Inconel 718 constitutes more than 50% of modern aircraft engines’ structural components [1]. Due to its superior properties, such as creep, oxidation, hot corrosion resistance, and high hot hardness, it withstands vigorous operating conditions in high-temperature engine cores [2]. The machining process for Inconel 718 includes drilling, face milling and turning. Among the all, face milling is known for a high-quality surface finish, producing more precise components for very minimal dimensional tolerance applications [3]. However, the typical machinability rating of Inconel 718 is between 0.09–0.3, which is less than 0.4, 1.2, and 1.9 for stainless steel 304, Al 6061 and 7075 alloys, respectively [4]. Hence, Inconel 718 is considered a hard-to-machine Ni-based alloy because of the high affinity to form a built-up edge (BUE), to react with tool’s elements, and to undergo precipitation hardening [5]. Such complex metallurgical properties facilitate tool wear mechanisms, which mainly include adhesion, abrasion, diffusion, and oxidation [6]. These mechanisms cause rapid tool deterioration, covering a spectrum of damage scales from micro-chipping to gross fracture [7]. More details of wear mechanisms and failure modes during face milling of Inconel 718 have been pursued in Ref. [8]. Flank wear is considered as a dominant failure mode that determines the tool life when face milling Inconel 718, and is primarily caused by the abrasion wear mechanism [9]. It evolves in three stages namely early rapid wear, uniform wear, and failure region [10]. The unprecedented failure modes in the failure region, such as chipping, flaking, BUE, and notching, affect the surface finish and dimensional tolerance of the components. As such, an optimal face milling process allows flank wear retaining in the uniform region and minimizes these unprecedented failure modes. The uniform flank wear width (VB) ranging between 200 to 500 µm is thus considered as minimum and maximum tool life, as presented by ISO 8688-1 standard.
Currently, the modern multi-layer-coated tools are widely applied to avoid rapid VB progression because they have a high hardness, elastic modulus, and plasticity [11], and thus replace uncoated or single-layered coated tools (Liu et al., 2022). The multi-layer-coated inserts with fine-crystalline TiAlN primary layers are known for Inconel 718 face milling operation [13]. On top of that, cutting condition optimization is a key to achieve the highest tool performance [14]. Research suggested that parameters of the cutting speed, feed rate, and axial depth of cut (ADOC) significantly affect tool performance in the Inconel 718 face milling operation (Xavior et al., 2016). Nevertheless, these parameters have synergistic effect to VB [16]. Owing to the lack of the physically-based mathematical correlation between the cutting parameters and VB, modulating these parameters via empirical methods is time-consuming and costly (Kosaraju et al., 2018). It is thus necessary to build up a model to predict VB progression in order to avoid unprecedented tool failure.
Recently, deep learning (DL) and machine learning (ML) model are considered as powerful methods to decipher and explore the complex underlying physics of the materials science and engineering [18], including quality prediction in manufacturing [19], effective charge in electromigration effect [20], dielectric constant and dissipation factor in low temperature co-fired ceramics [21], irradiation embrittlement in steel [22] etc. More relevantly, direct tool wear detection of physical vapor deposition (PVD)-coated carbide inserts by using a convolution neural network (CNN) model using image features is one of the approach to characterize failure occurrence [23]. In pursuing a direct VB prediction during facing milling, Kaya et al. [24] applied an artificial neuron network (ANN) model to predict VB of single-layered PVD-TiAlN coated inserts (i.e., R390-11 T3 08M-PL 1030) using the input features of cutting speed, feed, ADOC, time, force, and torque of a 5-axis CNC milling center. The model achieved a correlation coefficient (R2) of 0.99, and a mean relative error of 5.42% in testing the validation data set not used in fitting the model. Although it is shown that ML method is applicable in predicting VB, the model was built based on data where a single-layer-coated tool was used. To the best of the authors’ knowledge, there has been very few studies in developing VB prediction model using ML methods for PVD-multi-layer-coated inserts during face milling of Inconel 718. On the other hand, the existing physically-based models can neither decipher the underlying curvature of VB progression nor design better cutting conditions to minimize rapid tool failure [25]. Therefore, this research focuses on developing a ML model to extrapolate the VB progression of multi-layer PVD-TiAlN/NbN coated carbide inserts during face milling of Inconel 718. Unlike the NN-based models presented in previous research, this work used Gaussian kernel ridge regression (GKRR) model, which is powerful in predicting unknown data that is in the vicinity of given data in the training data set, to predict the VB progression. In addition, less hyperparameters are used in GKRR than conventional NN-based models, and therefore would help to eliminate overfitting issue over a small dataset and in the meantime speed up the training process. The present ML model was applied to design a new promising cutting condition which exhibited a good cutting efficiency. The model is thus applicable in both predicting and optimizing the tool performance for Inconel 718 face milling real operation, demonstrating the potential contribution to the intelligent manufacturing systems according to industry 4.0.