Mold element, mold comprising such a mold element, and housing for a

Abstract

A mold element (e.g., a mold core) for use in casting metal is manufactured by a process starting from a coated mold element having a coating (e.g., a core coating). The coated mold element is provided with a further coating and a bond coating on the further coating, the bond coating allowing the further coating to adhere to a metal casting made with the mold element, such as a cast housing for a turbomachine. A mold for producing a housing of a turbomachine comprises said mold element. A housing for a turbomachine includes an inner volute surface, wherein the inner volute surface includes a barrier coating bonded to the inner volute surface of the housing via an adhesive coating.

Description

TECHNICAL FIELD

[0001] The present utility model relates to a process for forming a mould element for casting metal, a method for casting a casing for a turbomachine, a mould element for use in casting metal, a mould for manufacturing a casting, a casing for a turbomachine, and a casting or a turbomachine casing manufactured according to the aforementioned process or method or using the aforementioned core or mould. The mould element is preferably, but not exclusively, a mould core, which can simply be referred to as a core. The present utility model is particularly, but not exclusively, applied to the manufacturing of a turbomachine turbine casing comprising one or more protective coatings. BACKGROUND

[0002] A turbocharger is a well-known device for supplying air to the intake of an internal combustion engine at a pressure (boost pressure) higher than atmospheric pressure. A conventional turbocharger basically comprises an exhaust-driven turbine mounted on a rotatable shaft located within a turbine casing. Rotation of the turbine causes rotation of a compressor wheel mounted on the other end of the shaft within a compressor casing. The compressor wheel delivers compressed air to the inlet manifold of the engine, thereby increasing engine power. Conventionally, the turbocharger shaft is supported by journal and thrust bearings, which include appropriate lubrication systems, and located within a central bearing housing connected between the turbine and compressor wheel casings.

[0003] In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine is mounted, an annular inlet passage defined between radially facing walls arranged around the turbine chamber, an inlet volute arranged around the inlet passage, and an outlet passage extending from the turbine chamber. The passages communicate with the chamber such that pressurised exhaust gas entering the inlet volute flows through the inlet passage, via the turbine, to the outlet passage and rotates the turbine.

[0004] Metal castings are typically made by introducing liquid metal into a mold and then allowing the liquid metal to solidify. For metal castings having complex shapes, such as those defining a volute or a cavity, it is often necessary to use a mold that includes a core. The core can be an integral part of the mold or can be a separate piece. Depending on the type of casting being made, there are various types of molds. For example, for parts having thin walls, high pressure die casting can be used. In high pressure die casting, liquid metal is provided under pressure into a mold cavity where it is held until it has sufficiently solidified. The mold, also referred to as a die, is then opened to release the casting. This cycle is then repeated to produce other castings. Another type of molding is sand molding, in which the mold and / or core is formed from sand that includes a binder additive to allow the mold to retain the desired shape. A surface coating can be provided to reduce or prevent damage to the surface of the casting from gases released due to decomposition of the binder additive in the mold as a result of the high temperatures involved in the metal casting. For example, acidic gases can be released from the mold and these acidic gases can cause damage to the surface of the casting. The casting can be released from the sand mold by breaking the sand mold and then finishing the casting to remove excess mold material and any casting irregularities after release from the mold. A housing for a turbomachine can be made by casting in a sand mold.

[0005] After release from the mold, the casting can need to be treated to provide a protective coating on the surface of the casting. While this can be simple in cases where the casting is a simple shape or can be simply submerged, for castings having complex shapes, particularly castings having internal surfaces, this is much more difficult. The additional difficulty of protecting complex shapes can bring additional costs and can still result in a less consistent protective coating being applied which can result in the protective coating flaking off or otherwise failing from the surface.

[0006] CN111230048A describes a method for manufacturing a cast component having an integrated thermal barrier coating. This is achieved by providing a core coated with a thermal barrier coating, disposing the core within a casting mold, casting metal around at least a portion of the core coated with the coating to form a cast intermediate, and then removing the core from the cast intermediate to form the cast component. An outer metal layer can be applied to the thermal barrier coating.

[0007] The present utility model is provided to address at least some of the deficiencies of the prior art. SUMMARY

[0008] The present utility model aims to provide new and useful methods and assemblies for use in the manufacture of castings, particularly housings for turbomachines, that include at least one functional layer, such as a protective layer or a thermal barrier layer.

[0009] In general, the present application proposes that a surface coating of a metal casting (e.g. a housing for a turbomachine) is provided by casting the metal casting in a mould, the mould comprising a bonding coating configured to allow a further coating to adhere to the casting, the further coating being located between the bonding coating and a mould element coating (e.g. a core coating). In the mould, the bonding coating is exposed to a cavity into which liquid metal is provided, such that the bonding coating is able to bond with the casting. The further coating is provided on the other side of the bonding coating. The further coating can be a functional coating, e.g. a protective coating or a thermal insulating coating. The further coating is provided between the bonding coating and the mould element coating, such that after the casting process has been completed and the mould has been removed, the mould element and the mould element coating are removed, leaving the further coating as a surface finish of the casting. In this way, complex shapes can be easily and reliably provided with a coating, which is particularly advantageous when the shape comprises an internal surface or volute which would otherwise not be possible to easily and reliably coat. The present application not only allows a functional coating to be provided as a surface finish of an internal portion of the casting (e.g. the interior of a volute), but also allows a functional coating to be provided as a surface finish of an external portion of the casting.

[0010] Accordingly, according to a first aspect of the present disclosure, there is provided a process of forming a mould element for casting a metal, the process comprising: a) providing a central mould element coated with a coating; b) providing a further coating on the central mould element coated with a coating; and c) providing a bonding coating on the further coating.

[0011] The mould element can be an integral part of the mould defining the internal shape of the casting or can be a separate element of the mould. In other words, the mould element can be one or more of a mould core, a cover, a drag or any other element of a complete mould. By providing a central mould element coated with a coating, the final surface of the casting, i.e. the surface of the casting once released from the mould with a further coating on the surface of the casting, is free from defects as the liquid metal cannot penetrate the mould and the gases released from the mould are prevented from damaging the final surface of the casting. The adhesive coating serves to provide adhesion of the material of the casting to the further coating which would otherwise easily fall off from the material of the casting, which would have significant negative consequences if this were to occur in use. Without the adhesive coating, the surface coating cannot properly adhere to the metal of the casting, which makes it unsuitable for use, particularly in the presence of the risk of damage of the further coating falling off from the rest of the metal casting. The order of the different coatings, i.e. the core coating, the further coating and the adhesive coating, is important as the further coating needs to be the outermost surface of the final casting and is provided during the casting process, not subsequently, by transferring the further coating and the adhesive coating from the core and / or the mould to the cast piece. In this context, the outermost surface is the surface finish of the casting, whether it is an external surface finish or an internal surface finish. Thus, the outermost surface can be provided on the internal surface and / or the external surface of the casting, it being understood that even the internal surface has an outermost layer, i.e. a surface layer or surface finish. The further coating is intended to protect the casting, e.g. either thermally or from dirt, and thus the further coating needs to be provided in a position where it can perform its desired function. This allows the further coating to be easily and reliably provided on the casting, particularly on the internal surface of the casting, whereas existing methods require any surface coating to be provided after removal from the mould and it is time consuming and difficult to provide a reliable and uniform surface coating, particularly in relation to internal surfaces where it is not possible to provide a uniform and reliable coating on such internal surfaces.

[0012] Providing the mould element coated with a coating can comprise providing a sand-based mould element with a water-based coating. The water-based coating is able to improve the surface finish of the final casting.

[0013] One or both of the further coating and the adhesive coating can be provided by spraying, e.g. thermal spraying, and / or dipping.

[0014] The other coating can be one selected from the group consisting of a thermal barrier coating, an anti-corrosion coating, a friction reducing coating, an oleophobic coating, a hydrogen embrittlement protection coating, an oxidation resistant coating, an erosion resistant coating, and a stain resistant coating. A thermal barrier coating is used to reduce the transmission of thermal energy. In one example, where a thermal barrier coating is applied to the interior surface of a housing of a turbomachine, the thermal barrier coating reduces the amount of thermal energy lost from the exhaust gas travelling through the housing. As less energy is lost, more energy is available to drive the turbine and thereby the compressor. Previously, it was not possible to coat the interior passages of a turbine housing with a thermal barrier coating because the material was applied by thermal spraying and it was not possible to ensure a consistent and uniform coating of the interior surface was stable using this technique. A fuel cell can use a turbine. Such a fuel cell can use a fuel that is typically at a low temperature, such as hydrogen or a hydrocarbon gas. Thus, while high temperatures can not be a problem, certain materials are susceptible to hydrogen embrittlement, and thus a coating can be selected to protect the underlying material from embrittlement.

[0015] The mold element can be in the shape of an interior passage of a turbomachine. A turbocharger, a particular type of turbomachine, uses exhaust gas to drive a turbine, which in turn drives a compressor, which then feeds compressed gas into an engine. The interior passage is shaped to maximize efficiency within dimensional limits, and thus the interior passage is a relatively complex shape. Such a complex shape does not allow for easy and reliable application of an interior coating, but the present disclosure allows for such an interior coating to be applied. The present disclosure also allows for a functional coating to be provided on an exterior surface of the casting.

[0016] According to a second aspect of the present disclosure, there is provided a method of casting a housing of a turbomachine having an interior coating, the method comprising: a) providing a multi-layered core having, in order, a central core, a core coating, another coating, and a bond coating; b) providing a casting mold comprising the multi-layered core, the casting mold and the multi-layered core defining a mold cavity; c) introducing molten metal into the mold cavity; and d) allowing the molten metal to solidify in the mold cavity to form a housing casting intermediate.

[0017] The method according to the second aspect of the present disclosure provides a method for providing an interior coating on a housing of a turbomachine. This is achieved by providing a multi-layered core having coatings in a predetermined order, such that when liquid metal is introduced, the liquid metal is able to form a bond with a bond coating, which in turn is bonded to another coating, which becomes the surface of the casting intermediate once removed from the mold.

[0018] The method can further comprise removing the central core and the core coating, leaving the adhesive coating and the further coating on the surface of the shell casting intermediate. Subsequently, the casting intermediate can be further machined to provide a finished casting.

[0019] The further coating can be selected from one of the group consisting of a thermal barrier coating, an anti-corrosion coating, a friction reducing coating, an oleophobic coating, a hydrogen embrittlement protection coating, an oxidation resistant coating, an erosion resistant coating and a stain resistant coating. The further coating is preferably a thermal barrier coating.

[0020] According to a third aspect of the disclosure, there is provided a mould element for use in casting a metal, the mould element comprising: a) a central mould element; b) a mould element coating on the central mould element; c) a further coating on the mould element coating; and d) an adhesive coating on the further coating.

[0021] As will be appreciated and as with other aspects of the disclosure, the mould element can be a mould core. The core can be a separate core, that is to say, the core is a separate element from the rest of the mould, or can be a monolithic core where the core is effectively integral with the rest of the mould.

[0022] The mould element coating can be a water-based mould element coating.

[0023] The adhesive coating can comprise iron. This is particularly advantageous where the body of the casting is of a ferrous metal, such that the adhesive coating shares a common metal, namely iron, to provide a strong connection between the casting and the adhesive coating. Where the body of the casting is of a material other than iron, for example aluminium, the adhesive coating preferably comprises the same material, for example aluminium. By having the same material in the body of the casting and the adhesive coating, a stronger bond is formed. Thus, the adhesive coating can comprise the material that forms the majority of the body of the casting.

[0024] As with other aspects of the disclosure, the further coating of the mould element can be one selected from the group consisting of a thermal barrier coating, an anti-corrosion coating, a friction reducing coating, an oleophobic coating, a hydrogen embrittlement protection coating, an oxidation resistant coating, an erosion resistant coating and a stain resistant coating.

[0025] According to a fourth aspect of the disclosure, there is provided a mould for producing a shell of a turbomachine, the mould comprising a mould element according to the third aspect of the disclosure.

[0026] According to a fifth aspect of the disclosure, there is provided a shell for a turbomachine, the shell comprising an internal volute surface, wherein the internal volute surface comprises a barrier coating adhered to the internal volute surface of the shell via an adhesive coating.

[0027] As described in relation to other aspects of the disclosure, it has previously not been possible to provide a sufficient barrier coating on the internal volute of a turbomachine housing, whether a thermal barrier coating, an anti-corrosion coating, a friction reducing coating, an oleophobic coating, a hydrogen embrittlement protection coating, an oxidation resistant coating, an erosion resistant coating or a fouling resistant coating, for the reason that it is difficult to ensure that such a barrier coating is applied uniformly within the internal surface of the casting.

[0028] According to a sixth aspect of the disclosure, there is provided a turbomachine housing manufactured according to the process or method of the first or second aspects of the disclosure.

[0029] It will be appreciated that features described in relation to one aspect of the disclosure are equally applicable to any other aspect of the disclosure unless those features are incompatible with each other. Thus, all features describing one aspect of the disclosure are equally applicable to any other aspect of the disclosure.

[0030] It will be appreciated that any of the above aspects can include one or more features of any of the other aspects, where appropriate. BRIEF DESCRIPTION OF DRAWINGS

[0031] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[0032] Figure 1 depicts one way in which a core can be formed according to the first aspect of the disclosure;

[0033] Figure 2 depicts a casting intermediate produced according to aspects of the disclosure;

[0034] Figure 3 depicts a cross-section of the casting intermediate before the sand core is removed; and

[0035] Figure 4 depicts a cross-section of a prior art casting having a thermal barrier coating applied directly to the casting. DETAILED DESCRIPTION

[0036] Figure 1A mold element in the form of a sand core 1 is depicted. It should be understood that alternative foundry core materials such as salt can be used and the present disclosure is not particularly limited to the exact core material used. The sand core 1 forms the center core of the final coated center core and the shape of the final casting. The core 1 can be a separate piece relative to the mold in which the casting is formed or can be an integral part of the mold. The core 1 includes a core coating 1'(not shown) that is configured to prevent another coating, which can be a barrier coating such as a thermal barrier coating, a corrosion protection coating, a friction reduction coating, an oil repellent coating, a hydrogen embrittlement protection coating, an oxidation resistant coating, an erosion resistant coating, or a stain resistant coating, from becoming rough or pitted due to partially penetrating into the material that comprises the core, which can be sand. In process step 2, another coating 3 is disposed on the coated center core 1. The other coating 3 is a material coating that is additional to the coating disposed on the center core material. This other coating 3 is the material that will be the outer surface of the final casting. In other words, the other coating 3 will be the surface finish or surface material of the casting, whether it is the surface material of the interior face of the casting or the surface of the exterior face of the casting. In some embodiments, the other coating 3 is a thermal barrier coating that is configured to reduce the rate of thermal energy transfer through the thermal barrier coating. In the case where the casting is a housing of a turbomachine, the thermal barrier coating serves to retain thermal energy in the exhaust gas flowing through the housing so that a greater amount of energy can reach the turbine. In process step 4, a bond coating 5 is disposed on the other coating 3. This bond coating 5 is a different composition than the other coating 3 and is configured to provide improved adhesion between the other coating 3 and the metal of the casting. Without the bond coating, it has been found that the other coating only poorly adheres to the casting metal, if at all. This makes it unsuitable for use in practice, particularly in cases where any material that falls off of the casting metal can cause downstream damage, for example to the turbine in a turbomachine.

[0037] Figure 2 A multi-layer core disposed within a mold 6 is depicted. Figure 1 The core and mold 6 together at least partially define a mold cavity 7 into which molten metal, which can be iron-containing, can be poured. When making a casting, molten metal is provided into the mold cavity 7 and allowed to solidify in the mold cavity 7 to form a casting intermediary. The core and mold material can then be removed to leave the casting. The casting includes the other coating 3, the bond coating 5, and the solidified metal casting 8. In the depicted example, the core, mold, and final casting are shown as circular for clarity, but the exact shape of these elements will depend on the desired shape of the final casting.

[0038] Figure 3 A cross section through a casting intermediate according to the present disclosure is depicted. It can be seen that the stack comprises in order or in succession a sand core 1, a core coating 1', a further coating 3, a bonding coating 5 and a solidified metal casting 8.

[0039] Figure 4 A cross section through a casting intermediate of the prior art is depicted. The stack comprises in order or in succession a sand core 1, a thermal barrier coating 3 and a solidified metal casting 8. There is no bonding coating between the thermal barrier coating 3 and the metal casting 8, nor is there a core coating 1' on the core 1.

[0040] The present disclosure provides a method and a device for providing a functional coating on a casting, and in particular for castings having a complex internal geometry, such as a volute of a turbine housing of a turbocharger. The present disclosure also provides a functional coating on an internal surface and / or an external surface of a casting. The present disclosure allows to provide a coating that is firmly bonded and uniform to the casting. This is achieved by providing a coated mold element, which can be a coated mold core or a cover or a drag or equivalent, and a bonding coating, neither of which is achieved in the prior art.

Claims

1. A mold element for use in casting metals, characterized in that, the mold element comprises: a) a central mold element; b) a mold element coating on the central mold element; c) a further coating on the mold element coating; and d) a bonding coating on the further coating, the mold element coating is a water-based mold element coating.

2. The mold element according to claim 1, characterized in that the bonding coating comprises the same metal as the material forming the body of the desired casting.

3. The mold element of claim 2, wherein the metal is iron.

4. The mold element of claim 2, wherein the metal is aluminum.

5. The mold element according to any one of claims 1 to 4, characterized in that the further coating is one coating selected from the group consisting of a thermal barrier coating, an anticorrosion coating, a friction-reducing coating, an oleophobic coating, a hydrogen embrittlement protection coating, an oxidation-resistant coating, an erosion-resistant coating, and a stain-resistant coating.

6. The mold element according to any one of claims 1 to 4, characterized in that the mold element is a core.

7. A mold for producing a casing of a turbomachine, characterized in that, the mold comprises the mold element according to any one of claims 1 to 6.

8. A casing for a turbomachine, the casing comprising an internal volute surface, characterised in that, the inner volute surface is made from the mold according to claim 7 and the inner volute surface comprises a barrier coating bonded to the inner volute surface of the housing via a bonding coating.

9. The casing for turbomachinery according to claim 8, characterized in that, the barrier coating is one coating selected from the group consisting of a thermal barrier coating, an anticorrosion coating, a friction-reducing coating, an oleophobic coating, a hydrogen embrittlement protection coating, an oxidation-resistant coating, an erosion-resistant coating, and a stain-resistant coating.

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