**3. Results and Discussion**

#### *3.1. Impact Tests*

Impacts performed at point I3 (Figure 4) did not provide any valid information because the base housing was disconnected from the tool in order to ensure that the ultrasonic vibrations were transmitted to the lathe [29].

According to the impact position, the best responses always corresponded to the vertical direction (Y), that is, the impact direction. Frequency response functions (FRF) of point P3 revealed poor coherence between excitation and response.

Signals measured at points P1 and P2, corresponding to the tests performed without contact between tool and specimen, revealed a natural frequency of around 1.5 kHz. Figure 6 presents the FRF and the coherence function corresponding to the response at point P1 with excitation at I2. Some low peaks also appear around 500 Hz and 900 Hz. The band between 2 and 3 kHz is also noticeable but its coherence is poor.

In the tests performed with contact between the tool and specimen, no differences were noted between the different test conditions (force and tool position). Consequently, the load and tool position did not affect results. In these cases, natural frequencies in the signals measured at point P3 are noticeable. New components in the band between 1.2 and 2.8 kHz appeared. The amplitudes were lower than those obtained in measurements without contact between the tool and specimen. Figure 7 presents the FRF and coherence corresponding to response at P1 and impact at I1, in the position near to the lathe point and with a load of 90 N.

Natural frequencies were always much lower than the assisting frequency, that is, 40 kHz constituted just 5% of this magnitude.

#### *3.2. Vibration Monitoring during the Burnishing Process*

Measurement signals acquired by tool accelerometers (Figure 1) and test bed accelerometers (Figure 2) during burnishing processes were processed and analyzed in time and frequency domains. The maximum analysis frequency was 4 kHz. Spectra are presented, in some cases, up to 2 kHz as no components appeared at higher frequencies.

**Figure 6.** FRF obtained at P1 with impact at I2.

The vibration behavior of the tool during the burnishing process was similar for both materials under all burnishing conditions. Components at 15.8 Hz and 32.1 Hz were noticeable when burnishing C45, while burnishing GJL250 resulted in noticeable components at 17 Hz and 34.2 Hz. This difference is due to the slightly different burnishing speeds and the relation between these components and the mechanical and electrical operation of the lathe. In all cases, these components are very small (μm/s RMS). According to the ISO 20816 [33] for vibrations in machines, the highest allowable vibration level is 0.28 mm/s RMS: all of the measured levels were considerably lower. Figure 8 presents the spectra in the Z direction (direction of burnishing force) for both materials with a burnishing force of 90 N and without vibration assistance.

In tests without vibration assistance, the amplitudes corresponding to points P1 and P2 in burnishing force direction (Z) were higher than amplitudes corresponding to the other two directions: burnishing feed direction (X) and vertical direction (Y). This result can be justified as these points are at the end of the tool and receive all the effort of the burnishing operation. In the case of C45, the amplitudes at point P3 were greater than at points P1 and P2, while in GJL250 the amplitudes at the three points were similar. At point P3, the highest amplitude was in the Y (vertical) direction, which may be due to point P3 being in the distal part of the tool at the time of burnishing, as shown in Figure 1b.

Figure 9 compares the spectra, measured with a burnishing force of 90 N for both materials, with and without vibration assistance, at point P3. This figure shows the Y-axis only because all directions demonstrated the same frequency peaks with very similar amplitudes. Additionally, signals measured without vibration assistance had similar behavior to those with vibration assistance.

**Figure 7.** FRF obtained at P1 with impact at I1, with a load of 270 N.

**Figure 8.** Vibration spectra measured at point 2, in Z direction (direction of the burnishing force), with a NVABB force of 90 N.

**Figure 9.** Comparison of spectra of both materials, using VABB and NVABB, measured at point P3, in Y (vertical) direction, and with a burnishing force of 90 N.

Two burnishing forces were used in tests with C45 steel. No differences between the signals monitored under these conditions were noted. To demonstrate this, Figure 10 presents the spectra corresponding to different points and measurement directions. All of these spectra correspond to measurements without vibration assistance.

Additionally, measurements without burnishing feed were carried out for both materials, under the same conditions previously specified. The duration of each of these measurements was 3 min. Analysis of these tests reveals similar results to those obtained with burnishing feed. The same frequency peaks appeared, with very small amplitudes. The only difference was the possible excitation of natural frequencies in the range between 800 Hz and 1400 Hz, but with very low amplitudes. Figure 11 presents an example of these signals.

The vibration measurements in the lathe test bed show normal behavior of the machine. Components that interfere with those obtained in the tool were not noted, but a possible excitation of the natural frequencies between 800 Hz and 1400 Hz was noticeable, but with very low amplitudes.

Figure 12 presents signals measured in the lathe test bed during the burnishing of the steel specimen. Comparing the signals measured with vibration assistance in the horizontal (H), vertical (V), and axial (A) measurement positions, a very similar behavior was noticeable between them, and the amplitudes were again very low. Via inspection of the non-vibration-assisted spectra measured in the horizontal direction at burnishing forces of 90 N and 270 N (Figure 12), no differences appeared between them; consequently, the burnishing force did not affect the horizontal vibrations of the lathe test bed. Additionally, no differences appeared between horizontal spectra with or without vibration assistance at both burnishing forces in Figure 12; therefore, there was no influence of the vibrating assistance in the lathe test bed. This is expected according to the tool design, lathe rigidity, and the test bed points where the measurements were performed.

**Figure 10.** Comparison of spectra of C45 with burnishing forces of 90 N and 270 N, without vibration assistance.

**Figure 11.** Measurements in burnishing without feed.

**Figure 12.** Spectra measured in lathe test bed at different loads and different measurement positions.

#### *3.3. Acoustic Emission Monitoring during the Burnishing Process*

Figure 13 presents acoustic emission time histories for different processes. Figure 13a presents the background noise. Figure 13b presents the acoustic emission signal during the burnishing process of the C45 steel specimen with a burnishing force of 90 N, without vibration assistance. Finally, Figure 13c presents the C45 burnishing with a burnishing force of 90 N, with vibration assistance. Figures 13a and 13b are very similar; therefore, the burnishing process without vibration assistance did not produce any acoustic emission. However, a clearly noticeable signal appears in Figure 13c, corresponding to the vibrationassisted burnishing process. This is due to the acoustic emission sensor detecting the assisting vibration signal. No acoustic emission apart from that produced by the vibration assistance appeared; consequently, the ball burnishing process did not produce any damage in the material.

Figure 14 presents the AE spectra corresponding to the VABB of both materials, burnished with different forces: (a) C45 steel specimen with 90 N, (b) C45 steel specimen with 270 N, and (c) GJL250 grey cast iron specimen with 90 N. In all figures, the frequency corresponding to the vibration assistance is the only noticeable peak. This is consistent with previous findings [20].

The vibration assisting frequency was very stable. Its variation of 100 Hz corresponds to 0.25% of the frequency. This level remained the same for both VABB of C45 steel; thus, the burnishing force did not influence it. The vibration level, in the case of GJL250 VABB, was lower those of C45 due to the high GJL250 internal damping [34].

**Figure 13.** AE time histories of C45 under different conditions: (**a**) background noise; (**b**) NVABB; (**c**) VABB.
