DE3873994T2 - METHOD AND METHOD FOR METAL CASTING. - Google Patents

Description

The present invention relates to a method and a device for pouring molten metal according to the preambles of independent claims 1 and 8, respectively.

Many of the components used in the manufacture of gas turbine engines are precision castings that must be formed using investment casting techniques such as lost wax casting. Other items, such as turbocharger components, can also be manufactured using such techniques.

In the gas turbine engine industry, there are three different types of metallurgical grain structures used to manufacture components. These structures are unidirectional (EA), directionally solidified (DS) with directionally oriented columnar grains, and single or monocrystalline (SX) components.

EA components were the first investment cast components to be used commercially in gas turbine engines. Both DS and SX components were later developments.

There are many, often conflicting, variables in the manufacture of an EA turbine blade, for example. The components often have a relatively thick and heavy root section at one end and a slightly less thick and less heavy shroud section at the other end, with the two sections often connected by a relatively long, thin-section wing section. In many cases, the part also has hollow channels inside it that cause the wall section of the wing section to become even thinner.

Due to their cross-sectional changes, typical components are very susceptible to the formation of pores during solidification.

In the case of EA components, there is little or no control of solidification other than that provided by the external runner and feeder system. The feeder system is usually arranged so that the last or furthest metal solidifies in an attempt to ensure the supply of liquid metal to the solidifying component. In the case of wings, secondary feeders are sometimes placed on thin cross sections in an attempt to keep liquid metal feedable and thus prevent or minimize the development of shrinkage porosity. However, such measures can cause unacceptable variations in grain size which can result in the part not being accepted. For example, engine manufacturers have established specifications for maximum acceptable porosity and its distribution in components. Considerable resources are therefore spent on non-destructive testing, etc. to identify components that do not meet this standard.

With respect to DE 36 03 310 A1, the applicant has voluntarily limited the scope of the present application with respect to Germany and filed separate claims for that Contracting State.

Chandley et al., in US 34 20 291, which forms the basis of the preamble of claim 8, propose a method for reducing or avoiding residual porosity by pressurizing the solidifying metal. He proposes a pressure vessel in which either the mold is heated or into which a heated mold can be inserted. The pressure vessel itself must, in the case of typical turbine component alloys, be enclosed in a vacuum melting and vacuum casting chamber. Molten metal is poured into the mold under vacuum, after which a closing part is attached to the pressure vessel in the closed position and the vessel is pressurized with a gas that does not react with the molten metal. A pressure of 1,000 to 4,000 psi (6.9 to 27.6 MPa) is prescribed for this, with 2,000 to 3,000 psi (13.8 to 20.7 MPa) is the preferred range. Although significant improvements in the integrity of the castings appear to have been achieved, the process has not been adopted by the industry. The reasons for this non-acceptance are mainly the complexity of using the apparatus described and also the possible safety hazard. This is because it is first necessary to heat the mold or vessel interior and place the mold in the vessel. Since the vessel and mold must be together inside a vacuum casting furnace, it is then necessary to close the vacuum casting furnace and pump down to the required pressure. In a typical furnace large enough to accommodate the vessel assembly, this may take more than 15 minutes. Once the required pressure is reached, the metal charge must be melted and brought to a constant temperature before pouring. This also takes approximately 10 minutes. After the metal has been poured into the mold, the pressure vessel must be sealed and pressurized. With the vessel insulated within the vacuum chamber this is extremely difficult to do and requires complicated remote-controlled mechanisms passing through the walls of the vacuum chamber. The time to seal the pressure vessel can take more than 3 minutes. The vacuum casting furnace is then occupied while the molten metal solidifies. It is therefore very difficult to carry out more than one or at most two pours per hour. Due to the relatively long time required to seal the pressure vessel, the metal has already partially solidified before pressure is applied and therefore the pressure required to achieve a given result is high; up to 4,000 psi The pressure vessel itself must be tested at pressures which may be several times the maximum designed operating pressure for safety reasons. Another variable is that the pressure vessel itself is heated due to heat radiated from the mold or mold heaters, which weakens the material from which the pressure vessel is constructed. There are therefore significant security risks.

DE-A-32 20 744 describes a vacuum casting furnace with an upper chamber for melting and casting metal and a lower chamber for cooling the mould, whereby the two chambers can be isolated from each other by a valve. A mould transport system is provided between the two chambers, but the lower chamber cannot be pressurised with an inert gas to the pressures necessary to reduce the porosity which develops during solidification, and there are no means of heating the mould in the upper chamber.

GB-A-13 56 890, which forms the basis of the preamble of claim 1, describes an apparatus for producing DS castings. The apparatus comprises a movable vacuum melting and vacuum pouring chamber serving two or more cooling chambers in order to improve the utilization of the melting plant. However, the apparatus has no means for pressurizing the cooling chambers since the components are manufactured by the DS process and are therefore essentially free of porosity, as explained above.

GB-A-21 63 684 describes a method and apparatus for treating and casting metal in an enclosed space. The apparatus in question is primarily intended for molten metals which are allowed to react with an alloying gas as part of the casting process. The base metal is melted together with some solid alloying additives in a pressurized chamber and then poured into a tundish having a gas permeable bottom through which the alloying gas bubbles to react with the molten metal in the tundish. After the metal has reacted with the gas, it is then poured by means of valve means into a mold in a second chamber which is then pressurized to prevent the solidifying metal from escaping the reacted gas.

An alternative process that has also been used is hot isostatic pressing. Some internal porosity may be included with this technique, but it is very expensive and has no effect on surface porosity. For some applications, some engine manufacturers will not accept components that have been treated in this way.

DS and SX components are manufactured using essentially the same types of vacuum furnaces. The molds are built on a water-cooled chill plate that is part of the end of a ram with controllable position and speed. The molds are uninsulated and are preheated in place in the casting chamber by radiant heat. The cold mold is raised to the casting position by the ram and heated to the desired temperature in the casting position by radiant heaters. Mold temperatures up to 1600ºC are achieved. The molten metal is poured into the mold where it is held in the molten state. Solidification is initiated by slowly retracting the mold on the ram from the radiant heat zone so that a substantially flat solid/liquid interface is created. Liquid metal is continuously fed into the solidifying metal so that shrinkage or gas porosity can never develop or become trapped since there are no closed pockets of liquid metal in the solidified metal. The effect of such a solidification process on the structure is to produce directionally oriented columnar grains in DS components or a single crystal in SX components, with the latter requiring special initial additional steps.

Although DS and SX components are consistently free from defects in porosity, they are only used in very difficult applications on some parts of some engines because of the advantages that come from the metallurgical structure produced. DS and SX components are expensive to manufacture due to the relatively slow solidification rate.

For the foreseeable future, the majority of investment cast gas turbine engine components will be of the EA type. For economic reasons, it is highly desirable to reduce the amount of scrap produced due to porosity in EA components. A typical scrap rate for EA components can be 30% and for some particularly difficult components the scrap rate rises to over 50%. It is also economically desirable to preheat the moulds in situ in the casting furnace to avoid the costly preheating step in external furnaces which is common in the types of furnaces used for EA castings.

It is also desirable to be able to produce EA, DS and SX components in the same furnace type with minimal effort for changeover.

According to a first aspect of the present invention, an apparatus for producing cast metal parts is provided. The apparatus comprises a casting chamber, means for melting and pouring the metal, which are arranged to fill a mold in the casting chamber, a mold chamber adjacent to the casting chamber and connected to it by a valve means of sufficient size through which the filled mold can be passed, a vacuum pump for generating a negative pressure in the casting and mold chambers, a mold transport means for transporting the filled mold from the casting chamber to the mold chamber and is characterized by a mold heating means present in the casting chamber and a pressure generating means connected to the mold chamber, with which the mold chamber can be independently pressurized with a gas up to a pressure of 7 MPa.

The formation of mold heating devices using, for example, resistance-heated molybdenum or induction-heated graphite heat radiators in the casting chamber avoids the need for external preheating furnaces and additional mold insulation and enables higher temperatures together with greater accuracy and adaptability of preheating temperatures.

The mold chamber may also have internal, variable thickness insulation aligned to selected areas of a particular mold to achieve different cooling rates in selected areas of the filled mold.

The mold chamber may also have radiant heating devices for targeted heating of parts of the filled mold.

In one embodiment of the device according to the present invention, the mold chamber is arranged under the casting chamber. The mold moving device can have a ram which is actuated, for example, by hydraulic or electrical devices and moves the mold vertically from the mold chamber to the casting chamber and back again through the valve devices.

The mold moving device may also include insulation between the mold and its seat on the mold moving device. In the case of manufacturing EA components, such insulation may be necessary to prevent excessively directed heat flow from the mold by conduction. For DS and SX components, direct contact between the mold and the cooling device connected to the mold moving device is necessary to promote directed heat flow from the mold.

The mold chamber is constructed as a pressure vessel and is separated from the furnace chamber by valve means including a fast acting isolation valve. For EA components, the mold containing the poured molten metal can be quickly returned to the mold chamber, the isolation valve closed, and the mold chamber pressurized with a fluid such as argon. The mold can be returned to the mold chamber, isolated, and pressurized within about 10 to 20 seconds after the metal has been poured. It has been found that because of this short period of time, the pressure required to achieve a significant reduction or elimination of residual porosity can generally be reduced to less than 1,000 p.s.i. (6.9 MPa) and for most applications to less than 500 p.s.i. (3.45 MPa). This has advantages both in terms of the safety of the process due to lower pressures, and in economic terms due to lower capital expenditure for equipment.

However, the device can be designed to withstand any pressure required, consistent with technical and safety requirements. There is no reason why the mould could not be designed to withstand, for example, 35 MPa, if this were desired.

The device can be used to produce DS and SX components, although in the case of such components the pressurization device is not required due to the solidification mechanism. However, this has the advantage of giving the manufacturer complete flexibility in that the molding device can be used to produce any type of component.

Since the mold chamber and its connected equipment are relatively compact, two or more mold chambers can be served by one casting chamber. In this way, a higher output of castings can be achieved, since the casting chamber is also available while the castings are solidifying in one mold chamber.

According to a second aspect of the present invention, a method for producing castings is provided. The method comprises the steps of melting a metal charge, pouring the metal into a mold in a casting chamber with a reduced pressure atmosphere, returning the filled mold to a mold chamber and pressure isolating the mold chamber from the casting chamber and is characterized in that the mold is heated in the casting chamber before pouring the metal and after returning the filled mold to the mold chamber, it is pressurized with a gas up to a maximum pressure of 7 MPa for at least part of the time required to solidify the metal in the mold.

In a preferred embodiment of the method, the time between the end of the pouring of the metal and the application of pressure can be less than 40 seconds. In a particularly preferred embodiment, this time can be less than 20 seconds. The faster the pressure is applied to the poured metal after pouring, the lower the pressure required to bring about a given reduction in porosity. This of course assumes other constant parameters, such as constant metal pouring temperatures and mold preheating temperatures. as well as consistent shape and construction.

The apparatus and method of the present invention can be used to cast a wide variety of metals and alloys, including, for example, copper and aluminum alloys as well as iron, nickel and cobalt based superalloys.

In order to better understand the present invention, examples will now be described by way of illustration with reference to the accompanying drawings. In these drawings:

Figure 1 shows a schematic section through a first embodiment of a device according to the present invention with a mold located therein, immediately before the pouring of molten metal;

Figure 2 shows the device from Figure 1 after pouring molten metal;

Figure 3 shows a schematic section through a second embodiment of a device according to the present invention with a mold located therein, immediately before the pouring of molten metal;

Figure 4 shows the device from Figure 3 after pouring molten metal;

Figure 5 shows a modification of a device similar to that shown in Figures 3 and 4, and

Figure 6 shows a modification of a mold vacuum/molding pressure chamber.

Reference is made to Figures 1 and 2, in which the same features are identified by the same reference numerals.

An apparatus for casting parts is shown generally at 10. The apparatus comprises a vacuum casting chamber 11 having an opening 12 connected to a vacuum pump (not shown). In the chamber 11, a Coil box assembly 13 containing induction heating coils (not shown) and a crucible 14; assembly 13 is mounted so that it can be tilted for pouring molten metal 15 in a known manner. The chamber structure also includes an opening 16 and a vacuum seal 17 to allow refilling of crucible 14 with new metal under vacuum. An opening 21 of sufficient size is provided in the bottom wall 20 of chamber 11 to allow a mold assembly 22 to pass through. Above and around opening 21 is disposed a mold heating chamber 23 which includes an outer insulating container 24 containing known radiant heating means 25 and suitable power and control means (not shown) attached thereto. An opening is provided in the top of the insulation vessel 24 with a pouring tube 27 therein to direct the molten metal into the mold 22 during pouring. A mold chamber 30 is provided below the vacuum chamber 11. The mold chamber 30 is secured in a sealing connection to the lower wall 20 of the pouring chamber 11. The chamber 30 can be isolated from the chamber 11 by means of the isolation valve 31 and a seal 32. The valve 31 can be moved between the open position (Figure 1) and the closed position (Figure 2) by suitable known remote control devices (not shown). Cooling channels 33 are provided around the chamber walls. The chamber 30 has a door 34 in one side wall to enable positioning and later removal of the mold 22, the door 34 being sealable against the chamber wall 35. Also provided in the chamber wall 35 is a vacuum pump port 36 which is connected to a vacuum pump (not shown) via a valve 37. A further port 38 is provided in the wall 35 for supplying fluid under pressure from a fluid supply source (not shown) via a valve 39. The mold 22 is mounted on a table 40 but is insulated from it by an insulating block 41. The table 40 itself is mounted on top of a movable ram 43 which slides in a sealed connection through the lower wall 44 of the chamber 30. The table 40 and the ram 43 are provided with channels 45 for cooling water. The ram 43 is hydraulically driven, but can also be electrically driven, and it moves the mold 22 between the chambers 11 and 30.

In operation, the chamber 11 is evacuated to a low pressure appropriate to the requirements of the alloy to be cast. The valve 31 is closed and the ram 43 and table 40 are returned to their lowermost position. The metal in the crucible 14 is melted and the temperature is raised and maintained at the desired level. The mold 22 and insulating block 41 are then placed on the table 40 through the door 34, after which the door 34 is closed and sealed and the chamber 30 is evacuated to the same or similar pressure as that in the chamber 11. At the appropriate pressure, the valve 31 is opened and the mold and associated supports are raised by the ram into the mold heating chamber 23 (Figure 1). The mold temperature is then raised to the desired temperature by the radiant heater 25. As soon as the mold 22 and the metal 15 are stabilized at the desired temperature, the metal is poured into the mold. Immediately after the metal has been poured, the plunger 43 is returned to the chamber 30, the valve 31 is closed, the valve 37 is also closed and the valve 39 is opened to supply pressurized fluid, which can be argon gas, for example (Figure 2). The chamber is quickly brought to a pressure of less than 7 MPa. The molten metal can thus solidify under gas pressure, which leads to the prevention of void formation in shrinkage porosity due to the pressurization, so that liquid metal is continuously supplied between the randomly growing grain structures of the solidifying mass. While solidification in the chamber 30 progresses, the crucible 14 can be refilled with new metal through the opening 16 and the vacuum seal 17 and the metal can be melted for the next mold.

Referring now to Figures 3 and 4, in which like features are designated by common reference numerals, a vacuum casting and pressure solidification apparatus is shown generally at 50. The apparatus includes a melting and pouring chamber 51, a mold heating chamber 52, and a mold vacuum/pressure chamber 53. All three chambers may be isolated from each other by suitable valves as described below. The melting and pouring chamber 51 includes a crucible 54 which is heated by induction coils 55. Metal can be charged into the crucible 54 by removing the opening cover 57. The chamber is evacuated via a line 56 which is connected to both a vacuum pumping device (not shown) and to the mold vacuum/pressure chamber 53 by means of a manifold 66. The chamber 51 can be isolated from the chamber 53 by a valve 58 in the line 56. The chamber 52 is evacuated via line 59 which is also connected to the vacuum pumping device (not shown). The chamber 51 can be isolated from the chamber 52 by the valve 60 mounted on a pivot arm. The chamber 52 can be isolated from the chamber 53 by the valve 62 mounted on a pivot arm. The chamber 52 accommodates mold heating devices 63 which include insulation 64 and radiant heating devices 65. The chamber 53 is secured to the manifold 66 by an intermediate seal 68. The manifold 66 is further provided with a conduit 69 and a valve 70 for supplying pressurized fluid from a pressurized fluid source (not shown), such as a gas cylinder. The chambers 52 and 53 have external water cooling pipes 71 and 72, respectively. A mold 75 is supported on a table 76 by means of an insulating block 77, the table being mounted on a movable ram 78. The table 76 and ram 78 are provided with internal water cooling channels 79. The ram is in sealed sliding connection with the lower cover 80 of the chamber 53. The chamber 53 itself is movable relative to the manifold 66 by means of a hydraulically operated mechanism (not shown) on which the chamber 53 and the associated ram 78 are mounted. The mechanism moves the chamber 53 and the associated device downward and outward to allow access to the interior of the chamber 53.

The operation of the device is essentially similar to that described with reference to the device of Figures 1 and 2.

The chamber 51 is initially closed by the valves 60 and 68 from the chambers 52 and 53 and evacuated via the pipe 56. The mold 75 is placed in the chamber 53 and the chamber is then placed against the seal 68. The evacuation of the chamber 53 can be carried out by opening the valve 58. The chamber 52 is also initially closed against the Chamber 53 is isolated by valve 62 and evacuated via line 59. After chambers 51, 52 and 53 have been evacuated to the desired pressure or pressures, valve 62 is opened and the mold is moved up from chamber 53 to chamber 52 on the ram 78 and associated table 76 and insulation 77. While the metal melts and is stabilized at a desired temperature, the mold is preheated to a desired temperature by radiant heater 65. When the metal is near its desired temperature, valve 60 is opened. After pouring the metal into the mold, the filled mold is immediately returned to chamber 53, valves 58 and 62 are closed and valve 70 is opened to permit pressurized fluid to flow into chamber 53 to a maximum pressure of 7 MPa. In this way, the metal in the mold can solidify under isostatic gas pressure.

Figure 5 shows a modification of the device of Figures 3 and 4 in which the chamber 53 can be separated from the chamber 52 and the manifold 66 without breaking the vacuum in the chambers 51 and 52 or reducing the pressure in the chamber 53.

The device 50 is essentially similar to that shown in Figures 3 and 4, but is additionally provided with a second manifold 90 which seals against the seal 68 of the manifold 66. The pressure fluid line 69 and the valve 70 are no longer attached to the manifold 66 in this embodiment and are replaced by a vacuum line 91 and a valve 92 which is connected to the vacuum pumping means (not shown). The manifold 90 is provided with a vacuum line 94 and a valve 95 which are connected to the vacuum pumping means (not shown) for independently evacuating the chamber 53. Also provided in the manifold 90 are a valve 97 with a pivoting arm, a pressure fluid inlet line 98 for pressurized fluid and a control valve 99, the latter two being connected to a pressure fluid source (not shown). The chamber 53 is sealed from the distributor 90 by a seal 100 and is held thereto by suitable clamps 101. The chamber/distributor assembly 53, 90 etc. is movable as a unit and can be released, while still under pressure, from the seal 68/manifold 66 by means of a hydraulically or electrically operated mechanism (not shown) which moves the chamber and manifold assembly containing the metal-filled mold away, replacing it with a second, similar chamber and manifold assembly containing a new, empty mold. In this way, production speed can be increased since the pouring apparatus is available while the metal solidifies.

Solidification of the metal in the mold occurs primarily through the mechanism of radiation to the walls of the mold chamber, from where the heat is dissipated through the cooling channels 72. In some cases it may be desirable to prevent rapid heat dissipation and to control the cooling rate. As shown in Figure 6, this can be achieved by means of insulating members 110 arranged in the mold chamber. Such members 110 can vary in shape, thickness, material, etc., depending, among other things, on the design and dimensions of the parts to be cast. Materials that can be used include, for example, stiffened ceramic fiber cloths made of aluminum oxide, silicon dioxide or zirconium dioxide.

Argon is probably the most commonly used pressure fluid, but other gases can be used, including helium, nitrogen and air, the choice being dictated primarily by the materials being cast. Although helium is more expensive than argon, it can be used to accelerate the cooling rate if this is desirable in certain cases.

Although the various embodiments of the apparatus described here are primarily intended for the manufacture of low porosity EA turbine components, they can also be used to manufacture DS and SX components. The apparatus can be equipped with known microprocessor-based control systems which control the heating produced by the radiant heaters, the retraction speed of the ram, etc. To manufacture the DS or SX components, the insulation between the mold and the ram table is removed and the mold is built up directly on the water-cooled stamp table or on a separate quenching plate in order to achieve directed heat dissipation and, as a result, a flat solid/liquid interface by the slow, controlled retraction of the filled mold from the mold chamber.

In this way, a device is provided that is completely flexible in the tasks it can perform, resulting in improved production efficiency and reduced costs.

It is also intended to use the technique and the device for the production of components other than those intended for gas turbine engines or turbochargers. For example, components can be cast from materials that do not require a vacuum for protection. Such components can be cast in air and placed in a pressure chamber for the solidification phase.

Claims (10)

1. Device for producing cast metal parts with a casting chamber (11), with devices (13) for melting and pouring the metal, which are set up to fill a mold (22) in the casting chamber, with a mold chamber (30) next to the casting chamber and connected to it by a valve device (31) of sufficient size through which the filled mold (22) can be moved, with a vacuum pump for generating a negative pressure in the casting and mold chamber, with a mold moving device (43) for transporting the filled mold from the casting chamber to the mold chamber, with a mold heating device (23) in the casting chamber and with a pressure generating device connected to the mold chamber, with which the mold chamber can be independently pressurized with a gas up to a pressure of 7 MPa, characterized by an insulation (41) between the mold and the mold moving device. 2. Device according to claim 1, characterized in that the mold chamber can be detached from the casting chamber and replaced by a second chamber. 25 3. Device according to claim 1 or 2, characterized by first (62) and second (97) valve devices which are arranged between the casting chamber (52) and the molding chamber (53) so that the two chambers can be separated while a negative pressure in the casting chamber and a positive pressure in the molding chamber are maintained. 4. Apparatus according to any preceding claim, further comprising insulating means (110) in the mold chamber to control the cooling rate in selected areas of the mold. 5. Device according to one of the preceding claims, characterized in that the mould chamber also has heating devices for selectively heating parts of the filled mould. 6. Device according to one of the preceding claims, characterized in that a separate mold heating chamber (52) is present, which can be isolated from the metal melting chamber (51) and the mold chamber (53) by valves (60, 62). 7. Method for producing castings with the process steps of melting a metal charge, heating a mold (22) which is located in a casting chamber (11), pouring the metal into the mold, whereby an atmosphere with negative pressure prevails in the casting chamber, returning the filled mold to a mold chamber (30), pressure-isolating the mold chamber from the casting chamber, pressurizing the mold chamber after the filled mold has been moved therein with a gas up to a maximum pressure of 7 MPa for at least part of the time period required for the metal in the mold to solidify, characterized in that the mold is pressurized within 40 seconds after the completion of the pouring of the molten metal. 8. A method according to claim 7, characterized in that the mold chamber is pressurized within 20 seconds after completion of the pouring of the molten metal. 9. Method according to one of the preceding claims 7 or 8, characterized in that the mold in the first chamber is heated to a temperature above the melting point of the alloy to be cast. 10. Method according to one of the preceding claims 7 to 9, characterized in that the metal to be cast is an iron, nickel or cobalt-based superalloy.