Tensile Tests – Acoustic Emission
Since the acoustic emission (AE) signals were monitored throughout the tests, they could be compared with the stress-strain curve to extract the deformation phases present in the material from the time-domain AE signal. In the frequency spectrum corresponding to the elastic phase of the AISI 4340 steel (Fig. 4), the highest peak appears at a frequency of 150 kHz, followed by the peaks at 160 kHz, 170 kHz and 130 kHz.
The frequency spectrum of the plastic deformation phase of the AISI 4340 steel (Fig. 5) showed excitation at the same frequencies as the elastic deformation, but with reduced amplitudes. According to Physical Acoustics [14], the excitation frequency band of steel in the elastic phase lies in the region of 40 kHz to 200 kHz, whereas for the plastic phase excitation appears in the region of 350 kHz to 450 kHz. The fact that the amplitudes decreased but did not reach zero is due to the presence of a region of elastic deformation in the specimen.
The frequency spectrum for the fracture of the AISI 4340 steel (Fig. 6) shows one frequency range of 90 kHz to 110 kHz and another of 140 kHz to 155 kHz. The excitation band at 90 kHz to 110 kHz appeared in preliminary machining tests on other materials carried out by the authors. This indicates that its elimination could compromise the evaluation of wear mechanisms and phase changes. The band at 140 kHz to 155 kHz was used as the band-stop filter for the analysis of the AE data obtained in the turning tests, the elimination of this band resulting only in the signal related to the wear mechanisms.
Turning tests – Acoustic Emission
The short-time Fourier transform (STFT) technique is able to identify variations in the excitation mechanism, since it monitors the signals in both domains (time and frequency). This analysis is interesting, as it verifies whether there is any variation in the frequency over the data collection time, since the acoustic emission signal is highly dynamic and sensitive to the excitation mechanism.
Based on studies published in the literature, several mechanisms excite different frequencies in the spectrum of the AE signal. In machining, because of the dynamic cutting action, the tool undergoes wear and the formation of the chip varies the shape and the phase of the material causing a variation in the AE spectrum. The frequency range observed in machining is from 70 to 115 kHz [17]. White layer formation excite frequencies above 60 kHz [18]; isothermal phase transformation excites a frequency range from 250 to 350 kHz [19]; mild adhesive wear frequencies up to 120 kHz [20]; sliding from 25 to 110 kHz [21]; dislocation movement from 10 to 220 kHz [22]; particle interaction from 120 to 350 kHz [23]; abrasive wear from 200 to 1000 kHz [15, 24]; crack propagation from 350 to 550 kHz [1, 18]; phase transformation from 350 to 550 kHz [19]; vacancies accommodation from 220 to 380 kHz [25]; dislocation annihilation excites the AE signal on 100 kHz [4]; Frank-Read dislocation on 1000 kHz [4]; plastic deformation on 50 kHz [4] and from 150 to 500 kHz [22]; elastic deformation excite the AE signal in the range of 25 to 250 kHz [22] and thermal noise from 10 to 100 kHz [4].
Due to the various tribological mechanisms deriving from the wear of the tool and since these mechanisms operate at different frequencies, the acoustic emission signals in the turning tests are discrete and time variant. Thus, the use of STFT is a relevant option to demonstrate these mechanisms and their joint performance in the evolution of cutting tool wear.
In this work, STFT is used to detect and map the mechanisms which influence tool wear. Analyzing the STFT signal generated in the AE signal in turning AISI 4340 steel with a cutting speed of 250 m/min, feed rate of 0.25 mm/rev and depth of cut of 0.75 mm in a tool without end coating (Fig. 7), it is noted that the peak of greater amplitude is located in the frequency of 253kHz in the time of 0.75s. This is probably due to the interaction of particles since it is a K-class end-of-life tool in contact with steel, which denotes a diffusive mechanism. It is also noticed the frequency range of 90kHz to 170kHz excited during the entire period acquired from the diffusive mechanism and the movement of discordances. A line in the 35 kHz range is also excited and denotes sliding mechanisms. There is also a low amplitude range in the order of 200 kHz to 230 kHz which can be attributed to abrasive tool wear. This is consistent because an end-of-life tool suffers from high contact with the part and this provides the mechanism of diffusion. In order not to make the work large and repetitive, we chose to condense the images of the STFT signals into the plate shape (Fig. 8b) and compare the start and end of tool life signals.
In order to facilitate the demonstration of STFTs results, the 3 axes (time (x), frequency (y) and energy (z)) were unified in order to demonstrate the signals more clearly.
The STFTs of the acoustic emission signal obtained during the first turning pass with a cutting speed of 200 m/min, feed rate of 0.10 mm/rev and depth of cut of 0.25 mm are shown in Fig. 8. In Figs. 8a and 8b, the uncoated tool presented an STFT signal in which there is an excitation with irregular periodicity at frequencies of 222 kHz, 267 kHz, 314 kHz, 358 kHz, 403 kHz and 447 kHz, which are excited by the machine-tool spindle rotation (589 rpm). It can also be noted that the signal energy threshold is much higher (1150 mV) than that at the beginning of the tool life, where these two phenomena can be attributed to the chip entanglement in the part.
In Fig. 9c, a stimulus appears at between 1.575 s and 1.595 s, which repeats at between 1.756 and 1.774 s, with excitation in the frequency range of 138 kHz to 171 kHz. The highest energy frequency is at 150 kHz, which reaches an energy level of 1265 mV. High energy levels are also observed at the frequencies of 32 kHz and 30 kHz (545.5 mV and 542.5 mV, respectively). In the signal referring to the end of tool life of the AlCrN-coated tool (Fig. 9d), a decrease in the signal energy (peak of 454.20 mV) can be noted. The frequency with the highest energy is 32 kHz followed by 56.25 kHz. Excitation in the frequency range of 2 kHz to 98 kHz is observed throughout the signal acquisition period. In addition, there is excitation in the range of 232 kHz to 288 kHz, but with less energy than the previously mentioned range. The frequency ranges of 370 kHz to 468 kHz also shows excitation, which occurs over a period of 0.06 s with a range of 0.02 s, reaching a peak at 443 kHz. According to Hase et al. [7], at frequencies above 100 kHz excitation is due to crack propagation and particle interaction, which accelerates the tool wear mechanisms.
In the case of the AE signals for the nanostructured AlCrN-coated tool under intermediate machining conditions (Figs. 9e and 9f), it can be noted that in most cases there are two well-characterized excitation ranges: at 75 kHz to 105 kHz and at 389 kHz to 475 kHz. In the former frequency range the highest peak amplitude appears at 91 kHz in most of the signals obtained for these tools. Ferrer et al. [21] attribute this range to slip friction and Wada and Mizuno [22] to mild abrasive wear. By imposing a surface with a lower coefficient of friction, as demonstrated by Mo et al. [26] in their studies comparing AlCrN with other coatings, the tool begins to show sliding and soon after, by imposing force, abrasive wear and therefore excitation at this frequency is observed.
The range of 389 kHz to 475 kHz is characteristic of crack propagation [27] and medium-intensity abrasive wear [7]. During the cutting process the tool undergoes abrasive wear resulting in the beginning of crack formation and propagation, where there is a loss of material from the cutting edge of the tool, as is evident in Fig. 10. Grooves can be observed on the clearance surface (two-body abrasive wear characteristic) and porosity at the central end of the clearance surface (three-body abrasive wear characteristic). In the central region of the tool, characteristic surface fracture is noted.
Although the excited frequency bands demonstrate the prevalence of abrasive wear, the nanostructured AlCrN-coated tools show the best performance in terms of acoustic emission drivers. On comparing the STFTs of the signals for all of the tools studied, with a cutting speed of 200 m/min, a feed rate of 0.10 mm/rev and depth of cut of 0.25 mm, as shown in Fig. 9, it can be noted that the signal with the least change from the beginning to the end of the tool life is that with the nanostructured-AlCrN coating. This tool also presented a longer life.
Temperature may act as an attenuator of AE signal amplitude [28]. However, an increase in temperature contributes to a decrease in the resistance of the tool and this causes an increase in the wear, which is an AE signal generator. The fact that the cutting mechanism studied is severe (hard turning) and the geometry of the tool has negative angles of inclination and output, together with the cutting parameters, cause the temperature to rise and the tool loses resistance, suddenly altering the AE signal. In this situation, the AE signal indicates excitations at distinct frequencies over a short time interval. Figure 11 illustrates qualitatively (but not quantitatively because of the variation in the emissivity of the material due to the temperature variation) the difference in the temperature of the chip at two moments. Firstly (Fig. 11a), at the beginning of the chip entanglement in the workpiece, the temperature reaches a threshold lower than that at the moment where there is complete entanglement and the temperature of the chip increases (Fig. 11b). This causes the wear mechanisms, and consequently the acoustic emission signals to change.
At the beginning of the tests, using a cutting speed of 200 m/min, a feed rate of 0.20 mm/rev and a depth of cut of 0.75 mm (Fig. 12), the uncoated tool (Fig. 12a) shows excitation in the frequency range of 90 kHz to 110 kHz, a characteristic that is attenuated in the end-of-life signal for the tool. At a frequency of 109 kHz there was strong excitation in the period from 1.32 s to 1.35 s. This lies within the range of excitation frequencies associated with adhesive wear and adhesion-pull out mechanisms, the most predominant in the case of the uncoated tool. This is verified by Fig. 12a, where it can be noted that the tool without coating presents a lower amount of deposited material, being strongly influenced by adhesion-pull out, which later generates abrasive wear.
At the end of the tool life, for the uncoated tool (Fig. 12b) there is a decrease in the signal energy in relation to the signal generated at the beginning of the tool life (Fig. 12a). The increase in temperature and decrease in the rate of wear tend to promote this, since there is an association between the AE signals and the increase in temperature, with a decrease in the energy of the signals [28] and the rate of the excitation mechanisms. In addition, for the uncoated tool, the very characteristic peaks in the frequency range for the abrasive wear and crack propagation mechanisms (350 kHz to 460 kHz) observed at the beginning of the tool life (Fig. 12a) become more sparse in the signal obtained at the end of the tool life. This indicates that these phenomena occur mainly during the early part of the tool life.
Signals for the AlCrN-coated tool (Figs 12c and 12d) showed excitation in a wide frequency range of 2.34 kHz to 119.50 kHz, with sparse excitation across the entire frequency spectrum. The excitation observed up to 120 kHz is attributed to adhesive wear [22], dislocation and adhesion-pull out mechanisms. In the SEM micrographs (Fig. 13b), it can be noted that the tool has a large amount of workpiece material adhered on both the flank and rake surfaces. This corroborates the finding that the signals are in the frequency range associated with adhesive wear.
Figure 13d shows a strong influence of abrasive wear and crack propagation (frequency range 420 kHz to 470 kHz), which can be observed on the SEM micrograph of the tool (Fig. 13b) through the formation of chippings and crater wear. In addition, the signal includes the influence of adhesive wear and adhesion-pull out, with a decrease in the energy of the signal in relation to the beginning of the cut, this being influenced by the increase in temperature, which attenuates the signals.
The signals for the nanostructured AlCrN-coated tool (Figs. 12e and 12f) were practically the same at the beginning and end of the tool life. The signals are strongly influenced by adhesive wear and pull out (15 kHz to 115 kHz range) and by abrasive wear and crack propagation (350 kHz to 450 kHz). The signal for the nanostructured AlCrN-coated tool at the end of tool life (Fig. 12f) was attenuated, as was the signals for the other tools, due to the greater chip-tool-workpiece contact that generated higher temperature and thus attenuated the AE signals at the end of the tool life. This also increases the adhesion of the workpiece material to the tool, as shown in Fig. 13 (“a” to “c”).
A cutting speed of 250 m/min, feed rate of 0.20 mm/rev and depth of cut of 0.75 mm was used as the maximum material removal rate (Figs. 14 and 15). The tools were able to perform only one pass before reaching the end-of-life criterion (0.6 mm of maximum flank wear) and the uncoated tool (Figs. 14a and 14b) showed the highest energy for the signal, indicating that the coating attenuates the tool degradation mechanisms during cutting. The uncoated tool excitation ranges were 32 kHz to 154 kHz and 375 kHz to 466 kHz for the beginning and end of the tool life, respectively. Note that the signals are practically the same under the two conditions, which indicates that at the beginning of the cut the tool is strongly affected by wear mechanisms and this continues until the end of the tool life.
The AlCrN-coated tool (Figs 14c 14d) shows excitation in a wide range of frequencies (2.3 kHz to 460 kHz) with a peak energy of 800 mV. As with the uncoated tool, for the AlCrN-coated tool the signals at the beginning and end of the tool life are practically the same, showing excitation in the period from 0.63 s to 1.23 s in the frequency range of 420 kHz to 423 kHz. This is associated with the excitation caused by the propagation of cracks or phenomena related to a phase change in the tool material, as reported by Hase et al. [7].
The nanostructured AlCrN-coated tool (Figs 14e and 14f) showed greater excitation in the ranges 10 kHz to 120 kHz and 320 kHz to 460 kHz when compared with the signals originated from uncoated tools. The first range corresponds to adhesive wear, pull out, and sliding while the second is related to abrasive wear and crack propagation. There was excitation at a frequency of 288 kHz throughout the cut for the tools coated with the AlCrN and nanostructured AlCrN. This frequency is associated with excitation by voids and particle interaction [22, 23, 29], which can be caused by the aggressiveness of the operation that causes this rearrangement in the coating structures.
The micrographs (SEM) in Fig. 15 show the clearance surfaces after turning with the coated tools. Figure 15a shows the presence of microcracks in the tool coated with AlCrN, which corroborates the appearance of excitation in the signal at the end of the tool life. As verified by Fig. 15b, no cracks are observed on the tool and abrasive wear is predominant.
Briefly, Table 2 sets the frequency ranges excited by the mechanisms observed in the tools.
Table 2. Wear Mechanisms and its AE frequencies in the turning.
Wear Mechanism
|
Frequency band observed
|
Adhesive wear
|
15 to 115 kHz
|
Sliding
|
35 kHz
|
Diffusion wear
|
40 to 120 kHz
|
Slip friction
|
75 to 105 kHz
|
Movement of dislocations
|
100 to 220 kHz
|
Abrasive wear
|
200 to 230 kHz
|
Particle Interaction
|
100 to 253 kHz
|
Crack propagation
|
420 to 470 kHz
|
Phase transformation
|
420 to 423 kHz
|