**1. Introduction**

In order to control several parameters of machining processes, different measurements (vibrations, energy consumption, airborne noise, acoustic emission, etc.) can be determined and processed using various signal-processing techniques.

Acoustic emission (AE) is one of the most frequently used measurements for this purpose. AE can be described as a set of elastic pressure waves generated by the rapid release of energy stored within a material. This energy dissipation is basically due to dislocation motion, phase transformations, friction, and crack formation or growth [1].

Different vibration and signal measurement techniques have been used in the past for the detection of failures in manufacturing processes [2]. Several authors [3–7] discuss how AE is related to the wear mechanisms of cutting tools. Pandiyan and Tjahjowidodo [8]

**Citation:** Fernández-Osete, I.; Estevez-Urra, A.; Velázquez-Corral, E.; Valentin, D.; Llumà, J.; Jerez-Mesa, R.; Travieso-Rodriguez, J.A. Ultrasonic Vibration-Assisted Ball Burnishing Tool for a Lathe Characterized by Acoustic Emission and Vibratory Measurements. *Materials* **2021**, *14*, 5746. https:// doi.org/10.3390/ma14195746

Academic Editor: Stanislaw Legutko

Received: 3 September 2021 Accepted: 28 September 2021 Published: 1 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

applied dynamic measurements to establish the fault thresholds in grinding wheels under different conditions, while Lopes et al. [9] monitored the condition of a grinding wheel. Wang et al. [10] and Zanger et al. [11] studied the relationship between the AE and chip size.

The quality requirements of industrial products are constantly increasing and machined components are no exception. For this reason, ultrasonic-assisted tools are now used to improve surface quality. Hence, new methodologies are required to study the vibratory behavior of these kinds of tools, as described in the previous paragraph.

Referring to finishing operations, the British scientist Griffith concluded in 1921 that the strength of materials with isotropic properties was much lower (between 10 and 20 times) than could be predicted theoretically and that this is due to the lack of continuity of the material; that is, the existence of defects [12]. These defects occur in the process of obtaining the components (metallurgical defects) or in the production process due to geometric details. Defects of the surface layers of machined components are especially dangerous. In these surface layers, three properties are especially important: surface hardness, roughness, and compressive residual stresses.

Ball burnishing is one the most suitable processes with which to improve these properties [13,14]. This process consists of the plastic deformation of irregularities in the target surface through the application of a controlled force by a sphere [15]. In recent years, the technical world has witnessed the birth of vibration-assisted ball burnishing (VABB). In this technique, the ball that compresses the target surface is subjected to a high-frequency vibration (between 20 and 40 kHz) which, in turn, is transmitted to the target surface [16]. This vibration of the surface material produces a lowering of its yield limit—a phenomenon called acoustoplasticity [17]. As a result, plastic deformation of the material is achieved with forces lower than those that would be necessary without vibration assistance. Consequently, VABB provides better results than conventional or non-vibration-assisted ball burnishing (NVABB) [18].

Different systems have been used to enhance ball burnishing with vibrations in a variety of different machining processes [16–18]. Most of these systems, including the one studied in this paper, use a resonant system characterized by a low amplitude movement (between 3 and 30 μm) [19]. This system, which is described by Jerez-Mesa [20], applies a high-frequency electrical charge to a piezoelectric stack, causing it to undergo a deformation which is then transmitted to the ball. This device is called a sonotrode [21].

With regard to burnishing, there are few studies where the application of AE is so direct. Dornfeld and Liu [22] concluded that AE helps to reveal information about the frictional behavior of the ball burnishing process, as it has a strong correlation with the kinetic friction coefficient and the texture surface profile. Their work also concluded that AE demonstrates how the burnishing process can be divided into four stages from a dynamics perspective. Only in the first two can positive results during burnishing be obtained. Strömbergsson et al. [23] observed that monitoring AE parameters during the burnishing process to confirm that the operation has performed its intended function is highly beneficial. For example, inspection of an AE signal in root mean square (RMS) representation for 5 min demonstrated that the decrease in the coefficient of friction (COF) stagnated after a time, and that the tribological behavior did not remain stable. Therefore, investigators should be made aware of the effects of excessive wear of the burnishing ball and how these can affect the finishing results. Salahshoor and Guo [24] used AE to monitor the burnishing process on a magnesium-calcium alloy.

In addition to the limited studies regarding the application of AE to burnishing, some studies apply this technique to the diagnosis of possible faults in contacts between solid bodies in relative motion—a case to which burnishing is easily applicable due to the way it takes place, and which is therefore considered relevant as an antecedent to this paper [25]. Tandon et al. [26] highlighted the effectiveness of AE for detecting failures in contacts between ball bearings, and concluded that it can detect the transfer of particles from the wear of the two surfaces in contact. They also concluded that AE is more effective than vibration analysis as it can detect errors before they occur. Hase et al. [27] concluded that

the frequency spectra of the AE signals measured during the tribological tests allowed them to determine the wear mechanism between the contact surfaces. Geng et al. [28] concluded that AE signals acquired at the highest sampling frequencies were more sensitive for the detection of the friction mechanisms between two contact surfaces than the evolution of the friction coefficient that characterizes this contact.

In this paper, a full vibration characterization of a ball burnishing process performed in a lathe is presented. This ball burnishing process was performed using a tool designed by the authors [29]. The main objective was to demonstrate that the machine and the tool do not present any resonance issues during their service that could result in possible hardware malfunctions. This dynamic analysis validates the suitability of the tool when it is attached to an NC lathe and is relevant to the eventual industrial users of the system. The designed tool is particularly intended for application in industries that manufacture elements with revolution symmetry that are subjected to high-cycle fatigue or in which a particular type of wear must be prevented. The adequateness of the system to transmit vibrations through the material is assessed.

A specific methodology was applied to validate the dynamic behavior of the tool by combining several techniques based on quantification of the normal and ultrasonic vibration ranges through static and dynamic measurements. In the static measurements, the frequency response functions of the tool were measured and, consequently, the natural frequencies were determined [30]. The dynamic measurements were used to characterize the burnishing process (vibration assisted or not) under operating conditions. In this case, acoustic emission was used to detect possible damage in the material during the VABB process.

The analyses included in the previous paragraph demonstrate the fundamental importance of the traditional techniques of static and dynamic vibration analysis as applied to the VABB process discussed here. The dynamic results derived from VABB applied to two different ferric alloys are described in order to evaluate different magnitudes under different burnishing conditions: two burnishing forces (90 N and 270 N) and the existence of vibration assistance (yes or no). The measured magnitudes were burnishing force, vibrations, and acoustic emission. This allowed us to characterize the process itself and the tool's ability to transmit vibratory assistance, as well as to detect possible damage in the specimens produced by this process. From the vibration measurements, an operational deflection shape (ODS) exercise was also performed.

This research is novel as, despite the fact that the vibration-assisted ball burnishing process is not new, a new tool that is capable of carrying it out was analyzed. This tool has a series of characteristics that make it unique from those on the market. Additionally, the AE technique applied for characterization and verification of the influence of the vibration assistance did not produce any negative effects on the process results. No reference to the use of AE for this purpose was found in the reviewed literature.
