WO2016149774A1

WO2016149774A1 - Method for manufacturing a centrifugal metal impeller and a centrifugal impeller obtained with such a method

Info

Publication number: WO2016149774A1
Application number: PCT/BE2016/000014
Authority: WO
Inventors: Aleksandr Pulnikov
Original assignee: Atlas Copco Airpower, Naamloze Vennootschap
Priority date: 2015-03-26
Filing date: 2016-03-17
Publication date: 2016-09-29

Abstract

Method for manufacturing a centrifugal impeller (1), whereby this impeller (1) comprises a base (7) with a hub (8), whereby the impeller (1) further comprises a number of protruding blades (4) that are affixed on the surface (6) of the hub (8), characterised in that the method comprises the following steps: - the provision of a metal impeller base (7) with a hub surface (6); - the layer-by-layer construction of the blades (4) on the hub surface (6) of the metal impeller base (7) with layers (10) of a metal making use of an additive manufacturing method, whereby the metal used for the layers (10) is compatible with the metal of the metal impeller base (7).

Description

Method for manufacturing a centrifugal impeller and a centrifugal impeller obtained with such a method.

The present invention relates to a method for manufacturing a centrifugal impeller.

The centrifugal impeller can be used in a centrifugal machine such as a turbocompressor , turbine or similar.

As is known, a centrifugal compressor element as used in turbocompressors consists of an impeller that is rotatably affixed in a housing with an axial inlet and a radial outlet, whereby the impeller is formed by a type of solid trumpet-shaped hub to bend the gas drawn in at the inlet from the axial direction to the radial direction at the outlet, and by blades that are affixed on the hub and which together with the hub and the housing define narrowing channels through which the gas is guided to compress it.

The impeller is provided with a central borehole to be able to attach the impeller to a drive shaft.

Centrifugal impellers are conventionally manufactured by making use of subtractive manufacturing methods, such as turning and milling.

This starts with a workpiece that is turned down to the outer shroud of the centrifugal impeller, called the ^'"shroud surface' . Then the aforementioned channels and blades are milled out.

The depth of the channels is typically determined by the ^hub surface' .

In order to increase the efficiency, the blades are preferably made as thin as possible.

As a result of this, 90% to 95% of the material between the shroud surface and the hub surface is removed.

This is of course a great waste and an inefficient use of material . It would thus be better to add the blades to a workpiece that is formed by the hub surface, whereby extra material is added to the workplace instead of removing material.

The material to be added in the form of blades would be around 10 to 20 times less than the material that has to be removed when a subtractive manufacturing method is used.

In this way costs and material are saved. However, there are a number of complications. For example the height of the blades in a centrifugal impeller is not constant. The inlet diameter is typically smaller and the height of the blades at the inlet is greater compared to the height of the blades at the outlet. This means that the chosen method for affixing the blades to the hub surface must leave sufficient room to be able to access the construction location.

There have already been attempts to manufacture a centrifugal impeller with an additive manufacturing method instead of a subtractive manufacturing method. For example, WO 2014/128169 describes a method for manufacturing an impeller making use of separate segments. This document describes these segments being welded to one another against a hub or attaching the segments to the hub by means of a mechanical fastener. The aforementioned segments are made by means of an additive manufacturing method. However, a great disadvantage of this approach is that there is no continuous connection between the segments and the hub such as with a subtractive manufacturing method, such that they are unsuitable for impellers that are used in machines with a high speed.

Indeed, such impellers are driven at high speeds of tens of thousands of revolutions per minute, whereby the linear circumferential speed at the outlet of the impeller can reach a few hundreds of metres per second.

Due to the high centrifugal forces that occur at such high speeds, very high stresses are generated that act on the joint between the segments and the hub. As a result there is a real risk of the failure of this aforementioned joint, such that not only the centrifugal impeller, but also the machine can be irreparably damaged. Moreover, the affixing of the last blades on the hub will be difficult, as the accessibility is limited due to the blades already present.

The purpose of the present invention is to provide a solution to at least one of the aforementioned and other disadvantages.

The object of the present invention is a method for manufacturing a centrifugal impeller, whereby this impeller comprises a base with a hub, whereby the impeller further comprises a number of protruding blades that are affixed on the surface of the hub, whereby the method comprises the following steps:

- the provision of a metal impeller base with a hub surface;

- the layer-by-layer construction of the blades on the hub surface of the metal impeller base with layers of a metal making use of an additive manufacturing method, whereby the metal used for the layers is compatible with the metal of the metal impeller base.

The aforementioned metal impeller base comprises a trumpet- shape whose diameter increases in the direction from one end to the other end, whereby the hub is provided with a central borehole to be able to attach the impeller to a drive shaft. The outer surface of the hub is called the hub surface .

The blades are affixed on the aforementioned hub base, more specifically on the hub surface.

The additive manufacturing method is 'laser cladding' for example. Hereby material to be added is supplied to the melt zone, i.e. the zone where material is desired to be added. The material can be applied in the form of a powder or a wire or a combination of both. Using a laser, the melt zone is heated locally, such that the powder and/or wire and the section of the underlying material melt. An advantage of a method according to the invention is that there is no or practically no loss of material.

Moreover, by building up the blades layer by layer, after having applied one or a few layers of one blade it will be possible to proceed to a next blade. In this way the necessary space and accessibility can always be guaranteed during the next phases of the manufacturing process.

Another advantage is that by building up the blades layer by layer, the stress in the blades will be distributed. The layer-by-layer construction using an additive manufacturing method will ensure that a continuous joint is formed as it were between the hub and the blades. As a result, the centrifugal impeller can be used in machines with a high speed, as it is resistant to the high centrifugal forces that are associated with this. The invention also concerns a centrifugal impeller manufactured according to a method according to the invention and a compressor or expander provided with such a centrifugal impeller.

With the intention of better showing the characteristics of the invention, a few preferred variants of the method according to the invention for manufacturing a centrifugal impeller and a centrifugal impeller thereby obtained are described hereinafter by way of an example, without any limiting nature, with reference to the accompanying drawings, wherein: figure 1 schematically shows a conventional method for manufacturing a centrifugal impeller;

figure 2 schematically shows a method according to the invention for manufacturing a centrifugal impeller; figures 3 and 4 show variants of a method according to the invention.

Figure 1 schematically shows a conventional method for manufacturing a centrifugal impeller 1.

This starts with an unworked workpiece 2 that is turned down to the shroud surface 3. This is a geometric surface that encloses the blades 4 of the impeller 2.

Then channels 5 are milled in the workpiece 2 to form the blades 4 by removing material . The bases of the channels are formed by the hub surface 6, this is the surface of the impeller 1 on which the blades 4 are attached. In a method according to the invention, as shown in figure 2, this starts with a workpiece that is defined by the hub surface 6.

A first step of the method thus consists of providing an impeller base 7 with a hub 8 with a hub surface 6, as shown in figure 2A. This hub 8 is provided with a central borehole 9 in order to be able to attach the impeller 1 to a drive shaft. This impeller base 7 is made of metal.

The impeller base 7 or the hub 8 can be manufactured by turning the workpiece down to the hub surface 6, by casting a workpiece or by another manufacturing method that yields the desired quality of the hub surface 6. For example, the impeller base 7 or the hub 8 can also be manufactured using an additive manufacturing method.

Then the blades are built up layer by layer on the hub surface 6 with layers 10 of metal, whereby use is made of an additive manufacturing method. This is shown in figures 2B-2D.

The metal used for the layers 10 is compatible with the metal of the impeller base and thus the hub surface. Ά possible additive manufacturing method is laser cladding.

To this end a metal that is compatible with the metal of the impeller base 7, and thus the hub surface 6, is supplied and melted locally using a heat source. As a result, a type of welding process is created as it were.

Due to the compatibility between the two metals, it is ensured that a bond is possible between the hub surface and 6 and the aforementioned layer 10.

However, this means that the composition of the metal supplied can be different to the composition of the metal of the hub surface 6.

The material that is supplied to the melt zone can be in the form of a powder or a wire or a combination of powder and wire. For the heat source use can be made of a laser or a plasma torch. But any heat source that can deliver heat to a localised point is suitable.

For the first layer 10 the hub surface 6 acts as a substrate to which this first layer 10 is attached. For each subsequent layer 10, the underlying layer 10 acts as a substrate to melt the next layer 10 thereon.

If the material is supplied as a powder, the supply of this powder can be interrupted, so that it can be worked in discrete steps so that material can be built up at discrete locations .

Moreover, with a powder it is possible to change the composition of the powder, for example per layer 10.

In such a case, but not necessary for the invention, the metal used for the layer-by-layer construction of the blades 4 can have a different composition per layer 10.

In this way the composition of the powder of the blade 4 can be changed or varied during the construction of the blade 4. It is also possible that the composition does not vary over the entire blade 4, but for example the composition of the powder is only changed at the location of the edge 11 of the blades 4 at the inlet side 12, where the blade 4 is exposed to erosion due to incoming dust particles and water droplets, for example by adding carbide particles.

The powder can be supplied by means of one or more nozzles that are affixed around the laser source for example, and which inject the powder at the desired location on the surface. By using different nozzles the composition of the powder can be adjusted, by providing each nozzle with a different type of powder and adjusting the flow rate of each nozzle. Preferably, but not necessarily, the substrate, i.e. the hub surface 6 or a previous layer 10, is heated or preheated for the application of a subsequent layer 10. As a result it will be possible to reduce the internal stresses in the centrifugal impeller 1.

An additive manufacturing method is preferably implemented in a protected atmosphere, for example an inert atmosphere or in a vacuum, in order to prevent excessive oxidation of the metal added.

Different methods can be applied to build up the blades 4 layer by layer using the principle of laser cladding described above or another additive manufacturing method.

Figure 2 shows a possible method, whereby each layer 10 extends in a direction parallel to the hub surface 6. This can clearly be seen in figures 2B, 2C.

This means that the construction direction of the blades 4 extends perpendicularly or as good as perpendicularly to the hub surface 6.

In this case for each blade 4 one, two, three or more layers 10 are applied, before one, two, three or more layers 10 are applied on the next blade 4.

For example, all first layers 10 of each blade can be first applied, after which the second layer 10 is applied on each of these first layers 10. By ensuring that two or more successive layers 10 of a blade 4 have a different length, the height of the blade 4 can be varied along the length. This means that one end 10a of the layer 10 is situated at the edge of the hub surface 6 at the inlet side 12, while the other end 10b of the layer 10 is either situated at the edge of the hub surface 6 at the outlet side 13 or at a location between the inlet side 12 and the outlet side 13, depending on the height at which the layer 10 concerned is located.

When passing from one blade 4 to the next, the application direction of the layer 10 can be changed from: from the inlet side 12 to the outlet side 13, to: from the outlet side 13 to the inlet side 12. This will reduce the time needed to pass from the one blade 4 to the next. It is of course also possible to preserve the application direction and for example to apply each layer 10 from the inlet side 12 to the outlet side 13, or vice versa.

Due to the successive application of the layers 10, it is possible to obtain and preserve good accessibility in order to apply the next layers 10.

If there is enough space available for the tools that are used for the additive manufacturing method, two or three layers 10 could be applied for each blade 4 before going to the next blade 4. When all layers 10 of the blades 4 have been applied, one or more additional layers could still be applied to smooth out the stepped transitions of the successive layers 10. It is also possible for the stepped transitions to be melted again, for example with a laser without powder being supplied, so that a smooth transition is obtained in this wa . The sides of the blades 4 can also be melted again to reduce the surface roughness.

It is also possible to apply an extra layer to the edges 11 of the blades 4 at the location of the inlet side 12. As already stated, this edge 11 of the blades 4 is exposed to incoming dust particles and water particles such that it is susceptible to erosion. By a suitable selection of the composition of this extra layer 10, the resistance to erosion can be increased.

In a subsequent optional step of a method according to the invention, the centrifugal impeller 1 can be subject to a heat treatment. By applying such a heat treatment, such as tempering, hardening, stress relieving or annealing, any internal stresses in the blade 4 that occur during the construction can be reduced. To finish off, the method can optionally comprise the step of working the edges and sides of the blades 4 by means of machining.

The top edge of the blade 4, i.e. the edge that coincides with the shroud surface 3 and hence is also called the shroud line', can be worked by means of turning or milling. The sides of the blades 4 are preferably worked by means of milling.

The impeller 1 thus formed can be used in a compressor or expander. As the stresses in the blades 4 are distributed and a continuous joint is formed between the hub surface 6 and the blades 4, the impeller 1 can also be used in a compressor or expander that has a high speed.

In the example of figure 2, the method was focused on the use of (metal) powder for laser cladding. This has the advantage that the material supply can be interrupted. A disadvantage is that laser cladding must be done in an inert atmosphere.

As already said, with laser cladding it is also possible to ensure the material supply using a (metal) wire.

In this case it is not necessary to provide an inert environment as with a wire the oxidation rate will be slower .

The process will also proceed more quickly when use is made of a wire. In this case it is not possible, or very difficult, to interrupt the material supply.

In this case continuous layers 14 will be used, i.e. a first uninterrupted layer 14 for all blades 4, whereby a subsequent uninterrupted layer 14 can be applied on this first layer 1 .

Figure 3 shows an example of such a method, where the first continuous uninterrupted layer 14 is applied to the hub surface 6 of an impeller base 7.

With such a method, after manufacturing the blades 4 it will be necessary to remove certain parts of the material added by means of milling in order to create the individual blades 4.

Figure 4 shows another variant of a method according to the invention. In this example, the successive layers 10 extend in a direction that deviates from the direction of the hub surface 6.

This means that the construction direction of the blades 4 deviates from the direction perpendicular to the hub surface 6.

In most cases this means that more layers 10 are required to fully build up a blade, which means that the construction of the blades 4 will take longer, for example also due to a longer transfer time from the one blade 4 to the next blade 4.

However, as a consequence of this the composition of the blades 4 can be better manipulated.

As is shown in figures 4B-4D, the layers 10 are built up from the outlet side 13 of the impeller 1 to the inlet side 12.

This means that the first layers are applied at the location of the outlet side 13, after which the layers 10 are increasingly shifted in the direction of the inlet side 12.

Such a method will ensure that there is always sufficient space to be able to apply the subsequent layers 10.

Although in the variants shown above the application is always transferred to the next blade 4 after one, two or three layers 10, it is also possible that only after all layers 10 of the first blade have been applied, the layers 10 of a next blade are applied. It is clear that the various optional steps and methods for building up the blades 4 layer 10 by layer 10, as discussed in the variant of figure 2, can also be applied in the other variants of figures 3 and 4. The present invention is by no means limited to the embodiments described as an example and shown in the drawings, but such a method and centrifugal impeller thereby obtained can be realised in different variants without departing from the scope of the invention.

Claims

Claims .

1. - Method for manufacturing a centrifugal impeller (1), whereby this impeller (1) comprises a base (7) with a hub

(8), whereby the impeller (1) further comprises a number of protruding blades (4) that are affixed on the surface (6) of the hub (8), characterised in that the method comprises the following steps:

- the provision of a metal impeller base (7) with a hub surface ( 6) ;

- the layer-by-layer construction of the blades (4) on the hub surface (6) of the metal impeller base (7) with layers (10) of a metal making use of an additive manufacturing method, whereby the metal used for the layers (10) is compatible with the metal of the metal impeller base (7 ) .

2. - Method according to claim 1, characterised in that the method also comprises the step of applying a heat treatment after building up the aforementioned blades (4).

3. - Method according to claim 1 or 2, characterised in that the method also comprises the step of working the edges or the sides of the aforementioned blades (4) by means of machining .

4. - Method according to any one of the previous claims, characterised in that the hub surface (6) and the layers (10) already applied on the hub surface (6) are heated before the next layer (10) is applied.

5. - Method according to any one of the previous claims, characterised in that after one, two, three or more layers (10) of a first blade (4) have been applied, one, two, three or more layers (10) of a next blade (4) are applied.

6. - Method according to any one of the previous claims, characterised in that two or more successive layers (10) of a blade (4) have a different length, so that the height of the blade (4) varies along the length of the blade (4).

7. - Method according to any one of the previous claims, characterised in that layers (14) that form one continuous layer (14) for all blades (4) are used to build up the blades (4) layer by layer.

8. - Method according to any one of the previous claims, characterised in that the successive layers (10) extend in a direction parallel to the hub surface (6) .

9. - Method according to any one of the previous claims 1 to 7, characterised in that the successive layers (10) extend in a direction that deviates from the direction of the hub surface ( 6) .

10. - Method according to claim 9, characterised in that the layers (10) are built up from the outlet side (13) of the centrifugal impeller (1) to the inlet side (12) .

11.- Method according to any one of the previous claims, characterised in that the metal used for the layer-by-layer construction of the blades (4) has a different composition for each layer (10).

12. - Method according to any one of the previous claims, characterised in that the hub (8) or impeller base (7) is manufactured making use of an additive manufacturing method .

13. - Centrifugal impeller, characterised in that the centrifugal impeller (1) is manufactured according to a method according to any one of the previous claims.

14. - Compressor, characterised in that the compressor is provided with a centrifugal impeller (1) according to claim 13.

15.- Expander, characterised in that the expander is provided with a centrifugal impeller (1) according to claim 13.