Turbine rotor

Abstract: A turbine rotor has a ceramic rotor and a metallic shaft (10). The ceramic rotor has a hub (22), a plurality of blades extending from the hub in generally radial directions, an axle (26) coaxially connected to the hub, and a boss coaxially connected to the hub at the opposite side of the axle. The metallic shaft is coaxially engaged to the axle of the ceramic rotor. The metallic shaft has an open end (11) having a sidewall having a main portion, a foremost end (15), a radially outer surface, and a radially inner surface forming a hollow for receiving the axle of the ceramic rotor. An average diameter at the radially outer surface of the foremost end is smaller than a diameter at the radially outer surface of most of the main portion so as to give a decreased thickness in the foremost end compared to the main portion. An engagement boundary (29) is located in the radially inner surface of the foremost end of the sidewall, and the engagement boundary refers to a boundary between (a) the area in which the radially outer surface of the axle of the ceramic rotor and the radially inner surface of the sidewall of the metallic shaft are in mutual contact and (b) the area in which they are not in mutual contact. Stress concentration at the engagement boundary is relaxed so as to prevent cracking at the shaft of the ceramic rotor and to increase the bonding strength between the ceramic rotor and the metallic shaft.

Description: Background of the Invention (1) Field of the Invention The present invention relates to a turbine rotor wherein a ceramic rotor is engaged to a metallic shaft. Turbine rotors are used in the turbocharger for an automobile engine. (2) Description of the Related Art The turbocharger of an automobile engine has a turbine having blades, an air compressor having blades, and a shaft connecting between them. An exhaust gas emitted from the engine rotates the turbine for driving the air compressor so as to increase an amount of air fed into the engine. The increased amount of air leads to an increased amount of fuel fed into the engine, resulting in a higher engine output. The turbine of the turbocharger has a plurality of blades so that an exhaust gas from engine pushes the blades to give a rotating power therein. The blades extend in radial directions while slightly twisted in a circular direction so as to allow smooth discharge of the exhaust gas. The turbine rotor of the present invention may serve as the turbine and the shaft of the turbocharger. To the other end of the shaft may be bonded to the rotor of an air compressor. The rotor of the turbine is exposed to an exhaust gas having high temperatures from the engine, and thus the rotor is required to withstand heat. The rotor is required to have a small weight for easy rotation. The rotor must have a sufficient mechanical strength to withstand a centrifugal force at high temperatures. Therefore, ceramics satisfying these requirements, such as Si 3 N 4 , SiC, sialon and the like, have been proposed to used in the rotor. A ceramic rotor, however, has a problem in bonding to a metallic shaft, particularly when the ceramic rotor is bonded to the metallic shaft by engagement, for example, by interference fit. In the interference fit, an engagement boundary of the ceramic rotor is subject to the compressive force of engagement, and stress concentrates in the engagement boundary so that the bonded body has low resistance to bending and torsional motions. Herein, the engagement boundary refers to a boundary between (a) the area in which a radially outer surface of the ceramic rotor is in contact with a radially inner surface of the metallic shaft and (b) the area in which a radially outer surface of the ceramic rotor is not in contact with the radially inner surface of the metallic shaft. Japanese Patent Publication No. 51895/1984 discloses a bonded body having a ceramic cylinder and a metal cylinder engaged with an outer surface of the ceramic cylinder. The metal cylinder has an open end having a sidewall forming a hollow for receiving the ceramic cylinder, and the sidewall has a decreasing thickness toward the end. The reference, however, does not disclose the turbine rotor nor the position of the engagement boundary. Japanese Patent Publication No. 35265/1991 discloses a bonded body wherein a ceramic member is engaged with a metallic member. In this body, an engagement boundary is located at an inner surface of the sidewall of the metallic member corresponding to the groove thereof so as to relax stress concentration. This bonded body may be a turbine rotor of turbocharger, as shown in Fig. 4. In Fig. 4, a turbine rotor 30 has a ceramic rotor 32 and a metallic shaft 36 engaged coaxially to the axle 34 of the ceramic rotor 32. The metallic shaft 36 has an open end having a sidewall 37 forming a hollow for receiving the axle 34 of the ceramic rotor 32. In a radially outer surface of the sidewall 37, a groove 38 for receiving a seal ring and groove 39 for receiving an oil slinger are formed. In Japanese Patent Publication No. 35265/1991, an engagement boundary 35 is located at the radially inner surface of the sidewall 37 corresponding to either groove 38 or groove 39. In Fig. 4, the engagement boundary is located at the radially inner surface of the sidewall 37 corresponding to the groove 38. The engagement boundary refers to a boundary between (a) the area in which the radially outer surface of the axle 34 of the ceramic rotor 32 and the radially inner surface of the sidewall 37 of the metallic shaft 36 are in mutual contact and (b) the area in which they are not in mutual contact. In the above structure, however, the grooves 38 and 39 had to be formed by mechanical working after the engagement or inserting the axle 34 of the ceramic rotor 32 to the open end of the metallic shaft 36 because the positions of the grooves 38 and 39 are determined by the position of the engagement boundary 35. Moreover, the axle 34 of the ceramic rotor 32 might rupture due to the load applied during mechanical working for forming the grooves 38, 39. Further, stress tends to concentrate at a thick portion in the side wall 37 which is not formed of the grooves 38 and 39, and a portion of the axle 34 of the ceramic rotor 32 corresponding to the thick portion might crack. DE 4 220 224 C discloses a turbine rotor which is attached to a metal journal by shrinkage fitting. The rotor includes a ceramic shaft having a diameter reducing in the direction toward the rotor hub, and the journal is deformed to fittingly cover the shaft. Summary of the Invention An object of the present invention is to solve the aforementioned problem. In the present invention, the engagement boundary is not located in the radially inner surface corresponding to one of the grooves but in a radially inner surface of the foremost end of the sidewall. The foremost end has an outer diameter smaller than most of the other portions of the side wall so as to give a decreased thickness therein, thereby relaxing the stress concentration thereof. According to the present invention, there is provided a turbine rotor according to claim 1. Preferably, the radially inner surface of the sidewall of the metallic shaft has a generally cylindrical shape; 0.7 ≤ a t ≤ 2.5 wherein "a" denotes a length of a radially outer surface of the foremost end in the axial direction of the metallic shaft; and "t" denotes to an average thickness of the foremost end. Further preferably, 1.0 ≤ a t ≤ 2.0. Preferably, 1. 0 ≤ d D ≤ 1.2 wherein "d" denotes to the average diameter at the radially outer surface of the foremost end; and "D" denotes to an average diameter at the radially inner surface of the sidewall. Further preferably, 1.05 ≤ d D ≤ 1.2. Preferably, the sidewall has a step in the radially outer surface thereof so that the foremost end extends from the step to an end surface of the foremost end. Preferably, the open end of the metallic shaft is comprised of a metallic material having an average thermal expansion coefficient of 6-8x10 -6 /°C in a temperature ranging from room temperature to 450°C, and a creep strength of not less than 500 MPa at 450°C over 200 hours. Preferably, the metallic shaft has a shaft portion bonded to the open end, and the open end is comprised of an age-hardening alloy having a Rockwell hardness H RC of at least 35. Preferably, the metallic shaft has a shaft portion bonded to the open end, and the metallic shaft is made by the steps of: subjecting the shaft portion to quenching and tempering so as to have a Rockwell hardness H RC of 35-45; and bonding the shaft portion to the open end. Preferably, the foremost end has an edge in the radially inner surface, and the edge is tapered or chamfered. Preferably, a groove is formed in the radially outer surface of the main portion of the sidewall. Brief Description of the Drawings Fig. 1 is a cross sectional view of the turbine rotor according to the present invention. This is an enlarged view of part A of Fig. 2. Fig. 2 is a side view of the turbine rotor according to the present invention, wherein a part of the turbine rotor is shown in a cross section. Fig. 3 is a cross sectional view of the foremost end of the metallic shaft of the turbine rotor according to the present invention. This is an enlarged view of part B of Fig. 1. Fig. 4 is a cross sectional view of a conventional turbine rotor. Fig. 5 is a cross sectional view of the foremost end of the metallic shaft of the turbine rotor according to the present invention. Fig. 6 is a cross sectional view of the foremost end of the metallic shaft of the turbine rotor according to the present invention. Detailed Description of the Preferred Embodiments Fig. 1 is enlarged part A of Fig. 2. In Fig. 2, a turbine rotor has a ceramic rotor 20 and a metallic shaft 10 engagedly bonded thereto. The ceramic rotor 20 has a hub 22, a plurality of blades 24 extending from the hub in generally radial directions, an axle 26 coaxially connected to the hub 22, and a boss 28 coaxially connected to the hub 22 at the opposite side of the axle 26. The blades 24 extend in radial directions while slightly twisted in a rotating direction of the rotor 20 so as to allow fluid pushing the blades 24 and to flow therefrom. The boss 28 has a radially outer surface formed of a plurality of grooves extending in the axial directions. The pitch in the grooves are typically constant. Preferably, the hub 22, the blades 24, the axle 26 and the boss 28 are integrally formed to give the ceramic rotor. The ceramic rotor may be obtained by a process including the steps of: molding a ceramic material including a ceramic powder and an organic binder to give a molded article having a desired shape as shown in Fig. 2; heating the molded article so as to substantially remove the organic binder; and sintering the molded article for densification. Preferably, between the step of heating the molded article and the step of firing the molded article, the molded article is subjected to isostatic pressing to reduce pores therein. The metallic shaft 10 has a shaft portion 18a having a larger diameter and a shaft portion 18b having a smaller diameter coaxially connected to the shaft portion 18a. The shaft portion 18a has an open end 11 coaxially connected thereto, and the shaft portion 18b has a closed end 19 coaxially connected thereto. In Fig. 1, the open end 11 has a sidewall 14 having a radially outer surface 14t and a radially inner surface 14s, both of which have a generally cylindrical shape. The radially inner surface 14s forms a hollow 12 for receiving the axle 26 of the ceramic rotor 20. The open end 11 of the metallic shaft 10 is coaxially engaged to the axle 26 of the ceramic rotor 20. The sidewall 14 has a main portion in the side of the metallic shaft 18a and a foremost end 15 in the side of an end surface 15c. The sidewall 14 has a step 15a in the radially outer surface 14t so that the foremost end 15 extends from the step 15a to the end surface 15c of the open end 11. An average diameter at the radially outer surface 15t of the foremost end 15 is smaller than a diameter at the radially outer surface 14t of most of the main portion in the sidewall 14 so as to give a decreased thickness in the foremost end 15 compared to the main portion in the sidewall 14. The most of the main portion of the sidewall 14 excludes grooves 16, 17 formed in the radially outer surface 14t of the sidewall 14. In Fig. 1, the radially outer surface 15t of the foremost end is generally parallel to the axial direction. However, the radially outer surface 15t of the foremost end may not be parallel to the axial direction. In Fig. 1, the step 15a extends in directions parallel to radial directions. However, the step 15a may not extend in directions parallel to radial directions. In the present invention, an average thickness "t" of the foremost end 15 is smaller than a thickness of most of the main portion of the sidewall 14. The thickness "t" in the foremost end 15 may be constant, as shown in Fig. 3. However, the thickness "t" in the foremost end 15 may not be constant. For example, a radially outer surface 15t of the foremost end 15 may be tapered. In the present invention, an engagement boundary 29 is located in a radially inner surface 15s of the foremost end 15 of the sidewall 14. The engagement boundary 29 refers to a boundary between (a) the area in which the radially outer surface 26s of the axle 26 of the ceramic rotor 20 and the radially inner surface 14s of the sidewall 14 of the metallic shaft 10 are in mutual contact and (b) the area in which they are not in mutual contact. Stress concentration tends to occur in the engagement boundary, and a foremost end 15 having a limited thickness relaxes the stress concentration. In Fig. 1, a radially outer surface 26s of the axle 26 is connected to an outer surface of the hub 22 at a connection 25, where a radius of curvature changes to give a small protrusion at the connection 25. However, such small protrusion may not be formed. The radially outer surface 26s of the axle 26 is tapered in a portion close to the connection 25. A radius of the curvature at the tapered surface 26s is larger than a radius of the curvature at the corner 15b of the foremost end 15. In Fig. 1, connection 25 is located at an imaginary extension of the radially outer surface 15t of the foremost end 15. However, connection may be located outside or inside of the imaginary extension of the radially outer surface 15t of the foremost end 15. In the radially outer surface 14t of the sidewall 14 are formed of a groove 16 for receiving a seal ring and a groove 17 for receiving an oil slinger. Both grooves 16, 17 are preferably formed in the entire circumference of the radially outer surface 14t. Grooves 16, 17 have transverse cross sections having a rectangular shape and a semi-circular shape, respectively. However, the shapes of the grooves are not limited in the present invention. In Fig. 5, a foremost end 45 has a radially outer surface 45t, which does not extend parallel to the axial direction. The radially outer surface 45t of the sidewall 44 is tapered so as to give the foremost end 45 having a smaller outer diameter. The sidewall is formed of a step 45a, which does not extend at the right angle to the axial direction. The angle at the step 45a and the axial direction preferably ranges from 90 degrees to 170 degrees. The engagement boundary 49 is disposed at a radially inner surface of the foremost end 45. In Fig. 6, a foremost end 55 has a radially outer surface 55t, which does not extend parallel to the axial direction. The radially outer surface 55t of the sidewall 54 is tapered so as to give the foremost end 55 having a smaller outer diameter. In Fig. 6, the sidewall 54 is not formed of a step, and a radially outer surface 54t of the sidewall is continuous to the radially outer surface 55t in the foremost end 55, being free of a step. An angle at the connection between the radially outer surface 54t of the sidewall 54 and the radially outer surface 55t of the foremost end 55 may range from 135 degrees to 175 degrees, and preferably range from 150 degrees to 175 degrees. The engagement boundary 59 is located in a radially inner surface 55s of the foremost end 55. Examples Example 1 In Example 1, a turbine rotor shown in Figs. 1-3 was made. Firstly, a method of making a ceramic rotor 20 is described. 100 parts by weight of silicon nitride powder having an average particle diameter of 1.0 µm were mixed with 2 parts by weight of SrO 2 powder, 3 parts by weight of MgO powder and 3 parts by weight of CeO 2 powder. To 100 parts by weight of the resulting mixture was added 20 parts by weight of an organic binder, consisting essentially of paraffin wax and having a melting point of 62°C. The resulting mixture was subjected to injection molding to give a molded article having a shape of the ceramic rotor 20 having a blade diameter of 65 mm. The molded article was completely buried in an inorganic powder including activated alumina as a main component. The molded article with the inorganic powder was heated in an electric kiln for removing the organic binder: the temperature was elevated from room temperature to 60°C at a rate of 1°C/hr and kept at 60°C for 30 hours; then elevated from 60°C to 180°C at a rate of 2°C/hr and kept at 180°C for 20 hours; further elevated from 180°C to 450°C at a rate of 3°C/hr and kept at 450°C for 10 hours; and finally, the kiln was cooled from 450°C to room temperature. Then the molded article taken from the inorganic powder was kept at 1,700°C in a nitrogen atmosphere for 1 hour for sintering the molded article. The axle 26 of the sintered article 20 was subjected to grinding with a #270 metal bonded diamond wheel and then to finishing work with a green carbide wheel so as to have an outer diameter of 12.003 to 12.008 mm. The kind and grade of the grinding wheel and the working conditions were determined so that the shaft of the rotor after the finishing step had a surface roughness Ra of 0.1 to 0.8 µm, a roundness of 0.006 µm or less and a cylindricality of 0.012 µm or less. The end of the axle 26 was further subjected to chamfering consisting of C 1 and R 1.5 mm. Then, a method of making a metallic shaft 10 is described. A round bar of 18 mm in diameter, composed of a heat-resistant alloy having low thermal expansion (a trade name of HRA 929 from Hitachi Metals, Ltd.) was cut so as to have a length of 20 mm, and then worked to a round bar of 18 mm in diameter and 15 mm in length. Separately, a round bar of 12 mm in diameter, composed of a structural alloy steel (SNCM 439) was subjected to cold forging to obtain a shaft having a shaft portion 18a, a shaft portion 18b, and an end 19. The resulting two round bars were coaxially bonded by friction welding. The bonded body was subjected to a heat treatment in an electric kiln at 780°C for 6 hours and then at 680°C for 8 hours to undergo aging in the HRA 929 portion so as to hardening thereof and to have a H RC hardness of 38-45. The bonded body was treated with a lathe so as to form a hollow 12 at the end 11 of the HRA portion. The radially outer surface 14t of the sidewall 14 was subjected to grinding so as to form a foremost end 15. The size of the foremost end 15 is shown in Table 1. The radially inner surface 14s of the sidewall 14 was worked so as to have an inner diameter of 11.923±0.02 mm, a surface roughness Ra of 0.1-0.8 µm and a roundness of 0.020 µm or less. The shaft 18a of the SNCM round bar was machined so as to have an outer diameter of 9.5 mm. Using a press-fitting machine, the axle 26 of the ceramic rotor 20 was inserted into the hollow 12 of the metallic shaft 10 to obtain a bonded body. The interference of the engagement was taken so that the outer diameter of the axle 26 was larger by 50-130 µm than the inner diameter of the radially inner surface 14s of the sidewall 14. The bonded body of the ceramic rotor 20 and the metallic shaft 10 was fitted to a lathe, and the metallic shaft 10 was subjected to rough working, followed by finishing working with a grinder. In the radially outer surface of the sidewall 14 were formed a groove 16 for receiving a seal ring and a groove 17 for receiving an oil slinger. The finishing working was applied to the ceramic rotor 20 in the shroud and tip at an end of each blade 24. Examples 2-20 Ceramic rotors and metallic shafts were made in the same manner as in Example 1 except for the size of the foremost end 15 in the open end 11 of each metallic shaft 10. Comparative Examples 1-5 Ceramic rotors were made in the same manner as in Example 1. However, metallic shafts were made without forming the foremost end; that is, the radially outer surface 14s of the sidewall 14 is free of any step, and the sidewall 14 has a constant outer diameter except for the grooves 16, 17. Evaluation A turbocharger including any one of the turbine rotor above was assembled. The turbocharger was fitted to a gasoline engine having a displacement of 2,000 cc. Using an engine dynamometer, the operating conditions of the engine were set so that: the turbine rotates 120,000 rounds per minute; and the temperature of the exhaust gas at the inlet of the turbine became 950°C. The assembly was continuously driven for 200 hours. After the operation, the turbine rotor was removed from the assembly, and its appearance was observed. Then a heat cycle test was conducted. The turbine rotor was heated in an electric kiln at 500°C for 5 minutes. Then, the rotor was taken out of the kiln and cooled to 100°C in one minute by blowing compressed air to the engaging portion of the ceramic rotor and the metallic shaft. A heat cycle consisting of the heating and the cooling was repeated 300 times. After the heat cycle test, a rotary bending test was conducted. However, the turbine rotor that the engagement portion ruptured in the heat cycle test did not undergo the rotary bending test. In the rotary bending test, the shaft 18a, 18b of the turbine rotor was fixed to a jig, and a given load was applied by an autograph to the end of the boss 28 of the ceramic rotor 20. In this condition, the ceramic rotor 20 was rotated at one turn, i.e. 360° at the rate of 1.5 round per minute. The load was started from 60 kgf and gradually increased each time by 20 kgf, and the test was continued until the axle 26 of the ceramic rotor 20 ruptured. Please note that 1 kgf is equivalent to 9.80665 newton. The results of tests are shown in Table 1. In the column of the heat cycle test and the rotary bending test in Table 1, "ruptured" refers that the axle 26 of the ceramic rotor 20 ruptured. In the column of the heat cycle test in Table 1, "cracked" refers that the sidewall 14 of the open end 11 of the metallic shaft 10 cracked. In the column of the rotary bending test in Table 1, "disengaged" refers that the axle of the ceramic rotor 20 was disengaged from the open end 11 of the metallic shaft 10. Examples 11 and 16 have about the same ratio "d/D" of about 1.08 and different ratio "a/t" of 2.00 and 2.55, respectively. Accordingly, the ratio "a/t" is preferably 2 or less. In comparative examples 1-5, each turbine rotor after the heat cycle test had cracks in the radially outer surface 14t of the sidewall 14 in its appearance. In the turbine rotor of the present invention, stress concentration at the engagement boundary is relaxed so as to prevent cracking at the shaft of the ceramic rotor and to increase the bonding strength between the ceramic rotor and the metallic shaft.

Claims: A turbine rotor comprising: a ceramic rotor (20) having a hub (22), a plurality of blades (24) extending from the hub in generally radial directions, an axle (26) coaxially connected to the hub and having a cylindrical radially outer surface, and a boss (28) coaxially connected to the hub at the opposite side from the axle; and a metallic shaft (10) coaxially engaged by press-fitting to the axle (26) of the ceramic rotor (20), the metallic shaft having at one end a hollow (12) receiving said axle (26) and bounded by a sidewall (14), having a cylindrical radially inner surface region contacting said outer surface of said axle (26), said radially inner surface and said outer surface being cylindrical over essentially the whole of their length in which they are in contact, wherein, as seen in axial cross-section, said sidewall (14) bounding said hollow (12) has, from its foremost end closest to said hub (22), (i) initially a first portion (15), (ii) adjoining said first portion (15), a second portion of greater radial wall thickness than said first portion and of greater outer diameter than said first portion, and (iii) a groove (16) in the outer surface of said sidewall spaced from said first portion, and wherein the end closer to the hub (22) of the engagement region of contact between the inner face of said sidewall (14) and the outer face of said axle (26) is located at the inner surface of said first portion (15) of the sidewall (14), and said inner surface of said first portion (15) curves away from the outer surface of said axle at said end of the engagement region. A turbine rotor according to claim 1, wherein: 0.7≤ a t ≤2.5 wherein a is the length of the radially outer surface of said first portion (15) of the metallic shaft; and t is the average thickness of said first portion (15). A turbine rotor according to claim 2, wherein 1.0≤ a t ≤2.0 A turbine rotor according to any one of claim 1 to 3, wherein 1.0≤ d D ≤1.2 wherein d is the average diameter of the radially outer surface of said first portion (15); and D is the average diameter at the radially inner surface of the sidewall (14). A turbine rotor according to claim 4, wherein 1.05≤ d D ≤1.2 A turbine rotor according to any one of claims 1 to 5, wherein the sidewall (14) has a step in its radially outer surface so that said first portion (15) extends from the step to the foremost end of the shaft. A turbine rotor according to any one of claims 1 to 6, wherein the end of the metallic shaft bounding said hollow is comprised of a metallic material having an average thermal expansion coefficient of 6-8x10 -6 /°C from room temperature to 450°C, and a creep strength of not less than 500 Mpa at 450°C over 200 hours. A turbine rotor according to any one of claims 1 to 7, wherein the metallic shaft has a shaft portion bonded to its end bounding said hollow, and the open end is comprised of an age-hardening alloy having a Rockwell hardness H RC of at least 35.

Inventors: KAWASAKI KEIJI, KAWASAKI, KEIJI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1996
Main classification: F01D5
IPC codes: F01D5, F02B39, F02B1, C04B37