This invention relates to a diffusion bonding method to be adapted to a superalloy based on Ni, Co, Fe or Ti.
Recently, gas turbine blades have been produced by joining structural members consisting of superalloys together in a predetermined configuration. In bonding the superalloys, however, they must have the same properties as those of the superalloy base metals and hence, no definite bonding method has yet been established. Fusion welding or high temperature brazing using a Ni-based brazing material has generally been employed. However, strict limitations exist in applying the fusion welding techniques to turbine structural members and a number of structural elements can not be adapted simply because of their complicated shapes.
Though brazing offers a number of advantages in comparison with fusion welding, it has very limited application because the brazed joint portion has relatively low strength and its melting point is low. Diffusion bonding, which involves the movement of the atoms between the contact base metals, is known as a bonding method having high reliability. To practise this method, however, severe requirements must be satisfied. That is, the surface coarseness of the joint surfaces must be minimized, the joint surfaces must be completely cleaned and a non-joint interface must be eliminated. To satisfy the last mentioned requirement, diffusion treatment must be carried out under high pressure contact conditions for an extended period of time.
To facilitate the finish of the joint surface and to improve the joint strength, there have been proposed a method which arranges a powder insert material between the base metal surfaces to be joined and carries out diffusion bonding until the insert material finishes the reaction with the base metals (U.S. Pat. No. 3,632,319) and a method which places a foil-like insert material between the base metals to be joined and carries out diffusion bonding until the insert material finishes the reaction with the base metals (U.S. Pat. No. 3,678,570).
When the foil is used as the insert material in accordance with the prior art methods, two interfaces exist between the base metals and the insert material and when the powder is used as the insert material, an extremely great number of interfaces exist between them. The greater the number of the interfaces between the joint surface of the base metals and the reactive metallic insert material, the severer becomes the pre-treating condition of the base metal surface and the longer becomes the time required for diffusion treatment. Incidentally, diffusion treatment is carried out for about 24 hours in order to obtain the joint portion having substantially the same characteristics as those of the base metals.