Method for manufacturing closed impeller

Abstract

In a closed impeller (1) made of an aluminum alloy, a plurality of blades (3) arranged in the circumferential direction connect a hub (2) and a shroud (4). First, electric discharge machining is performed on impeller material to form a plurality of flow passages (5) between the blades (3). Then, a surface of a flow passage that is the inner peripheral surface of each flow passage (5) in the impeller material is polished to remove a modified layer formed on the surface of the flow passage by the electric discharge machining, and then the surface of the flow passage is further polished.

Description

Closed impeller manufacturing method

[0001] The present disclosure relates to a method for manufacturing a closed impeller having a hub, a shroud, and a plurality of blades.

[0002] Centrifugal turbomachines, such as centrifugal compressors, centrifugal pumps, and radial turbines, are known. Centrifugal compressors and centrifugal pumps apply work to a fluid to increase its pressure, while radial turbines recover power from the high-pressure fluid. These centrifugal turbomachines often use a closed impeller in which a hub and a shroud are connected by a plurality of blades arranged in the circumferential direction.

[0003] For example, Patent Document 1 discloses a method for manufacturing a steel closed impeller. In Patent Document 1, the closed impeller is referred to as a "one-piece impeller." In the manufacturing method of Patent Document 1, first, an impeller material is subjected to electrical discharge machining to form multiple flow passages between circumferentially arranged blades. Next, the flow passage surfaces, which are the inner circumferential surfaces of each flow passage in the impeller material, are polished to remove affected layers formed on the flow passage surfaces by electrical discharge machining. Finally, the flow passage surfaces of the impeller material are polished to reduce the surface roughness of the flow passage surfaces.

[0004] Patent No. 5787638

[0005] In centrifugal turbomachines, there is a demand for rotating closed impellers at high speeds. In particular, in centrifugal compressors for hydrogen, the maximum peripheral speed of the closed impeller can be 500 m / s or more. In this case, it is desirable to use an aluminum alloy, which is lightweight and has high strength, as the material for the closed impeller.

[0006] Therefore, an object of the present disclosure is to provide a manufacturing method suitable for manufacturing a closed impeller made of an aluminum alloy.

[0007] The present disclosure provides a method for manufacturing a closed impeller made of an aluminum alloy, in which a hub and a shroud are connected by a plurality of blades arranged circumferentially, by performing electrical discharge machining on an impeller material to form a plurality of flow passages between the plurality of blades, polishing the flow passage surfaces, which are the inner surfaces of each of the plurality of flow passages in the impeller material, to remove any altered layers formed on the flow passage surfaces by the electrical discharge machining, and then further polishing the flow passage surfaces.

[0008] According to the present disclosure, a manufacturing method suitable for manufacturing a closed impeller made of an aluminum alloy is provided.

[0009] Fig. 2 is a front view of a closed impeller obtained by a manufacturing method according to one embodiment. Fig. 3 is a cross-sectional view taken along line II-II in Fig. 1. Fig. 4 is a view showing a state after a rough machining step in the manufacturing method. Fig. 5 is a perspective view of an impeller material after the rough machining step. Fig. 6 is a view showing a state after an electric discharge machining step in the manufacturing method.

[0010] 1 and 2 show a closed impeller 1 obtained by a manufacturing method according to one embodiment. The closed impeller 1 has a generally disk-like shape with a through-hole 11 in the center. A rotary shaft is fitted into the through-hole 11.

[0011] Inside the closed impeller 1, a plurality of flow paths 5 are formed that open in the axial direction of the closed impeller 1, i.e., in the direction of extension of the center line 10, around the through hole 11, and that open on the end face facing radially outward of the closed impeller 1.

[0012] More specifically, the closed impeller 1 includes a hub 2 that forms the through hole 11 and the back surface of the closed impeller 1, and an annular shroud 4 that covers the hub 2 from one axial side of the closed impeller 1 and forms the peripheral portion of the surface of the closed impeller 1.

[0013] Furthermore, the closed impeller 1 includes a plurality of blades 3 arranged in a circumferential direction between the hub 2 and the shroud 4. The hub 2 and the shroud 4 are connected by the blades 3.

[0014] In the following description, for convenience of explanation, the front side of the closed impeller 1 in the axial direction, i.e., the shroud 4 side, will be referred to as the upper side, and the back side, i.e., the hub 2 side, will be referred to as the lower side.

[0015] The closed impeller 1 is made of an aluminum alloy. Although the aluminum alloy is not particularly limited, for example, an Al-Cu alloy specified as a 2000 series alloy in JIS (Japanese Industrial Standards) H 4000 or an Al-Zn-Mg alloy specified as a 7000 series alloy in JIS H 4000 is used.

[0016] The hub 2 includes a cylindrical portion 22 that forms the through hole 11, and a disk portion 21 that extends radially outward from the cylindrical portion 22. The cylindrical portion 22 penetrates the annular shroud 4 and constitutes a central annular portion on the surface of the closed impeller 1. In this embodiment, the disk portion 21 extends radially outward from the center of the cylindrical portion 22. However, the length of the cylindrical portion 22 may be shortened, and the disk portion 21 may extend radially outward from the lower end of the cylindrical portion 22.

[0017] The upper surface of the disk portion 21 facing the shroud 4 is curved so as to shift upward as it moves radially inward, and the outer peripheral surface of the upper part of the cylindrical portion 22 is curved so as to shift radially outward as it moves downward. The outer peripheral surface of the upper part of the cylindrical portion 22 and the upper surface of the disk portion 21 are smoothly connected to each other and form a continuous curved surface that widens in diameter downward. The curved surface guides axial flow into radial flow when the closed impeller 1 is used in a centrifugal compressor or centrifugal pump, and guides radial flow into axial flow when the closed impeller 1 is used in a radial turbine.

[0018] In this embodiment, the lower surface of the disk portion 21 has an upwardly recessed depression 23 at a position adjacent to the cylindrical portion 22. However, the lower surface of the disk portion 21 may be entirely flat without having the depression 23.

[0019] The shroud 4 is in the form of a plate that is curved 90 degrees along the curved surface of the hub 2, and has an inner surface that is a curved surface that faces opposite the curved surface of the hub 2, an outer surface that faces the opposite side from the hub 2, an axial end face that faces upward between the radially inner base end of the inner surface and the radially inner base end of the outer surface, and a radial end face that faces radially outward between the radially outer end of the inner surface and the radially outer end of the outer surface.

[0020] In this embodiment, the radially inner portion of the outer surface of the shroud 4 has a straight cylindrical shape, and the other portions have curved surfaces. However, the outer surface of the shroud 4 may be entirely curved. Also, in this embodiment, the axial end face of the shroud 4 is located below the upper end face of the cylindrical portion 22 of the hub 2, but the axial end face may be located at the same height as the upper end face of the cylindrical portion 22.

[0021] The blades 3 divide the annular space curved at 90 degrees between the hub 2 and the shroud 4 into the above-mentioned multiple flow passages 5. Each blade 3 is perpendicular to the curved surface of the hub 2 and the inner surface of the shroud 4. Each blade 3 is also curved in one circumferential direction toward the radially outer side of the closed impeller 1.

[0022] In this embodiment, the inner edge of each blade 3 located on the radially inner side of the closed impeller 1 is parallel to the radial direction of the closed impeller 1 and is located lower than the axial end face of the shroud 4. However, the inner edge of each blade 3 may be located at the same height as the axial end face of the shroud 4. Alternatively, the inner edge of each blade 3 may be inclined downward toward the radially inner side.

[0023] The outer edge of each blade 3 located radially outward of the closed impeller 1 constitutes the end face of the closed impeller 1 together with the outer end face of the disk portion 21 of the hub 2 and the radial end face of the shroud 4 .

[0024] Next, a manufacturing method of the closed impeller 1 according to this embodiment will be described. The manufacturing method includes a rough machining step, a heat treatment step, an electric discharge machining step, a first polishing step, a finish machining step, and a second polishing step. The rough machining step, heat treatment step, electric discharge machining step, first polishing step, finish machining step, and second polishing step are performed in this order. However, the order of the finish machining step and the second polishing step may be reversed.

[0025] In the rough machining step, as shown in Fig. 3, a disk-shaped impeller material 6 made of an aluminum alloy is roughly machined by cutting using a milling machine or the like to form the shape shown by the solid line in Fig. 3. Specifically, the peripheral edge of the impeller material 6 is formed into a flange shape, and a through hole 11 is formed in the center of the impeller material 6. Note that the shaping of the peripheral edge into a flange shape, the formation of the through hole 11, and the formation of the annular groove 61 and recess 62 described below may be performed in any order.

[0026] The flange shape into which the peripheral edge of the impeller material 6 is molded has excess material remaining on the front, back, and outer peripheral surfaces relative to the final shape, which is the shape of the closed impeller 1. In other words, the portion radially outward and on the front side of the planned flow path formation portion, which is a circumferentially continuous region including all of the flow paths 5, is the shroud internal portion 40 with excess material added to the shroud 4, and the portion radially inward and on the back side of the planned flow path formation portion is the hub internal portion 20 with excess material added to the hub 2. The hub internal portion 20 includes a cylindrical portion internal portion 220 with excess material added to the cylindrical portion 22, and a disk portion internal portion 210 with excess material added to the disk portion 21.

[0027] As shown in FIG. 4 , an annular groove 61 is formed in the axial end face 6 a of the impeller material 6 on the axial opening side of the flow passages 5, the annular groove 61 being continuous in the circumferential direction about the center line 60 of the impeller material 6 so as to pass through the axial openings of all of the flow passages 5, and a plurality of recesses 62 are formed from the annular groove 61 and recessed into the axial openings of all of the flow passages 5.

[0028] In other words, the portion located between adjacent recesses 62 is a blade-enclosed portion 30, which is the blade 3 plus excess material. Each recess 62 is recessed obliquely to follow the blades 3 located on both sides of the corresponding flow path 5. The depth of the recess 62 is determined so that, in a cross section including the center line 60 of the impeller material 6, the radius of a circle inscribed so as to contact the bottom of the recess 62 in the plate-like portion of the impeller material 6 including the above-mentioned portion intended to form the flow path, as shown by the dashed line in Figure 3, is equal to or less than a tolerance. The tolerance is a value corresponding to the strength to be achieved in the subsequent heat treatment process.

[0029] In the thermal processing step, the roughly processed impeller material 6 is heat-treated. Examples of heat treatment include solution treatment and age hardening treatment. For example, the heat treatment is T4, T5, T6, etc. as specified in JIS H 0001.

[0030] In the electrical discharge machining process, electrical discharge machining is performed on the impeller material 6 to form the flow passages 5 between the blades 3, as shown in Fig. 5. In this embodiment, the electrical discharge machining is performed in two stages: a first half and a second half. In the first half of the electrical discharge machining, the radially extending portion of the flow passage 5 is formed from the radially outer side of the impeller material 6, and in the second half of the electrical discharge machining, the remaining portion of the flow passage 5 is formed from the axial direction of the impeller material 6. However, depending on the shape of the flow passage 5, the electrical discharge machining may be performed in one stage.

[0031] In the first half of the electrical discharge machining, a first electrode shaped according to the radially outer portion of the flow path 5 is used, and while the first electrode is brought from the radially outer side close to the position where the flow path is formed in the impeller material 6, an arc discharge is generated between the first electrode and the impeller material 6, thereby removing a part of the impeller material 6. By repeating this process, a radially extending portion of the flow path 5 is formed.

[0032] After the first half of the electrical discharge machining, an unmachined region remains adjacent to the recess 62. The unmachined region corresponds to a portion where the direction of the flow path 5 changes significantly. The unmachined region is removed in the second half of the electrical discharge machining.

[0033] In the latter half of the electrical discharge machining, a second electrode having a shape corresponding to the portion where the direction of the flow path 5 changes significantly is used, and an arc discharge is generated between the second electrode and the impeller material 6 while the second electrode is brought from within the recess 62 close to the position where the flow path is formed in the impeller material 6, thereby removing a portion of the impeller material 6. By repeating this process, the entire unmachined region is removed, and the remaining portion of the flow path 5 is formed.

[0034] In the first polishing step, the flow path surface, which is the inner circumferential surface of each flow path 5 of the impeller material 6, is polished to remove an altered layer formed on the flow path surface by electrical discharge machining. The polishing in the first polishing step can be performed by chemical polishing, blasting, or the like. Chemical polishing is performed by immersing the impeller material 6 in a chemical solution. The blasting is, for example, shot blasting or sand blasting.

[0035] Chemical polishing is sometimes called etching. When chemical polishing is performed, for example, a caustic soda solution with a concentration of 20 g / L to 50 g / L may be used as the chemical solution. In this case, the impeller material 6 is immersed in the 40°C chemical solution for about 30 minutes. During immersion, in order to achieve uniform wall thinning, it is desirable to remove reaction bubbles generated by the chemical reaction, for example, by bubbling or shaking. As a pre-process of chemical polishing, the impeller material 6 may be immersed in a neutral detergent solution to remove oil, or as a post-process of chemical polishing, the impeller material 6 may be immersed in a nitric acid / hydrogen peroxide aqueous solution to remove smut, which is a metal additive remaining on the surface layer.

[0036] In the finishing process, excess material remaining in the rough machining process is removed by cutting, etc. As a result, the final shape of the closed impeller 1 is obtained as shown by the two-dot chain line in Figure 5.

[0037] In the second polishing step, the flow path surface of the impeller material 6 after the removal of the altered layer is further polished to reduce the surface roughness of the flow path surface. The polishing in the second polishing step can be performed by MMP (Micro Mashing Process), manual polishing, blasting, barrel polishing, or the like. MMP is performed by flowing aggregates of microtools along the flow path surface. The blasting in the second polishing step is, for example, a jet processing method in which particles of an elastic material are collided with the flow path surface in a sliding manner.

[0038] The degree of reduction in the surface roughness of the flow path surface in the second polishing step is preferably such that the arithmetic mean roughness Ra specified in JIS B 0601 is 1.0 or less, and more preferably such that Ra is 0.4 or less.

[0039] According to the present disclosure, a manufacturing method suitable for manufacturing a closed impeller 1 made of an aluminum alloy is provided.

[0040] <Modifications> The present disclosure is not limited to the above-described embodiment, and various modifications are possible without departing from the gist of the present disclosure.

[0041] For example, the formation of the recess 62 in the rough machining step can be omitted. Simply forming the annular groove 61 can allow heat to sufficiently penetrate into the interior of the plate-shaped portion of the impeller material 6 in the heat treatment step, when the thickness of the plate-shaped portion including the planned flow path formation portion of the impeller material 6 is thick. In contrast, if the recess 62 is formed in the rough machining step, heat can sufficiently penetrate into the interior of the plate-shaped portion of the impeller material 6 in the heat treatment step, even when the thickness of the plate-shaped portion including the planned flow path formation portion of the impeller material 6 is even thicker.

[0042] Depending on the thickness of the closed impeller 1, the heat treatment step may be carried out first, and then the rough machining step may be carried out.

[0043] <Summary> In a first aspect, the present disclosure provides a method for manufacturing a closed impeller made of an aluminum alloy, in which a hub and a shroud are connected by a plurality of blades arranged circumferentially, the method comprising: performing electrical discharge machining on an impeller material to form a plurality of flow passages between the plurality of blades; polishing the flow passage surfaces, which are the inner circumferential surfaces of each of the plurality of flow passages in the impeller material, to remove any affected layers formed on the flow passage surfaces by the electrical discharge machining; and then further polishing the flow passage surfaces.

[0044] According to the above configuration, a manufacturing method suitable for manufacturing a closed impeller made of an aluminum alloy is provided.

[0045] In a second aspect, before the impeller material of the first aspect is subjected to electric discharge machining, the impeller material may be roughly machined to form a peripheral portion of the impeller material into a flange shape, and the roughly machined impeller material may be heat treated, and in the rough machining of the impeller material, a circumferentially continuous annular groove may be formed in the axial end face of the impeller material on the axial opening side of the plurality of flow paths so as to pass through the axial openings of the plurality of flow paths. With this configuration, when the thickness of the plate-shaped portion including the portion where the flow paths are to be formed in the impeller material is thick, it is possible to allow heat to sufficiently penetrate into the interior of the plate-shaped portion in the heat treatment process.

[0046] In a third aspect, in the second aspect, a plurality of recesses may be formed in the rough machining of the impeller material, the recesses being recessed from the annular groove into the axial openings of the plurality of flow paths. With this configuration, even if the thickness of the plate-like portion including the portion in the impeller material where the flow paths are to be formed is even greater, heat can be sufficiently introduced into the interior of the plate-like portion in the heat treatment step.

[0047] As a fourth aspect, in the third aspect, for example, each of the plurality of recesses may be recessed obliquely so as to follow the blades located on both sides of the corresponding flow passage.

Claims

1. A method for manufacturing a closed impeller made of an aluminum alloy, in which a hub and a shroud are connected by a plurality of blades arranged circumferentially, comprising the steps of: performing electrical discharge machining on the impeller material to form a plurality of flow passages between the plurality of blades; polishing the flow passage surfaces, which are the inner surfaces of each of the plurality of flow passages in the impeller material, to remove any altered layers formed on the flow passage surfaces by the electrical discharge machining; and then further polishing the flow passage surfaces.

2. A method for manufacturing a closed impeller as described in claim 1, wherein, before performing electrical discharge machining on the impeller material, the impeller material is roughly machined to form the peripheral portion of the impeller material into a flange shape, and the roughly machined impeller material is heat treated, and during the rough machining of the impeller material, a circumferentially continuous annular groove is formed in the axial end face of the impeller material on the axial opening side of the multiple flow paths so as to pass through the axial openings of the multiple flow paths.

3. A method for manufacturing a closed impeller according to claim 2, wherein a plurality of recesses recessed from the annular groove into the axial openings of the plurality of flow paths are formed in the rough machining of the impeller material.

4. A method for manufacturing a closed impeller according to claim 3, wherein each of the plurality of recesses is recessed obliquely so as to follow the blades located on both sides of the corresponding flow passage.

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