Pulsed magnetic field treatment of TiAlSiN-coated milling tools for improved cutting performances

In this study, a pulsed magnetic treatment was attempted to improve the cutting performance of the TiAlSiN-coated WC-12wt%Co cemented carbide end mills, and the effects of the strength of the pulsed magnetic field on the cutting forces, the cutting vibrations, the tool wear, the machined surface roughness, and mechanical properties were investigated. It is found that the cutting performances of the coated tools are successfully improved with a relatively lower cutting force and less wear area. The average resultant cutting force Fxyave\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${Fxy}_{ave}$$\end{document} decrease by 14.53% in the last machining process when the optimum processing parameters of 0.5 T magnetic field are used, accompanying a maximum decrease of 46.8% in the cutting vibration. The maximum reductions of 57.65% and 25.4% in the flank wear and the average surface roughness of the workpiece are obtained respectively after the treatment. Both the hardness and toughness of the cemented carbides are slightly improved with the imposition of the field. The improvements in the cutting performance of the tool are attributed to the enhanced adhesion strength between the coating and matrix, which is caused by the increased compressive residual stress induced by the PMT.


Introduction
With the development trend of high-speed, high-precision, and high-efficiency cutting in the modern machining industry, cutting tools require increasingly exceptional cutting performances to maintain an excellent machining accuracy, a good surface finish, and a high-efficiency and low-cost production [1]. Previously, many studies were conducted to improve the cutting performance of cutting tools and they mainly focused on reducing tool wear, extending tool life, and improving the surface quality of processed workpieces. The commonly used method in the past was the heat treatment [2] and it usually took dozens of hours to improve the overall microstructure during the heat treatment, thereby improving the physical and mechanical properties of cutting tools to enhance their cutting life. Although the heat treatment process could treat samples on a large scale, it is usually time-consuming [3] and incurs a high cost. In recent years, as an emerging technology in manufacturing research, the pulsed magnetic treatment (PMT) receives enormous attention since it can modify the material to improve the microstructures and performances of the materials in a very short time [4]. Many investigations then used the PMT to treat cutting tools of different materials such as HHS [5], cemented carbide [3], ceramic metal [6], and super-hard materials [7] for enhanced cutting performances. The modification effects induced by the PMT on the cutting performance are usually associated with an increased surface hardness, a reduction in friction, an extended wear life, a delayed crack growth, and an improved residual stress distribution [3]. Ma et al. [5] treated M42 HHS micro-cutting tools using the PMT and found that lattice distortion and the precipitation of a large number of dispersed carbides occurred in the tool material. Bataineh et al. [8] applied pulsed magnetic fields to HHS twist drills and demonstrated an increase in tool lifetime by as much as 35% in aggressive cutting conditions. Similar to the HHS tool, the introduction of the PMT renders the cemented carbide tool with an increasing dislocation density, which tended to strengthen the matrix and improve the cutting performance. Liu et al. [3] studied the effect of PMT on the milling performance of cemented carbide end mills, and found that pulsed magnetic treatment tools could significantly reduce the milling force and reduce wear by 35% when milling a low-carbon steel. The significant improvement in the milling performance was attributed to the increase in the Vickers hardness and transverse rupture strength (TRS). Yuan et al. [9] studied the relation between the coupled fields and the cutting performance of the cemented carbide tool and it is believed that residual stress, magnetostriction effect, and the field-induced defects were closely associated with the tool life. Besides the cutting tools, the PMT is found to be effective in the modification of other materials. Fahmi et al. reported an increased fatigue life of low-carbon steels for cyclic loading after the treatment. Cai and Huang [10] showed that the pulsed magnetic field could reduce the residual stress of Cr4Mo4V steel and Yip and To [7] found that the super-hard diamond tools treated by pulsed magnetic field could reduce the adhesive wear and improve the machined surfaces of titanium alloy.
Previous literatures indicate that the PMT can improve the microstructures and performances of the cutting tools. However, to the best knowledge of the author, there are scarce investigations on the treatment of the coated tool using the pulsed field. Therefore, in the presented study, the PMT was introduced to treat the TiAlSiN-coated WC-12wt%Co cemented carbide end mills to evaluate the effects of the pulsed field on the cutting performance of the tool. The results obtained are expected to provide insights for the potential applications of this post-treatment in the industry of coated tools.

Cutting tool and workpiece material
Milling experiments were performed using commercially replaceable TiAlSiN-coated end mills of WC-12wt%Co cemented carbides (Dongguan FULLANTI Tools Co., Ltd, China). The tool geometry and the key properties of deposited TiAlSiN layer are summarized in Table 1. The workpiece employed in the milling experiment is AISI 316L stainless steel with a size of 20 × 10 × 3 cm 3 and the mechanical properties of AISI 316L stainless steel are shown in Table 2. To evaluate the interface properties between the coating and the matrix, a TiAlSiN coating is

Pulsed magnetic field treatment
A schematic diagram of magnetic field treatment process with a 0.5 field intensity is given in Fig. 1 and the direction of pulsed magnetic field is perpendicular to that of the milling tool. The pulsed magnetic field with a specific waveform was introduced by adjusting the coil current and the TiAlSiN-coated cemented carbide end milling tools (WC-12wt%Co) were magnetized 20 times with intensities from 0.5 to 1.5 T with a pulse interval of 10 s.

Milling experiments
The milling experiments were conducted on AISI 316L stainless steels using a TEMA VL1060A CNC vertical machining center and the milling parameters are compiled in Table 3. A typical linear cycle cutting method was employed in this study and the distance of each feeding was 100 mm. After each cutting was completed, the tool returned to the starting point, and this feeding process was repeated 20 times along the axial direction until the axial cutting depth reached 6 mm.

Measurement and processing of the cutting force signal
A Kistler 9257B three-way dynamometer with a Kistler 5019A multi-channel charge amplifier was installed on the AISI 316L stainless steel workpiece through four bolts and was used to obtain the cutting force component values in three directions ( F x , F y , F z ) during the cutting process at a sampling frequency of 5 kHz. Figure 2 shows the experimental step and note that the axial cutting depth ( a p = 0.3mm ) is less than the radial cutting depth ( a e = 2.0mm ) and the tool diameter ( D = 4.0mm ); therefore, the axial cutting force F z is much smaller than the horizontal component force (the feed force F x , the cross-feed force F y ), and can be considered negligible in this study. The resultant cutting force F xy and Fxy ave can be calculated by Eq. (1) (N is the number of cutting force data points).
Milling is a typical multi-edge intermittent cutting process. During the cutting process, the periodic changes of cutting force and chip thickness will produce the cutting vibrations, which can be manifested and reflected by the fluctuation of the cutting force component during the machining process [11]. In the experiment, the signal characteristic value (feature energy) was extracted to reflect the tool wear, as suggested by the report [6]. In the present work, the discrete horizontal cutting force signals were decomposed over j = 3 levels using Daubechies 5 wavelets, which produced 2 3 different sets of coefficients j m (t) for a data vector with n data points, where m = 1, 2, …, 2 3 , and the length of where x m,k is the amplitudes of the discrete points of the reconstruction signal S j,m (m = 1, 2, …, 2 3 ; k = 1, 2, …, n; n is the number of discrete sampling points of the signal).

Characterization
After milling experiments, a scanning electron microscope (SEM) equipped with an energy dispersion spectrometry (EDS) was used to analyze the final morphology and elemental distribution of the flank worn surface of the TiAl-SiN-coated milling tools. The surface topography of the workpiece and surface roughness (Ra) values along the feed direction was measured by a BRUKER Contour GT-K, and the residual stress developed in the TiAlSiN coating was characterized by X-ray residual stress analysis equipment (Japan Pulstec's model μ-X360). The adhesion strength between the TiAlSiN coating and the cemented carbide substrate was tested by Rockwell indentation with a load of 60 kg, and the indentation morphology was observed by an optical microscope (OM). Furthermore, a scratch tester (MFT-4000) was employed to quantitatively analyze the adhesion strength between the TiAlSiN coating and the WC-12%wtCo cemented carbide substrate under applied load from 0 up to 150 N with a loading speed of 100 N/ min. The scratch tracks were examined by an OM to identify the coating failure morphologies and the critical load was confirmed by both the scratch tracks. Vickers hardness and indentation fracture toughness values were obtained using a Vickers hardness tester on WC-12wt%Co cemented carbide samples with the size of 20 × 5 × 5mm 3 . At least five indentation points are tested for the reliable result.

Cutting forces
Cutting force is an indicator of the state of tool wear during machining and Fig. 3 shows the average cutting force Fx ave , Fy ave , and Fxy ave during the steady-state cutting for the untreated and treated samples. It can be seen that there is hardly appreciable change in the cutting force of Fx ave for all samples with the increasing cutting length due to a very short period in the X direction. Contrary to the Fx ave , the cross-feed cutting force Fy ave for all tools increases progressively with the increase of the cutting distances (Fig. 3b) and the cutting force for the untreated sample increases more sharply than that for treated samples. In addition, 0.5 T treated sample exhibits the slowest increases in the cutting force. It is widely known that tool wear increases with increasing cutting distances, and the tool cutting edges become blunted and lose their original geometries gradually. The deterioration of the tool geometry leads to an increase in the cutting force [12] and therefore, it is expected that the wear area for the untreated sample will be significantly higher than that for the treated samples. The variation of the resultant cutting force Fxy ave with the cutting distance shown in Fig. 3c is almost the same as that of the Fy ave (Fig. 3b). This is mainly due to the fact that the cutting period in the Y direction is much longer than that in the X direction and the feed in the Y direction during the cutting process is much larger than that in the X direction. As a result, the F y is much greater than F x during the cutting period. Similar to what observed in Fy ave , the tool treated with a 0.5 T field presents the lowest value in the resultant cutting force and the values of Fxy ave for the 0.5 T and 1.0 T treated tools are 14.53% and 3.61% lower than the value of Fxy ave for the untreated tool respectively in the last feed.

Cutting vibrations
As the tool wears, the cutting process becomes unstable and the cutting vibration increases. Based on the wavelet packet decomposition (WPD), the main cutting force F y during the last feed was decomposed and reconstructed in the time-frequency domain, and the static cutting force characteristics and dynamic cutting vibration characteristics related to tool wear characteristics were extracted through high and low frequency separation. In addition, the wavelet packet coefficient energy value (feature energy) of the m − th signal band calculated according to Eqs. (1) and (2) and the total feature energy for all tools during the last feed were used as the indicator of tool wear and cutting stability as illustrated in Fig. 4. It can be seen that for all tools, the feature energy in the E1 and E2 bands accounts for more than 95% of the total feature energy and the feature energy is mainly distributed in the first frequency band (E1). The reconstructed signal in E1 band implies the static cutting forces and the reconstructed signal in the high frequency band (E2) suggests the dynamic cutting vibrations caused by periodic cutting during milling process. The feature energies in the E1 band for all treated tools are lower than the feature energy for the untreated tool. In addition, tools treated by 0.5 T and 1.0 T fields exhibit significant lower energies compared to the untreated tool. These differences in feature energies of the static cutting forces correspond exactly to the variation of the cross-feed cutting force Fy ave in Fig. 3c. Comparing the feature energy of the second frequency band (E2), it can be found that the degree of the dynamic cutting vibrations has decreased after PMT and in particular, the cutting vibration energy for the tool treated by a 1.0 T field is the lowest, which is 46.8% lower than that of the untreated tool. For the total feature energy, since the feature energy of the static cutting forces is much greater than that of the dynamic cutting vibrations, the tool treated by the 0.5 T field still has the lowest energy, and is decreased by 27.09% as compared to the untreated tool during the last feed. These results indicate the PMT provides a more stable cutting process at a lower vibrational energy, as reported previously by Yuan et al. [9].

Tool wear
As an important indicator of the milling performance of the tool [13], flank wear is examined and their morphologies are shown in Fig. 5. As each end mill is comprised of two long cutting edges and two short cutting edges, typical failure modes of the cutting edge are provided for the analysis. It can be seen from the Fig. 5 that nose wear (the normal flank wear), build-up edge (BUE), and chipping and fracture of the cutting edge are noted after the cutting process. In the case of untreated tools (Fig. 5.a1), it can be observed obviously that there is a large area of coating delamination and flaking occurred on the flank face and the severe unevenness of the coating along the cutting edge may in turn initiate ploughing instead of shearing during the cutting and led to the catastrophic tool fracture, as shown in Fig. 5.b1. It is expected that the poor cutting edge integrity and non-uniform of the coating along the cutting edge lead to greater ploughing forces, which in turn, cause immediate catastrophic fracture of the tool [14]. In the sharp contrast, the tool treated by the 0.5 T field exhibits the significantly smaller wear area of the coating and slight chipping wear, which suggests that a significant portion of TiAlSiN coating still retains on the end mill surface. As tribo-oxidation behavior causes the TiAl-SiN coating to form an alumina surface protective layer with high chemical and thermodynamic stability, which critically helps prevent the stainless steel from sticking to the tool, coatings remaining on the flank face will reduce the amount of the BUE formed during the cutting process. The increase in the strength of the field treatment to1T and 1.5 T leads to increasing flaking area of the coating and the occurrence of the chipping. However, compared with the untreated tool, the chipping area of the cutting edge of the tool treated by 1.0 T and 1.5 T field is reduced. In general, the untreated tool presents severe tool wear and the significant distortion of cutting edge shapes weakens the strength of the cutting edge, resulting in deteriorated cutting performance and inferior service life. In contrast, the treated tools exhibit relatively less wear and maintain a better cutting edge integrity. It can be inferred that the degree of wear of tools field is reduced after the PMT and consistent with the result of the cutting force (Fig. 3). What should be noted is that the relative movement between the workpiece and the tool promotes the cyclic removal of BUE, and the formation and tearing off of the BUE during cutting usually lead to the coating delamination or flaking, and the machining forces instability, which results in cutting edge chipping [15,16], as evident in Fig. 5.a1. Moreover, the cyclic formation and disappearance of the BUE caused by its periodic fractures will further lead to the cutting instability, which causes the cutting vibration and different tool wears [17].
VB max and VB max values of four cutting edges for each tool are compiled in Fig. 6 based on SEM images to further evaluate the cutting performance of tools. According to ISO standard, the flank wear value of 0.3 mm is used as the standard of tool wear failure. Note that the differences of the VB max values for the same tool are very large as four cutting edges in the end mill possess varying lengths. It can be seen that the VB max values of tools treated by pulsed magnetic field decrease dramatically in comparison with that of untreated tool. In particular, the shape of most cutting edges for the tool treated by the 0.5 T field remains relatively intact and presents the smallest wear area, which results in a 57.65% decrease in the VB max .

Surface roughness
Surface roughness is one of the most important factors affecting the appearance of the product, reliability, and functionality [18]. The average surface roughness (ASR, Ra) is a common measure for evaluating the surface texture, and the three-dimensional surface morphology and average surface roughness of the workpiece are shown in Fig. 7. It can be observed that the surface exhibits a typical milling surface morphology with a semicircular ridge formed by the circular motion of the cutting edge in the feed direction. The treated tools all show a relatively smoother surface morphology and smaller surface roughness. As indicated previously, the loss of the flank edges (catastrophic cutting edge fracture) under The wear widths of the flank face ( VB max and VB max ) after different treatments dynamic vibrations tends to cause higher radial roughness (Ra) values and lead to more noticeable feed mark embossments [19]. Therefore, the better workpiece surface finish is attributed to the better preservation of the sharpness of the cutting edge. In particular, surface morphology produced by the 0.5 T treated tool shows the smoothest surface, and compared with the untreated tool, the average surface roughness of the tool treated by the 0.5 T field is reduced by 25.4%. As the smaller profile height between surface peaks and valley can translate into a smoother and stronger machined surface with better dimensions and mechanical properties [19], the difference in the surface roughness is attributed to the fact that a less wear tool with a better integrity of the cutting edge tends to introduce a smaller ploughing effect and a better surface quality.

Residual stress
The above results indicate that 0.5 T field is the optimum processing parameter for improving the cutting performance of the coated tool. Therefore, the parameter was adopted for further comparison analysis. In most applications, a key factor in the service life and performance of coated components is the adhesion of the coating to the substrate. The premise of the protective function of surface coatings on cutting tools is that the coating has a good adhesion strength to the tool substrate during the service process [20]. The above results clearly indicate PMT can indeed improve the cutting performance of the coated tools and the improvement in the performance is closely associated with the adhesion strength of the TiAlSiN coating to the substrate. The indentation test is employed to preliminarily evaluate the adhesion strength of the interface between the TiAlSiN coating and the cemented carbide substrate and the result is provided in Fig. 8. It can be seen that a large area of the coating spallation around the indent is observed for the untreated sample, in addition to several large and long cracks and many short cracks. In comparison, the degree of the coating spallation and the formation of cracks are greatly reduced after the PMT, implying a better adhesion strength of the coating to the substrate induced by the pulsed field. For further analysis, the scratch experiment is employed to quantitatively analyze the adhesion strength (bonding force), and the scratch morphologies are shown in Fig. 9. At the beginning of the scratch, the load is very low and the scratch is smooth due to the defects such as droplets and pits on the coating surface. As the load increases, transverse cracks begin to occur at the edges of the coating. Lc2 in Fig. 9 represents the load value where the coating is continuous cracking along the scratch track which shows the real adhesion [21]. For the WC-12wt%Co sample deposited with TiAlSiN coating, a value of Lc2 equal to 83.24 N is obtained before the treatment (Fig. 9a), whereas Lc2 achieves the value of 92.02 N (Fig. 9b) after PMT, which indicates a 10.55% increase in the bonding force and enhanced adhesion between the coating and this substrate with the introduction of the pulsed field. According to the previous literature [22], the residual stress developed in the coating structure is an important factor affecting the adhesion between a coating and a substrate. In addition, since the compressive stress counteracts the crack initiation and propagation in the coating, they are considered advantageous for loaded tools, and a higher compressive stress implies a longer service life of coated tools [23]. The residual stress developed in the TiAlSiN coating of the tools is examined and the results are provided in Fig. 10. It can be clearly seen that the residual stress developed in the TiAlSiN coating of the untreated sample is a compressive residual stress with a magnitude of 421Mpa; after PMT (0.5 T), the compressive residual stress increases to 679Mpa, representing a remarkable 61% improvement. The increase in the residual compressive stress is expected to enhance the bonding strength between the TiAlSiN coating and the cemented carbide substrate, which is consistent with the adhesion strength result in Fig. 8. Due to the enhancement of the adhesion strength after PMT, more TiAlSiN coating remains on the cutting edge to participate in subsequent cutting with less substrate exposure, which will be beneficial for the reduction of friction and thermal energy dissipation at the cutting zone. It is obvious that the reductions of cutting force and cutting heat effectively slow down the accumulation of deformation and internal damage in the cemented carbide substrate. As mentioned before, a higher temperature and a higher cutting force are the main reasons that cause the adhesion of work piece material onto the tool flank face, thus accelerating the tool wear. This enhancement of the adhesion strength will indirectly lead to the alleviation of BUE formation and shedding, which can avoid further damage to the non-shedding coating and the aggravation of the cutting vibrations. These factors (lower BUE formation and lower progressive build-up), coupled with the alleviation of the internal damage in the cemented carbide substrate while cutting, lead to the reduction of the probability of tool edge chipping and fracture, which can cause a lower ploughing force and a relatively small cutting vibration. Therefore, the enhancement of the adhesion strength of the TiAlSiN coating after PMT reduces the formation of BUE and the cutting vibrations, and chipping and fracture of the cutting edge are less prone to occur, resulting in a longer tool life and a better cutting performance.

Mechanical properties of the cemented carbides substrate
The Vickers hardness and fracture toughness of the cemented carbides matrix before and after the PMT are compiled in Fig. 11 to assess the mechanical properties of the substrate material. It can be seen in the figure that the Vickers hardness of the carbide cemented material exhibits a marginal increase after PMT, while the indentation fracture toughness is increased substantially by 8% with the imposition of a pulsed magnetic field. Previous reports indicate that one of the major underlying mechanisms is such that the introduction of the pulsed magnetic field tends to promote the formation of the dislocations within the cemented carbides to strengthen the material. As a result, hardness of the materials presents a slight increase. Fracture toughness is a characterization of the ability of a material to prevent crack propagation. When a crack is formed, it tends to propagate along a path with the least resistance and usually presents a fast progress rate within the brittle materials. As the pulsed field strengthens the cemented carbides matrix, it is expected that the resistance to the crack path is enhanced, which reduces the crack growth rate. Accordingly, not only can the pulsed field alter the interface residual stress, but also can strengthen the substrate material to improve the toughness while maintaining the hardness. As a result, chipping and fracture of the cutting edge are less prone to occur after the PMT treatment.

Conclusion
This work used a pulsed magnetic field treatment to enhance the service life and cutting performance of the TiAlSiN-coated WC-12wt%Co cemented carbide tool and the improved performances were evaluated by cutting forces, the cutting vibrations, the tool wear, and the machined surface roughness of the workpiece. The obtained results indicate that 0.5 T field is the optimum processing parameter, under which the average resultant cutting force Fxy ave decreases by 14.53% in the last machining process. A reduction of 57.65% in the flank wear VB max is obtained after the PMT, with a reduction of 46.8% in the cutting vibration. The better cutting edge integrity of field treated tool weakens the ploughing effect on the surface of the workpiece, thereby obtaining a better surface finish of the workpiece. The compressive residual stress developed in the interface between the TiAlSiN coating and the cemented carbides substrate is considered the main reason to enhance the cutting performance. These obtained results demonstrate that the proposed PMT is an effective and fast way to improve the lifetime and cutting performance of the coated tool, and can be beneficial to the future industrial application of the low strength, easyto-produce, and controllable magnetic fields.
Funding This work was supported by the National Nature Science Foundation of China (no. 51975390) and partially supported by Sichuan University and Yibin Municipal People's Government University and City strategic cooperation special fund project (grant no. 2020CDYB-29) and the Key Research and Development Program of Science and Technology Department of Sichuan Province (no. 2020YFS0575, no. 2021KJT0012-2021YFS0067).

Data availability Not applicable.
Code availability Not applicable.

Declarations
Ethics approval The authors declare that this manuscript was not submitted to more than one journal for simultaneous consideration. Also, the submitted work was original and has not been published elsewhere in any form or language.
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Competing interests
The authors declare no competing interests.