SUMMARY

Inconel 718 is a high nickel content superalloy possessing high strength at elevated temperatures and resistance to oxidation and corrosion. It is the most commonly used nickel base superalloy and is utilized in a variety of aerospace applications. The non-traditional manufacturing process of wire-electrical discharge machining (EDM) possesses many advantages over traditional machining during the manufacture of Inconel 718 parts. However, certain detrimental effects are also present. The top layer of the machined surface is melted and resolidified to form what is known as the recast layer. This layer demonstrates microstructural differences from the bulk workpiece, resulting in altered material properties. An experimental investigation was conducted to determine the main machining parameters which contribute to recast layer formation in Inconel 718. It was found that average recast layer thickness increased with increasing energy per spark, peak discharge current, current pulse duration, and open-voltage time and decreasing sparking frequency and table feed rate. Over the range of parameters tested, the recast layer was observed to be between 5 and 10 µm in average thickness, although highly variable in nature. Surface roughness of the cut parts showed an increase with increasing energy per spark. Electron Probe Microanalysis revealed the recast layer to be alloyed with elements from the wire electrode. X-ray diffraction testing showed the residual tensile stresses evident near the cut surface to decrease with increasing energy per spark. Additionally, nano-indentation hardness testing indicated that the recast layer is reduced in both hardness and elastic modulus compared to the bulk material. Vibratory tumbling was examined as a simple and low cost post-processing tool for recast layer removal. Results were promising when using pre-formed ceramic abrasive media for large surfaces and fine grained aluminum oxide media for small internal features.