Ultrasonic-Assisted Face Turning of C45 Steel: An Experimental Investigation on Surface Integrity

1. Introduction

Face turning is a widely used machining process for generating flat surfaces that are perpendicular to the rotational axis of a workpiece. It is commonly applied to components such as flanges, piston end faces, punches, and other parts requiring high flatness and surface uniformity. The key parameters that govern the face turning process include cutting speed (v_c), depth of cut (a_p), and feed rate (f_r). Among these, cutting speed plays a critical role, exerting a highly variable and difficult-to-control influence on cutting force, the formation of built-up edges, and the resulting surface quality as it strongly depends on the characteristics of the material being machined [1,2,3]. Unlike cylindrical turning, where the cutting diameter remains constant, face turning involves a continuous change in cutting diameter (D_w) along the radial feed direction. As a result, the cutting speed varies depending on the position of the insert’s nose on the workpiece surface and is no longer a fixed value. It is calculated using the following equation:

$${\mathit{v}}_{\mathit{c}}={\displaystyle \frac{\mathit{\pi }{\mathit{D}}_{\mathit{w}}\mathit{n}}{1000}}$$

(1)

Here, D_w represents the instantaneous diameter at the cutting point, and n is the spindle speed. As the cutting tool moves radially toward the center of the workpiece, the effective diameter decreases, leading to a corresponding reduction in cutting speed. Conversely, at the outer edge of the workpiece, the cutting speed is significantly higher due to the larger diameter. This variation in cutting speed across the face area results in inconsistencies in the cutting mechanism and surface quality. In particular, when machining near the center, the cutting speed can become very low, often leading to increased surface roughness and degraded surface quality.

While face turning is commonly encountered in practical machining tasks, especially for producing flat surfaces on rotational parts, it has received less in-depth research compared to cylindrical, conical, or thread turning. This is partly due to its relatively specific application scope and the complexities introduced by the variation in cutting speed across the machined surface. In components such as pipe flanges, where the center contains a large hole, the reduction in cutting speed near the center is acceptable, allowing surface quality to be maintained. However, for solid shafts or punch tip that require machining all the way to the center, the issue becomes more critical, as the cutting speed near the center approaches zero, often resulting in poor surface finish and inconsistent cutting behavior.

Ultrasonic-assisted turning (UAT) has been demonstrated to effectively improve surface finish and reduce cutting forces, particularly for machining difficult-to-cut materials such as nickel-based alloys [4,5], composite materials [6,7,8], and even low-carbon steels [9]. The fundamental principle of UAT involves superimposing high-frequency ultrasonic vibration onto the cutting tool, resulting in an intermittent cutting mechanism that contributes to the observed improvements. When the vibration is applied in the direction of the cutting speed, the instantaneous cutting speed v_ins oscillates around the conventional cutting speed v_c​, with an amplitude of 2πAf, where A is the vibration amplitude and f is the vibration frequency [10]. This relationship is described by the following equation:

$$v_{ins}=v_c+2\pi Afcos(2\pi Aft+\phi )$$

(2)

If the conventional cutting speed is lower than 2πAf, there will be time intervals during which the instantaneous cutting speed becomes zero or negative, resulting in intermittent cutting. This phenomenon can significantly reduce cutting forces and enhance surface quality; however, the material removal rate (MRR) is reduced due to the lower cutting speed, leading to longer machining times [11,12]. Most recent studies apply UAT within low cutting speed region to maximize reductions in cutting forces and surface roughness [13,14,15]. Nevertheless, this restricts its applicability in industrial settings, where productivity is a critical factor. Recent research has begun to explore UAT at cutting speeds exceeding the critical threshold 2πAf, but these studies have primarily focused on cylindrical turning [16] or face turning at low spindle speeds [17,18], without specifically addressing surface quality in high-speed ultrasonic-assisted face turning

Therefore, this work aims to investigate the effect of ultrasonic vibration applied in the direction of cutting speed on surface homogeneity during face turning of C45 steel workpiece, within a spindle speed range of up to 1300 rpm. In particular, the experiments focus on evaluating improvements in surface roughness and structural characteristics near the central region of the workpiece, where machining quality is often significantly degraded due to extremely low cutting speeds.

2. Experimental Setup and Results

2.1. Experimental Setup

To investigate the effect of ultrasonic vibration on surface integrity in face turning, a series of experiments were conducted on a retrofitted conventional lathe (OKK Ramo). The experimental setup consisted of a vibration module delivering longitudinal ultrasonic excitation at 20 kHz frequency and 10 µm amplitude directly coupled to the turning insert. C45 steel shafts were used as workpiece material, while carbide inserts with a nose radius of 0.8 mm were selected from Dormer Pramet to ensure consistency across tests. The cutting parameters (spindle speed, feed rate, and depth of cut) were varied systematically as listed in Table 1. Surface roughness was measured using a 2D roughness tester (Mitutoyo SJ-301), and surface integrity was observed via digital microscope (Keyence VHX 7000). Each roughness and vibration measurement were repeated three times to ensure repeatability.

After the face turning process was completed, the samples were sectioned as illustrated in Figure 1, and surface roughness was measured as shown in Figure 2.

2.2. Surface Roughness

The resulting R_a and R_z values are presented in Figure 3. From the graphs, the following observations can be made: In the conventional turning (CT) method, surface roughness exhibited minimal variation with spindle speed in the range of 700 to 1300 rpm. This is because roughness was measured in the radial direction, which is perpendicular to the tool path. Since the actual cutting speed gradually decreases from the outer diameter toward the center, and the speeds within this range are insufficient to overcome built-up edge (BUE) formation - the primary cause of poor surface finish of medium carbon steel [19,20]. In the ultrasonic-assisted face turning (UAFT) method, surface roughness was strongly influenced by both cutting speed and feed rate. When the cutting speed was lower than the critical speed (defined as v_critical = 2πAf), a significant reduction in roughness was observed [7,13,21,22]. However, when the cutting speed exceeded this threshold, the improvement in roughness compared to CT became marginal, although the resulting surface finish remained acceptably fine [23].

The lowest mean R_a value obtained in the UAFT method was 0.91 µm, corresponding to the cutting parameters of n = 700 rpm and f_r = 0.1 mm/rev, representing a 71% reduction compared to the equivalent value in CT. However, the standard deviation remained relatively high at 0.15 μm, as the cutting speed at this level was entirely within the typical BUE formation zone. At low cutting speeds (typically below 60 m/min), the temperature at the rake face falls within the steel's "blue-brittle" range, where ductility is reduced and shear failure is more likely [24]. Under high shear stress and hydrostatic pressure, material begins to accumulate on the rake face, forming an initial BUE layer. As cutting continues, new material slides along the tool but partially adheres, causing the BUE to gradually thicken. Once the adhesive limit is exceeded, the BUE detaches and may transfer to the machined surface, leading to increased roughness. Although UAFT effectively mitigates BUE, the phenomenon remains a rapidly recurring cyclic process at low speeds, which compromises the uniformity of the surface finish.

Conversely, at n = 1300 rpm and f_r = 0.1 mm/rev, although R_a did not reach the minimum, the standard deviations of R_a and R_z were only 0.006 µm and 0.30 µm, respectively. These low variations indicate a highly consistent surface quality at high spindle speeds, primarily due to the significant reduction in the BUE phenomenon due to both high cutting speeds and assisted vibration.

An important observation lies in the ratio between R_a and R_z, which serves as an indicator of the surface texture characteristics. In conventional turning (CT), this ratio fluctuated around 15–16.3%, whereas in ultrasonic-assisted face turning (UAFT), it ranged from 17.9% to 20.3%. This suggests that surface asperities generated by CT tend to exhibit more irregular and sharper peaks, leading to a relatively higher R_z compared to R_a. In contrast, UAFT tends to produce more uniform and rounded peak formations, resulting in a higher R_a/R_z ratio. This inference is further supported by the roughness profiles shown in Figure 4 and is consistent with the findings of Thomas M. et al. [25], where a ratio of R_max/R_a = 4.4 corresponds to a saw-tooth profile composed of regular, periodic peaks—indicative of stable cutting without built-up edge (BUE) formation. Conversely, when the surface profile appears more random and irregular, a higher ratio of R_max/R_a = 6.3 is observed, reflecting the presence of significant BUE effects. These results highlight that analyzing the relationship between R_a and R_z (or R_max) provides not only a quantitative measure of surface roughness but also valuable insight into the underlying cutting mechanisms and stability of the machining process.

Notably, in Figure 4(b), where the roughness probe scanned from the center outward in a CT sample, the first half (corresponding to lower cutting speeds) exhibits an unstable and irregular roughness pattern. In contrast, the second half (corresponding to higher cutting speeds) shows more periodic and consistent peak formation. In comparison, Figure 4(a) for the UAFT sample demonstrates uniformly spaced roughness peaks across the entire profile. The distance between successive peaks closely matches the feed rate f_r, implying that the BUE formation was effectively suppressed even in regions with low cutting speed, thereby confirming the stabilizing effect of ultrasonic assistance.

2.3. Surface Topography

Microscopic observations using the Keyence VHX-7000 digital microscope further support the previously discussed findings. In the CT method at a spindle speed of n = 1300 rpm, the surface near the center of the workpiece (Figure 5a) exhibits distinct tool marks corresponding to the feed rate (f_r). However, the peak-to-valley height is considerably large, and the surface displays protrusions and depressions irregularly distributed along the cutting direction, indicative of material adhesion and unstable chip formation. These characteristics suggest the presence of a built-up edge (BUE), which contributes to increased surface roughness and the formation of surface irregularities.

Toward the outer region of the workpiece (Figure 5b), the height of the surface asperities is significantly reduced. Nevertheless, abnormal surface features oriented along the cutting speed direction are observed—commonly referred to as “adhered micro particle” by R. Tan [26]. This phenomenon is typically associated with lateral material deformation along the cutting edge and frequently occurs in conventional turning at high cutting speeds.

In contrast, surface observations of the samples machined using UAFT (Figure 6) show no signs of adhered micro particles. The peak-to-valley height is smaller than in CT and approaches the theoretical value calculated by the formula [27]:

$R_a={\displaystyle \frac{{0.0321f}_r^2}{R_c}}$ (3)

where R_a Arithmetic Mean Height, f_r is the feed rate, and R_c is the tool nose radius.

Additionally, theoretical and experimental studies, such as the one conducted in [23], have demonstrated that the ultrasonic vibration introduces periodic structures on the surface, with a wavelength given by:

$$w_i={\displaystyle \frac{10^6\times v_c}{60\times f}}$$

(4)

Here, $w_i$ represents the wavelength of the periodic structures formed on the machined surface by UAFT along the cutting direction, expressed in µm, and was found to exhibit negligible deviation from experimental results reported in [23]. In this equation $v_c$, denotes the cutting speed (in m/min), and f is the frequency (in Hz) of the assisted ultrasonic vibration.

3. Conclusions

The effect of ultrasonic vibration applied in the cutting speed direction on surface quality during face turning of C45 steel was systematically investigated. The experimental results revealed that ultrasonic assistance significantly improved surface roughness, particularly in the area near the center of the workpiece where the cutting speed is extremely low. The lowest R_a value obtained in UAFT was 0.91 µm at a spindle speed of 700 rpm and a feed rate of 0.1 mm/rev, representing a reduction of up to 71% compared to CT.

The improvement in surface quality at low cutting speeds is especially noteworthy, as CT under such conditions typically results in severe surface degradation, with high R_a and R_z values primarily due to the formation of BUE. However, with the application of ultrasonic vibration, BUE formation was significantly suppressed. Surface topography observations indicated that, in the UAFT method, roughness peaks appeared at regular intervals corresponding to the feed rate, and the machined surfaces exhibited uniform microstructures, attributed to the modulation of instantaneous cutting speed by ultrasonic vibration.