The effect of Electro Spark Deposition on the microstructure and mechanical properties of IN718

Abstract

The Electro Spark Deposition is a technique of continuing interest to repair or coat the surface of expensive components used in the aerospace industry. A potential use for this technique is to repair damaged turbine blades made with nickel-based superalloys, such as IN718. Nickel-based superalloys are susceptible to hot cracking and post weld heat treatment cracking during conventional processes, due to the high heat input that contributes to issues mainly observed in the heat affected zone of the components. An interesting approach to reduce thermal issues in nickel-based superalloys is to reduce the heat input during welding or posterior treatments. In this case, the use of the ESD technique to repair expensive parts made with nickel-based superalloys is a possibility, since this process has the advantage to generate minimal heat input onto the substrate. In order to better understand the inherent defects of this method and the microstructure and properties achieved through the ESD process, a systematic study of the effects of the main variables in this process is a necessity, including but not limited to the voltage, capacitance and spark frequency. In this work, IN718 deposited using ESD with different pulse energy and frequency, achieved by varying the voltage, capacitance and pulse frequency, was systematically investigated in terms of its microstructure, microhardness and tensile properties. It has been observed that the increasing pulse energy and frequency in the ESD process promotes the formation of internal voids and intermetallic phases that may be undesirable in the process of repairing or coating of components. The presence of intermetallic phases in a certain amount and distribution is believed to increase the susceptibility for cracking of the deposited layers. Tensile samples repaired with conditions that present less internal defects showed ductility closer to the ductility observed for the parent material.