pumping through the duct 117. The flange 119 located within the chamber 103 allows the duct 117 to be connected to a low- pressure chamber that is itself located within the casting chamber 103 during use. This will be further described below with reference to Figure 4.

In the present embodiment, the outlet of the pumping equipment 125 is connected via pipework 126 to an inlet port 127 of the casting chamber 103. Consequently, in operation gas evacuated through the duct 117 is recycled back into the chamber 103. In order for the gas recycled back into the chamber 103 to remain relatively dry, the pumping equipment 125

comprises a roots blower.

The chamber 103 also has an air inlet port 131 and associated valve 132. As will be described below, this valve may be opened to let air into the chamber, or allow gas to escape from the chamber, to bring the pressure within the chamber to ambient atmospheric pressure.

The apparatus comprises an induction melting unit comprising induction heating coils 123, located in the chamber 103, connected to a suitable power supply (not shown) as is known in the art. Means are provided for supporting a crucible 122 within the induction heating coils 123, such that metal located within the crucible may be melted by operation of the induction melting unit.

The induction heating coils are mounted in an elevated position such that they are positioned above the floor of the chamber 103 to form a space in which a mould may be located. In an alternative embodiment, the induction melting unit is replaced by alternative electrical heating means, such as a vacuum arc remelter, for melting metal within a crucible.

Figure 2

Mould apparatus 201 for use within the casting apparatus 101 is shown in Figure 2.

The mould apparatus comprises a mould box 202 which has continuous walls 203 extending upward from a floor 204. A gas permeable panel 205 is mounted within the box 202 such that it extends between the four walls of the box parallel with the floor 204. Thus the panel 205, the floor 204 and walls 203 define a low-pressure chamber 206. An outlet port 207 is provided for the low-pressure chamber 206 in one of the walls 203.

The panel 205 supports a particulate material 208, such as sand, and a ceramic shell mould, for example mould 209, is located within the particulate material. The panel 205 in the present case is formed from a mesh screen. The

mesh screen defines apertures that have sufficiently small dimensions to prevent the passage of the particulate material, while allowing for the passage of gas.

The mould apparatus 201 is provided with wheels, or rollers, 212 mounted under the floor 204 to facilitate movement into and out of the casting chamber 103 during use.

An impermeable membrane 210 is located on the upper surface of the particulate material 208. One or more apertures defined in the membrane 210 allow parts of the mould 209, such as a pouring cup 211, to extend through.

In alternative embodiments, particulate materials other than sand are used to surround a ceramic shell mould. Such particulate materials include molochite.

In an alternative embodiment, the impermeable membrane is omitted from the upper surface of the sand.

Figures 3 and 4

A method of casting a metal object, using the apparatus 101 of Figure 1, is outlined in the flowchart of Figure 3, and the apparatus 101 is shown being used in accordance with said method in Figure 4. The first step 301 of the method is to prepare mould apparatus, such as that shown in Figure 2, suitable for receiving molten metal to form a casting. The step 201 of preparing the mould is described in detail below with reference to Figure 5.

The mould apparatus 201 including the mould 209 is then loaded into the casting chamber 103 at step 302, for example through the main door

104. The outlet 207 of low-pressure chamber 206 is connected to the flange 119 of the duct 117, for example, by a flexible vacuum hose 401.

At step 303 the crucible 122 in the casting chamber 103 is loaded with metal 406 which is to be melted and poured into the mould 209. The crucible 122 may be loaded via crucible door 105.

It should be noted that steps 302 and 303 may be performed in either order. I.e. the crucible may be loaded before the mould is positioned in the casting chamber.

At step 304 the chamber doors (104 and 105) are sealed and valve 116 is opened so that air is pumped from the casting chamber 103 to form a vacuum. Typically, before the following step (step 205) is performed, the chamber 103 is pumped out until the pressure within the chamber is less than about 1x10 ^"1 mbar (10 Nm ^"2 ).

Once a sufficiently low pressure is obtained in the casting chamber 103, the induction heating equipment 123 is activated so that the metal 406 within the crucible 122 is melted at step 205. To ensure that the metal flows to all parts of the mould, particularly where the mould has small passageways, the metal is heated at step 205 to a temperature above its melting point. With the metal at its required temperature, the valve 116 is closed at step 306 such that further evacuation of the chamber is prevented.

The valve 111 is then opened to back-fill the chamber 103 with a non- oxidising gas. For example, where the metal being cast is stainless steel, the gas may be argon.

The gas is supplied to the chamber 103 until a pressure in the range of half to one atmosphere (0.5 x 10 ⁵ to 1.0 x 10 ⁵ Nm ^"2 ) and typically 0.9 atmospheres (0.9 x 10 ⁵ Nm ^"2 ) is obtained. The valve 121 is then opened at step 307 such that the pressure in the low-pressure chamber 206 is reduced, typically to a value of 500mm of mercury (about 6.7x10 ⁴ Nm ^'2 ), and non- oxidising gas is drawn through the mould and sand by pumping equipment 125.

The pressure differential, from the casting chamber 103 to the mould box and grains of sand 208, has the effect of locking the sand grains together to enhance the support that the sand provides for the mould 209. Furthermore, because of the limited permeability of the shell mould, a pressure gradient is created across the shell wall, with the non-oxidising gas

on the interior of the wall and a relatively low pressure on the exterior, sand side, of the wall. This holds the shell onto the rigid sand to provide yet further support.

It may be noted that, in the present embodiment, the gas drawn through the mould by pumping equipment 125 is returned to the casting chamber 103 via inlet port 127.

The molten metal in the crucible 122 is then poured into the mould at step 308. During pouring, a partial vacuum is formed within the unfilled spaces of the mould due to gas escaping through the mould 209, particulate material 208, low-pressure chamber 206 and the duct 117. Consequently, the pressure within the unfilled spaces of the mould becomes less than the pressure of gas within the casting chamber 103, and this pressure differential assists the flow of molten metal into the unfilled spaces of the mould.

It may be noted that the impermeable membrane 210 prevents the passage of gas from the chamber 103 to the upper surface of the particulate matter 208. Consequently, the membrane 210 prevents unnecessary loss of the gas through the mould apparatus 201.

In conventional casting processes using vacuum induction melting, pouring is performed under vacuum. However, in the present case, pouring is done at or near atmospheric pressure and this has a beneficial effect on soundness and yield of the castings produced.

After filling the mould, the valve 121 is closed at step 309 to prevent further evacuation of gas, and the valve 111 is also closed. The casting is then allowed to solidify and cool in the non-oxidising atmosphere within the chamber 103, so that adverse oxidation of the casting is prevented. When the casting is sufficiently cool, the air inlet valve 132 is opened at step 310 to allow pressure in the chamber 103 to equalise with the ambient atmospheric pressure. The door 104 is opened and the mould is removed from the casting chamber. The casting may then be removed from the mould. It should be noted that in the present embodiment the molten metal is

poured into a mould that has not been pre-heated. However, even though passageways in the mould have relatively narrow dimensions, the molten metal is able to flow into all parts of the mould before solidifying. The reason for this is the above described pressure differential caused by the pressure of the argon and the partial vacuum generated in the mould itself.

In the above described method, the metal is melted under vacuum at step 305, and then a non-oxidising gas is allowed into the chamber 103 at step 306. For some metals, this has the advantage of de-gassing the metal as it is melted under vacuum. However, in an alternative method the chamber 103 is filled with a non-oxidising gas before the metal is melted at step 305. This can be advantageous when de-gassing is undesirable, or where the metal is an alloy containing elements of relatively low melting points, such as aluminium, that would tend to vaporise under vacuum and adversely affect the composition of the alloy in the casting. It should be understood that the whole process takes place in a single

_^ chamber, using a cold mould. Thus, the mould is positioned within the chamber before it is evacuated and before the metal is melted by the induction heating equipment. Consequently, the manoeuvring of the mould into the chamber is simplified, and the use of ceramic moulds large enough to produce very large castings having a mass of 1000kg or more is made possible.

Figure 5

The step 301 of preparing mould apparatus is shown in further detail in the flowchart of Figure 5. At step 501 a thin walled ceramic shell mould is produced. This is achieved by injecting either wax or an expandable polymer, such as polystyrene, into a mould to form a pattern. In the case of polystyrene, polystyrene spheres are located within a mould and heated such that they expand and fuse together to form a substantially solid structure.

The pattern is then assembled with the running, gating and feeder system to form an assembly. The assembly is then coated with a series of

uniform layers of stucco comprising a slurry of a refractory material and binder, to which a grog is applied. Typically, for small castings, between 5 and 6 layers of stucco are applied and dried in this way, and for large castings 8 or 9 layers are used. The pattern, formed of wax or polystyrene, is then removed by melting, or burning off, before the shell is fired to harden it.

After cooling, the finished shell mould may be cleaned and stored for subsequent use.

The shell mould (as shown for example at 209 in Figure 2) is placed within the mould box (202) and particulate material, such as sand (208) is placed around it at step 102. The mould box is then vibrated at step 103, typically at a frequency of 40-50 Hz and displacement of 0.045mm RMS (root mean square) for a period of 90 seconds. The vibration causes the sand to flow into intimate contact with the shell and compacts the sand to a high bulk density. Thus, the sand provides substantial support to the thin ceramic shell mould during further steps of the casting procedure. (As described above, at step 307 this support is further enhanced by the action of the pressure differential caused by reduced pressure in low-pressure chamber 206.). The mould apparatus 201 is then ready for use in the apparatus 101.

Typically, between 5 and 9 layers of stucco are applied to the pattern to produce the ceramic shell. Consequently, the ceramic shell is too thin to be self-supporting during the casting operation. However, as described above, the shell is supported by compacted sand during casting.

The relatively thin wall of the ceramic shell, when compared to the ceramic shell used in conventional vacuum induction-melting processes, allows the shell to be sufficiently light for manual handling. In addition, the thin shell has a relatively low thermal capacity enabling it to be used without the need for pre-heating before casting. Thus, manual handling is further facilitated by the shell being used cold.

A lighter, cold shell is also easier to clean, and to keep clean, than the relatively heavy and very hot shell of conventional vacuum induction melting

processes. This enables inclusions to be reduced.

It may also be noted that the thinner shell of the present method allows the casting to contract on solidification and cooling, thereby reducing the prospect for hot-tearing to occur. Thin ceramic shells are also cheaper and faster to produce than the thicker walled shell used in conventional vacuum induction melting processes, and they allow the logistics of the whole casting operation to be improved.

Figure 6 In an alternative method of casting to that described above, the ceramic shell mould is replaced by a sand mould. Sand mould apparatus 601 is shown being used, within the apparatus 101 of Figure 1 , for casting a metal object in Figure 6.

The sand moulding apparatus 601 is similar to mould apparatus 201 in that it comprises a mould box 602 which has continuous walls 603 extending upward from a floor 604. A gas permeable panel 605 is mounted within the box 602 such that it extends between the four walls of the box parallel with the floor 604. Thus, the panel 605, the floor 604 and walls 603 define a low- pressure chamber 606. An outlet port 607 is provided for the low-pressure chamber 606 in one of the walls 603.

The panel 205 supports a granular material 608, such as sand, which defines an internal cavity 621 and feeding and gating channels 626, etc., as are known in the art.

The mould box itself is formed in three parts: a lower part 622 containing the low-pressure chamber 606; a middle part 623 comprising a lower portion of the sand 608 defining the mould; and an upper part 624 that contains an upper portion of said sand, separated from the lower portion of sand by a parting line 625. The upper part 624 and middle part 623 are essentially the same as a conventional sand mould box. However, the middle part 623 and lower part 622 are configured to seal together so that gas

pressure in the low-pressure chamber 606 may be reduced, thereby applying a vacuum to the sand mould.

The mould is prepared in a substantially conventional manner, thus the middle part 623 of the mould box 602 is partially filled with sand before patterns defining the mould, gating system, risers, etc. are located in the middle part of the mould box near to the parting line. The upper part 624 of the box 602 is then located above the middle part 623 before it too is filled with sand. The sand is compacted by vibration and/or other mechanical means. The upper part 624 is then lifted off, and the pattern carefully removed from the sand before the upper part is replaced to form the completed mould shown in Figure 6.

A gas impermeable membrane 631 is located on the upper surface of the granular material 608. The membrane 631 defines apertures providing access to the mould, for example to allow molten metal to be poured into the mould. Like the membrane 210 of Figure 2, in use, the membrane 631 resists a flow of gas from the chamber 103 through the upper surface of the granular material, to prevent unnecessary loss of gas. In the present embodiment the membrane 631 is formed of a plastics material, but other materials may be used that are able to withstand the temperatures generated in the casting procedure.

In the present example, the sand used to form the mould is pre-mixed with a binder, such as sodium silicate, but in alternative embodiments an organic binder, such as a binder based on phenolic urethane, alkaline phenolic, or furan resin, is used. Consequently, the mould that is produced is relatively stable during removal of the pattern and during casting.

The use of the apparatus 101 and sand mould apparatus 601 is similar to that of the apparatus 101 and mould apparatus 201 described above. Therefore, the steps 302 to 310 described with reference to the flowchart of Figure 3 also apply. It may be noted that, during casting, molten metal poured into the sand mould heats the binder in the neighbouring layer

of sand to high temperatures. This causes evaporation and/or chemical breakdown of components of the binder, and consequently a large quantity of gas is generated. If the pouring were performed under vacuum, the gas that is produced would expand violently, preventing the correct flow of metal into the mould. It is also possible that the expanding gases could expand at such a high rate as to destroy the mould. This problem is exacerbated by the high pouring rate of a mechanically operated pouring system, such as is used in the present invention. However, in the present embodiment, the pouring is performed in the presence of a gas atmosphere at or near atmospheric pressure, and consequently expansion of the gases generated from the binder is much reduced. In addition, the generated gases are drawn through the sand 608 and low-pressure chamber 606 by the action of the pumping system 115, and therefore their affect on the casting process is further reduced. In an alternative to the method described with reference to Figure 6, a sand mould is prepared using methods that are known for producing castings in air. The sand mould is then located in the casting chamber 103 of apparatus 101. Metal is loaded into the crucible 122 and the chamber is evacuated of air until the pressure within the chamber is less than about 1x10- ¹ mbar (10 Nm ^"2 ).

When a sufficiently low pressure is obtained in the casting chamber 103, the induction heating equipment 123 is activated so that the metal within the crucible 122 is melted. With the metal at its required temperature, the valve 116 is closed so that further evacuation of the chamber is prevented. The valve 111 is then opened to back-fill the chamber 103 with a non- oxidising gas, such as argon.

The gas is supplied to the chamber 103 until a pressure in the range of half to one atmosphere (0.5 x 10 ⁵ to 1.0 x 10 ⁵ Nm ^"2 ) and typically 0.9 atmospheres (0.9 x 10 ⁵ Nm ^"2 ) is obtained. The molten metal is then poured into the sand mould and allowed to cool before subjecting the casting to air

and removing the casting from the chamber.

This alternative method does not apply a vacuum directly to the mould box during pouring of the metal, and consequently the removal of evolved gas from the interior of the mould is not assisted. Nevertheless, the casting process is performed in a substantially non-oxidising atmosphere.

Figure 7

In a further alternative method of casting to that described above, the ceramic shell mould is replaced by a sand mould. Sand mould apparatus 701 is shown being used, within the apparatus 101 of Figure 1 , for casting a metal object in Figure 7.

The sand mould apparatus 701 is essentially the same as mould apparatus 201 but the ceramic shell mould is replaced by a conventional sand mould 702. Thus the mould box 202 contains particulate material 703 which contains and supports a conventional sand mould 702 containing granular material 704.

The upper surface of the sand mould 702, like previous embodiments, is provided with an impermeable membrane 732.

The lower surface of the sand mould rests on the particulate material

703, and in operation, when a vacuum is formed in chamber 206 gas is drawn through the cavity of the sand mould 702, through its granular material

704, and through the particulate material 703.

Apart from the preparation of the mould apparatus, the process for casting using the apparatus shown in Figure 7 is substantially as described with respect to Figure 3.