Nickel-based superalloys are typical materials used in components of aeroengines and gas turbine machinery. The strength properties of these alloys at high temperatures are crucial not only to the performance (e.g., power generation efficiency, energy consumption, and greenhouse gas emissions) of aeroengines and industrial gas turbines, but also to machinability during component manufacturing. This study comparatively evaluated the surface integrity of two superalloys, AD730^® and Inconel 718 (IN718), during high-speed finishing turning using cubic boron nitride (CBN) tools. IN718 is a conventional superalloy used for the hot section components of aeroengines and industrial gas turbines, while AD730^® is a novel superalloy with enhanced high-temperature mechanical properties and good potential as a next-generation superalloy for these components. High-speed turning tests of two superalloys were conducted using a CBN cutting tool and jet stream cooling. The achieved surface integrity of the AD730^® and IN718 superalloys was characterized and analyzed to assess the comparability of these alloys. 

The novel wrought nickel-based superalloy, AD 730™, is a good candidate material for turbine disc applications at high temperatures beyond 650 °C. The present study focuses on the machining performance of this newly developed alloy under high-speed turning conditions with advanced PCBN tools. Meanwhile, the machined surface integrity as influenced by cutting speed and feed rate was also investigated. The surface integrity was thoroughly characterized in terms of surface roughness and morphology, machining-induced plastic deformation, white layer formation, and residual stresses. It has been found that the cutting speed and feed rate had a strong effect on the cutting forces and resultant surface integrity. The cutting forces required when machining the alloy were gradually reduced with increasing cutting speed, while at 250 m/min and above, the flank tool wear became stronger which led to increased thrust force and feed force. A higher feed rate, on the other hand, always resulted in higher cutting forces. Increasing the cutting speed and feed rate in general deteriorated the surface integrity. High cutting speeds within the range of 200–250 m/min and a low feed rate of 0.1 mm/rev are preferable in order to implement more cost-effective machining without largely reducing the surface quality achieved. The formation of tensile residual stresses on the machined AD 730™, however, could be of a concern where good fatigue resistance is critical.

AD730® is a new type of nickel-based superalloy, developed by Aubert & Duval, with high temperature properties exceeding conventional superalloys such as Alloy 718. This paper presents the tool life model for this superalloy for the purpose of process optimization and manufacturing cost estimation. The tool life modelling is based on comprehensive machining tests, using the Colding model. The cutting tool material used is CBN 170, a grade specially designed and optimized for machining of nickel-based superalloys.

A higher gas turbine efficiency can be achieved by increasing the operating temperature in hot sections. AD730™ is a recently-developed wrought/cast nickel-based superalloy which can maintain excellent mechanical properties above 700 ℃. However, machining of AD730™ could be a difficult task like other nickel-based superalloys. Therefore, studies are needed with respect to the machinability of this new alloy.

In this paper, high-speed turning was performed on AD730™ using polycrystalline cubic boron nitride (PCBN) tools and coated tungsten carbide (WC) tools at varied cutting speeds. The surface integrity was assessed in two important aspects, i.e., surface and sub-surface plastic deformation and residual stresses. The PCBN tools generally showed better performance compared with the WC tools since it led to reduced machining time without largely compromising the surface integrity achieved. The optimal cutting speed was identified in the range of 200-250 m/min when using the PCBN tools, which gives rise to a good combination of machining efficiency and surface integrity. The further increase of the cutting speed to 300 m/min resulted in severe and deep plastic deformation. Meanwhile, a continuous white layer was formed at the machined surface. When turning with the WC tools, the increased cutting speed from 80 m/min to 100 m/min showed very little effect with respect to the plastic deformation on the machined surface. It was found that tensile residual stresses were developed on all machined surfaces no matter when the PCBN or WC tools were used, and the surface tension was generally increased with increasing cutting speed. The tensile layer might need to be modified by e.g., post-machining surface treatments such as shot peening, if taking good fatigue performance into consideration.

The current study focused on the surface integrity issues associated with broaching of Inconel 718 and the structural stability of the broached specimen at high temperatures, mainly involving the microstructural changes and residual stress relaxation. The broaching operation was performed using similar cutting conditions as that used in turbo machinery industries for machining fir-tree root fixings on turbine disks. Thermal exposure was conducted at 723 K, 823 K, and 923 K (450 A degrees C, 550 A degrees C, and 650 A degrees C) for 30, 300, and 3000 hours, respectively. Surface cavities and debris dragging, sub-surface cracks, high intensity of plastic deformation, as well as the generation of tensile residual stresses were identified to be the main issues in surface integrity for the broached Inconel 718. When a subsequent heating was applied, surface recrystallization and alpha-Cr precipitation occurred beneath the broached surface depending on the applied temperature and exposure time. The plastic deformation induced by the broaching is responsible for these microstructural changes. The surface tension was completely relaxed in a short time at the temperature where surface recrystallization occurred. The tensile layer on the sub-surface, however, exhibited a much higher resistance to the stress relief annealing. Oxidation is inevitable at high temperatures. The study found that the surface recrystallization could promote the local Cr diffusion on the broached surface.

Subsurface microstructural alterations and residual stresses caused by machining significantly affect component lifetime and performance by influencing fatigue, creep, and stress corrosion cracking resistance. Assessing the surface quality of a machined part by characterizing subsurface microstructural alterations and residual stresses is essential for ensuring part performance and lifetime in aero-engines and power generators. This comparative study characterizes and analyzes subsurface microstructural alterations and residual stresses in Inconel 718 subjected to high-speed machining with PCBN and whisker-reinforced ceramic cutting tools. Effects of cutting tool materials and microgeometry on subsurface deformation, microstructural alterations, and residual stresses were investigated. Surface and subsurface regions of machined specimens were investigated using X-ray diffraction, electron channeling contrast imaging, and electron back-scatter diffraction to characterize microstructural alterations and measure deformation intensity and depth.

Inconel 718 is a nickel based superalloy that is widely used as a turbine disc material in gas turbine industries. This study details the effect of thermal exposure on the residual stresses produced when broaching Inconel 718. The chosen parameters for broaching in this study are similar to those used when manufacturing turbine discs. The broaching operation produced a high level of tensile residual stresses at the broached surface. A layer with tensile residual stresses was formed in the sub-surface region, followed by a layer several times thicker with compressive residual stresses. Thermal exposure was conducted at 550 °C. The depth distributions of residual stresses after thermal exposure are presented and discussed in this paper. Complete relaxation of the surface tensile residual stresses was observed after 30 h thermal exposure, whereas the 3000 h thermal exposure influenced both the surface and sub-surface residual stress states.

Gas turbine discs operate mostly at high temperature gradients and are subjected to mechanical loads simultaneously. The high thermal and mechanical loads eventually could result in degradation and damages in disc material, thereby increasing the risk of disc failure. In this study, a damage analysis was performed in a retired gas turbine disc made of Inconel 718. Oxidation attack and microstructural degradation as the consequence of the high service temperature have been found to be the main damages that take place in the non-contact area of the retired disc. In the blade/disc contact area, fretting fatigue occurs, with a result that cracks initiate from the oxide/metal interface and propagate in the disc alloy parallel to the sliding direction of fretting, consequently reducing the stability and safety of the disc. Meantime, oxygen diffuses into the fretting fatigue cracks, thereby exacerbating the oxidation attack. A multi-layered scale with periodic formation of the Fe-oxide/spinel layer and the metallic layer is formed on the contact surface. In both contact and non-contact area, recrystallization and α-Cr precipitation take place in the surface layer of the disc alloy. The locations where α-Cr precipitates are commonly considered to be the natural sites for mechanical weakness.