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Article

Influence of Nano-Lubricants on Edge Cracking and Surface Quality of Rolled Mg/Al Composite Foils

by
Guang Feng
1,2,3,*,
Ning Wang
1,
Zhongxiang Li
1,
Shaoyong Du
1 and
Zhaopeng Li
1
1
College of Mechanical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
2
Engineering Research Center of Advanced Metal Composites Forming Technology and Equipment, Ministry of Education, Taiyuan 030024, China
3
TYUT-UOW Joint Research Centre of Advanced Forming and Manufacturing Technology, Taiyuan 030024, China
*
Author to whom correspondence should be addressed.
Lubricants 2025, 13(12), 518; https://doi.org/10.3390/lubricants13120518
Submission received: 25 October 2025 / Revised: 20 November 2025 / Accepted: 28 November 2025 / Published: 29 November 2025
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Abstract

This study systematically investigates the effect and mechanism of a TiO2 nano-lubricant on edge cracking and surface quality during the rolling of Mg/Al composite foils. Initial friction and wear tests identified an optimal nano-lubricant concentration of 3.0 wt.%, at which the system achieved a minimum average coefficient of friction of 0.067. Subsequent rolling tests using this concentration showed that the nano-lubricant reduced rolling force by 5.39–7.54% compared to dry conditions. It also significantly suppressed the initiation and propagation of edge cracks. Furthermore, the surface roughness parameters Ra and Rz were reduced by 16.5% to 24.0%, and the height profile fluctuation range was reduced by 33% to 45%, resulting in a smoother and more uniform surface morphology. The analysis of the underlying mechanism indicates that the superior performance originates from the synergistic effects of the rolling effect, the mending effect, the polishing effect, and the protective film effect. This work establishes that the use of a 3.0 wt.% TiO2 nano-lubricant is a viable strategy for fabricating high-quality Mg/Al composite foils with minimal defects. It thereby offers both theoretical and practical guidance for the advanced rolling of bimetallic composites.

1. Introduction

Magnesium alloys offer several advantageous properties, including low density, high specific strength, and excellent thermal conductivity. In contrast, aluminum alloys are highly malleable, corrosion-resistant, and exhibit good electrical conductivity. By combining magnesium and aluminum, researchers can fabricate magnesium/aluminum (Mg/Al) composite foils that harness the benefits of both metals. The resulting foils are not only lightweight and strong but also offer excellent conductivity and corrosion resistance. These combined properties make Mg/Al composite foils a highly promising material for electronic components in Micro-Electro-Mechanical Systems [1,2,3]. However, the widespread application of Mg/Al composite foils is impeded by the poor intrinsic formability of magnesium alloys. This limitation primarily arises from their hexagonal close-packed (HCP) crystal structure, which results in restricted plasticity at room temperature [4].
The predominant technique for fabricating Mg/Al composite foils is roll bonding. However, this method frequently encounters challenges such as edge cracking and poor surface quality [5,6]. Edge cracking refers to the initiation of cracks at the edges of a material during the rolling process, which subsequently reduces the yield of the final product [7]. To suppress edge cracking in magnesium alloys during the rolling process, Tian et al. [8] employed width-limited rolling. In this technique, specially shaped rolls are used to constrain the sheet’s width, inducing compressive stress at the edges, diminishing tensile stress along the rolling direction, and facilitating uniform stretching across the sheet. They revealed that this approach effectively suppressed edge cracking. Sun et al. [9] investigated the influence of pulsed electric current on edge cracking in magnesium alloy sheets. They found that the application of pulsed current resulted in an increased formation of shear bands, which subsequently reduced the strain within each band, thereby suppressing the occurrence of edge cracking. This crack-suppressing effect was more pronounced at higher current densities. Huang et al. [10] demonstrated that the introduction of small bumps along the edges of thin magnesium sheets prior to rolling enhanced the metal flow in the rolling direction at those edges. This alteration also modified the stress state at the edge during the rolling process, thereby suppressing the adverse effects of uneven metal flow on edge cracking. Xie et al. [11] investigated edge cracking in strip steel through experiments and analyses using atomic force microscopy and scanning electron microscopy. They concluded that the rate of crack formation and propagation is influenced by both friction and surface roughness. Their findings also revealed that decreasing the friction coefficient can effectively suppress the formation of small surface cracks. Ballarini et al. [12] investigated the relationship between the fracture load of various materials and their friction coefficients. Their findings indicated that the fracture load increases with an elevated friction coefficient in their experimental context. It is crucial to distinguish this from the rolling process, where a higher friction coefficient can generate greater frictional forces at the sheet-roll interface, which may increase the shear stress and thereby the likelihood of edge cracking. Zafošnik et al. [13] showed that friction during the rolling process significantly influences the strain and stress distribution within the steel strip, thereby affecting the propagation of edge cracks. Similarly, Maiya et al. [14] identified surface roughness as a critical factor in the formation of edge cracks during steel strip rolling, demonstrating that employing refined surface treatment processes can effectively suppress their development. Given the critical roles of friction and surface roughness in edge cracking, the application of lubricants becomes essential in the rolling process. However, conventional oil-based lubricants can cause environmental contamination. As lubrication technology advances, water-based lubricants containing nanoscale particles are increasingly utilized in tribology and lubrication [15,16].
Given the importance of surface conditions, nano-lubricants have emerged as a promising surface treatment technology. By synergistically combining multiple lubrication mechanisms—including the rolling effect, mending effect, protective film effect, and polishing effect—they significantly enhance lubrication performance, reduce friction, improve material surface quality, and suppress the initiation and propagation of edge cracks [17,18,19]. Amnon et al. [20] demonstrated that lubricants reduce mill load during the rolling of low-carbon steel. The lubricant acts by separating the rolls from the workpiece, which in turn reduces friction and wear, thereby enhancing the surface quality. Xia et al. [21] employed a TiO2 nano-lubricant during the rolling of 304 stainless steel. They found that its application reduced the rolling force and improved the surface roughness. Xie et al. [22] added SiO2 nano-lubricants during the rolling process of magnesium alloy sheets. Their findings indicated that the application of nano-lubricants significantly enhanced the surface quality. Sun et al. [23] reported that a water-based TiO2 nano-lubricant significantly diminished the rolling force and enhanced the surface quality of stainless steel, identifying a concentration of 3.0 wt.% as the most effective. Similarly, Ma et al. [24] observed improved surface quality in copper foils with TiO2 nano-lubricants, also identifying an optimal concentration of 3.0 wt.%. Wu et al. [25] revealed through experimental studies that water-based lubricants containing TiO2 nanoparticles exhibit excellent tribological properties. The lubrication mechanism is attributed to the rolling and repairing effects of the nanoparticles. Furthermore, due to their cost-effectiveness, TiO2 nano-lubricants are widely adopted in rolling processes. These findings provide a robust foundation for the use of nano-lubricants in single-metal systems. However, their application in the rolling of bimetallic composite materials remains scarce.
In summary, substantial advancements have been achieved in process methodologies and auxiliary techniques aimed at suppressing edge cracks in magnesium alloys during rolling. Extensive research has been conducted on the role of nano-lubricants in reducing the coefficient of friction, enhancing surface quality, and decreasing rolling forces. However, existing research focuses predominantly on single-metal materials, with relatively few studies conducted on Mg/Al composite foils. Due to the considerable differences in plastic deformation behavior between magnesium and aluminum alloys, challenges such as edge cracking and uneven thickness distribution frequently arise during rolling as a result of incompatible deformation [4]. Nevertheless, the existing literature predominantly emphasizes the effects of nano-lubricants on individual magnesium alloy materials. For example, Xie et al. [26] demonstrated that applying nano-lubricants during the cold rolling of magnesium foils effectively reduces rolling forces and improves surface quality. Wang et al. [27] found that nano-lubricants improve the tribological properties of magnesium alloys by forming a lubricating film that reduces the depth and width of wear scars, thereby enhancing surface integrity. Gürgenç et al. [28] investigated the effects of nano-lubricants on the tribological performance of magnesium alloys. Their findings showed that the incorporation of nano-additives significantly enhanced both the wear resistance and surface integrity of these alloys. They emphasized that lubrication conditions are critical to the tribological behavior of magnesium alloys. They further noted that reducing friction forces can extend the service life of magnesium alloy components by decreasing shear stress and strain on the material surface, thereby suppressing crack initiation. However, the application of nano-lubricants in the rolling of composite materials—particularly regarding their mechanisms for improving interfacial friction conditions to facilitate the deformation of bimetallic materials and subsequently suppress edge cracking—has yet to be systematically explored.
Therefore, this study aims to investigate the mechanism by which TiO2 nano-lubricants affect edge cracks and surface quality during the rolling of Mg/Al composite foils. The tribological properties of lubricants with different concentrations will first be evaluated through friction and wear tests. Subsequently, rolling experiments will be conducted to examine variations in rolling force, statistically analyze the average depth, width, and density of edge cracks, and measure the thickness distribution and surface roughness of the foils. Finally, the lubrication mechanism of the nano-lubricant will be elucidated by combining the above results with elemental distribution analysis. This work ultimately seeks to provide a theoretical basis for fabricating high-quality Mg/Al composite foils.

2. Materials and Methods

2.1. Materials

The materials for the rolling experiment were AZ31B magnesium alloy with dimensions of 150 × 15 × 0.1 mm3 and 5052 aluminum alloy with dimensions of 150 × 15 × 0.05 mm3. Their chemical compositions are provided in Table 1. Prior to rolling, the material surfaces were cleaned with alcohol to eliminate the influence of initial surface conditions. The side of the magnesium alloy intended for bonding with the aluminum alloy was ground with sandpaper to remove the oxide layer, thereby facilitating roll bonding.

2.2. Preparation of Titanium Dioxide Nano-Lubricant

The preparation procedure of the titanium dioxide (TiO2) nano-lubricant is shown in Figure 1. First, sodium dodecylbenzene sulfonate (SDBS) was added to deionized water as a dispersant. The mixture was sheared using a high-speed disperser (by IKA Corporation, Staufen, German) at 7200 rpm for 10 min. Subsequently, TiO2 nanoparticles were added at their respective concentrations. The mixture was stirred at the same speed for another 10 min. Next, the thickener sodium polyacrylate (PAAS) was added. Stirring continued at the same speed for 30 min to ensure the solution was fully mixed. The mixture was then subjected to ultrasonication for 30 min. This process ensured even dispersion of the nanoparticles and prevented agglomeration. In this study, TiO2 nano-lubricants with concentrations of 1.0 wt.%, 3.0 wt.%, 5.0 wt.%, and 7.0 wt.% were prepared.

2.3. Friction and Wear Tests

Friction and wear tests were conducted on magnesium foils using an MFT-5000 (RTEC Corporation, San Jose, CA, USA) multi-environment material wear resistance and lubrication testing system to measure the coefficient of friction (COF). The magnesium foils were cut into specimens with dimensions of 30 × 15 × 0.1 mm3. Each specimen was fixed onto a ceramic plate, which was then secured to the reciprocating module of the testing system. The experiments were performed at room temperature under dry conditions and with TiO2 nano-lubricant at concentrations of 1.0 wt.%, 3.0 wt.%, 5.0 wt.%, and 7.0 wt.%. For tests under nano-lubrication conditions, the nano-lubricant was applied by pouring it into a rectangular tray until the specimen surface was fully submerged. Prior to each test, the specimen and instrument were wiped with alcohol, and an identical GCr15 bearing steel ball with a diameter of 9.5 mm was used as the friction pair for each experiment. The reciprocating friction and wear test consisted of two stages. The first stage involved using a pressure sensor to determine whether the friction ball made contact with the specimen. In the second stage, the tests were conducted under a 10 N normal load for 120 s at 2 Hz reciprocating frequency.

2.4. Micro Rolling Test

A four-high micro rolling mill with a maximum rolling force of 40 kN was used for the experiments, as shown in Figure 2. Figure 3 shows the surface morphology and three-dimensional profile of the work rolls employed in the experiments. The work rolls had a diameter of 22 mm and a barrel length of 44 mm. Given that the 3.0 wt.% nano-lubricant demonstrated the lowest COF in our friction and wear tests, it was selected for the subsequent rolling experiments. Therefore, the rolling tests were conducted under both dry conditions and with the application of a 3.0 wt.% concentration of TiO2 nano-lubricant, at reduction rates of 32%, 37%, and 42%. The rolling speed was set at 0.25 m/min. Prior to each rolling pass, the nano-lubricant was evenly sprayed onto the surfaces of the upper and lower rolls. Three repeated tests were performed under each condition to obtain average values and minimize experimental errors. Several samples were sectioned from the rolled foils for subsequent characterization. Given the poorer formability of magnesium compared to aluminum, the analysis primarily focused on the quality of the magnesium alloy side of the composite foils.

2.5. Theoretical Basis for Rolling Force Reduction by Nano-Lubricant

Variations in rolling force serve as a key indicator for evaluating the effectiveness of nano-lubricants, making it essential to establish the theoretical basis for lubrication-induced rolling force reduction. The rolling of Mg/Al composite foils involves both plastic deformation and elastic deformation at the entry and exit zones. Therefore, the Bland-Ford-Hill model [29], which incorporates both types of deformation, is commonly employed for calculating rolling force in the cold rolling process.
F = F P + F e
F P = Q F k m − ξ W R ( h i n − h o u t )
F e = 2 3 1 − v 2 E k m h o u t h i n − h o u t k m − ξ W R ( h i n − h o u t )
Q F = 1.08 − 1.02 r + 1.79 r μ 1 − r R h o u t
ξ = α t i n + β t o u t
In the formula, F is the rolling force; Fp and Fe are the rolling forces in the plastic and elastic zones, respectively; QF is the influence coefficient for external friction on the rolling force; km is the mean deformation resistance; W is the foil width; R is the flattened roll radius; hin and hout are the entry and exit foil thicknesses, respectively; ν is Poisson’s ratio; E is Young’s modulus; μ is the COF; r is the reduction rate; tin and tout are the entry and exit unit tensions, respectively; α and β are the influence coefficients for the entry and exit tensions, respectively.
As the model indicates, during the rolling of Mg/Al composite foils under the same reduction rate, if the nano-lubricant could reduce the COF, the influence coefficient of external friction on rolling force would consequently decrease. This led to a lower rolling force in the plastic deformation zone, thereby reducing the total rolling force. The theoretical reduction in rolling force due to decreased COF provided the basis for evaluating nano-lubricant performance in the subsequent rolling experiments.

2.6. Characterization and Analytical Approaches

The rolling force was monitored using a force sensor (MTS Systems Corporation, Minneapolis, MN, USA) installed on the rolling mill. The surface morphology and 3D profile of the rolled foils were characterized using a KEYENCE VK-X1000 3D laser scanning microscope (manufactured by Keyence Corporation, Okayama, Japan). The average depth, average width, and linear density of edge cracks were measured, and thickness distributions along both the rolling and transverse directions were analyzed to generate height profile images. Surface roughness was evaluated by measuring three distinct regions of the foils: the head, middle, and tail sections. The macroscopic distribution of the nano-lubricant on the foil surfaces and additional information regarding the microscopic distribution of titanium elements were obtained using a JEOL-IT500 scanning electron microscope (SEM) (JEOL, Tokyo, Japan) equipped with an energy dispersive spectroscopy (EDS) detector (Oxford Ltd., Oxford, UK). These analyses provided insights into the underlying lubrication mechanism.

3. Results and Discussion

3.1. Variations in COF Under Different Lubrication Conditions

The results of the friction and wear tests are presented in Figure 4. The average COF was determined from the steady-state region of each friction curve. Under dry conditions, the average COF of the magnesium foil was 0.42. With the application of nano-lubricants, the average COF values were 0.095, 0.067, 0.078, and 0.11 at concentrations of 1.0 wt.%, 3.0 wt.%, 5.0 wt.%, and 7.0 wt.%, respectively. As shown in Figure 4, compared to the unlubricated condition, the presence of the nano-lubricant significantly reduced the COF of the magnesium foil. When the nano-lubricant concentration increased from 1.0 wt.% to 3.0 wt.%, the COF decreased, reaching a minimum at 3.0 wt.%. This can be attributed to the fact that TiO2 nanoparticles partially converted sliding friction into rolling friction during the wear process, and the mending effect of the nanoparticles also contributed to the reduction in the COF. Moreover, an increase in nanoparticle concentration allows more particles to enter the contact zone, which consequently enhances both the rolling bearing and mending effects. However, when the nano-lubricant concentration increased to 5.0 wt.% and 7.0 wt.%, the COF rose, and the curve exhibited more pronounced fluctuations. This is because nanoparticle agglomeration occurred at higher concentrations, which increased particle size and hindered other nanoparticles from entering the contact zone, thereby exacerbating wear and leading to an increase in the COF [30,31]. Overall, the curve at the 3.0 wt.% concentration demonstrated the smallest fluctuation amplitude, remained relatively stable over an extended period, and exhibited the lowest COF. Consequently, the nano-lubricant at a 3.0 wt.% concentration provided the optimal lubrication performance and was selected for subsequent rolling experiments.

3.2. Variation in Rolling Force Under Different Rolling Conditions

Figure 5 shows the variation in rolling force after the rolling tests under different reduction rates, with data recorded directly by the rolling mill. When the reduction rate increased from 32% to 42%, the rolling force rose from 4.45 kN to 5.42 kN under dry conditions, and from 4.21 kN to 5.04 kN under nano-lubricated conditions. The results demonstrate that the application of the nano-lubricant under the same reduction rate effectively reduced the rolling force, achieving a reduction of 5.39% to 7.54%. This is attributed to the improved lubrication conditions provided by the nano-lubricant during the rolling of the Mg/Al composite foils, which lowered the COF, reduced power consumption during rolling, and consequently decreased the rolling force [32]. These findings are consistent with the theoretical findings derived from the Bland-Ford-Hill model, demonstrating the effectiveness of the nano-lubricant during the rolling process. Furthermore, the reduction in rolling force, which lowers interfacial shear stress, contributes to the suppression of edge crack initiation.

3.3. Influence of Various Rolling Conditions on Foil Edge Quality

Figure 6 compares the edge morphology on the magnesium alloy side of the rolled Mg/Al composite foils under dry and nano-lubricated conditions at reduction rates of 32%, 37%, and 42%. The corresponding quantitative data of average crack depth, width, and linear density under different lubrication conditions are summarized in Table 2, Table 3 and Table 4.
Based on the data from Table 2, Table 3 and Table 4, under dry conditions, the edge cracking severity escalated with increasing reduction rate. At a 32% reduction rate, the average depth and width of the edge cracks were 199.4 μm and 33.6 μm, respectively, with a linear density of 4 cracks per millimeter. The corresponding morphology of the magnesium side, depicted in Figure 6a, exhibited numerous edge cracks with limited transverse propagation. However, no significant coalescence between adjacent cracks was observed, resulting in a moderate overall edge quality. When the reduction rate increased to 37%, the average crack depth and width increased to 288.2 μm and 160.1 μm, respectively, while the linear density decreased to 3 cracks per millimeter. As shown in Figure 6c, the foil edge exhibited a slightly wavy profile, and the cracks underwent substantial transverse propagation, resulting in poorer edge quality compared to that at the lower reduction rate. At a 42% reduction rate, the average crack depth and width increased significantly to 628.8 μm and 275.1 μm, respectively, while the linear density decreased to 1.5 cracks per millimeter. Figure 6e shows a prominently wavy foil edge, where the cracks underwent severe transverse propagation. The low crack density indicates that adjacent cracks had coalesced into larger ones, leading to severely compromised edge quality. In summary, the data from Table 2, Table 3 and Table 4, corroborated by the morphological evidence in Figure 6a,c,e, confirm non-uniform deformation at the foil edge under dry conditions, with cracks propagating preferentially in the transverse direction. As the reduction rate increased, the average depth and width of the edge cracks increased substantially, their transverse propagation was aggravated, and they progressively coalesced into larger cracks. These severe defects critically degrade the serviceability of the rolled foil and significantly reduce its utilization rate.
Table 2, Table 3 and Table 4 demonstrate a significant improvement in edge quality under nano-lubricated conditions compared to dry conditions. At a reduction rate of 32%, no visible edge cracks were observed. The morphology of the magnesium side edge, presented in Figure 6b, showed a smooth and intact profile, indicating good edge quality. When the reduction rate increased to 37%, minor edge cracks emerged, with an average depth and width of 64.4 μm and 7.5 μm, respectively, and a linear density of 1 crack per millimeter. As shown in Figure 6d, the foil edge remained relatively smooth with only very fine cracks, which corresponds to a satisfactorily good edge quality. At the highest reduction rate of 42%, the average crack depth and width increased to 114.4 μm and 32.9 μm, respectively, with a linear density of 2.5 cracks per millimeter. Figure 6f reveals the presence of edge defects, where the cracks began to propagate in the transverse direction, resulting in an overall moderate edge quality. The statistical data from Table 2, Table 3 and Table 4, combined with the morphological observations in Figure 6b,d,f, demonstrate that under nano-lubricated conditions, edge cracks began to appear as the reduction rate increased. Although the number of cracks gradually increased, their transverse propagation was limited, and they showed no tendency to coalesce. This evidence confirms that the presence of the nano-lubricant effectively suppressed the initiation and propagation of edge cracks during the rolling of the Mg/Al composite foils, thereby enhancing the edge quality and improving the rolling yield. However, the nano-lubricant could not completely eliminate edge cracks at higher reduction rates. Therefore, a reduction rate of 37% is identified as the optimal condition for minimizing edge cracking.
During cold rolling, edge cracking initiates with the formation of micro-cracks at the foil edge, which subsequently propagate transversely. As evidenced in Figure 6a,c,e, the crack width is greatest near the edge, as this is the initiation site. Initially, the grains exhibit a parallel, layered structure aligned with the rolling direction. The edge grains, however, experience less mechanical constraint and undergo less work hardening than the interior grains during rolling. This difference in deformation behavior aggravates crack propagation not only in the transverse direction but also along the rolling direction [33]. As shown in Figure 6, the transverse propagation distance of edge cracks varies along the rolling direction. This phenomenon is consistent with a slip band-induced propagation mechanism [34]. Specifically, when a crack reaches a critical length, its advancement is facilitated by the formation of slip bands ahead of the crack tip. The associated stress concentrations from dislocation pile-ups within these bands promote further crack growth, resulting in an alternating propagation along activated slip systems. As shown in Figure 6c,e, the edge cracks propagate longitudinally at an angle of approximately 45° to the rolling direction, which aligns with the direction of maximum shear stress. Furthermore, an increase in the reduction rate raises the rolling force and, consequently, the shear stress. This elevated stress intensifies the longitudinal propagation of the cracks. As a result, adjacent cracks may coalesce into larger ones, explaining the observed decrease in crack density and increase in average crack width at higher reduction rates.
Crack propagation during rolling is predominantly governed by interfacial friction, through its control of the contact conditions and its alteration of the internal strain and stress fields within the Mg/Al composite foils. This, in turn, influences the magnitude of the secondary tensile stress at the edges, which subsequently governs the propagation rate and direction of edge cracks. As shown in Figure 6, under dry conditions, edge cracks propagate deeply into the foil in the transverse direction. However, when nano-lubricant is present, cracks are confined to the immediate edge region with minimal further propagation. This stark difference can be explained by the role of initial surface defects under dry conditions: these defects act as high-friction sites upon roll contact. The resulting elevated friction intensifies the stress concentration at these locations, transferring greater pressure to the edge. Once the local stress exceeds the material’s yield strength, plastic deformation occurs, which reduces the resistance to crack propagation and promotes the growth and coalescence of initial defects into larger edge cracks [35,36]. However, when the nano-lubricant is applied, the nanoparticles mitigate friction by partially converting sliding friction into rolling friction. The consequent reduction in shear stress suppresses shear deformation and alleviates stress concentration at the foil edge, thereby effectively inhibiting the initiation of edge cracks. Furthermore, even when small defects or micro-cracks form at higher reduction rates, nanoparticles carried by the lubricant can fill these surface imperfections. This “mending effect” reduces stress concentrations at the crack tips, mitigates excessive plastic deformation, and suppresses micro-crack propagation. In summary, the nano-lubricant effectively suppresses both the initiation and propagation of edge cracks, leading to an improved material utilization rate.
Figure 7 shows the height profile fluctuations on the magnesium-side edge of the Mg/Al composite foil after rolling at reduction rates of 32%, 37%, and 42% under both dry and nano-lubricated conditions. The height profile fluctuation ranges under dry and nano-lubricated conditions at various reduction rates are as follows: at 32%, 2.30 μm in transverse and 2.31 μm in rolling direction versus 1.36 μm and 1.23 μm; at 37%, 2.72 μm and 2.76 μm versus 1.53 μm and 1.59 μm; at 42%, 3.28 μm and 3.32 μm versus 1.93 μm and 2.07 μm. The nano-lubricant suppressed these fluctuations, achieving a reduction range of 33% to 45%. A comparison of the two curves in Figure 7 indicates that the nano-lubricated condition yields a smaller height profile fluctuation range, a smoother curve, and a more uniform height distribution than the dry condition. Furthermore, the fluctuation range increases with the reduction rate. This trend is attributed to inhomogeneous deformation between the edge and center regions during rolling, which directly manifests as post-rolling surface height variations. As the reduction rate increases, this deformation inhomogeneity is amplified, leading to a greater fluctuation amplitude in the foil surface height.
The aforementioned issues of inhomogeneous deformation and stress concentration can be effectively mitigated by applying a TiO2 nano-lubricant. During the rolling of Mg/Al composite foils, the high pressure induces slight bending of the upper and lower rollers, thereby creating an uneven distribution of rolling force along the transverse direction. Furthermore, the friction between the rollers and the foil is unstable. Localized areas of high friction impede metal flow, which in turn reduces elongation along the rolling direction. These factors collectively lead to uneven deformation in both the transverse and rolling directions of the foil after rolling [37,38]. Surface roughness also induces stress concentration, which is detrimental to edge formation. This is because micro-cracks typically initiate from rough surface regions, where uneven topography promotes stress concentration. This concentrated stress subsequently causes sharp local fluctuations in the foil surface height profile. Figure 7 reveals sharper peaks, characterized by abrupt height changes, in the height profile under dry conditions. These peaks are caused by stress concentration. Both uneven edge deformation and stress concentration are key factors contributing to edge cracking. The application of TiO2 nano-lubricant mitigates frictional energy dissipation and stabilizes the friction coefficient. The nanoparticles form a uniform lubricating film at the roll/foil interface, which not only reduces but also homogenizes the friction coefficient across the foil surface. Consequently, high-friction zones are eliminated, promoting more uniform metal flow. The resultant reduction in rolling force minimizes roller bending, thereby improving the uniformity of deformation across the foil’s transverse direction. Additionally, the reduced interfacial shear resistance enables more coordinated plastic deformation along the rolling direction [39]. The nano-lubricating film ensures a more uniform surface roughness for the rollers to contact, thereby mitigating stress concentration and preventing the formation of sharp peaks in the height profile. Moreover, the lubricant’s high-pressure viscosity causes it to thicken within newly formed micro-cracks. This behavior isolates the crack tip from high-stress pulses and reduces the local stress concentration [11]. Consequently, the nano-lubricant promotes a more uniform stress distribution during rolling, resulting in more homogeneous foil deformation and a reduced propensity for edge cracking.

3.4. Variation in Foil Surface Quality Under Different Rolling Conditions

Figure 8 shows the surface roughness values, averaged from three separate measurements, on the magnesium side of the rolled Mg/Al composite foils at reduction rates of 32%, 37%, and 42%. To avoid areas prone to edge cracks, stress concentration, and other defects that can compromise measurement reliability, the surface roughness was measured in the center of the foil, where deformation and stress distribution are more uniform. As shown in Figure 8, as the reduction rate increases from 32% to 42%, the surface roughness values increase under both dry and nano-lubricated conditions. Under dry conditions, Ra and Rz rise from 0.142 μm and 2.16 μm to 0.181 μm and 3.07 μm, respectively. Correspondingly, under nano-lubricated conditions, they increase from 0.116 μm and 1.76 μm to 0.154 μm and 2.56 μm. This trend indicates that a higher reduction rate deteriorates the surface finish, which is attributed to the generation of more microcracks during severe cold rolling deformation. A comparison in Figure 8 demonstrates that the nano-lubricant effectively reduces surface roughness. Specifically, at 32%, 37%, and 42% reduction rates, Ra is reduced by 22.2%, 24.0%, and 18.2%, and Rz by 18.4%, 18.6%, and 16.5%, respectively, compared to dry conditions. This indicates that the improvement in surface quality is less pronounced at the 42% reduction rate than at the lower reduction rates. This attenuation of the beneficial effect at the highest reduction rate suggests a compromised effectiveness of the lubrication mechanism under extreme deformation conditions. The underlying mechanism is that an excessively high reduction rate raises the rolling force, which in turn intensifies plastic deformation to a degree that can disrupt the integrity of the nanoparticle lubricating film. Consequently, direct contact between the roller and the foil occurs, leading to surface quality deterioration. This demonstrates that the effectiveness of the nano-lubricant progressively diminishes with increasing reduction rate and may ultimately fail under excessively high reduction rates. Consequently, the reduction rate must be maintained within an optimal range to ensure effective nano-lubricant performance. This conclusion aligns with the results in Figure 6, where the nano-lubricant could not completely suppress edge cracks at higher reduction rates. Therefore, a 37% reduction rate is identified as the optimum for maximizing lubricant effectiveness.
Figure 9 shows the surface morphology and 3D profile of the magnesium surface after rolling under dry conditions at various reduction rates. The surface morphologies in Figure 9 exhibit a pronounced dependence on the reduction rate. At the lower reduction rate of 32%, the surface shows only minor variations. At the higher reduction rate of 42%, however, the formation of continuous microcracks is readily apparent, resulting in pronounced deterioration of surface quality. Figure 10 shows the surface morphology and 3D profile of the magnesium surface under nano-lubricated conditions at different reduction rates. At a 32% reduction rate, the foil surface is smooth and flat. However, as the reduction rate increases to 42%, the surface becomes significantly rougher. The superior surface quality achieved with the nano-lubricant is evidenced by a direct comparison of Figure 9 and Figure 10, which reveals a more uniform, flatter surface with significantly reduced height variations and the elimination of microcracks. These findings confirm the efficacy of the TiO2 nano-lubricant in enhancing surface quality, a conclusion validated by the roughness measurements presented in Figure 8.

3.5. SEM-EDS Analysis

Figure 11 shows the SEM and EDS mapping of the magnesium side after rolling under different reduction rates. The distribution of titanium, revealed by EDS mapping, serves as direct evidence for the distribution of TiO2 nanoparticles. Figure 11 demonstrates that a higher reduction rate results in an increased number of nanoparticles being deposited on the magnesium surface. This phenomenon is attributed to two primary mechanisms. Firstly, as the reduction rate increases, the foil undergoes more severe plastic deformation during rolling, which increases the contact area between the nanoparticles and the foil surface, thereby promoting the adhesion of a greater number of nanoparticles. Secondly, when the reduction rate becomes excessively high, microcracks may form on the foil surface, providing sites where nanoparticles can readily accumulate [40]. Additionally, the accelerated material deformation at higher reduction rates may prevent the nanoparticles from having sufficient time to achieve uniform dispersion between the roller and the foil, leading to localized aggregation of nanoparticles. The agglomerated and non-uniform distribution of nanoparticles prevents the lubricating film from forming effectively, thereby compromising its ability to suppress cracks.

3.6. Lubrication Mechanism

The lubrication mechanisms of the nano-lubricant can be summarized as four primary types, as shown in Figure 12. The first mechanism is the polishing effect, as illustrated in Figure 12a. Under rolling pressure, the hard nanoparticles in the lubricant act as a polishing material, which performs micro-cutting on the surface asperities. The TiO2 nanoparticles used in this study have high hardness and can therefore effectively remove surface asperities on the foil [41]. The protective film effect, the second mechanism, corresponds to the principle shown in Figure 12b. During rolling, the nano-lubricant forms a protective film that prevents direct contact between the roller and the material surface, thereby minimizing wear and scratches [42]. The rolling effect, the third mechanism, corresponds to the principle shown in Figure 12c. The nano-lubricant fills the gap between the roller and the foil, thereby converting sliding friction into rolling friction and reducing both the COF and the resultant friction force [43]. The mending effect, the fourth mechanism, corresponds to the principle shown in Figure 12d. During rolling, the nano-lubricant can enter the microcracks and other surface defects that readily form on the foil, thereby filling and mending them [44]. In summary, the synergistic effects of nanoparticles significantly enhance the rolling performance. The polishing effect produces a smoother foil morphology by reducing surface roughness. As evidenced by the titanium distribution in Figure 11, nanoparticles form a protective film that improves surface quality. Simultaneously, the rolling effect lowers the coefficient of friction, consequently reducing rolling force and suppressing edge crack initiation. Furthermore, the mending effect inhibits crack propagation through the filling of surface defects by nanoparticles.

4. Conclusions

This study systematically investigated the role of a TiO2 nano-lubricant in suppressing edge cracking and improving the surface quality of Mg/Al composite foils during rolling. The tribological properties, rolling force, edge crack characteristics, surface roughness, and lubrication mechanism were thoroughly examined. The principal conclusions are summarized as follows:
  • The TiO2 nano-lubricant significantly reduced the COF during the sliding contact of magnesium foils. An optimal concentration of 3.0 wt.% was identified, at which the system achieved a minimum average COF of 0.067. Both lower and higher concentrations resulted in less effective lubrication, with the deterioration at high concentrations attributed to nanoparticle agglomeration.
  • The application of the 3.0 wt.% nano-lubricant during the rolling of Mg/Al composite foils effectively reduced the rolling force by 5.39% to 7.54%, which is consistent with the predictions of the Bland-Ford-Hill model. This reduction in rolling force led to a decrease in interfacial shear stress, a key driver for edge crack initiation.
  • The nano-lubricant profoundly suppressed the initiation and propagation of edge cracks on the magnesium side. Under dry conditions, edge cracks underwent severe transverse propagation and coalescence, leading to deteriorated edge quality at higher reduction rates. In contrast, the nano-lubricant confined cracks to the immediate edge region, limited their transverse propagation, and prevented their coalescence. Furthermore, by promoting more coordinated metal flow and improving deformation uniformity, the nano-lubricant reduced the height profile fluctuation range by 33% to 45% in both the transverse and rolling directions, contributing to superior dimensional stability.
  • The surface quality of the rolled foils was significantly improved by the nano-lubricant, which reduced the surface roughness parameters Ra and Rz by 16.5% to 24.0% compared to dry conditions. This improvement is attributed to a more uniform deformation and the elimination of microcracks, as directly evidenced by the smoother surface morphology and reduced height profile fluctuations.
  • The superior performance is attributed to the synergistic operation of four lubrication mechanisms: the rolling effect and mending effect, which were primarily responsible for reducing the COF and rolling force and for suppressing crack propagation, respectively; and the polishing effect and protective film effect, which collectively enhanced the surface finish. SEM-EDS analysis provided direct evidence of the TiO2 nanoparticle distribution, confirming the formation of a lubricating film and the filling of surface defects.
In conclusion, the application of a 3.0 wt.% TiO2 nano-lubricant at an optimal reduction rate of 37% is a highly effective strategy for fabricating high-quality Mg/Al composite foils with significantly suppressed edge cracking and a superior surface finish, thereby providing a theoretical basis and practical guidance for the rolling of bimetallic composites and demonstrating the significant potential of nano-lubricants in advanced manufacturing processes.

Author Contributions

Conceptualization, G.F. and N.W.; methodology, G.F.; validation, N.W., Z.L. (Zhongxiang Li) and S.D.; resources, G.F.; writing—original draft preparation, N.W.; writing—review and editing, G.F.; supervision, Z.L. (Zhaopeng Li); funding acquisition, G.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China under Grant No. 52475392 and 52074190, the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province under Grant No. 20220007, the Research Project Supported by Shanxi Scholarship Council of China under Grant No. 2023-043, and the Fundamental Research Program of Shanxi Province under Grant No. 202403021211208.

Data Availability Statement

All relevant data are included in this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Preparation procedure of TiO2 nano-lubricant.
Figure 1. Preparation procedure of TiO2 nano-lubricant.
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Figure 2. Four-high precision micro rolling mill.
Figure 2. Four-high precision micro rolling mill.
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Figure 3. Surface morphology and 3D profile of the rolls.
Figure 3. Surface morphology and 3D profile of the rolls.
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Figure 4. The COF curves of the magnesium foil under different lubrication conditions.
Figure 4. The COF curves of the magnesium foil under different lubrication conditions.
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Figure 5. Variation in rolling force with reduction rate.
Figure 5. Variation in rolling force with reduction rate.
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Figure 6. Edge morphology on the magnesium side of the rolled foils under different rolling conditions: (a,c,e) dry; (b,d,f) with nano-lubricant, at reduction rates of 32%, 37%, and 42%, respectively.
Figure 6. Edge morphology on the magnesium side of the rolled foils under different rolling conditions: (a,c,e) dry; (b,d,f) with nano-lubricant, at reduction rates of 32%, 37%, and 42%, respectively.
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Figure 7. Height profile images of the magnesium-side edge of the foil under different lubrication conditions (a) reduction rate 32%; (b) reduction rate 37%; (c) reduction rate 42%.
Figure 7. Height profile images of the magnesium-side edge of the foil under different lubrication conditions (a) reduction rate 32%; (b) reduction rate 37%; (c) reduction rate 42%.
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Figure 8. Surface roughness of the magnesium side of the foil after rolling under different reduction rates (a) Ra, (b) Rz.
Figure 8. Surface roughness of the magnesium side of the foil after rolling under different reduction rates (a) Ra, (b) Rz.
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Figure 9. Surface morphology and 3D profile of the magnesium side of the foil after rolling under dry conditions at different reduction rates (a) 32% (b) 37% (c) 42%.
Figure 9. Surface morphology and 3D profile of the magnesium side of the foil after rolling under dry conditions at different reduction rates (a) 32% (b) 37% (c) 42%.
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Figure 10. Surface morphology and 3D profile of the magnesium side of the foil after rolling under nano-lubricated conditions at different reduction rates (a) 32% (b) 37% (c) 42%.
Figure 10. Surface morphology and 3D profile of the magnesium side of the foil after rolling under nano-lubricated conditions at different reduction rates (a) 32% (b) 37% (c) 42%.
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Lubricants 13 00518 g011aLubricants 13 00518 g011b
Figure 11. SEM and EDS mappings of the magnesium side of the foil after rolling under different reduction rates (a) 32% (b) 37% (c) 42%.
Figure 11. SEM and EDS mappings of the magnesium side of the foil after rolling under different reduction rates (a) 32% (b) 37% (c) 42%.
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Figure 12. Lubrication mechanisms of the nano-lubricant: (a) Polishing effect, (b) Protective film effect, (c) Rolling effect, (d) Mending effect.
Figure 12. Lubrication mechanisms of the nano-lubricant: (a) Polishing effect, (b) Protective film effect, (c) Rolling effect, (d) Mending effect.
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Table 1. Chemical composition of AZ31B and 5052 (wt.%).
Table 1. Chemical composition of AZ31B and 5052 (wt.%).
MaterialMgCuCaMnSiAlZnCrFe
AZ31B Mg alloyOthers0.010.040.80.073.21.2--
5052 Al alloy2.2–2.80.1-0.10.25Others0.10.15–0.350.4
Table 2. Average Edge Crack Depth under Dry and Lubricated Conditions (μm).
Table 2. Average Edge Crack Depth under Dry and Lubricated Conditions (μm).
Reduction Rate32%37%42%
Dry199.4288.2628.8
Lubricant064.4114.4
Table 3. Average Edge Crack Width under Dry and Lubricated Conditions (μm).
Table 3. Average Edge Crack Width under Dry and Lubricated Conditions (μm).
Reduction Rate32%37%42%
Dry33.6160.1275.1
Lubricant07.532.9
Table 4. Linear density of edge cracks under dry and lubricated conditions (cracks/mm).
Table 4. Linear density of edge cracks under dry and lubricated conditions (cracks/mm).
Reduction Rate32%37%42%
Dry431.5
Lubricant012.5
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Feng, G.; Wang, N.; Li, Z.; Du, S.; Li, Z. Influence of Nano-Lubricants on Edge Cracking and Surface Quality of Rolled Mg/Al Composite Foils. Lubricants 2025, 13, 518. https://doi.org/10.3390/lubricants13120518

AMA Style

Feng G, Wang N, Li Z, Du S, Li Z. Influence of Nano-Lubricants on Edge Cracking and Surface Quality of Rolled Mg/Al Composite Foils. Lubricants. 2025; 13(12):518. https://doi.org/10.3390/lubricants13120518

Chicago/Turabian Style

Feng, Guang, Ning Wang, Zhongxiang Li, Shaoyong Du, and Zhaopeng Li. 2025. "Influence of Nano-Lubricants on Edge Cracking and Surface Quality of Rolled Mg/Al Composite Foils" Lubricants 13, no. 12: 518. https://doi.org/10.3390/lubricants13120518

APA Style

Feng, G., Wang, N., Li, Z., Du, S., & Li, Z. (2025). Influence of Nano-Lubricants on Edge Cracking and Surface Quality of Rolled Mg/Al Composite Foils. Lubricants, 13(12), 518. https://doi.org/10.3390/lubricants13120518

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