FIELD OF INVENTION

The present invention relates to a method of manufacture of an impeller for a centrifugal compressor. BACKGROUND ART

An impeller for a centrifugal rotating machine, such as a centrifugal compressor, is generally composed of an approximately disk shaped main body having a front and a rear surfaces, with respect to the direction of the fluid flow, provided with a central circular hole that fits with a hub attached to a rotating axis, a shroud having a front and a rear surfaces that is arranged at an interval outward of the main body and a series of blades that couple the front surface of the main body and the rear surface of the shroud. A series of passages are formed by the front surface of the main body, the rear surface of the shroud and the side surfaces of the blades, the passages serving as a flow passage for compressing air. Generally the blades follow a curved line bending in both radial and axial direction. The front surface of the main body and the rear surface of the shroud bend accordingly to accommodate the curvature of the blades. The air flow enters the impeller tangentially to the front surface of the main body and exits from the impeller in an axial direction. A shrouded impeller may be manufactured from one single solid piece of material or it may instead be constructed from two or more components, which are attached together to form the finished product. In general the various components are fixed by welding. The impeller can be constructed from one single solid piece by full milling, but this process is very difficult and time consuming due to the limited accessibility given the complex shape of the flow passages. Electrical Discharge Machining (EDM) process is also an option for the single piece fabrication by means of electric spark metal-removal erosion. In this process an electric spark is used as the cutting tool to erode the single monolithic disc to produce the finished part to the design shape. The metal-removal process is performed by applying a pulsating (ON/OFF) electrical charge of high-frequency current through the electrode to the work piece. This technique involves the step of machining the single piece by premilling followed by EDM roughing and EDM finishing. For the reason set before, special shaped electrodes and special shaped tools must be used to conform to the complex passages design. The process is very time consuming and many working days are necessary to produce a single impeller. The impeller can be produced also by fabricating first two components i.e. by obtaining the blades by means of milling the main body or the shroud and subsequently by welding the two parts. With welding filling the internal junction between blades and main body or shroud given the lack of good accessibility is very difficult and welding defects easily occur in the fixing and thus the risk of cracking and deformation, i.e. thermal distortion, due to locally high temperatures, can become very high. Besides the surfaces of the flow passage will not have in general the smoothness required. In definitive it is very hard by using state of the art techniques to obtain an impeller with the high quality and reliability required. For these reasons a method to make an impeller having higher quality and that results in shorter production time and lower costs is highly needed. SUMMARY OF THE INVENTION

Accordingly a main aim of the present invention is to provide a new method of manufacture of an impeller that is simple and requires shorter machining times. Another aim of the present invention is a method of manufacturing an impeller having high qualities and reliability. A further objective of the invention is to provide an impeller for centrifugal compressor that enables advanced manufacturing technique.

Yet another scope of the invention is a method that allows the making of an impeller having complex geometries.

The present invention, therefore, achieves the objectives discussed above by means of a method of manufacture of an impeller for a centrifugal rotating machine that, according to claim 1 , is composed by an almost disk shaped main body having a front and rear surface, a shroud having a front and a rear surfaces arranged spaced from the front surface of the main body, a plurality of blades, attached to the front surface of the main body joining the main body with the shroud, the side surfaces of the blades, the front surface of the body and the rear surface of the shroud forming flow passages for compressing air, wherein the method comprises making single segments of impeller consisting of a single blade, a section of the body and the corresponding section of the shroud, and then joining together the plurality of segments to form the impeller.

Another aspect of the invention provides for an impeller for a centrifugal rotating machine according to claim 1 1. Advantageously the single segments can be produced using methods of additive manufacturing like Direct Metal Laser Melting (DMLM). Advantageously the single segments are joined together by welding or friction welding. The single sectors can also not be joined to each other, but be attached to the hub or directly to the shaft by means of mechanical junctions or welding. Advantageously the single sectors, or the entire impeller can be attached to the hub by mechanical joints like dovetail joints.

BRIEF DESCRIPTION OF THE FIGURES

Further characteristics and advantages of the invention will become more apparent in light of a detailed description of a preferred, but not exclusive, embodiment of an impeller for rotating machines, illustrated by way of non- limitative examples , with the aid of the accompanying drawings where:

Fig. 1 is a perspective view of an impeller according to the invention;

Fig. 2 is an explosed view of a cross section of an impeller before being fixed to the hub;

Fig. 3 represents a transverse section of the impeller joined to the hub; Fig. 4 represents a single segment of the impeller mounted on the hub;

Fig. 5 is a perspective view of a segment of an impeller according to another embodiment of the invention;

Fig. 6 is a perspective view of an impeller containing the segments shown in Fig. 5.

The same reference numbers in the figures identify the same elements or components.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION With reference to Figure 1 and 2 there is shown a perspective representation of a 3D impeller generally indicated with reference numeral 100. The impeller 100 is formed by a disk shaped main body 3 having a rear surface and a front surface 7. The front surface 7 starting from an outer circular portion almost flat and perpendicular to the rotating axis Z of the machine gradually curves towards the central region in the direction of the axis Z, the central region being a circular aperture adapted to mate with the external surface of the hub 1 or of the rotor shaft. A shroud 5 roughly following the shape of the front surface 7 of the body 3 is separated from this front surface by the blades 4. Air passages 6 through which the air is compressed are formed by the internal surface of the shroud, the front surface 7 of the body 3 and the lateral surfaces of the blades 4. In this type of impeller for a centrifugal compressor, the air flow enters the passages tangentially to the flat surface of the body towards the rotational axis and exits in an almost vertical direction parallel to said rotational axis from the circular passage formed between the front surface 7 of the body and the internal surface of the shroud. In Fig. 2 there is shown a sectional exploded view of this type of impeller with the hub 1 shown as a separated element to be inserted in the central region of the body 3. The hub 1 is attached to the rotor shaft of the machine. Fig. 3 shows the final assembly of the impeller with the insertion of the hub 1 . The blades 4 and thus the air passages 6 have generally a complex shape. As seen from Fig. 1 and 2 they follow a curved path not only in the axial direction but they are also backswept relative to the direction of rotation of the impeller. These characteristics of the impeller make very complex and laborious the work of milling and welding even when using special tools suitable to be inserted to operate in those air passages 6. These difficulties are detrimental to good accuracy, surface roughening and time. The present invention is based on the principle of segmentation in order to overcome these drawbacks. Fig. 4 is a representation of such technique. The idea consists essentially in making single segments of impeller, each segment for example containing a single blade, and then composing them to assemble the impeller by welding or by a mechanical locking system. The segments can be joined to each other or can be fixed to the shaft separately. In fig 4 reference 100' indicates one of these segments to be welded to the hub 1 . The segment is composed by a segment of shroud 5 and a segment of the main body 3 joined by a blade 4. In Fig. 4 it is also possible to see the complex shape of the blades and of the passages. By segmentation the problem of accessibility would be completely resolved. The impeller can be made by joining together the single segments 100' by external laser welding, and then joining the welded segments to the hub or directly to the rotor shaft by CMT deposition. It follows a step of heating and turning to arrive to the finished product.

It is also possible to fix the single segments one by one to the hub as shown in Fig. 4 or directly on the rotor shaft. The segment 100' can be attached to the hub, or directly to the rotor shaft, also by means of a mechanical locking system as shown in Fig.5 and Fig. 6. In Fig. 5 there is shown a segment having on the side facing the external surface of the hub a dovetail key 8 with inclined side walls. These dovetail keys 8 slidingly engage into it a corresponding mating dovetail groove 10, shown in Fig. 6, formed on the external surface of the hub. Fig 6 shows a perspective sectional view of an impeller made by using this mechanical locking system. The impeller can be assembled on the hub as a single piece with the segments joined together by welding and then sliding the hub into the central part or by sli ding the segments one by one into the dovetail grooves of the hub. When using the mechanical locking system the single segment can be provided with a groove 9 on the rear flat side surface of the body 3, Fig. 5, Fig. 6, that when the impeller is assembled forms a circular groove that can be provided with a ring to fasten the segments together. The manufacturing of a single blade segment is not only easy, fast and cheap, but also enables the use of advanced manufacturing technologies e.g. the use of composite material or techniques like Direct Metal Laser Melting (DMLM), where a laser fuses metal powder into a solid, three-dimensional part in an additive, layer by-layer process. Compared with the many days necessary to make an impeller by means of state of the art technology, for example by Electrical Discharge Machining (EDM), that requires a phase of Premilling followed by EDM roughing and finishing, the assembling of prefabricated single blade segments takes only few hours with a considerable savings of time and costs, without considering the improvement in quality. Making the impeller by assembling single blade segments, given the simplicity of fabrication and the high versatility of the process, enables also the design and fabrication of advanced high performance flow path of innovative geometries, that at the moment are not yet feasible.