⚡ electrical engineering

Surface Roughness Evolution and Tool-Wear Mechanisms in Prolonged Milling of Ti6Al4V ELI under Sunflower-Oil and MoS₂-Enhanced MQL Environments

This study demonstrates that while sunflower oil-based MQL significantly improves surface finish during prolonged milling of Ti6Al4V ELI, the addition of MoS₂ nanoparticles primarily enhances tool preservation by suppressing built-up edge and delamination rather than further reducing surface roughness.

Original authors: Deniz ÇOBAN ÖZKAN, Fikret Sönmez

Published 2026-08-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Deniz ÇOBAN ÖZKAN, Fikret Sönmez

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine trying to carve a piece of metal that is as tough as steel but refuses to let heat escape. This is the daily reality for engineers working with a specific titanium alloy known as Ti6Al4V ELI. It is a material prized for its incredible strength and its ability to live inside the human body without causing harm, making it a favorite for aerospace parts and medical implants. However, its very strength makes it a nightmare to machine. When a cutting tool tries to slice through it, the metal does not conduct the heat away; instead, the heat stays trapped right at the point of contact. This intense friction causes the metal to stick to the tool, forming a rough, jagged lump that ruins the smoothness of the cut and wears the tool down rapidly. To solve this, manufacturers usually spray a flood of liquid coolant, but this creates a mess of waste and health hazards. A cleaner alternative is a method called minimum quantity lubrication, which sprays a tiny, precise mist of oil directly onto the cutting edge. The question researchers have long debated is whether adding special microscopic particles to this mist can make it even better, or if the oil alone is enough.

In a recent study, researchers at Manisa Celal Bayar University set out to find the answer by putting these ideas to a rigorous test. They did not just run a quick cut and check the results; they subjected their tools to a grueling, continuous machining session lasting eighty-four minutes. This extended duration was crucial because it allowed them to watch how the tools and the metal surfaces changed over time, rather than just seeing a snapshot of the beginning. They tested five different environments: cutting with no lubrication at all, using a standard industrial cutting fluid, using a mist of sunflower oil, and then adding a specific type of nanoparticle called molybdenum disulfide to both the industrial fluid and the sunflower oil. The goal was to see which combination kept the tool sharp and the metal surface smoothest as the cutting process wore on.

The results were striking and revealed a clear hierarchy of performance. When the researchers cut the titanium without any lubrication, the tool suffered immediately. The cutting edge fractured, the protective coating peeled off, and the metal surface became incredibly rough. In contrast, the use of any lubrication strategy improved the situation significantly. The most successful outcome for creating a smooth surface came from using a mist of pure sunflower oil. This natural oil produced a surface roughness of 0.159 micrometers, a massive improvement over the dry cutting method, which resulted in a roughness of 0.645 micrometers. The researchers found that the sunflower oil was naturally excellent at wetting the metal and reducing friction, acting as a superior lubricant right from the start.

Interestingly, the addition of the molybdenum disulfide nanoparticles did not make the surface any smoother than the sunflower oil alone. The data showed no statistical difference in the final smoothness of the metal between the pure sunflower oil and the sunflower oil mixed with nanoparticles. This finding challenges the common assumption that adding more advanced particles always leads to a better finish. However, the story changed when the researchers looked at the tools themselves. While the nanoparticles did not improve the surface texture, they played a vital role in protecting the cutting tool. Under the microscope, the tools used with the nanoparticle-enhanced oil showed significantly less damage. The nanoparticles helped prevent the metal from sticking to the tool and forming those damaging lumps, and they kept the tool's protective coating from peeling away.

The study also uncovered a subtle but important truth about how machining works over time. The effectiveness of the lubrication was not constant; it changed as the tool wore down. The researchers discovered that the relationship between the type of oil used and the quality of the surface depended heavily on how many passes the tool had already made. A lubricant that worked perfectly at the beginning might behave differently after the tool had been cutting for an hour. This means that judging a lubrication method by a single, short test can be misleading. The true performance of a lubricant is a dynamic story that unfolds over the entire duration of the job.

Ultimately, the research suggests that for machining this difficult titanium alloy, a simple, sustainable approach may be the best. Sunflower oil, a common vegetable oil, proved to be a highly effective and environmentally friendly alternative to industrial chemicals, capable of producing the smoothest surfaces on its own. The addition of nanoparticles, while not necessary for surface smoothness, offered a distinct advantage for preserving the tool itself, reducing wear and extending its life. The findings point toward a future where sustainable, plant-based lubricants can replace harsh industrial coolants, offering a cleaner way to manufacture the high-strength components that power our aircraft and medical devices.

Technical Summary: Surface Roughness Evolution and Tool-Wear Mechanisms in Prolonged Milling of Ti6Al4V ELI under Sunflower-Oil and MoS₂-Enhanced MQL Environments

Problem Statement
Ti6Al4V ELI (Grade 23) is a critical material for aerospace and biomedical applications due to its high strength-to-weight ratio and biocompatibility. However, its machinability is severely compromised by low thermal conductivity, high chemical affinity with cutting tools, and the tendency to form built-up edges (BUE). These factors lead to rapid tool degradation and poor surface integrity. While Minimum Quantity Lubrication (MQL) offers an environmentally friendly alternative to flood cooling, and nanoparticle-enhanced MQL (NFMQL) has shown promise, significant research gaps remain. Specifically, there is limited data on the long-term tribological behavior of Ti6Al4V ELI (as opposed to the more common Grade 5), the combined application of sunflower oil and MoS₂ nanoparticles, and the statistical interaction between lubrication strategies and machining progression (pass number) over extended durations. Most existing studies focus on short-duration experiments, failing to capture progressive wear mechanisms and coating degradation.

Methodology
The study employed a face milling operation on Ti6Al4V ELI (Grade 23) using a 4-flute AlCrN-coated carbide end mill. The experiments were conducted under a sustained 84-minute protocol involving six successive axial passes to remove a total of 6 mm of material. Five distinct cooling/lubrication environments were evaluated:

Dry cutting.
Conventional MQL using a general-purpose cutting fluid (Werte Fastcut Ultra).
MQL with MoS₂-enhanced general-purpose fluid (0.5 vol.%).
MQL using sunflower oil.
MQL using sunflower oil with MoS₂ nanoparticles (0.5 vol.%).

To ensure consistency, a dedicated tool was used for each lubrication condition. Surface roughness ( $R_a$ and $R_z$ ) was measured after every pass using a profilometer, while tool wear was monitored via a toolmaker's microscope. Post-experiment analysis included Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) to characterize wear mechanisms and coating integrity. The MoS₂ nanoparticles were further characterized using SEM, FTIR, XRD, and XPS to confirm phase purity and crystallite size. Statistical analysis was performed using Two-Way ANOVA and Tukey HSD tests to evaluate the main effects of lubrication and pass number, as well as their interaction on surface roughness.

Key Contributions
This research makes three primary contributions to the field of sustainable machining:

Extended Duration Protocol: It provides the first systematic comparison of five lubrication environments for Ti6Al4V ELI under a prolonged 84-minute milling protocol, allowing for the observation of steady-state wear and progressive surface evolution.
Statistical Interaction Analysis: The study utilizes Two-Way ANOVA to statistically resolve the interaction between lubrication strategy and pass number, demonstrating that the effectiveness of a lubrication method is not static but evolves as tool wear progresses.
Differentiation of MoS₂ Benefits: It clarifies the specific role of MoS₂ nanoparticles, distinguishing between their impact on surface finish versus tool preservation, a distinction often blurred in previous literature.

Results

Surface Roughness: Dry cutting resulted in the poorest surface quality ( $R_a = 0.645$ µm) due to severe edge fracture and BUE formation. Sunflower oil-based MQL yielded the best surface finish ( $R_a = 0.159$ µm), representing a 75% reduction compared to dry cutting. The addition of MoS₂ nanoparticles to sunflower oil did not yield a statistically significant further improvement in surface roughness ( $R_a = 0.166$ µm).
Statistical Findings: Two-Way ANOVA confirmed that both lubrication condition and pass number significantly affect surface roughness. Crucially, a highly significant interaction effect was found ( $p < 0.001$ , partial $\eta^2 = 0.822$ ), indicating that the performance of a lubrication strategy changes depending on the machining pass. For instance, while dry cutting initially produced the worst results, the final pass showed a temporary reduction in roughness attributed to the burnishing effect of a severely worn edge, rather than improved cutting performance.
Tool Wear and Mechanisms: Dry cutting caused severe edge fracture and coating delamination, with maximum flank wear reaching 252 µm. SEM-EDX analysis revealed that MoS₂ nanoparticles effectively suppressed BUE formation and coating delamination. While MoS₂ did not significantly improve surface finish, it contributed to tool preservation by forming a tribofilm that reduced direct metal-to-metal contact and material adhesion. However, under the prolonged thermal stress of the 84-minute protocol, crater wear remained significant due to the low thermal conductivity of the alloy and the limited convective cooling of MQL, suggesting MoS₂ is more effective against adhesive wear than thermally activated diffusion wear.
Nanoparticle Characterization: XRD analysis confirmed the hexagonal phase of MoS₂ with an average crystallite size of 40.74 nm, while SEM showed particle sizes in the 200–400 nm range, consistent with a polycrystalline structure. FTIR and XPS verified the presence of characteristic Mo–S bonds without significant oxide formation.

Significance and Claims
The authors claim that this study establishes sunflower oil-based MQL as a superior, sustainable alternative to conventional cutting fluids for the finish milling of Ti6Al4V ELI, offering excellent surface integrity and reduced tool wear. The paper posits that while MoS₂ nanoparticles do not necessarily enhance surface roughness beyond what high-quality vegetable oils can achieve, their primary value lies in extending tool life by suppressing BUE and coating damage. The research underscores that evaluating machining performance requires considering the dynamic interaction between lubrication and tool wear progression over time, rather than relying on single-point measurements. The study concludes that for applications where tool life is the primary constraint, the addition of MoS₂ to sunflower oil is beneficial, whereas for surface finish optimization, sunflower oil alone is sufficient.

More like this