PSI - Issue 53

A. Teixeira et al. / Procedia Structural Integrity 53 (2024) 352–366 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Inconel 718 is a nickel-based superalloy, that is widely used in the aircraft, nuclear industry and automotive (Pedroso et al., 2024; Sebbe et al., 2024) due to its exceptional thermal resistance and the ability to retain its mechanical properties at high temperatures (De Bartolomeis et al., 2021; A. Thakur & Gangopadhyay, 2016), even when compared to other nickel-based alloys (Hong et al., 2001). These components are usually produced via machining processes, primarily turning and milling. Nickel based superalloys are known to be difficult to cut materials (Hosseini & Popovich, 2019) due to their high shear strength, high work hardening tendency, highly abrasive carbide particles in the microstructure, strong tendency to weld and form a built-up edge and low thermal conductivity (Buddaraju et al., 2021). This is particularly true for Inconel 718 alloy, that causes high amounts of tool wear for low cutting lengths, usually leading to premature tool failure. When machining this alloy, the material tends to adhere to the cutting tools’ surface (Sousa et al., 2023), generating high cutting force values during the process (Toubhans et al., 2020) and causing considerably high cutting temperatures, estimated to reach at values around 1100 ºC (Agmell et al., 2020). This high amount wear can also be attributed to the abrasive behaviour that this alloy exhibits during cutting. This is due to the presence of hard carbides in the metallurgical structure of Inconel 718, mainly Titanium Carbides (TiC) and Niobium Carbides (NbC), as evidenced in the study performed by Polvorosa et al., 2017. In the study, the authors also evaluate wear mechanisms present in cutting tools after machining, reporting on abrasive wear and built-up-edge. These wear phenomena, can generate mechanical, metallurgical, and topographical changes to the material’s surface, thus influencing the surface integrity of the machined part (Cantero et al., 2018). As such, machining Inconel 718 can prove to be quite challenging, with high amounts of tool wear being observed, which can negatively impact the quality of the produced parts, not only in terms of high values of surface roughness, but also in terms of causing residual stresses to the machined surface layer, due to plastic deformation and high heat generation (Dudzinski et al., 2004). To mitigate these problems, coated carbide tools can be used, as these can highly improve the machining process. For example, TiAlN-based coatings (Sousa, Da Silva, et al., 2021) are usually employed in highly demanding machining applications, improving surface quality as well as reducing tool-wear (increase in tool-life (Sousa & Silva, 2020)), or even by reducing the generated cutting forces during the process. These cutting forces provide relevant and valuable information regarding the performance of the machining process and its respective stability. Cutting forces generated during the machining process can even be related to the machining surface quality of the workpiece as well as the wear sustained by the employed machining tools (Sousa et al., 2020; Sousa, Silva, et al., 2021). The evaluation of tool-wear and tool-wear mechanisms is quite important, particularly from the process optimization standpoint (Pedroso et al., 2023). It provides useful information on the cutting behaviour, enabling not only the understanding of the cutting process as well as for the adjustment of the tool itself, either geometry or coating (Sebbe et al., 2024). The study to optimize the machining processes for Inconel 718, namely milling and turning, is still quite relevant, particularly due to the high specificity and precision of the parts required by the industries where it is applied (Sousa et al., 2023). Additionally, there seems to be a gap in the literature regarding studies performed about finishing turning operations performed on Inconel 718. Machining studies are usually performed by optimizing the machining parameters (Ribeiro da Silva et al., 2021), focusing on improving machining production quality, minimizing tool-wear (extending tool-life) or processing time. Studies such as these are commonly carried out experimentally, however, these can be quite expensive, due to equipment and consumable usage (such as tools or cutting fluids). As such, the simulation of this process is quite an appealing subject, as it mitigates the cost problems associated with practical testing (Paturi et al., 2021). Furthermore, these simulations can provide useful information regarding developed cutting forces (Kumar et al., 2017), wear (Yadav et al., 2015), and cutting temperature, with some limitations being felt due to the dynamic of the turning process, as mentioned by Rajashekhar Reddy et al., 2017. In this work an experimental approach towards machinability assessment of the Inconel 718 nickel-based alloy is led, through cylindrical turning instrumented tests using TiAlSiN coated WC-Co tools. The conducted experiments consisted of using two values of feed-rate, 0.1 and 0.2 mm/rev, and three different values of cutting speed, 50, 100 and 150 m/min and evaluating their influence on the sustained tool-wear, developed cutting forces and machined surface quality. Additionally, tool-wear mechanisms were identified and evaluated. Numerical simulations of the turning process were performed to compare and validate the experimental results. The numerical approach was conducted using the finite element method in DEFORM ® 2D/3D commercial software.

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