Process For Making A Turbine Wheel And Shaft Assembly

Abstract

A process for making a turbine rotor includes steps of providing a shaft made of a first metal composition and having an integral pin projecting axially therefrom, providing a mold that defines a cavity for investment casting a turbine wheel, disposing the pin within the cavity of the mold, pre-heating the mold and the pin to a mold temperature, pouring a molten second metal composition into the cavity of the pre-heated mold such that the molten second metal composition envelopes the pin within the cavity, and controlling the process so that the pin acts as a chill member causing the second metal composition around the chill member to solidify with a finer equiaxed grain structure than would be the case without the chill member. The turbine wheel is joined to the shaft via the casting of the turbine wheel about the pin.

What is claimed is: 
     
         1 . A process for making a turbine rotor, comprising steps of:
 providing a shaft made of a first metal composition, one end of the shaft having an integral pin projecting axially therefrom;   providing a mold that defines a cavity into which a molten second metal composition is to be poured for investment casting a turbine wheel, the cavity being configured for defining a hub portion of the turbine wheel and for defining blades extending from the hub portion;   disposing the pin within the cavity of the mold;   pre-heating the mold and the pin to a mold temperature within a range between a predefined minimum mold temperature and a predefined maximum mold temperature;   pouring the molten second metal composition into the cavity of the pre-heated mold such that the molten second metal composition envelopes the pin within the cavity, wherein the second metal composition differs from the first metal composition; and   controlling the process so that the molten second metal composition at the time of pouring is at a metal temperature exceeding the maximum mold temperature, the pin thus acting as a chill member causing the second metal composition around the chill member to solidify with a finer equiaxed grain structure than would be the case without the chill member,   the turbine wheel being joined to the shaft via the casting of the turbine wheel about the pin.   
     
     
         2 . The process of  claim 1 , wherein the pin is disposed in a region of the cavity that is configured for defining the hub portion of the turbine wheel. 
     
     
         3 . The process of  claim 1 , further comprising, prior to the disposing step, treating an outer surface of the pin to remove any oxide layer and foreign substances thereon. 
     
     
         4 . The process of  claim 1 , wherein the pre-heating step comprises providing a furnace and disposing the mold and the pin within the furnace, and operating the furnace so that an internal temperature within the furnace is within said range. 
     
     
         5 . The process of  claim 4 , wherein the predefined maximum mold temperature is selected to be below the solidus temperature for the second metal composition. 
     
     
         6 . The process of  claim 1 , wherein the second metal composition is selected from the group consisting of nickel-based superalloys and cobalt alloys. 
     
     
         7 . The process of  claim 6 , wherein the second metal composition is selected to be a nickel-based superalloy comprising (in wt %):
 chromium 8-15;   molybdenum 0-5.5;   niobium +tantalum 1-3;   aluminum 5.4-6.5;   titanium 0-1.25;   carbon 0-0.2;   boron 0-0.1;   zirconium 0-0.1;   silicon 0-1;   manganese 0-0.1;   iron 0-5;   unavoidable impurities; and   nickel balance.   
     
     
         8 . The process of  claim 7 , wherein the second metal composition is selected to be a cobalt alloy comprising (in wt %):
 chromium 25-30;   molybdenum 0-1;   tungsten 2-15;   carbon 0.25-3.3;   iron 0-3;   nickel 0-3;   silicon 0-2;   manganese 0-1;   unavoidable impurities; and   cobalt balance.   
     
     
         9 . The process of  claim 7 , wherein the first metal composition of the shaft is a steel. 
     
     
         10 . The process of  claim 9 , wherein the pin includes an outer coating of nickel.