Method of producing radial type ceramic turbine rotor

A method of producing a radial type ceramic turbine rotor having a shaft, a hub connected thereto, a blade portion radially extending from the hub whose end on an air exit side extends from an end of the blade portion on the air exit side, comprises steps of injection molding a ceramic material into a ceramic rotor body and machining extra thicker portions of the molded rotor to a predetermined shaped turbine rotor. According to the invention, the molded rotor is formed by the injection molding so as to be at least 0.5 mm thicker than a predetermined thickness at a fillet between the hub and the blade portion on the air exit side and the molded rotor is then machined by a diamond wheel to remove the extra thicker portions until the thicker fillet becomes to the predetermined thickness to from a predetermined shaped turbine rotor which is free from cracks to have a high strength.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of producing a high strength radial type 
ceramic turbine rotor which is free from cracks in the proximity of a 
connection between a hub and a blade portion on an air exit side of the 
rotor. 
2. Description of the Prior Art 
Recently, silicon ceramic materials such as silicon nitride, silicon 
carbide, sialon and the like have been noticed as structural materials for 
gas turbine, diesel engines and the like, because of their light weight 
and superior heat and thermal shock resistance. Particularly, radial type 
turbine rotors made of these ceramic materials have been highlighted for 
applications to gas turbine rotors or turbo charger rotors for 
automobiles, because these ceramic rotors are lighter and are capable of 
being used at much higher temperatures than metal turbine rotors and are 
superior in thermal efficiency to the metal rotors. Moreover, because the 
radial type turbine rotors are complicated in shape, they are usually 
molded by an injection molding process, or the like, which forces the 
ceramic materials into narrow or curved portions or corners of the 
complicated shapes. 
In order to form the rotors by an injection molding of the ceramic 
materials, a great amount of plasticizer such as resin, wax or the like 
must be added into the ceramic material. When the injection molded ceramic 
rotor body is heated or sintered to remove the resin or wax, the shape in 
the proximity of a connection between a hub and a blade portion on an air 
exit side so rapidly changes that the resin or wax added as the 
plasticizer is not uniformly removed. Accordingly, the molded body is not 
uniform in density, so that locally different shrinkages occur in the body 
during sintering, resulting in tensile forces which lead to cracks, 
particularly in the connection between the hub and blade portion on the 
air exit side. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved method of producing 
a radial ceramic turbine rotor of high mechanical strength which 
eliminates the above disadvantages of the prior art methods. 
This object can be achieved by the method according to the invention of 
producing a radial type ceramic turbine rotor consisting of a shaft, a hub 
connected threreto, a blade portion radially extending from said hub whose 
end on an air exit side extends from an end of said blade portion on the 
air exit side, said method comprising the steps of injection molding a 
ceramic rotor body having a thicker fillet than a predetermined thickness 
at a connection between said hub and said blade portion on the air exit 
side, and machining said thicker fillet portion of said molded rotor body 
until said thicker fillet equals its predetermined thickness. 
The invention will be more fully understood by referring to the following 
detailed specification and claims taken in connection with the appended 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates one example of a ceramic turbine rotor having a thicker 
fillet portion according to the present invention. The ceramic turbine 
rotor 1 consists of a shaft portion 2, a hub 3, and blade portion 4 
radially extending from the hub 3. A gas is exhausted by the rotor in a 
direction shown by an arrow in the drawing in which direction an end of 
the hub 3 extends somewhat from an end of the blade portion 4. A reference 
numeral 5 illustrates an arcuate fillet formed at a circular connection 
between the hub 3 and the blade portion 4. 
The ceramic turbine rotor is produced in the following manner. The rotor 
consists of a material selected from silicon nitride, silicon carbide, 
sialon or substances which convert to these materials by sintering. This 
starting material is mixed with a plasticizer such as a resin, wax or the 
like and further preferably is added and mixed with at least one sintering 
aid such as Y.sub.2 O.sub.3, MgAl.sub.2 O.sub.4, MgO, CeO.sub.2, SrO or 
the like in case of the silicon nitride. Sintering aids such as Be, Al, B, 
C or the like can be added to silicon carbide to obtain a raw material for 
molding. This prepared raw material is injection molded by the use of 
injection molding dies to obtain a molded rotor body whose fillet 5 is 
thicker by an amount l, as shown in FIG. 1 by solid lines, than the 
predetermined thickness of an ultimate rotor product as shown by the 
broken lines in FIG. 1. The value l at the maximum thickness is not less 
than 0.5 mm, preferably not more than 5 mm, more preferably in the order 
of 1-3 mm. In this case, the fillet is preferably concave in a 
longitudinal section passing through a rotating axis of the rotor. 
However, it may be flat or convex. 
The molded rotor body is heated up to 300.degree.-600.degree. C., to remove 
the plasticizer, such as the resin and wax or the like, at a heating rate 
slower than 100.degree. C./hr, preferably slower than 10.degree. C./hr. 
These heating conditions are dependent upon the kinds and amounts of 
plasticizer contained in the body. After the removal of the plasticizer, 
hydrostatic pressure is applied to the rotor body after presintering. 
The presintering is effected at 800.degree.-1,200.degree. C. in order to 
facilitate handling the rotor body and give it a requisite strength for 
machining. After the plasticizer is removed, hydrostatic pressure is 
applied, the molded rotor body is covered with an elastic bag and then a 
hydrostatic pressure of 500.degree.-5,000 kg/cm.sup.2 is applied to the 
covered rotor body to increase the density of the rotor body. Thereafter, 
the molded rotor body is sintered for 10-200 minutes at a temperature of 
for example, 1,600.degree.-2,200.degree. C., which is sufficient to sinter 
the body completely, depending upon the raw material. When the starting 
material is silicon nitride, silicon carbide or sialon, or the starting 
materials are substances which produce these materials, important 
parameters for obtaining good sintering results are not only the sintering 
temperature, but also the sintering. For example, a nitrogen atmosphere is 
used for the silicon nitride and an argon atmosphere is used for the 
silicon carbide. 
Thereafter, the sintered rotor body is machined to reduce the thicker 
fillet portion to its predetermined size and shape. The machining may of 
course be effected after the heating for removing the plasticizer or after 
the presintering. Jigs or tools may be suitably selected according to 
materials, shapes or sintered degrees of the injection molded bodies. For 
example, conventional cutting tools or diamond wheels may be used for 
machining the bodies after heating for de-plasticizing or presintering, or 
diamond wheels may be preferably used after sintering. The machining of 
the thicker fillet portions is preferably made so as to form the 
aforementioned fillet whose radius of curvature, in section, is greater 
than 0.5 mm in an ultimately finished condition. 
According to the invention, a perfect rotor without any cracks can be 
obtained because of less localized difference in contraction due to 
uniform density resulting from the uniform removal of the plasticizer and 
a gradual variation in thickness obtained by injection molding a thicker 
fillet at the connection between the hub and blade portion on the air exit 
side of the rotor. Moreover, the larger the radius of curvature of the 
fillet of a finished rotor body, the thinner the fillet of a molded rotor 
body can be. On the contrary, the smaller the radius of curvature of the 
fillet, the thicker the fillet should be. At any rate, the thickness l of 
the fillet must be more than 0.5 mm. 
If the fillet of an injection molded rotor body is less than 0.5 mm, the 
aforementioned uniform removal of the plasticizer and hence uniform 
density of the body cannot be achieved to obviate the differential 
contraction at the fillet and the rotor will contain undesirable cracks. 
The effect of the invention will be explained with reference to examples 
hereinafter. 
EXAMPLE 1 
For preparing a raw material for injection molding, the following materials 
were mixed and kneaded, 100 parts by weight of powder Si.sub.3 N.sub.4 
having an average grain diameter 0.5 .mu.m, 3 parts by weight of MgO, 2 
parts by weight of SrO and 2 parts by weight of CeO.sub.2 as sintering 
aids and 15 parts by weight of polypropylene resin as a plasticizer. The 
thus obtained material was injection molded with dies to form ceramic 
turbine rotors each having a thickness l of 2 mm at the fillet for 
producing radial type turbine rotors each having a maximum blade diameter 
60 mm (after firing) and a radius of curvature 0.5 mm of the fillet at a 
connection between a hub and blade portion on an air exit side of the 
rotor. Thereafter, the molded rotor bodies were heated to 500.degree. C. 
at a heating rate 5.degree. C./hr and further heated at 500.degree. C. for 
10 hours to remove the plasticizer. The bodies were then sintered in a 
nitrogen atmosphere at 1,720.degree. C. for 30 minutes. No crack occurred 
in the fillets or at any other portion in the sintered rotor bodies. The 
rotor bodies were ground by diamond wheels to remove the thicker fillet 
portion of each rotor until the radius of curvature of each fillet was 0.5 
mm. In this manner, radial type ceramic turbine rotors were obtained, each 
of which was a perfect rotor without any cracks having a predetermined 
shape and a 0.5 mm radius of curvature at the fillet between the hub and a 
blade portion on the air exit side of the rotor. In order to compare 
therewith, the same raw material was injection molded with dies having 0.5 
mm rounded edges which directly corresponded to the final fillet size and 
rotor bodies without thicker fillets were produced. Cracks having a length 
of about 5 mm occurred in ends of hubs after removal of the plasticizer 
therefrom. 
EXAMPLE 2 
A raw material for injection molding was prepared by mixing and kneading 
100 parts by weight of powder SiC having an average grain diameter 0.2 
.mu.m, 2 parts by weight of boron, 2.5 parts by weight of carbon black and 
10 parts by weight of wax as a plasticizer. The raw material was injection 
molded with dies to obtain rotor bodies 1, 2 and 3 having respective 
thicknesses of 2, 1 and 0 mm at fillet portions threof. Each of the radial 
type ceramic turbine rotors each had a maximum blade diameter of 110 mm 
(after firing) and a radius of curvature of 2 mm at the fillet portions 
between the hub and blade portions on an air exit side of the rotor. 
Thereafter, the molded rotor bodies were heated to 400.degree. C. at a 
heating rate 3.degree. C./hr and further heated at 400.degree. C. for 5 
hours to remove the plasticizer. The bodies were then sintered in an argon 
atmosphere at 2,100.degree. C. for 60 minutes. The sintered rotor bodies 
Nos. 1 and 2 were ground by diamond wheels to remove the thicker fillet 
portions until radii of curvatures at the fillets became 2 mm, to obtain 
rotors without any cracks. Furthermore, the sintered rotor bodies No. 3 
which did not have thicker fillet portions, exhibited cracks of about 3 mm 
in the proximity of the fillets. 
In order to carry out rotating tests, the rotors Nos. 1, 2 and 3 were 
dynamically balanced up to a dynamic unbalance of 0.05 g.cm. After metal 
shafts had been secured to the rotors, they were again adjusted up to a 
dynamic unbalance of 0.0005 g.cm. The rotors were then tested on a 
rotation testing machine progressively increasing its rotating speed. Both 
the rotors Nos. 1 and 2 did not rupture even at 70,000 RPM, while the 
rotors No. 3 commonly ruptured at approximately 40,000 RPM. 
As can be seen from the above description, according to the invention, a 
rotor body is formed by injection molding so as to have a thicker fillet 
than a predetermined thickness at a connection between a hub and a blade 
portion on an air exit side of the rotor and then machined to remove the 
thicker portions to obtain a high strength ceramic turbine rotor 
completely precluding cracks, which is suitable for a turbo-charger rotor 
for diesel and gasoline engines and a rotor for gas turbine engines and 
very useful for the industry. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that the foregoing and other changes in form and detail 
can be made therein without departing from the spirit and scope of the 
invention.