materials Article The Accuracy of Finishing WEDM of Inconel 718 Turbine Disc Fir Tree Slots Jan Burek 1 , Robert Babiarz 1 , Jaroslaw Buk 1, * , Pawel Sulkowicz 1 and Krzysztof Krupa 2   Citation: Burek, J.; Babiarz, R.; Buk, J.a.; Sulkowicz, P.; Krupa, K. The Accuracy of Finishing WEDM of Inconel 718 Turbine Disc Fir Tree Slots. Materials 2021, 14, 562. https://doi.org/10.3390/ma14030562 Academic Editor: Yong-Cheng Lin Received: 28 December 2020 Accepted: 20 January 2021 Published: 25 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Mechanical Engineering and Aeronautics, Department of Manufacturing Techniques and Automation, Rzeszów University of Technology, 35-959 Rzeszów, Poland; jburek@prz.edu.pl (J.B.); robertb@prz.edu.pl (R.B.); sulkowicz@prz.edu.pl (P.S.) 2 Faculty of Mechanical Engineering and Aeronautics, Department of Materials Science, Rzeszów University of Technology, 35-959 Rzeszów, Poland; krupa@prz.edu.pl * Correspondence: jbuk@prz.edu.pl Abstract: Servicing aircraft engines sometimes requires manufacturing only a single piece of a given part. Manufacturing a turbine disc using traditional methods is uneconomical. It is necessary to use a different machining method recommended for small lot production. One of the proposed methods is WEDM (wire electrical discharge machining). The article presents the results of the research on finishing WEDM of Inconel 718 turbine disc fir tree slots. The influence of infeed, mean gap voltage, peak current, pulse off-time, and discharge energy on the shape accuracy, surface roughness, microcracks, and the white layer thickness were determined. Mathematical models were developed based on the DoE (Design of Experiment) analysis. The statistical significance of the models was verified with the ANOVA (Analysis of Variance) test. The machining parameters control methods that allow achieving the required shape accuracy, surface roughness, and surface layer condition were presented. The obtained surface roughness was Ra = 0.84 μm, the shape accuracy of the slot in the normal-to-feed direction was Δd = 0.009 μm, the profile shape accuracy was Δr = 0.033 μm, and the thickness of recast (white) layer was approximately 5 μm. Keywords: WEDM; finishing machining; Inconel 718; fir tree slot 1. Introduction Electrical discharge machining (EDM) is an alternative to traditional machining meth- ods. The method consists of removing the material due to electrical discharges between the tool and the workpiece. As a result, EDM can be utilized in machining conductive materials regardless of their hardness [1,2]. EDM is widely used in manufacturing injection molds and in the automotive industry [3]. The continuous development of this machining method and innovations in the design of EDM machine tools allow applying the method in the medical sector and aerospace industry as well [4–6]. Wire electrical discharge machining (WEDM) is a type of electrical discharge machining in which the tool is an electrode in the form of a wire. The most commonly used is a brass wire of a diameter of 0.02 to 0.5 mm. The machining is conducted in a dielectric liquid and consists of removing the material as a result of melting and evaporation [7–9]. WEDM is a trepan type technology in which the erosion of the small amount of the machined material allows removing significant blank volume [10]. The occurrence of microcracks in the surface layer and the formation of a recast (white) layer [11] on a workpiece surface due to high temperature are notable shortcomings of the electrical discharge machining [12]. Considering the above, the method has not been commonly used in the manufacturing of critical machine parts, which include, i.e., an aircraft engine turbine rotor [13]. Depending on the accuracy and surface roughness, the machining of rotors may be only rough or can include a few finishing passes [14]. Finishing machining allows applying lower electrical parameters, which results in a lower discharge energy and reduced heat impact on the Materials 2021, 14, 562. https://doi.org/10.3390/ma14030562 https://www.mdpi.com/journal/materials