Evaluation of WEDM Performance Characteristics of Inconel 706 for Turbine Disc Profile Application

Abstract

Nickel-iron-based superalloys are categorized as an exceptional class of structural material. These superalloys contain both nickel and iron as base elements and characterized by the high phase stability of FCC austenitic matrix. These superalloys exhibit excellent mechanical properties such as high tensile strength, excellent creep, improved fatigue life, good surface stability, resistance to degradation in corrosive and oxidizing environments. Therefore, these superalloys are best suited for manufacturing of gas turbine components. The machining of these superalloys has become an active area of research due to their growing demands in aircraft and power generation turbines. These superalloys typically constitute around 40–50 % of the total weight of an aircraft engine and most extensively used in the combustor and turbine sections of the engine where elevated temperatures are maintained during operation. The turbine disc is amongst the most critical components in an aero engine which includes a number of complex slots to fix the turbine blades. The combined assembly of turbine disc and blade is located in a hot gas stream from which mechanical power is extracted to drive the compressor, gearbox and other accessories of aero engine. Since Inconel 718 was being used in manufacturing of turbine disc in aircraft engines for more than 35 years, with the new arrival of advanced gas turbines with working firing temperature of 1260 °C, it became necessary to develop the advanced superalloy (i.e., Inconel 706) with improved fabricability along with high mechanical strength. Machining of this superalloy to a very close tolerance and producing a high surface finish is essential for achieving superior performance of turbine disc. Owing to high quality requirement of turbine disc such as complex profile slots along with high dimensional accuracy (within the range of ±5 µm) and also excellent surface finish (surface roughness less than 0.8 µm), conventional machining process seems to be ineffective for turbine disc slot production. The conventional machining of nickeliron-based superalloys also exhibits poor machining performance due to high chemical affinity, strong work-hardening tendency.To overcome these issues, non-conventional machining methods such as laser beam machining (LBM), electrochemical machining (ECM), abrasive water jet machining (AWJM), and electrical discharge machining (EDM) are effectively implemented for machining of these superalloys. However, there are certain issues with nonconventional machining processes such as micro-cracking, poor surface quality, low dimensional accuracy and significant recast layer formation in the LBM process; more chance of corrosion due to acidic electrolyte, comparatively low MRR and require special shaped electrode in the ECM process; impingement of abrasive particles into matrix, crack propagation and burr formation at the edge in the AWJM process; less recast layer formation compare to LBM and require special shaped electrode in EDM process. To develop the gas turbine components, an efficient manufacturing process is required for aerospace and power generating industries. Wire electrical discharge machining (WEDM) is an advanced version of EDM capable of manufacturing components with intricate shapes and sharp edge profiles, which is difficult to be obtained by other machining process. Moreover, it eliminates the need of special shaped electrode and reduces the recast layer thickness (RLT) significantly with the use of low discharge pulse. Additionally, WEDM is more efficient than the EDM in terms of flexibility and offers low residual stresses on the machined component. In the past few years, WEDM allowed success in the production of gas turbine components which required complex shaped profiles with high precision. The high degree of dimensional accuracy and better surface quality of the machined components make WEDM valuable. The main aim of this study is to evaluate the WEDM performance characteristics of Inconel 706 for turbine disc application. To achieve the feasibility in manufacturing of turbine disc profile slots, the current research work has been divided into four parts. In first part, one factor at a time approach was used to understand the effect of various control parameter such as pulse on time, pulse off time, servo voltage, wire feed, servo feed and flushing pressure on WEDM performance characteristics. In second part, the effect of wire materials and wire diameters on WEDM performance characteristics (i.e., cutting speed, surface roughness, surface topography, recast layer,subsurface microhardness, microstructural and metallurgical changes) have been evaluated by considering the significant control parameters and different discharge mode. In third part, turbine disc profile slots were machined successfully on Inconel 706 superalloy as per the standard of gas turbine industries. Moreover, the various WEDM performance characteristics of profile slots such as cutting speed, surface roughness, subsurface microhardness, surface topography, recast surface, crystal structure, residual stresses, profile accuracy, microstructural and elemental changes have been evaluated. In fourth part, the modeling and optimization of WEDM performance characteristics have been carried out by considering optimum wire material as well as optimum wire diameter. The mathematical models for MRR and SR have been developed using response surface methodology (RSM) followed by backward elimination method. Then, teaching learning based optimization (TLBO) algorithm was used for individual as well as multi-objective optimization. Finally, Pareto optimal solutions have been obtained at different weightage which might be beneficial to gas turbine manufacturing industries. The manufactured turbine disc profile slots have shown low level of tensile residual stresses (less than 850 MPa), average surface roughness less than 0.8 µm, profile accuracy within the range of ±5 µm, almost negligible recast layer, minimum hardness alteration, no micro cracks, and no thermal alterations while using hard brass wire of diameter 150 µm followed by appropriate trim cut strategy.