Effect of Ultrasonic Surface Rolling on the Fretting Wear Property of 7075 Aluminum Alloy

Abstract

This paper investigates the effect of ultrasonic surface rolling (USR) on the fretting wear properties of the 7075 aluminum alloy. A white light interferometer, Vickers hardness tester, and X-ray diffractometer were employed to comparatively analyze the variations in surface roughness, hardness. and grain size before and after the USR treatment. The fretting tests were carried out under oil lubricated and dry fretting conditions, using a ball-on-flat contact tangential fretting tester. The worn surface morphology, wear debris, and chemical composition were analyzed using an optical microscope (OM), a scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS), etc. The results revealed that after USR treatment, the surface roughness was reduced by 90%, the hardness was increased by 13HV, and the grains were refined. Moreover, the wear was reduced under oil lubrication conditions but increased under dry fretting conditions. It can be concluded that the microstructure and mechanical properties of the 7075 aluminum alloy surface can be enhanced by the USR treatment. The improved fretting performance in oil should be attributed to the increased surface hardness, which helps reduce abrasive wear and plastic deformation. However, under dry fretting conditions, the wear was increased due to the presence of hard debris particles that peel off from the USR-treated surface, leading to aggravated abrasion.

1. Introduction

Lightweight design of equipment structure is of great significance for energy saving and emission reduction. Aluminum alloys are widely used in aerospace, transportation, construction, medical equipment, and other fields due to their high specific strength, low density, low temperature resistance, and easy processing [1,2]. However, aluminum alloy components are often faced with heavy loads, poor lubrication, corrosion, and other harsh service environments. Compared with steel and its products, aluminum alloy has serious deficiencies in wear and corrosion resistance, which limits its application scope [3,4]. Studies show that the use of appropriate surface engineering techniques such as carburization [5,6], selective laser melting [7,8,9], and plasma nitriding [10,11] are effective means to enhance the wear resistance of aluminum alloy surfaces. However, the above methods require high-temperature heat treatment, which means high energy consumption.

Ultrasonic surface rolling is a highly efficient and environmentally friendly chip-free processing technology. It utilizes the combined effects of applied normal load and low-amplitude, high-frequency ultrasonic vibration to induce plastic deformation on the rolled surface. As a result, not only the grains are refined, but the microstructure, surface roughness, microhardness, and wear resistance of the material are also improved [12,13,14,15]. Extensive research has been conducted by scholars to study the impact of ultrasonic surface rolling strengthening on the microstructure and tribological properties of aluminum alloys. Gong et al. [16] investigated the impact of rolling time on the surface integrity and wear resistance of Cr12MoV steel. The findings demonstrated that the increase in rolling time resulted in a further increase in the thickness of the plastic deformation layer and a decrease in surface carbide content. These effects were found to be advantageous in inhibiting crack initiation and propagation, effectively addressing the issue of layered spalling and enhancing the material’s wear resistance. Wang et al. [17] investigated the effect of USR on the microstructure and wear properties of a selective laser melted Ti-6Al-4V alloy and found that the enhancement of wear resistance and the suppression of spalling crack emergence were related to the improvement of hardness and shear resistance. Zhang et al. [18] carried out USR treatment on 25CrNi2MoV steel under different static pressures and investigated its microstructure and tribological behavior and found that the wear mechanism of the UT samples was adhesive wear and that of the USR samples was abrasive wear. Wu et al. [19] studied the effect of textures produced by ultrasonic surface rolling on tribology performance. It was found that the improvement in friction and wear properties was attributed to the synergistic effect of the substrate strengthening, reduction of contact area, and entrapment of wear debris. Dang et al. [20] investigated the surface integrity and wear behavior of 300M steel subjected to the ultrasonic surface rolling process, and it was found that the improvement achieved by USRP tends to saturate with increasing processing passes. Xiaohui Zhao et al. [21] studied the effect of ultrasonic surface rolling processing and subsequent recovery treatment on the wear resistance of magnesium alloy AZ91D and found that the wear resistance of the AZ91D Mg alloy depends not only on hardness but also on toughness. The literature research indicates that ultrasonic surface refinement (USR) is an effective technique for enhancing the surface microstructure and properties of aluminum alloys. However, the existing research on the tribological properties of USR surfaces primarily concentrates on sliding wear, with limited studies investigating fretting wear properties. Furthermore, there is a lack of research considering the influence of lubrication conditions on these properties.

This paper investigates the effect of USR treatment on the fretting wear properties of the 7075 aluminum alloy. Microscopic analysis methods are employed to investigate alterations in the surface microstructure and the mechanical properties before and after USR treatment. Fretting wear tests were carried out using a self-developed tangential fretting tester with the ball-on-flat contact configuration. The friction behavior as well as the wear mechanism are comparatively analyzed in detail. The findings of this study can provide guidance for reducing the fretting wear damage of aluminum alloy surfaces by surface mechanical strengthening technology.

2. Materials and Methods

2.1. Specimens

The sample material used in this study was 7075 aluminum alloy, provided by the Aluminum Corporation of China. The composition of the alloy is as follows: 1.20–2.00 wt% Cu, ≤0.40 wt% Si, ≤0.30 wt% Mn, 0.26 wt% Fe, 0.18–0.28 wt% Cr, 5.10–6.10 wt% Zn, 0.02 wt% Ti, and 2.10–2.90 wt% Mg. Its mechanical properties are summarized in

Table 1. Mechanical properties of 7075 aluminum alloy.

Materials	HV	σys (MPa)	σuts (MPa)	E (GPa)
7075	150	455	524	71

. The material was processed into test specimens with a diameter of Φ = 70 mm and a height of h = 10 mm. The upper specimens used in the study were Si3N4 ceramic balls with a diameter of Φ = 6 mm and a surface roughness of Ra = 0.025 µm. Their surface hardness was 1500 HV. According to Archards’ model, the wear rate is negatively correlated with the hardness of a material [22]. So, the focus of the investigation was primarily on the worn surface of the aluminum alloy, as its hardness is considerably lower compared to that of the ceramic balls.

2.2. Ultrasonic Surface Rolling

Prior to USR treatment, the surfaces of the 7075 aluminum alloy samples were polished using sandpapers of different grit sizes (180#, 400#, 800#, 1500#, and 2000#) in a sequential manner. Following this, the surfaces were ultrasonically cleaned in an alcohol bath to remove contaminants. Figure 1 illustrates the schematic diagram of the USR processing. A tungsten–cobalt carbide cylindrical roller with a length of 7 mm and a diameter of 12 mm was used as the rolling head. The static pressure was set at 0.2 MPa, the amplitude at 6 μm, and the frequency at 27.5 KHz. The rolling process was conducted four times at a feed rate of 2000 mm/min.

2.3. Fretting Wear Tests

The fretting wear experiments were conducted using a self-developed tangential fretting tester, which employed the ball-on-flat contact configuration [23] (Figure 2). Prior to installing the ceramic ball upper specimen and the aluminum alloy lower specimen, the specimens were ultrasonically cleaned in an alcohol bath for 5 min to remove surface contaminants. After cleaning, the specimens were securely fixed in the specimen holders and loaded with a dead weight. The reciprocating relative movement between the lower and upper samples was driven by a precision electric motorized stage (EPSH100G, Wuhan Red Star Yang Technology Co., Ltd., Wuhan, China). The stage was equipped with a servomotor (Panasonic, MHMF012L1C2, 100 W, 0.32 Nm), and the motion of the stage was closed-loop, controlled with the help of a grating ruler (model: JCXE-DF; resolution ratio: 0.1 μm), thus realizing a motion accuracy of 1 μm. The relative movement amplitude, frequency, and number of cycles were controlled by setting the servo motor parameters, and the friction force was measured by a force sensor and transmitted to the computer in real-time by a data acquisition card. The experiments in this paper were carried out under oil-lubricated (ISO VG46) and dry fretting conditions, as well as three fretting amplitudes of D = 50 μm, 150 μm, and 250 μm. For all experiments, the normal load was set at P = 30 N, the number of cycles N = 20,000, the frequency f = 2 Hz, the humidity at 60 ± 2%RH, and the temperature at 20 ± 2 °C. The experiments under each parameter were repeated more than 3 times to ensure the reliability of the experimental results.

2.4. Analysis Methods

In this paper, the microtopography and hardness of surfaces before and after undergoing ultrasonic surface rolling (USR) treatment were analyzed using the Ultra-Depth Three-Dimensional Microscope (Olympus DSX 510, Shanghai optical instrument Factory, Shanghai, China), field emission scanning electron microscope (FESEM, FEI, Nova NanoSEM400, FEI Company, Hillsboro, OR, USA), and micro-Vickers hardness tester (HV-1000A, under a load of 0.1 N and a loading time of 10 s). Additionally, the physical phase and grain size analysis of the surface before and after USR treatment were conducted using an X-ray diffractometer (SmartLab SE, Rigaku Corporation, Akijima, Japan). Following the fretting wear experiments, the friction coefficient was calculated and analyzed. The worn-surface morphologies, as well as the collected wear debris, were observed using OM and FESEM. Furthermore, the valence analysis of the elements in the wear region was performed using X-ray photoelectron spectroscopy (XPS, AXIS SUPRA+, Shimadzu Corporation, Shanghai, China).

3. Results and Discussion

3.1. Surface Morphology and Phase Analysis

Figure 3 presents the SEM and OM morphologies of the specimen surfaces before and after USR treatment. The SEM image in Figure 3a reveals the presence of deep ‘peaks’ and ‘valleys’ on the untreated surface, along with traces of sandpaper sanding. In contrast, the USR-treated specimen surface (Figure 3b) appears much flatter and smoother. This is attributed to the high frequency impact of the rolling process, which induces significant plastic deformation of the surface ‘peaks’ and ‘valleys’. As a result, the surface microdamage is effectively eliminated and the machining marks are reduced, leading to improved surface quality. Additionally, the measurements indicate a reduction in surface roughness from Sa = 0.23 μm to 0.022 μm, representing a decrease of approximately 90%. Furthermore, the surface microhardness increases from 170 HV to 183 HV.

Figure 4 shows the XRD patterns of surfaces before and after USR treatment. It can be seen that the peak shape of the USR-treated surface is similar to that of the untreated surface; no new impurity diffraction peaks were generated, but the peak intensity and half-peak width (FWHM) were increased. The XRD patterns show strong diffraction peaks at 2θ = 38.44, 44.66, and 78.16, which correspond to the FCC solid solution phases (111), (200), and (311), respectively (refer to PDF#04-0787 in Al). The diffraction peaks are shifted to the left, and these shifts are caused by spatial changes in the crystal surface due to plastic deformation. The half-peak widths of the (111), (200), and (311) diffraction peaks in the XRD pattern were used to find the grain size using Scherrer’s formula (1).

$$D=\frac{k\lambda }{\beta cos\theta }$$

(1)

where k, λ, and θ represent the constant (0.89), the wavelength of the X-rays (0.15405 nm), and the diffraction angle of the XRD, respectively. D* and σ represent average grain size and standard deviation, respectively.

The average grain size (D*) calculated from the XRD spectra represents the average size of a large area. Therefore, the XRD method provides an objective reflection of the changes in the average grain size. The calculation results are presented in

Table 2. Grain size analysis parameters of the surface before and after USR.

Specimens	Crystal Ceil	2θ/Degree	d/Å	FWHM	D/nm	D*	σ
UT	111	39.06	2.34	0.21	39.13	37.95	1.18
	200	44.68	2.027	0.23	36.77
USR	111	38.54	2.33	0.25	33.29	33.3	0.016
	200	44.82	2.021	0.26	33.32

, which shows that the average grain sizes of the specimens before and after USR are 37.95 nm and 33.3 nm, respectively. This suggests that the grains appear to have been refined after undergoing USR.

3.2. Friction Coefficient and Wear Volume

Figure 5 illustrates the variation of friction coefficient with number of cycles under both oil lubricated and dry fretting conditions. It can be observed that D = 50 μm reaches a steady state after 1000 cycles in the dry fretting condition, which is similar to the oil lubrication case, both with short break-in periods, whereas D = 150 µm and D = 250 µm reached a steady state after 7500 cycles. The friction coefficient exhibits a sharp increase and then decreases within the first 1000 cycles, gradually reaching stabilization (Figure 5a). In the stable stage, the fluctuation of the friction coefficient is small. This can be attributed to the small tangential movement resistance and low wear rate of the friction pair due to the protection of lubricating oil film. Overall, the friction coefficient of the USR-treated surface is lower than that of the untreated surface, possibly due to its lower roughness, which reduces the adhesion between the localized micro-convex bodies on the upper and lower surfaces.

The results in Figure 5b indicate that the friction coefficient exhibits an initially increasing and then stabilizing tendency under the dry fretting condition. The fluctuation of the friction coefficient is mainly attributed to the insufficient protection of the lubricant film, which intensifies wear as well as the generation and overflow behavior of wear debris. Additionally, the fluctuation of surface roughness has a significant impact, resulting in a relatively large fluctuation in the contact state of the friction [24]. In addition, it can be seen that the fluctuation of the friction coefficient at D = 150 μm and 250 μm is larger than that at D = 50 μm. This can be attributed to the increase in the displacement amplitude, leading to more drastic wear and fluctuations in the surface contact state.

Figure 6 and Figure 7 show the three-dimensional morphology and the two-dimensional cross-sectional profile of worn surfaces. It can be seen that all the wear marks are U-shaped. According to the typical characteristics of fretting worn surfaces (adhesive wear and plastic deformation in partial slip regime; existence of crack initiation and propagation in the mixed slip region; severe material removal in the gross slip region [23]), it can be inferred that in this paper, all the fretting tests run in the gross slip regime. Moreover, different degrees of bulging can be observed at the edge of the wear scar, primarily caused by the extrusion deformation of the material in the contact area due to reciprocating contact stress. Furthermore, it is evident that when lubricated with oil, the wear pits on the surface are reduced in size after undergoing USR treatment (Figure 6), and the width and depth of wear scar contours are also diminished (Figure 7). This indicates an improvement in wear resistance after USR treatment under oil lubrication conditions. Conversely, when subjected to dry fretting, the area of pits on the worn surface increases after USR treatment (Figure 6), and the width and depth of wear scar contours also increase. This suggests that USR treatment exacerbates wear under dry friction conditions.

To more intuitively reflect the effect of USR on the wear degree, the wear volume was calculated using the Ultra-Depth Three-Dimensional Microscope software by integrating the volume of the abrasion pits with the unworn surface as the reference plane. Figure 8 indicates that the wear degree increases with the increase in displacement amplitude. Overall, under oil lubrication conditions, the wear volume decreased by about 40% after the USR, but under dry fretting conditions, the wear volume increased by about 20%. From the above phenomena, it can be inferred that the fretting wear properties of the ultrasonic rolling surface are significantly affected by the lubrication conditions. In this paper, the USR-treated 7075 aluminum alloy surfaces were more suitable for fretting under oil rather than under dry conditions.

3.3. Wear Mechanism

Figure 9a indicates that the UT surface under oil lubrication conditions exhibits a relatively smooth worn area with mainly plastic deformation characteristics. However, the central area shows a more pronounced furrow feature. So, it can be concluded that the wear mechanisms of the 7075 aluminum alloy surface under oil lubrication conditions are primarily plastic deformation and abrasive wear. For the USR-treated surface (Figure 9b), the entire wear area is relatively flat, and the furrow characteristics are significantly reduced, possibly because its higher surface hardness improves the anti-abrasive wear ability. From Figure 9c,d, it can be seen that the main features of the UT and USR specimen surfaces under dry fretting conditions are plastic deformation and delamination, and the presence of furrows can also be observed at the ends of the fretting region, suggesting that the wear mechanisms are mainly plastic deformation, delamination, and abrasive wear. In contrast, plastic deformation is more severe on the UT surface, while abrasive wear and delamination are more serious on the USR surface.

The wear debris on the dry fretting UT and USR worn surfaces were collected and observed by the Ultra-Depth Three-Dimensional Microscope (Figure 10). It can be seen that the wear debris is mainly formed by fine particles and is black in color. This is because after peeling off the base material, the flaky wear debris was repeatedly crushed and ground, and then it oxidized with the surrounding oxygen [25]. In the debris generated on the worn surface of USR-treated sample, not only fine particles but also a flaky debris with metallic color can be observed. The possible reason is that the hardness and brittleness of the surface are improved after USR treatment. The material peeled off from the surface is difficult to be crushed due to its high hardness, and it will scratch the surface during the reciprocating motion of the friction pair, thereby intensifying the abrasive wear [26], which is also the reason for the increase in wear after USR treatment under dry fretting conditions.

The elements (

Table 3. The content of the elements detected on the worn surfaces.

Specimens	O	C	Al	Fe	Gu	Zn
UT-oil	39.66	41.75	15.69	1.74	0.46	0.7
USR-oil	40.95	39.31	17.17	1.86	0.39	0.31
UT-dry	43.02	35.38	19.54	1.32	0.48	0.26
USR-dry	42.3	35.99	19.82	1.13	0.37	0.38

) and chemical states of the worn surfaces were analyzed by X-ray photoelectron spectroscopy. The binding energy was calibrated using C1s = 284.8 eV, and a fine spectrum analysis was carried out for the two most abundant metal elements on the worn surface, Al and Fe. As depicted in Figure 11, the type of substances produced on the surface in terms of Al 2p and Fe 2p does not show a significant difference with the use of USR. However, the lubrication conditions have a pronounced effect. Specifically, as shown in Figure 11a, the peaks of Al 2p on both lubricated surfaces indicate the presence of oxide (Al₂O₃) as well as Al monomers inherent in the material itself. However, under oil lubrication conditions, the worn surface generates AlO (OH) (corresponding to 76.59 eV) due to the presence of OH- ions [27,28]. In Figure 11b, the substances generated by Fe 2p under both lubrication conditions are mainly Fe₂O₃ and Fe₃O₄, while there is also the presence of FeS (713.06 eV and 71.54 eV) as well as Fe monomers (718.04 eV) under oil lubrication conditions. The presence of sulfides is beneficial to reduce the adhesion of surface materials and reduce the friction coefficient, which is one of the important reasons why oil lubrication is beneficial to reduce micromotion damage.

4. Conclusions

• Compared with the untreated specimens, the USR treatment results in a 90% reduction in surface roughness and a 13HV increase in microhardness. The XRD analysis showed that the grains were refined after USR treatment.
• Under oil lubricated conditions, the wear mechanism mainly consists of abrasive wear, plastic deformation, and oxidation. The friction coefficient is reduced and the amount of wear is decreased after the USR treatment. This is due to the increase in surface hardness and the decrease in roughness, which has a certain inhibitory effect on surface wear and plastic deformation. Moreover, after USR, the grain refinement leads to more boundaries, which makes it easier for the oil to penetrate into the material, generating more FeS to reduce the surface adhesion of the material and lowering the coefficient of friction.
• Under dry fretting conditions, the main wear mechanisms are abrasive and oxidative wear. The wear volume and friction coefficient of the USR-treated surfaces are increased, and this should be attributed to the exacerbation of abrasive wear by the hard abrasive particles exfoliated from the USR surfaces.