Method of making a turbine engine component

DETAILED DESCRIPTION The present invention relates to a new and improved method of making a turbine engine component with joints between airfoils and a shroud ring free of bonds to enable thermal expansion to occur between the airfoils and the shroud ring.
This is accomplished by forming heat resistant layers around the airfoils.
Each of the heat resistant layers has a melting temperature which is greater than the melting temperature of the material forming the airfoil around which the layer extends.
When molten metal is poured into a mold and flows into a shroud ring mold cavity, the molten metal engages the heat resistant layers.
At this time, the molten metal is at a temperature which is below the melting temperature of the heat resistant layers.
Therefore, fusion bonds do not form between the heat resistant layers and the molten metal as the metal solidifies.
Although the heat resistant layers could be formed in many different ways on airfoils having many different compositions, it is preferred to form the heat resistant layers on nickel-chrome superalloy airfoils.
This is done by heating a portion of the airfoil which is to be exposed to molten metal.
Thus, the portion of the nickel-chrome superalloy airfoil which is engaged by the molten shroud ring metal is heated to a temperature above 1,093.
degree.
C.
in an atmosphere containing oxygen (air).
This results in the formation of a chromium sesquioxide (Cr.
sub.
2 O.
sub.
3) layer having a characteristic green oxide color, around the end portion of the airfoil.
Simultaneously with the forming of the green chromium sesquioxide layer on the outside of the airfoil, a heat resistant inner layer is formed.
This inner layer results from a depletion of chromium and other elements, from the nickel-chrome superalloy metal forming the airfoil.
Although the inner layer has a lower melting temperature than the green chromium sesquioxide outer layer, the inner layer has a higher melting temperature than the nickel-chromium superalloy metal forming the airfoil