Microstructural characteristics of the built up layer of a precipitation hardened nickel based superalloy by electrospark deposition

Buildup of precipitation hardened nickel base superalloys by electro spark deposition due to the low heat input of the process has many attractions. Characterization of the microstructure of the ESD built up layer of IN738LC over an as-cast base metal is accomplished in this work. The grain structure and solidification texture of the coating are investigated by orientation imaging microscopy (OIM), optical and scanning electron microscopy. It is shown that the deposited layer is formed mainly through epitaxial nucleation and growth on the base metal structure while discontinuities acting as nucleation sites produce fine grains with independent orientations. It is shown that such independent grains can have a significant role in improving the integrity of the ESD built up layer, since they can act as crack arrest sites and make the coating more resistant to the propagation of liquation and solidification fissures. Moreover, it is found that nanosized γ′ precipitates exist in the coating indicating the high tendency of γ′ for precipitation even in the extremely high cooling rates involved in the ESD process. Hardness measurements indicated a higher hardness for the built up layer which is attributable to the finer microstructure of the coating.