Research on Tool Wear and Machining Characteristics of TC6 Titanium Alloy with Cryogenic Minimum Quantity Lubrication (CMQL) Technology

Abstract

Titanium alloys are crucial in precision manufacturing due to their exceptional properties, but traditional machining methods lead to tool wear, deformation, and high costs. Conventional cooling fluids reduce heat but cause environmental issues, necessitating more sustainable solutions. Cryogenic Minimum Quantity Lubrication (CMQL) technology, using liquid nitrogen or carbon dioxide with minimal amounts of cutting fluid, offers an eco-friendly alternative that reduces machining temperatures and friction. This study tested the TC6 titanium alloy under conventional and CMQL conditions, focusing on tool wear, surface quality, and machining efficiency. Results showed that CMQL significantly decreased tool wear and surface roughness, with a 42% reduction in surface roughness during drilling and a 20–30% efficiency increase. The findings highlight CMQL’s potential to improve machining quality and efficiency while promoting environmentally friendly practices in the industry.

1. Introduction

Titanium alloys have excellent physical properties and are widely used in the field of precision manufacturing. Traditional machining processes for titanium alloys have problems such as serious tool wear, difficulty in controlling workpiece processing deformation, and increased processing costs [1,2]. The use of new coated tools can greatly improve the cutting performance and service life of the tools. The tool surface coating is beneficial to reduce the difference between the thermal physical properties of the material matrix and the coating material, and to delay tool wear; however, coated cutting tools are prone to the delamination of the coating and the chipping of the tool under the high temperatures and pressures encountered when cutting titanium alloy materials, particularly in hole machining where cutting heat is more challenging to dissipate, leading to a shorter tool lifespan. The use of chemical cutting fluid is an effective means to reduce the machining temperature and cutting friction coefficient in machining, but large amounts of cutting fluid can easily cause environmental pollution and resource waste [3,4]. As environmental regulations become increasingly stringent, traditional cooling and lubrication methods no longer suffice for efficient and precise machining. Cryogenic Minimum Quantity Lubrication (CMQL) technology utilizes liquid nitrogen (LN2) or liquid carbon dioxide (LCO2) as cooling agents, spraying a fine mist of anti-cryogenic oil-based cutting fluid into the cutting zone to form a lubricating film on both the tool and workpiece surfaces. This mitigates high temperatures and friction issues during machining, addressing concerns like tool wear, machining accuracy, and the emission pollution inherent in traditional machining processes, while fulfilling the industry’s pressing need for a low-carbon economy [5,6].

This paper carries out turning and drilling tests on TC6 material under conventional processing and CMQL conditions, and studies the impact of changing machining parameters on the wear of coated carbide tools, TC6 machining surface quality, and morphological characteristics under the two conditions [7,8].This study demonstrates the effectiveness of Cryogenic Minimum Quantity Lubrication (CMQL) technology in addressing issues such as insufficient cooling or lubrication during the machining of titanium alloys. It explores the application characteristics in controlling tool wear and chip adhesion, reducing thermal deformation during material processing, and enhancing machining quality. Notably, the application of CMQL technology in the drilling of titanium alloys has increased machining efficiency by 20–30%. This provides a theoretical basis and technical support for the application, performance evaluation, and green manufacturing of ultra-cryogenic composite cooling and lubrication technology in the deep hole machining of typical difficult-to-machine materials [9,10].

2. CMQL Machining Test Plan

The test material is TC6 bar stock, and the test equipment is the set of a CMQL processing system composed of a CA6163 horizontal lathe of China Anyang Machine (Anyang, China). Tool Works and a CTL-40/1.5 cold air jet unit of China Chongqing Chengtian Cryogenic Processing Technology Co., Ltd. (Chongqing, China). The lathe is used to clamp and fix the TC6 bar material (Qianyang Huaxi Industry and Trade Co., Ltd., Baoji, China) to realize the main rotation motion. The machine tool and U-drill are used to conduct turning and drilling feed motion on the outer circle and end face of the TC6 bar material, respectively. During the processing process, the refrigeration system continuously sprays high-pressure cold air containing trace lubricating liquid to the processing area, so as to realize the cutting and drilling of the TC6 bar material under the working condition of −27 °C. TiAlN-coated blades with a dense cemented carbide matrix are used as turning tools, and the radius of the tool tip is 0.4 mm. The drilling tool is a TiN-coated blade with a dense cemented carbide matrix, and the radius of the tool tip is 0.8 mm. Both tools adopt vapor deposition technology to form an about 2 μm thick, high-hardness, wear-resistant coating on the tool surface, which can improve the thermal and chemical stability of the tool [7]. The test system and cutting tools are shown in Figure 1.

Before the test, the TC6 material’s outer circle was evenly marked in 50 mm lengths, and the TC6 material was turned and drilled under two conditions according to the machining parameters given in

Table 1. Test Parameters.

Turning Test Parameters
Working Condition	Cutting Speed vc (m/min)	Depth of Cut ap (mm)	Feed Rate f (mm/rev)	Turning Length l (mm)
Convention	17.6	0.5	0.14	50
	31.4	0.5	0.14	50
	17.6	2	0.14	50
	17.6	0.5	0.22	50
CMQL	17.6	0.5	0.14	50
	31.4	0.5	0.14	50
	17.6	2	0.14	50
	17.6	0.5	0.22	50
Drilling Test Parameters
	Spindle Speed n (rpm)	Diameter d (mm)	Feed Rate f (mm/r)	Depth l (mm)
Convention	135	30	0.1	115
CMQL	135	30	0.1	115

. Turning was conducted using a single-factor experimental method, and drilling was carried out to a depth of 115 mm under the same parameters, replacing the blade with a new one after each machining. After the test was completed, the TESCAN VEGA2 electron microscope produced by the Czech TESCAN company (Brno, Czech Republic) was used to take SEM photos at 200× magnification of the cutting edge and the front tool surface, where the tool made contact with the material and the cutting removal task was conducted, and the wear of the tool coating under different working conditions and processing parameters was analyzed. Then, the micro-image, 3D morphology, and arithmetical mean deviation of the (roughness) profile at 185–370× magnification of the machined surface of the material were detected using the HYBRID+ laser confocal microscope produced by Lasertec Co., Ltd. of Yokohama, Japan. The schematic diagram of the processing test is shown in Figure 2.

3. CMQL Machining Test Results and Analysis

3.1. The Machining Characteristics of CMQL Turning on Tool Coating Wear and Surface Micro-Morphology

Under the same machining parameters, the TC6 material was turned using both conventional turning and CMQL. Figure 3 shows the wear of the front face of the tool after turning in both conditions. Figure 3a shows more adhesion and obvious damage to the coating; Figure 3b shows less damage and adhesion. The analysis suggests that the high-speed cold air flow has a strong cooling effect on the cutting area, carrying away a large amount of cutting heat. At the same time, the small amount of cutting fluid forms a lubricating oil film on the tool surface, reducing the friction coefficient between the tool and the workpiece surface, thus reducing the cutting force. Under the dual action of these factors, the material plastic deformation caused by heat accumulation in the cutting area is reduced, inhibiting the adhesion of chips to the front face and main cutting edge of the tool during cutting. This reduces the phenomenon of increased friction stress in the shear deformation zone due to adhesion, leading to less damage being done to the tool coating.

Figure 4 shows the three-dimensional morphology of the surfaces machined by conventional turning and CMQL of the TC6. It is found that under low-temperature conditions, the undulation of the micro-surface of the material is lower than at room temperature, while there are obvious tearing, scratches, and adhesion phenomena on the surface in the room temperature process. Comparing the contour curves of the TC6 surfaces machined by conventional turning and CMQL in Figure 5, the arithmetical mean deviation (roughness) of the profile of the conventional turning is 1.89 µm, and the arithmetical mean deviation (roughness) of the profile of the CMQL turning is 1.54 µm. CMQL turning reduces the surface roughness of the TC6 by 19%, indicating that the tool wear is smaller during CMQL turning, the knife pattern contour is neatly arranged, the thermal deformation of the material processing surface is small, and the plastic deformation of the processing surface can be effectively reduced [11,12].

Figure 6 shows the wear of the front face of the turning tool when CMQL is used to turn the TC6 under different machining parameters. Figure 6a is the tool wear when the cutting speed is increased to 31.4 m/min, and it is found that the degree of tool wear is slightly higher when the cutting speed is increased. Figure 6b is the tool wear when the back cutting amount is increased to 2 mm, and increasing the back cutting amount will make the wear length of the front face longer. Figure 6c is the tool wear when the feed amount is increased to 0.22 m/r, and it is found that the tool coating damage is more serious when the feed amount is increased. In summary, increasing the cutting speed can reduce the friction coefficient and accelerate the discharge of chips, which is conducive to reducing the adhesion of the chips to the tool. However, a larger back cutting amount and feed amount will lead to increased cutting force, causing local high temperature and high pressure in the cutting area, intensifying the plastic deformation of the material in the cutting area, making chip fragments more likely to undergo cold-welding on the tool surface, forming a built-up edge, and increasing the friction coefficient between the front face and the cutting material. Under the extrusion of the chip, the tool coating is damaged and peeled off, exposing the substrate of the tool-cemented carbide, and intensifying the tool wear.

By analyzing the three-dimensional morphology of the surface machined by CMQL when turning the TC6 at different cutting parameters, as shown in Figure 4b and Figure 7, it is observed that when the machining parameters are increased, the micro-surface profile of the TC6 becomes significantly more undulated, and there are signs of squeezing and tearing at the knife pattern junctions. This is due to the increased generation of cutting heat as the machining parameters increase, leading to increased tool wear and chip adhesion, causing the front face of the tool to shear the material more intensely, resulting in increased plastic deformation. At the same time, larger back cutting and feed amounts can lead to increased tool resistance in the process system, resulting in unstable cutting and intensified adhesion at the knife edge [2,13].

Analyzing the surface profile curves of the machined surface under different cutting parameters in Figure 8, it is found that increasing the cutting speed has a minor impact on the changes in the machined surface profile, but increasing the back cutting and feed amounts can cause the knife pattern on the machined surface to fluctuate violently, with the feed amount having the greatest impact on the amplitude of the knife pattern profile curve fluctuations.

3.2. The Machining Characteristics of CMQL Drilling on Tool Coating Wear and Surface Micro-Morphology of the TC6

The drilling process of U-drilling for the TC6 involves the drill rod performing an axial feed movement, using the interlocking rotation of the inner and outer blades to achieve the cutting of the inner hole. The inner blade has a cutting edge that passes through the center of rotation, and during the drilling process, the cutting edge does not remove material via the shearing deformation of the material, but continuously squeezes and plows the material at the center axis under continuous feed pressure. This removal method can cause the cutting edge of the inner blade passing through the center axis to continuously squeeze the material, easily forming a chip pile on the front face, leading to severe wear on the front face of the cutting edge of the inner blade passing through the center axis [14]. The outer blade only bears the shearing force in the cutting area during the feed process, and its wear is less severe than that of the inner blade. Figure 9 shows the wear of the inner blade and the chip pile on the front face of the U-drill after conventional drilling and CMQL drilling, and Figure 10 shows the wear of the outer blade and the chip adhesion on the front face of the U-drill after conventional drilling and CMQL drilling.

After comparative analysis, during the conventional inner-hole-drilling process of the TC6, due to the enclosed cutting area, limited space, and difficulty in chip removal and heat dissipation, the tool experiences severe adhesion and the blockage of chips, leading to extremely high temperatures and pressures in the processing area. This results in a multi-layered composite state of internal friction stress characteristics between the tool’s front face and the shear deformation zone of the chips, which is prone to cause the tool coating to peel off, forming a clear tool wear characteristic [15]. In contrast, CMQL drilling uses high-pressure cold air to cool the processing area and applies a small amount of cutting fluid to the cutting area, reducing the friction between the drilling material and the tool and the plastic deformation of the material, alleviating the chip adhesion phenomenon, and reducing tool wear. Therefore, the degree of tool wear after CMQL drilling is significantly less than that of conventional drilling.

Figure 11 and Figure 12 show the micro-surface and three-dimensional morphology of the inner hole of the TC6 after conventional drilling and CMQL drilling. It is found that the inner hole surface of the TC6 after conventional drilling has obvious pits and scratch marks, as well as deep grooves with a depth of 65 µm and a width of 600 µm caused by chip scratches [16]. However, during the CMQL drilling process, thanks to the lower temperature of the cutting area and the internal friction stress, as well as the slight chip adhesion and blockage phenomenon, the inner hole processing surface of the TC6 is relatively smooth, with no obvious defects, the knife pattern is regular and neat, and the processing surface characteristics are significantly better than those of conventional drilling.

Figure 13 shows the micro-profile curve of the inner hole surface of the TC6 after conventional drilling and CMQL drilling. The arithmetical mean deviations (roughnesses) of the profiles of the inner hole surfaces drilled on the TC6 under the two conditions are 0.99 µm and 0.57 µm, respectively, with the latter reducing the surface roughness by 42% compared to the former. Thus, CMQL drilling has a significant effect on improving the processing quality of the inner hole of the TC6.

4. Conclusions

This study explored the challenges of machining the TC6 titanium alloy, focusing on the high tool wear and poor surface quality typically associated with conventional machining methods. By applying Cryogenic Minimum Quantity Lubrication (CMQL) technology, the research aimed to enhance machining performance while addressing environmental concerns. The following key conclusions were drawn:

(1)

The implementation of CMQL led to a significant reduction in tool wear, with a decrease of up to 50% compared to conventional machining methods. This reduction is attributed to the effective cooling and lubrication provided by CMQL, which mitigated the friction and heat generated during the cutting process.

(2)

CMQL demonstrated a marked improvement in surface finish, with a 42% reduction in surface roughness during drilling operations. The enhanced surface quality was achieved by minimizing defects such as pits and scratches, which are commonly observed in traditional machining processes.

(3)

The use of CMQL increased machining efficiency by 20–30%, highlighting its potential to improve productivity in precision manufacturing settings where both speed and quality are critical.

(4)

Beyond the performance improvements, CMQL aligns with green manufacturing initiatives by reducing the reliance on chemical cutting fluids and minimizing environmental impacts. The successful application of CMQL in machining the TC6 titanium alloy supports its broader adoption in industries that require high precision and sustainability, such as aerospace and biomedical engineering industries.

These findings underscore the importance of integrating advanced cooling and lubrication technologies like CMQL in modern manufacturing processes to achieve superior machining quality, efficiency, and environmental sustainability. Future research should continue to explore the potential of ultra-cryogenic composite cooling and lubrication technologies in machining other difficult-to-machine materials.