Turbine rotor for supercharger and manufacturing method thereof

1. A turbine rotor for a supercharger comprising:

a TiAl turbine wheel and a carbon steel shaft joined to each other via an Ni brazing filler metal at a brazed part distanced from a back face of the turbine wheel so that a turbine wheel outer diameter ratio calculated by “a distance from the back face of the turbine wheel to the brazed part” / “an outer diameter of the turbine wheel” is within a range of from 0.07 to 0.1, wherein a maximum temperature at the brazed part is less than 60% of a melting point of the Ni brazing filler metal.

2. The turbine rotor for a supercharger according to claim 1 , further comprising:

a back plate at a back face side of the turbine wheel along the back face with a gap between the back plate and the back face so as to prevent an exhaust gas that leaks from an inlet side toward an outlet side of the turbine wheel from flowing into a joint part of the Ni brazing filler metal.

3. The turbine rotor for a supercharger according to claim 1 ,

wherein the brazed part is disposed on a position such that the turbine wheel outer diameter ratio is substantially 0.08.

4. A manufacturing method of manufacturing a turbine rotor for a supercharger where a TiAl turbine wheel and a carbon steel shaft are joined to each other via an Ni brazing filler metal, comprising the steps of:

measuring an outer diameter of the turbine wheel;

setting a distance from a back face of the turbine wheel to a brazed part so that a turbine wheel outer diameter ratio calculated by “a distance from the back face of the turbine wheel to the brazed part” / “an outer diameter of the turbine wheel” is in a range of from 7 to 10%; and

joining the TiAl turbine wheel and the carbon steel shaft via the Ni brazing filler metal at a position away from the back face by the distance set in the setting step.