Introduction

Nickel-based superalloys are extensively used in aerospace, marine engineering, oil and gas refineries, nuclear power plants, and heat exchangers due to their outstanding heat resistance, chemical stability, and wear resistance1,2. These materials exhibit excellent corrosion resistance, maintain mechanical strength at elevated temperatures, and retain hardness even under extreme operational conditions3,4. Among these, Monel-400 has gained significant attention for its superior mechanical performance, including excellent toughness and strength even at sub-zero temperatures. However, Monel-400 poses notable challenges in machining, which are distinctly different from the general properties of nickel-based alloys. The alloy’s low thermal conductivity leads to excessive heat generation during cutting, while its work-hardening tendency contributes to rapid tool wear and unstable cutting conditions5. These issues are further aggravated by mechanical load fluctuations and chip sliding, complicating tool-workpiece interaction and reducing machining performance6. Initially, dry machining was explored as a sustainable alternative, but it resulted in elevated cutting zone temperatures, promoting the formation of built-up edge (BUE) and accelerating tool degradation. The combination of poor heat dissipation and work-hardening behavior significantly deteriorated both tool life and surface integrity7. Conventional mineral-based coolants were subsequently adopted to address these challenges, offering better thermal control, tool life, and dimensional accuracy. However, their usage raised environmental and occupational concerns, including health hazards and high costs associated with disposal and recycling8. These persistent challenges have underscored the need for optimized cooling and lubrication strategies tailored to Monel-400, particularly approaches that strike a balance between machining performance, tool longevity, and sustainability.

Numerous research endeavors have been undertaken to mitigate adverse effects, with a prominent emphasis on Minimum Quantity Lubrication (MQL), also referred to as near-dry machining9,10. Vegetable-based oils and synthetic esters, known for their superior biodegradability, are the most commonly employed cutting fluids in MQL applications11. To reduce cutting temperatures, and forces while enhancing machining quality, minimal quantities of lubricant are precisely delivered to the cutting zone in the form of fine droplets. External MQL devices equipped with multiple nozzles are employed in machining operations where the machining zone remains fully accessible. Yıldırım et al.12 conducted an investigation into the machining of Waspaloy using coated carbide tools under MQL conditions, demonstrating that vegetable-based oils exhibited superior performance over other lubricants in minimizing cutting forces and prolonging tool life. In recent times, Saraf et al.13 explored the use of micro-pillar textured tungsten carbide inserts—fabricated via Reverse Micro-EDM—for machining additively manufactured Ti6 Al4 V. Their study showed that under MQL medium, textured tools significantly enhanced machinability by reducing chip-tool contact, cutting forces, flank wear, and improving surface finish. Nevertheless, a notable drawback of MQL lies in the propensity of oil particles to combust and evaporate before effectively reaching the cutting zone, primarily due to the substantial thermal energy generated at elevated cutting speeds14. Meanwhile, water vapor is an eco-friendly and cost-effective cooling and lubrication medium in machining superalloys, offering efficient heat dissipation and minimal environmental impact. Few studies highlight its potential in reducing tool wear and enhancing surface quality while promoting sustainability. For an example, Fan et al.15 investigated Inconel 718 machining under water vapor + air cooling, finding that oxides like Cr2O3 and Fe2O3 reduce tool wear by lubricating the tool-chip interface, lowering cutting temperatures, and enhancing tool life compared to dry cutting. However, Water vapor can lead to increased tool corrosion and may not provide sufficient lubrication under high-speed machining conditions. Additionally, its effectiveness in heat dissipation can be limited in extreme machining environments. Cryogenic cooling has emerged as one of the most efficacious techniques for promoting sustainability in machining processes. Commonly employed cryogenic coolants include liquid nitrogen (LN2) and liquid carbon dioxide (CO2). In a representative study, Inconel 718 was utilized as the workpiece material, undergoing machining with coated carbide cutting tools. Cryogenic machining demonstrates a sustainable alternative, delivering superior surface finish with reduced roughness across various cutting parameters16. During machining, heat is generated primarily due to the plastic deformation of the metal in the shearing zone, friction between the chip and the rake face, and interaction between the workpiece and the flank face. The application of cryogenic coolants significantly mitigates friction at these contact points, leading to a considerable reduction in cutting zone temperature and enhancing tool life. Despite the evident performance enhancements afforded by cryogenic environments, inconsistent outcomes have been observed, largely due to insufficient lubrication17. Additionally, cryogenic systems can be expensive to set up and require a steady supply of cryogenic fluids, which adds to the cost. Safety is also very important—there are risks like frostbite, pressure build-up, and low oxygen levels in enclosed spaces. To use these systems safely, proper ventilation, insulated pipes, and protective clothing are necessary.

Previous authors have explored Cryo-MQL in machining of superalloys due to its combined benefits of enhanced cooling from cryogenic fluids (like liquid nitrogen or CO₂) and lubrication from minimal bio-based oils. This hybrid approach addresses the critical challenges of high cutting temperatures, rapid tool wear, and poor surface finish typically encountered in machining superalloys. For an instance, Shokrani et al.18 developed a hybrid Cryo-MQL system for machining Ti-based superalloy, extending tool longevity by 30 times in contrast to traditional flooded cooling. Danish et al.19 underscored the requirement for machining techniques that supports contemporary ecological and economic requirements. Their research on Ni-based superalloy machining under eco-friendly lubricating conditions identified LN2 as the most effective coolant, minimizing costs, energy consumption, cutting forces, and tool degradation while enhancing surface finish, thereby promoting aerospace sustainability. Schoop et al.20 investigated the machining of superalloys under flooded, and sustainable lubricating mediums, revealing that the Cryo + MQL system significantly diminished tool wear (by 4–5 times), whereas cryogenic machining yielded the finest surface quality. Yildirim et al.21 explored superalloy turning in MQL, cryogenic, and Cryo-MQL conditions, determining that Cryo-MQL was the most efficient in decreasing cutting temperatures, tool deterioration, and surface roughness. Danish et al.22 reported Inconel 718’s poor thermal conductivity using a Cryo-MQL system featuring separate nozzles for LN2 and vegetable oil. This approach led to a 61%, 37%, and 20% drop in surface roughness, cutting temperature and tool wear as compared to dry machining. SEM images illustrated adhesion and abrasion wear, while Cryo-MQL enhanced microhardness and grain refinement. Gajrani et al.23 introduced a hybrid MQL-cryogenic lubrication technique for machining titanium alloys. Cryo-MQL reduced cutting forces by 27% and surface roughness by 46% relative to dry machining. Tool morphology examination indicated a diminished contact length in sliding-sticking areas, and surface elemental mapping revealed less workpiece adhesion on the tool rake face. Gupta et al.24 explored hybrid cryogenic MQL cooling/lubrication techniques, including Ranque-Hilsch Vortex tube assisted MQL (RHVT + MQL), and N2 + MQL, for turning Ti-6 Al-4 V with CVD-coated carbide inserts. The study shows that N2 + MQL enhances tool wear, surface quality, chip morphology, and reduces specific cutting energy, achieving superior performance and sustainability compared to dry machining. Naik et al.25 developed the intermittent spraying system using a time-regulated pulsating fluid delivery technique. Their Pulse-assisted Cryo + MQL method proved economical, generating smoother surfaces with minimum tool wear and reduced cutting forces. Further, SEM and EDX studies validated its efficiency in machining titanium alloys. Korkmaz et al.26 assessed various cooling strategies for turning Inconel 601, including dry cutting, MQL, nanofluid based MQL (NMQL), cryogenic cooling, and cryogenic-assisted NMQL. Cryo-NMQL exhibited the best performance, substantially reducing flank wear and sustaining continuous chip formation by decreasing tool-chip friction. Dry cutting resulted in coarser grain structures and increased plastic deformation, whereas cryogenic and NMQL cooling promoted grain refinement and enhanced microhardness. Recently, Sarikaya et al.1 explored the machinability of the cobalt-based Haynes 25 superalloy using sustainable cooling and lubrication methods. Their findings show that hybrid cooling systems, especially those combining nanofluids and cryogenic CO₂, significantly improve machining performance by reducing tool wear, cutting temperature, and power consumption, offering a sustainable approach to enhancing machining efficiency and tool life.

This study investigates the role of MQL and cryogenic cooling in enhancing the machinability of superalloys compared to conventional dry cutting. MQL and NMQL offer superior lubrication, and cryogenic cooling ensures efficient heat dissipation, yet each technique has its inherent limitations. Cryogenic cooling, while effective at lowering temperatures, lacks lubricity, and MQL alone may fall short in mitigating the significant thermal loads encountered during machining nickel-based alloys. To address these limitations, hybrid lubrication-cooling strategies present a promising solution. However, most previous studies have concentrated on well-explored alloys such as Inconel 718 and Ti-6 Al-4 V, focusing primarily on tool wear, surface integrity, and thermal effects. In contrast, this study shifts the focus to Monel 400, a nickel-copper alloy known for its high thermal conductivity, significant work-hardening tendency, and poor machinability. These characteristics pose substantial challenges in cutting operations, making it a prime candidate for exploring novel machining strategies. Recognizing the need for a sustainable and effective approach, this study introduces a novel hybrid MQL + CO₂ system—integrating sunflower oil-based MQL with cryogenic carbon dioxide—to simultaneously provide lubrication and cooling. This innovative system is specifically designed to overcome the machining challenges of Monel 400, a material that has not been extensively studied in hybrid machining research. The study systematically evaluates cutting force, tool wear, surface finish, and surface morphology under dry, MQL, and cryogenic CO₂ conditions, highlighting the practical improvements in tool life and part quality. Furthermore, a multi-objective response surface methodology (MORSM) is employed to optimize cutting velocity, feed rate, and axial depth of cut, aiming to enhance machining efficiency and product quality. The findings of this study are highly relevant to industries where Monel 400 and similar superalloys are critical, such as aerospace, marine, and chemical processing, where high performance and cost-efficiency are paramount. By focusing on a challenging yet widely used alloy and offering a sustainable hybrid solution, this research contributes to the development of more efficient, eco-friendly machining processes.

Experimental details

Turning operation

Monel-400, a material known for its propensity to undergo work hardening during machining, is widely employed in high-performance applications owing to its exceptional corrosion resistance and mechanical properties. To evaluate its machinability, this experimental study meticulously selected four different lubrication/cooling environments—dry, MQL, cryogenic CO₂, and a hybrid MQL + CO₂ approach. The machining trials were conducted using a lathe machine (HMT, NH26), equipped with PCBN inserts securely mounted on an appropriate tool holder to ensure optimal cutting efficiency. Cylindrical rods, each measuring 250 × 50 mm, were utilized as the work material for these experiments. The chemical composition of Monel-400, as obtained from the material test certificate provided by the supplier, along with the corresponding error margins for each constituent element, is detailed in Table 1. The experimental parameters and setup are comprehensively outlined in Table 2. The selection of initial machining parameters was based on a combination of preliminary trials and insights from prior literature, ensuring a balanced assessment of tool performance, material removal, and surface integrity under varying cooling conditions.

Table 1 Elemental composition of Monel-400.
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Table 2 Details of the experimentation.
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The dry machining condition was executed without any external coolant or lubricant, highlighting a sustainable approach by entirely eliminating the use of additional fluids. Under the MQL condition, a minimal amount of sunflower oil was atomized using compressed air and delivered through a 3 mm nozzle aimed at the rake face of the cutting tool. In this setup, the sunflower oil serves as the lubricant while the compressed air functions as the coolant. This method offers a substantial reduction in environmental impact due to the significantly reduced oil usage. In the cryogenic CO₂ condition, liquid CO₂ is stored at approximately 57 bar and released through a 5 mm nozzle targeting the rake side of the cutting tool. A pressure regulator ensures controlled delivery, optimizing cooling efficiency by effectively managing the extreme temperatures generated during machining. While cryogenic cooling provides excellent heat removal performance, safety measures must be strictly enforced to prevent asphyxiation risks and to handle pressurized gases safely within the industrial shop floor environment. To end, the hybrid MQL + CO₂ condition integrates the benefits of both lubrication and cryogenic cooling. In this approach, the simultaneous spraying of MQL and CO₂ forms frozen oil particles that enhance the overall lubri-cooling effect. While this method improves tool life and surface finish, the associated costs of dual systems and safety precautions must be carefully evaluated for industrial adoption.

Design of experiment

For the present work, turning operations were conducted in two stages. Initially, four experiments were carried out under constant machining conditions, as presented in Table 2, to identify the most effective lubricating medium. Subsequently, twenty-seven experiments were performed using the selected lubricating medium to optimize the machining parameters. Taguchi’s L27 orthogonal array27 was employed to design these experiments. To incorporate the principle of randomization and reduce systematic bias, the order of the experimental runs was randomized. Furthermore, each experiment was replicated three times to ensure reproducibility and account for experimental variability. The repeatability of the measurements was confirmed by reporting the standard deviations for key experimental parameters such as surface roughness (0.0219 μm), cutting forces (2.2974 N), and tool wear (0.0299 mm), with a 95% confidence level. Cutting speed, feed rate, and depth of cut were selected as input variables, while resultant cutting force, tool wear, and surface roughness were taken as performance characteristics. The machining parameters and their corresponding levels are detailed in Table 3. The selected ranges for cutting speed, feed rate, and depth of cut were determined based on a combination of literature review and preliminary experiments, ensuring the chosen levels reflect practical relevance and promote optimal machining performance.

Table 3 Machining parameters and their levels.
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Response measurement

In this study, the cutting force applied to the Monel-400 workpiece was divided into three components: Fx, Fy, and Fz, with the resultant force being determined by combining these components. A quartz dynamometer (Kistler 9272 B) was employed to measure the cutting forces. The dynamometer, paired with a multi-channel charge amplifier (5080 A) and a computer running Kistler’s software, enabled the measurement and recording of the cutting forces. The data acquisition system facilitated force measurement at a sampling rate of up to 10 kHz, ensuring accurate capture of dynamic force variations during machining. Subsequently, tool wear, resulting from continuous friction at the tool-workpiece interface, was assessed through optical microscopy, with maximum flank wear (VBmax) used due to its highly non-uniform nature. Tool rejection criteria followed ISO 3685 standards, and Scanning Electron Microscopy (Hitachi, TM3000) was employed to analyze wear patterns on the tool. Energy Dispersive X-ray (EDX) analysis provided further insight into the adhesion status at the wear sites. Finally, surface roughness (Ra), a critical measure of machining precision, was evaluated using a 3D profilometer (Taylor Hobson) in accordance with ISO 4287 guidelines. To minimize measurement error, surface roughness was recorded at three different locations on the workpiece, and the average value was taken for further analysis. The flow diagram of the contemporary study is shown in Fig. 1.

Fig. 1
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The flow diagram of the contemporary study.

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Machining tribology of Monel-400

Surface-induced properties play a critical role in the overall performance and longevity of machining processes. The reduction of wear, achieved through the optimization of operational parameters, is essential to maintaining efficiency and tool life. In cutting operations, plastic deformation is induced in the material being machined, which is subsequently sheared off as chips. The interaction between the cutting tool and the workpiece generates significant forces, leading to localized deformations within the machined zone and causing microstructural alterations in the material28. Figure 2 presents a schematic depiction of the mechanical phenomena occurring at the interface between the tool and workpiece. This encompasses the primary, secondary, and tertiary shear zones, which emerge due to the interaction between the tool and work material. Machining operations involve various heat dissipation mechanisms, including conduction, convection, and radiation29. The thermal energy produced during the cutting process is allocated among the materials within the cutting zone, including the work material, tool, tool holder, and the chips. From a sustainability standpoint, prolonging tool longevity by minimizing wear through the utilization of cutting fluids and reducing dimensional inaccuracies is crucial30. In this investigation, Monel-400 is turned in dry, MQL, CO2, and MQL + CO2 mediums. The intricate mechanism of surface-based tribological interactions and their impact on the turned surface are illustrated in Fig. 2(a-d). Under dry cutting conditions (Fig. 2a), friction at the tool-workpiece junction intensifies due to the lack of a coolant, resulting in heightened tool wear and thermal stresses. The MQL condition (Fig. 2b) entails the deposition of fine sunflower oil particles in the machined surface. These fine particles adhere to both the tool and workpiece, creating a thin lubricating layer that reduces friction and improves heat dissipation from the heated surfaces until thermal saturation is reached. Cryogenic cooling (Fig. 2c), on the other hand, significantly alters the thermal behavior by enhancing the heat dissipation near the cutting area, thus improving thermal conductivity and generating a more pronounced thermal gradient. This cooling mechanism also leads to improved microstructural integrity of the machined surface, as it reduces thermal distortion and prevents phase transformations. When both cryogenic cooling and MQL are employed simultaneously (Fig. 2d), the interaction of chilled air and lubricated oil droplets results in a combined cooling and lubrication effect. The forced convection provided by the lubricated cold air regulates the temperature, further reducing the friction-induced heat. The frozen oil droplets remain in the cutting zone until they reach the saturation temperature, at which point they evaporate, continuing the cycle of cooling and lubrication. This dual-action technique not only controls the temperature more effectively but also improves the tribological properties of the machined surface, contributing to enhanced surface finish and prolonged tool life.

Fig. 2
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Tool-work interaction under various lubrication/cooling conditions.

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Results and discussion

Influence of lubrication/cooling conditions

Resultant cutting force

During the machining process, the cutting tool undergoes varying forces depending on the workpiece material and the cutting trajectory. Abrupt shifts in contact and movement can result in force variations, increasing friction and elevating temperatures within the cutting zone. This can affect the workpiece’s surface quality and, in severe cases, cause tool failure31. In this study, a dynamometer measured cutting forces to analyze how different machining conditions affect force behavior. The recorded force patterns are shown in Fig. 3.

Fig. 3
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Pattern of cutting forces during machining operation.

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The resultant cutting force (Fr) was calculated by combining the peak radial force (x-axis), tangential force (y-axis), and feed force (z-axis), as shown in Eq. (1). The data is presented in Fig. 4. Testing under different lubrication and cooling conditions showed that dry cutting produced the highest force at 337 N. Cryogenic CO₂ cooling reduced it to 302 N (10.38% decrease), while MQL with sunflower oil lowered it further to 288 N (14.54% decrease). The biggest reduction occurred with the MQL + CO₂ method, bringing the force down to 271 N (19.58% decrease). A similar study by Gajrani et al.23 also found that a hybrid lubri-cooling approach significantly reduced cutting forces in machining titanium-based superalloys.

$$F_r=\:\sqrt{{F}_{x}^{2}\:+\:{F}_{y}^{2}\:+{F}_{z}^{2}}$$
(1)

Where x, y, and z represent the Cartesian coordinate axes.

Fig. 4
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Variation of resultant cutting force with lubrication/cooling conditions.

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The observed reduction in cutting forces under Cryogenic CO₂, MQL, and MQL + CO₂ conditions, compared to dry machining, can be attributed to several interrelated physical mechanisms that affect the tribological and thermal behavior at the cutting interface. In Cryogenic CO₂-assisted machining, the extremely low temperature of the CO₂ jet leads to a rapid and continuous cooling of the cutting zone. This results in a significant decrease in the thermal load on both the tool and the workpiece. Consequently, thermal expansion is minimized, which in turn reduces the contact area and friction between the tool and work material. The suppressed thermal softening of the workpiece maintains its mechanical integrity, leading to a stable shear zone with reduced plastic deformation and cutting resistance. In MQL using vegetable-based oils such as sunflower oil, a thin lubricating film is deposited at the tool–chip and tool–workpiece interfaces. This film effectively lowers the coefficient of friction, reducing the mechanical energy needed to shear the material. Furthermore, the oil film acts as a thermal barrier, limiting heat transfer and thus maintaining a lower temperature in the cutting zone. The presence of MQL also mitigates the formation of built-up edge (BUE) by minimizing material adhesion on the tool surface, thereby enhancing cutting efficiency and reducing force fluctuations. Additionally, the continuous supply of fine oil mist helps in chip evacuation and debris removal, lowering the possibility of abrasive wear and tool damage, which further contributes to reduced cutting forces. The hybrid strategy of Cryogenic CO₂ combined with MQL leverages the advantages of both cooling and lubrication. The CO₂ jet rapidly removes heat from the cutting area, preventing thermal degradation of the lubricant and ensuring the oil retains its viscosity and lubricating capacity even under high-temperature conditions. Simultaneously, the lubricant facilitates smooth material removal with minimal interfacial friction. This synergy leads to a stabilized machining process with improved thermal control, reduced tool–workpiece contact stress, and suppression of tool wear mechanisms such as adhesion, abrasion, and diffusion. Collectively, these effects result in a substantial reduction in the cutting forces required during machining, enhancing tool life and surface integrity.

Tool wear

The machining characteristics, including cutting temperature and machined surface texture, are significantly affected by cutting tool degradation. The advancement of tool wear is determined by aspects such as the tool composition, coating variety, machining parameters, and lubrication or cooling techniques17. This section examines how different lubricants affect tool wear under fixed machining conditions, focusing on the maximum flank wear after 25 min of cutting. A key issue is notch wear, which is especially severe when machining nickel-based superalloys and plays a major role in tool degradation. Although studies highlight the importance of notch wear, its exact causes are not fully understood. However, research shows that a well-planned lubrication strategy can help reduce notching. Factors like mechanical stress, high temperatures, work-hardening, and microchips contribute to its formation and growth. Figure 5 shows how tool wear changes over time with different lubricants. The hybrid lubri-cooling method, combining Cryogenic CO2 and MQL, is the most effective in reducing wear. Compared to dry machining, tool wear decreases by 6.74% with Cryogenic CO2, 16.85% with MQL, and 19.10% with the MQL + CO2 combination.

Fig. 5
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Progression of tool wear with machining time.

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The observed reduction in tool wear when using Cryogenic CO₂, MQL, and their hybrid application (MQL + CO₂) compared to dry machining can be attributed to distinct and synergistic physical mechanisms involving enhanced cooling and lubrication at the tool–workpiece interface. Cryogenic CO₂, owing to its rapid expansion and phase change upon release, efficiently absorbs the heat generated during cutting. This drastic temperature drop not only reduces thermal stress and softening of the cutting tool but also suppresses the formation of BUE, which is a major contributor to tool wear. By maintaining a lower and more stable cutting zone temperature, Cryogenic CO₂ helps preserve the hardness and structural integrity of the cutting edge. On the other hand, MQL with sunflower oil acts through boundary lubrication, where a thin, adherent oil film reduces direct metal-to-metal contact. This film minimizes adhesive wear and friction-induced micro-cracking, leading to smoother chip flow and reduced tool flank and crater wear. The bio-based nature of sunflower oil also contributes to better wettability and tribo-chemical interactions, forming protective tribofilms that further inhibit wear mechanisms.

The combined use of Cryogenic CO₂ and MQL results in a hybrid lubri-cooling environment where cryogenic cooling rapidly removes heat while the oil-based lubrication reduces friction and adhesion. This dual mechanism mitigates both thermal and mechanical wear phenomena simultaneously, thereby significantly lowering notch wear, reducing diffusion-related degradation, and preserving the tool’s cutting geometry for extended periods. In contrast, dry machining lacks both cooling and lubrication support, leading to elevated cutting temperatures, higher friction, and increased wear due to oxidation, abrasion, and adhesion. These factors collectively accelerate tool wear and reduce machining efficiency. Hence, adopting sustainable lubrication strategies such as MQL, Cryogenic CO₂, and especially their hybrid application, is highly effective for minimizing tool wear, improving surface finish, and enhancing tool life—particularly in the context of high-performance machining of hard-to-cut materials like superalloys.

Tool wear mechanism

The SEM micrographs presented in Fig. 6 offer comprehensive insights into the prevailing wear mechanisms observed under various lubricating conditions, including Dry, MQL, Cryogenic CO₂, and hybrid MQL + CO₂ environments. Across all tested conditions, adhesive wear is prominently observed, characterized by the development of BUE and BUL. These features suggest the strong tendency of workpiece material to adhere to the cutting insert due to elevated interface temperatures and intense contact stresses during machining. The elemental composition of these adhered layers is confirmed by EDX analysis, corroborating the findings of Habeeb et al.32, who similarly documented the formation of BUE and BUL in the machining of superalloys. Ezugwu et al.33 further emphasized that microscopic chips can adhere to the cutting edge and rake face, promoting localized plastic deformation and accelerating tool wear.

The persistence of adhesive wear, even under advanced lubrication conditions such as MQL + CO₂, is attributed to the insufficient thermal diffusion capacity of lubricants in the extreme-temperature zones typically encountered in superalloy machining. These materials exhibit high-temperature strength and low thermal conductivity, which concentrate heat at the tool-workpiece interface. This thermal buildup intensifies metallurgical bonding between the tool and chip material, thereby enhancing the formation and stability of BUE/BUL, despite the presence of lubrication. Simultaneously, abrasive wear emerges as another dominant mechanism, manifested by distinct grooves and scratches aligned with chip flow direction on the tool surface. This abrasive action results from hard carbide particles present in Monel 400, which interact abrasively with the tool during chip evacuation. The grinding effect caused by these hard inclusions exacerbates tool degradation, especially when the lubrication is unable to form a robust protective tribofilm. Although the use of hybrid cooling/lubrication techniques, particularly MQL + CO₂, offers some mitigation in terms of reducing cutting temperature and marginally slowing wear progression, they do not fundamentally alter the governing wear mechanisms. Adhesive and abrasive wear continue to dominate due to the intrinsic material properties of superalloys and the high mechanical and thermal loads in the cutting zone. Understanding these persistent wear mechanisms is vital for advancing tool material design, coating technologies, and lubricant formulations, aiming to enhance tool life and minimize unscheduled downtimes in high-performance machining operations.

Fig. 6
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SEM analysis of cutting tools under different lubrication and cooling conditions.

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Surface roughness & morphology

Surface texture significantly affects mechanical components by impacting fatigue resistance, friction, wear, and impact strength34. This study assessed surface roughness following the ISO 4287 standard. Each condition underwent three trials, and the arithmetic mean was calculated to obtain the Ra value. The results, shown in Fig. 7, highlight surface roughness variations under different cooling and lubrication methods. Dry machining, performed without cooling or lubrication, had the highest roughness (0.89 μm). The MQL method provided better surface quality than dry machining and the Cryogenic CO2 method. MQL uses a fine mist of compressed air and sunflower oil to create a lubrication tribofilm, which improves surface quality, reduces wear, and minimizes heat effects. MQL improved surface roughness by 39.32% compared to dry machining. The MQL + CO2 method, combining cryogenic cooling with MQL, achieved the lowest roughness (0.47 μm). Compared to dry machining, surface roughness was reduced by 39.32% with Cryogenic CO2, 17.98% with MQL, and 47.19% with MQL + CO2. Research by Ross et al.35 supports these findings, showing that dry machining results in the roughest surfaces, while hybrid lubrication methods produce the smoothest. This study confirms that the best surface quality is achieved through combined lubri-cooling techniques.

Fig. 7
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Surface topology and variation of Ra with lubrication/cooling mediums.

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The microscopic images (Fig. 8) reveal significant differences in surface quality under various lubrication and cooling conditions, particularly in terms of feed marks, chatter marks, and chip adhesion. In dry machining, the absence of lubrication leads to high friction and elevated cutting temperatures, resulting in deep and irregular feed marks that indicate unstable material removal. The presence of chatter marks suggests fluctuations in cutting forces due to vibrations caused by tool-workpiece interaction. Additionally, chip adhesion is prominent, as high temperatures promote material build-up on the surface, forming a built-up edge that further deteriorates surface quality. In contrast, MQL improves the surface finish by reducing friction, leading to more uniform feed marks and eliminating chatter. However, MQL provides limited cooling, meaning minor chip adhesion can still occur due to localized heat generation. Cryogenic CO₂ cooling effectively lowers the cutting temperature, minimizing thermal expansion and helping achieve more uniform feed marks than in dry machining. However, chip adhesion persists to some extent due to the lack of strong lubrication, causing occasional surface defects. The best surface quality is achieved with the hybrid MQL + CO₂ cooling strategy, where lubrication and cooling work together to reduce friction and temperature simultaneously. This results in mild and evenly spaced feed marks, the absence of chatter marks, and minimal chip adhesion, leading to the smoothest machined surface among all methods. In short, dry machining produces the roughest surface with the most defects, while hybrid lubri-cooling ensures superior surface integrity by effectively controlling thermal and mechanical damage.

Fig. 8
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Surface morphology of machined workpieces in different lubrication and cooling conditions.

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Statistical analysis and optimization framework

Probability distribution of machining responses

Before utilizing the collected machining response data, an appropriate probability distribution analysis was conducted to ensure its usability. Typically, data generated from any system tend to follow a normal distribution. Therefore, it is crucial to verify whether each machining response—resultant cutting force, tool wear, and surface roughness—adheres to this assumption. The empirical cumulative distribution (ECD) function is a statistical tool used to assess whether a dataset conforms to a specific probability distribution36. Since this study focuses on normal distribution, the collected data were plotted using the ECD method, as shown in Fig. 9. In these plots, the x-axis represents the machining responses, while the y-axis indicates the corresponding percentiles. The stepped (stair-like) lines depict the cumulative progression of data values, whereas the smooth curve represents the expected normal distribution function. From the plotted results, it is evident that all machining response data align reasonably well with the normal distribution. Specifically, the mean values for resultant cutting force, tool wear, and surface roughness are 271.38, 0.29 and 0.72, respectively, with standard deviations of 2.2974, 0.0299, and 0.0219. The coefficient of variation (Cv), calculated as the ratio of the standard deviation to the mean, is 0.85%, 10.16%, and 3.03%, respectively, for these machining responses. The relatively low Cv values indicate minimal data dispersion, confirming the reliability of the collected machining response data for further analysis.

Fig. 9
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Empirical cumulative distribution of machining responses.

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Effect of machining parameters on responses

The analysis for Fig. 10 focuses on the resultant cutting force and how it is influenced by cutting speed, feed, and depth of cut. Increasing cutting speed generally reduces the cutting force due to the thermal softening of the workpiece, which makes material removal easier. However, at excessively high speeds, the reduction in cutting force becomes less significant, as increased tool wear leads to higher friction and resistance. Feed rate has the most substantial impact on cutting force, as higher feed introduces more material per revolution, requiring greater force to maintain cutting action. The increased force led to higher power usage as well as additional stress applied to the cutting tool that shortens the tool life. Depth of cut enhances cutting force since increased tool-workpiece contact leads to higher resistance thus requiring extra mechanical load. More force becomes essential to efficiently shear materials when chip thickness increases because of deeper cutting. Hence, the machining process becomes unstable because of multiple negative effects such as tool deflection and vibrations that harm the total machining performance.

Fig. 10
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Effect of machining parameters on resultant cutting force.

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The Fig. 11 examines tool wear, which is significantly influenced by cutting speed, feed, and depth of cut. At moderate cutting speeds, tool wear remains controlled due to stable thermal conditions and efficient chip evacuation, extending tool life. However, at very high cutting speeds, excessive heat generation leads to diffusion wear and oxidation, weakening the tool material and accelerating degradation. This can also result in built-up edge formation, which negatively impacts both tool performance and surface quality. Feed rate plays a crucial role in tool wear, as higher feed increases friction and mechanical stress on the tool, leading to abrasive and adhesive wear. Excessively high feed rates can also cause tool chipping or premature failure due to sudden force variations. Depth of cut contributes to tool wear by increasing tool-workpiece contact, leading to more heat build-up and mechanical stress. A greater depth of cut results in severe tool engagement, which can accelerate flank wear, crater wear, and even catastrophic tool failure. Optimizing these machining parameters is essential for minimizing tool wear while maintaining productivity and machining efficiency.

Fig. 11
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Effect of machining parameters on tool wear.

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The Fig. 12 focuses on surface roughness and how it is affected by cutting speed, feed, and depth of cut. Higher cutting speeds generally lead to improved surface finish, as they reduce tool-workpiece interaction time, minimizing deformation and producing finer cuts. However, at excessively high speeds, excessive heat generation can cause thermal damage, built-up edge formation, and surface smearing, leading to a decline in surface quality. Feed rate has the most dominant effect on surface roughness, as increasing feed leaves deeper tool marks on the machined surface, resulting in a rougher finish. This makes feed optimization crucial for achieving the desired surface smoothness. Depth of cut plays a relatively moderate role in surface roughness, as deeper cuts increase cutting forces and material deformation, which can lead to vibrations and chatter, affecting surface quality. However, its effect is less significant compared to feed and cutting speed. Achieving optimal surface roughness requires balancing these parameters to minimize tool marks, avoid excessive heat generation, and ensure a smooth, high-quality machined surface.

Fig. 12
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Effect of machining parameters on surface roughness.

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Analysis of variance

To improve machining accuracy and optimize process settings, it is essential to measure how each input factor affects machining outcomes. This helps understand how different parameters interact and influence key metrics like cutting force, tool wear, and surface roughness. A common method for this analysis is the Analysis of Variance (ANOVA)37, which was performed in this study at a 95% confidence level to determine the percentage contribution of each factor. ANOVA provided detailed statistical insights, including Degrees of Freedom (DF), Adjusted Sum of Squares (Adj SS), Adjusted Mean Squares (Adj MS), F-value, and P-value. The F-value indicates the relative impact of each factor, while the P-value shows statistical significance. A bar chart (Fig. 13) visually represents these contributions. For resultant cutting force, cutting speed, feed rate, and depth of cut contributed 25.41%, 40.13%, and 32.86%, respectively. Tool wear was mainly influenced by cutting speed (30.79%), feed rate (50.31%), and depth of cut (16.90%). Surface roughness was affected by cutting speed (45.62%), feed rate (32.87%), and depth of cut (20.57%). Since all P-values were below 0.05, the statistical significance of these parameters was confirmed. Among them, feed rate and cutting speed were the most influential, aligning with findings from Yildirim et al.38. Note that the observed dominance of cutting speed on surface roughness and feed rate on tool wear in this study contrasts with conventional findings, where feed rate is typically more influential on surface finish and cutting speed on tool wear. This deviation can be attributed to the specific cutting conditions employed. Higher cutting speeds may have generated excessive heat, deteriorating surface integrity and resulting in rougher surfaces. Conversely, higher feed rates likely imposed greater mechanical loads and friction on the tool, accelerating wear. Thus, the combined effects of thermal and mechanical loads under the given experimental parameters led to this inverse trend.

Fig. 13
figure 13

ANOVA-based percentage contribution of machining parameters.

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Multi-objective optimization

Optimization aims to achieve the best performance while adhering to given constraints, either by maximizing or minimizing specific parameters39. One such technique, MORSM, combines statistical and mathematical tools to analyze complex problems involving multiple interacting variables40. It is widely applied in mechanical engineering, especially when multiple input factors, such as cutting speed, feed rate, and depth of cut, affect performance in ways that are challenging to measure or model traditionally41,42. MORSM uses regression models to predict the relationship between input parameters and output responses like cutting force, tool wear, and surface roughness. The coefficients of these regression models are derived from experimental data, typically through least squares regression, which minimizes the sum of squared deviations between predicted and actual values. The model assumes linearity and additivity of effects but also incorporates interaction and higher-order terms to capture more complex, non-linear behaviours. These assumptions help the model provide accurate predictions across a wide range of machining conditions.

In this study, MORSM is employed to optimize the machining process by identifying the best conditions that minimize cutting force, tool wear, and surface roughness. The optimization graph, shown in Fig. 14, illustrates the RSM-based process for turning operations, where the best parameters are selected to meet target functions. The experimental validation of the MORSM model under recommended conditions demonstrates strong agreement between predicted and actual results, as seen in Table 4. The composite desirability score of 0.84 highlights the model’s effectiveness in solving complex machining problems quickly. However, applying these optimal conditions across different production settings may require further experiments to ensure consistency and reliability. This step is essential for adapting the parameters to various production scales, making the optimization results more robust and practical for industrial applications.

$${R_a} = {\text{ }}0.6279{\text{ }} + {\text{ }}0.002103{\text{ }}{v_c} + {\text{ }}0.281{\text{ }}f{\text{ }} - {\text{ }}0.0081{\text{ }}{a_p} - {\text{ }}0.000008{\text{ }}{v_c}^2 + {\text{ }}1.942{\text{ }}{f^2} + {\text{ }}0.01541{\text{ }}{a_p}^2 - {\text{ }}0.00707{\text{ }}{v_c}f{\text{ }} - {\text{ }}0.000412{\text{ }}{v_c}{a_p}$$
(2)
$${F_r} = {\text{ }}293.54{\text{ }} - {\text{ }}0.6006{\text{ }}{v_c} + {\text{ }}35.88{\text{ }}f{\text{ }} + {\text{ }}0.203{\text{ }}{a_p} + {\text{ }}0.003354{\text{ }}{v_c}^2 + {\text{ }}68.6{\text{ }}{f^2} - {\text{ }}0.031{\text{ }}{a_p}^2 - {\text{ }}0.3588{\text{ }}{v_c}f{\text{ }} + {\text{ }}0.00468{\text{ }}{v_c}{a_p}$$
(3)
$$V{B_{max}} = {\text{ }}0.14633{\text{ }} + {\text{ }}0.000547{\text{ }}{v_c} - {\text{ }}0.3611{\text{ }}f{\text{ }} + {\text{ }}0.09211{\text{ }}{a_p} + {\text{ }}0.000002{\text{ }}{v_c}^2 + {\text{ }}0.200{\text{ }}{f^2} - {\text{ }}0.02600{\text{ }}{a_p}^2 + {\text{ }}0.006333{\text{ }}{v_c}f{\text{ }} - {\text{ }}0.000100{\text{ }}{v_c}{a_p}$$
(4)
Fig. 14
figure 14

Optimization process using MORSM for sustainable machining.

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Table 4 Comparative assessment between the experimental and RSM predicted values.
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Conclusions

This study provides a comprehensive analysis of the machining characteristics of Monel-400 under various lubrication and cooling conditions, underscoring the novelty and effectiveness of the hybrid MQL + CO₂ approach in achieving superior machinability. By comparing dry machining, MQL, Cryogenic CO₂, and MQL + CO₂ environments, the results reveal that the MQL + CO₂ combination significantly outperforms other methods, showing reductions in cutting force, tool wear, and surface roughness by 19.58%, 19.10%, and 47.19%, respectively, compared to dry machining. This synergistic enhancement highlights the unique potential of combining lubrication with effective cooling, improving cutting performance, reducing tool wear, and offering a promising pathway toward sustainable machining solutions. The use of MORSM for multi-objective optimization further validated the optimal machining parameters, ensuring efficient, precise, and sustainable operations with a composite desirability of 0.84. This optimization framework is crucial for industries seeking to balance performance and cost-efficiency while meeting environmental and sustainability goals.

Although the findings demonstrate substantial advancements, the study’s focus on Monel-400 presents an opportunity to expand the research to other nickel-based superalloys and diverse machining setups. Broadening the scope of materials and cutting conditions will help establish the versatility and robustness of the MQL + CO₂ method across various manufacturing applications. Additionally, the experimental results, while promising, are limited to specific cutting parameters, suggesting that further validation under a broader set of conditions is needed to ensure applicability in real-world environments. The observed wear mechanisms, including adhesive and abrasive wear, remain significant challenges even under the best cutting conditions. This indicates a critical area for innovation, such as exploring advanced tool coatings or next-generation lubrication techniques, to further reduce wear and extend tool life.

Future research should focus on exploring the scalability and adaptability of the MQL + CO₂ approach to more advanced materials, alternative lubricants, and cooling systems. Integrating real-time monitoring and predictive modeling with advanced sensors and AI could optimize the machining process dynamically, enabling real-time adjustments and improving adaptability to various machining conditions. These advancements would solidify the broader impact of this study, providing a significant contribution to sustainable and efficient machining technologies in aerospace, automotive, and other high-precision manufacturing sectors.