Gas turbine engines typically have stators to direct the compressed high temperature gas against the turbine blades. Stators are comprised of an annular array of airfoils or vanes interposed between inner and outer shroud rings. Usually, the three components are cast from the same material making the vane integral with the shroud rings at the top and bottom edges of the vanes. During transient conditions, such as start up and shut down of the engine, the gas temperature rapidly changes. Because a larger portion of the vane relative to the shrouds is exposed to the gas, it respond more quickly to the changes in gas temperature. Thus, when heated faster then the shrouds, the vanes become susceptible to large thermal compressive stress because the vanes want to expand but are constrained by the shroud rings. Similarly, when cooled, a large tensile stress is created across the vane which wants to contract.
The thermal stresses are particularly high in the thin, trailing and leading edges. The cyclic nature of the thermal stresses make the vanes highly susceptible to low cycle fatigue cracking. Therefore, it is desirable to have vanes with good low cycle fatigue properties which tend to be expensive.
Bicasting is another method of forming a turbine stator. This method includes casting shroud rings around the tip and root edges of prefabricated vanes. The advantage to bicasting is that the vanes and shroud rings can be formed from materials having different compositions and crystallographic structure. This permits the use of single crystal or columnar grained crystallographic vanes which have low elastic modulus and good low cycle fatigue properties in the direction of primary stress.
U.S. Pat. No. 4,728,258 discloses a bicast turbine stator having a vane configured for mounting with a slip joint between the vane and the shroud ring to accommodate the thermal expansion of the vanes. The slip joint is produced by printing or stamping through the shroud ring which reduces its strength. Also, with slip joints, the hoop stress in the shroud ring must be carried by the portions of the ring surrounding the slip joint and adjacent the leading and trailing edges of the vanes. Not only does this reduce the amount of material available for carrying the hoop stress but compounds the problem by producing large stress concentrations at the leading and trailing edges.
U.S. Pat. No. 5,069,265 discloses a bicast turbine stator in which the shroud ring is strengthened by the addition of a rail which carries a portion of the hoop stress. A space is maintained between the rail and the shroud ring to accommodate the thermal expansion of the vanes. However, the rail adds weight to the shroud ring, increases the thermal mismatch between the vanes and the shroud ring, and increases the thermal stress in the shroud ring.
Thus disadvantages to slip joints are reduced material available in the shroud ring for carrying hoop stress, stress concentrations adjacent the vanes leading and trailing edges, radial space taken by the shroud rings and rails, increased weight, increased thermal mismatch between the vanes and shroud rings, and increased thermal stress in the shroud rings.
Accordingly, there is a need for a stator vane that when bicast to shroud rings increases the stator's structural integrity, and reduces thermal stresses.