# Effect of different cryogenic lubrication methods on machinability of Ti6Al4V

Gabriel de Paiva Silva, Yessine Ayed, Bruno Lavisse, Guénaël Germain

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Gabriel de Paiva Silva, Yessine Ayed, Bruno Lavisse, Guénaël Germain. Effect of different cryogenic lubrication methods on machinability of Ti6Al4V. ESAFORM, May 2025, Paestrum, Italy. pp.1817-1825, 10.21741/9781644903599-195. hal-05343717

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# Effect of different cryogenic lubrication methods on machinability of Ti6Al4V

Gabriel DE PAIVA SILVA¹,ª*, Yessine AYED¹,ᵇ, Bruno LAVISSE¹,ᶜ and Guénaël GERMAIN¹,ᵈ

¹Arts et Metiers Institute of Technology, LAMPA - 2 Bd du Ronceray, 49035 - Angers, France

ªgabriel.depaiva_silva@ensam.eu, ᵇyessine.ayed@ensam.eu, ᶜbruno.lavisse@ensam.eu, ᵈguenael.germain@ensam.eu

**Keywords:** Ti6Al4V, Cryogenic Machining, Tool Wear, Cutting Forces

**Abstract.** In machining industry, there is a growing interest in cryogenic cooling techniques, because of their environmental benefits, including reduced toxicity, safer operation, and lower environmental impact compared to conventional cutting fluids. The titanium alloy Ti6Al4V, which is commonly used in aerospace, automotive and biomedical industries, presents low machinability and often requires abundant use of cutting fluids to inhibit tool wear. This study investigates the machinability of Ti6Al4V, comparing conventional lubrication (water-oil emulsion) with two cryogenic fluids: liquid nitrogen ($LN_2$) and liquid carbon dioxide ($LCO_2$). Longitudinal turning tests were conducted and tool life, wear mechanisms, and cutting forces were evaluated for each lubrication condition. The tool life provided by emulsion, $LN_2$ and $V_c$ were 8.2 min, 17.7 min and 9.9 min, respectively. Adhesion was identified as the predominant wear mechanism across all conditions. Overall, the results suggest that the cryogenic coolants can effectively increase tool life and reduce cutting forces in comparison with conventional lubrication, however, further optimizations of the delivery system of the cryogenic coolants are still necessary.

**Introduction**

The titanium alloy Ti6Al4V presents excellent corrosion resistance and strength-to-weight ratio, being widely used in industries such as aerospace, automotive and biomedical. However, this material is known for its low machinability. Machining of Ti6Al4V often imposes challenges associated with rapid tool wear and high cutting forces and temperatures [1]. The poor machinability of titanium and its alloys is associated with its low thermal conductivity, high chemical reactivity, low modulus of elasticity, high hardness at high temperatures, and peculiar hardening characteristics [2].

A major concern in the modern machining industry is the environmental impact caused by the use of traditional cutting fluids made from mineral oils. The use of such fluids, which are largely used for cooling and lubrication during machining of hard-to-cut alloys such as Ti6Al4V, has been questioned as regards to health, economic and environmental issues [3,4]. There is a growing need to implement more efficient and sustainable practices in machining operations, by minimizing environmental impact and improving cutting performance.

Cryogenic machining can be an environmentally friendly alternative to conventional lubrication [5,6]. In these processes, conventional cutting fluids are replaced by a cryogenic fluid, which is a gas liquefied at high pressure that, when released to the atmosphere, cools down the cutting zone and evaporates instantaneously, returning to the atmosphere and leaving no residue or contamination on the part, chips, machine tool or operator [7]. There are different strategies used in cryogenic machining in terms of the application of the cryogenic coolant. Some of the most common cryogenic fluids used in machining are liquid nitrogen ($LN_2$) and liquid carbon dioxide ($LCO_2$), which reach the temperatures of -196°C and -78°C, respectively [8]. A definition proposed by the Cryogenic Society of America affirms that “cryogenics” refers to temperatures below 120

<figure><img src="image_2.png" /></figure>
K or 153°C [9]. However, several researchers have used the terms “cryogenics” or “cryogenic machining” to refer to temperatures below 0°C, therefore including LCO₂ applications [10–12]. The cryogenic coolants can also be combined with other lubricating techniques, for example, acting as a solvent for cutting fluids applied via MQL (minimum quantity lubrication) [8,13,14].

The use of cryogenic cooling in machining has been an important topic of research, because it is cleaner, safer, non-toxic and causes less environmental impact than conventional cutting fluids [15]. Furthermore, cryogenic machining can significantly extend tool life by reducing the cutting temperature and minimizing tool wear mechanisms [16–18]. However, cryogenic fluids are still rarely used industrially, since they tend to have high operating costs, problems related to safety, and scarce availability of data about their effects on machinability [19]. Therefore, this work aims at investigating the machinability of Ti6Al4V under two cryogenic lubrication methods: liquid nitrogen (LN₂) and liquid carbon dioxide (LCO₂). Tool wear and cutting forces will be analyzed and the results will be compared with conventional lubrication with water-oil emulsion.

## Experimental methodology

The machined material is a cylindrical workpiece of Ti6Al4V with diameter D = 130 mm and length L = 310 mm. Longitudinal turning tests were carried out using the CNC lathe Leadwell LTC25iL (2500 rev/min, 24kw). The machine tool is equipped with a system for measuring the cutting forces, consisting of a Kistler 9257B piezoelectric dynamometer, a HBM CD600 charge amplifier and a HBM MX440A 24 bit A/D converter. Three lubrication conditions were tested: conventional lubrication with a water-oil emulsion; liquid nitrogen (LN₂); and liquid carbon dioxide (LCO₂). The cutting fluid is the soluble oil Ecocool CS+, which is mixed with water in a concentration of 6%. For the tests with conventional lubrication, the lubricant was applied directly from the reservoir of the lathe, with a pression of 5 bar, whereas for the cryogenic tests with LN₂ and LCO₂, the fluids were stored in tanks with pressure in the order of 10 bar and 60 bar, respectively.

The cutting tools are uncoated H13A tungsten carbide inserts CNMG 12-04-08 SM from Sandvik. The tool and tool holder assembly provides a total rake angle of 9°, clearance angle of 6° and cutting-edge angle of 95°. A nozzle holder was designed for conducting the cooling fluid (emulsion, LN₂ or LCO₂) to the cutting zone. In the nozzle holder, there is a threaded hole where nozzles with different diameters can be connected. The hole directs the jet toward the nose of the tool with an angle of incidence of 15°. After each machining interval, the cutting tool was placed on a support and observed in a LEICA Z16 APO stereo microscope with a magnification of 40× so that the maximum flank wear VBmax could be tracked according to standard 3685:1993 using a micrometer screw gauge (resolution 0.01 mm). The experimental set-up for the machining tests can be observed in Fig. 1, which illustrates the dynamometer and the tool holder in the CNC lathe, as well as the nozzle holder, an example of nozzle, the cutting insert, and the support used to measure VBmax. For qualitative analysis of the wear mechanisms of the cutting tools, micrography images were obtained using a scanning electron microscope (SEM).
<figure><img src="image_3.png" /><figcaption>Figure 1 – Experimental set-up for the machining tests.</figcaption></figure>

The cutting conditions used in all machining tests are: cutting speed $V_c$ = 140 m/min; depth of cut $a_p$ = 0.5 mm; and feed rate $f$ = 0.1 mm/rev. These cutting parameters were chosen based on the approximate tool life obtained in tests with emulsion and LN₂, performed in previous works with the same methodology [20], therefore, a repeatability test was performed only for LCO₂. For the tests with emulsion and LN₂, the nozzle with diameter 1.4 mm was used. For the tests with LCO₂, the diameter of the nozzle was smaller, of 0.8 mm, because this fluid is stored under the highest pressure (70 bar), so it was expected that a larger diameter would waste the LCO₂ unnecessarily.

Table 1 shows the approximate delivery flow rate for each cooling strategy.

<table><caption>Table 1– Fluid delivery parameters.</caption><thead><tr><td>Lubrication condition</td><td>Nozzle diameter (mm)</td><td>Fluid Pressure (bar)</td><td>Flow rate (l/min)</td></tr></thead><tbody><tr><td>Emulsion</td><td>1.4</td><td>5</td><td>2.0</td></tr><tr><td>LN<sub>2</sub></td><td>1.4</td><td>10</td><td>3.2</td></tr><tr><td>LCO<sub>2</sub></td><td>0.8</td><td>70</td><td>1.9</td></tr></tbody></table>

## Tool wear

The curves of tool wear evolution are shown in Fig. 2. The test with conventional lubrication was used as a comparison reference, resulting in a tool life of 8.2 min. It can be remarked that machining with LN₂ provided a substantial increase in tool life, reaching 17.7 min, which is more than double than machining with emulsion. As for LCO₂, the tool life was 8.0 min and 7.4 min in the two tests (difference of 8.1 %). The resolution of the micrometer screw gauge was used as the associated measuring error.

<figure><img src="image_4.png" /><figcaption>Figure 2 – Tool wear curves.</figcaption></figure>
As can be deduced from Fig. 2, the jet of LCO₂ was not able to increase tool life. The LCO₂ delivery system could be optimized for more efficient lubrication, in order to ensure that the jet of gas reaches the zone of interface between the tool and the workpiece. Masson et al. [21] investigated the applicability of supercritical CO₂ jet in machining. The authors concluded that the distance and the angle of the cryogenic fluid supplier play crucial roles for efficient cooling of the cutting zone. Besides, the gas of CO₂ has lower lubrication capacity than LN₂, even though the temperature reached is low. According to An et al. [22], lubrication is more important than cooling to restrain tool wear in machining of Ti6Al4V.

The rake faces of the inserts were observed with the electron microscope to investigate tool wear mechanisms, as exemplified in Fig. 3, Fig. 4 and Fig. 5. EDS analysis was performed to highlight the presence of workpiece material on the tools, as shown in Fig. 6. It is known that the low temperatures reached in cryogenic machining of Ti6Al4V inhibit the diffusion process responsible for the crater wear [23]. In fact, crater wear was not observed, neither with emulsion nor cryogenic assistance. However, it can be seen from Figs. 3 to 6 that LN₂ or LCO₂ did not avoid severe adhesion of workpiece material to the tool. The main type of wear observed is adhesion. All cutting tools presented similar wear behavior, with adhesive layers of titanium on the tungsten carbide matrix and zones with accumulated workpiece material on the flank face. Microchips were also observed, especially on the tools that machined with LN₂.

<figure><img src="image_5.png" /><figcaption>Figure 3 – Tool condition for machining with emulsion.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4 – Tool condition for machining with LCO₂.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 5 – Tool condition for machining with LN₂.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6 – EDS analysis for each machining condition: a) LN₂; b) LCO₂; c) emulsion.</figcaption></figure>

## Cutting forces

For each machining interval, the forces acting on the tool were measured in the three directions: cutting force (tangential direction), feed force (axial direction) and passive force (radial direction). The average forces in the first machined interval (when the tool was new) were compared to the average forces at the end of tool life (VBmax close to 0.3 mm), as shown in Fig. 7.

<figure><img src="image_9.png" /><figcaption>Figure 7 – Average cutting forces for new tool and worn tool.</figcaption></figure>

As the tool wears, cutting forces increase due to the intensified friction and resistance encountered by the worn tool edge [1]. From Fig. 7, it can be seen that, for the cutting tool in new condition, the force in the tangential direction is always the highest. At VBmax close to 0.3 mm, on the other hand, the radial force becomes the highest for all cutting conditions. This happens because the radial component is the most sensible to the advancement of tool wear, since it acts directly on the area of normal contact between the chip and the tool, which increases as the tool becomes worn.

For the new tools, the cutting forces with LN₂ were up to 17% higher than with conventional lubrication. This is probably because the jet of LN₂ cools down not only the cutting tool, but also the workpiece, leading to alterations in the mechanical properties of the Ti6Al4V alloy. The hardening and embrittlement of the workpiece material induced by the low temperatures can lead to an increase in cutting forces [20]. Interestingly, this decrease in temperature does not seem as significant for LCO₂, in which the cutting forces did not increase considerably in comparison with emulsion, which is logical considering that LCO₂ has lower cooling capacity.
Even though LN₂ provided the longest tool life, the smallest forces were obtained by LCO₂ after the tool was worn. It is possible that further modifications in the delivery system of the cryogenic coolants could provide better results in terms of decreasing cutting forces. Parameters such as clearance angle and cutting-edge geometry need to be optimized for efficient reduction in cutting forces during cryogenic machining [24]. The supply of coolants also plays an important role in the suppression of fluctuation of output variables in cryogenic machining of Ti6Al4V, as suggested in the works of Liu et al. [25] and Rodriguez et al. [26].

## Conclusions

In this work, machining of Ti6Al4V alloy was carried out. Conventional lubrication with water-oil emulsion was compared with two techniques of cryogenic assistance: LN₂ (liquid nitrogen) and LCO₂ (liquid carbon dioxide). The following conclusions can be drawn:

* Tool life with LN₂ (17.7 min) is more than two times longer than with emulsion (8.2 min), highlighting the great potential of LN₂ to be used as an eco-friendly substitute for conventional cutting fluids.

* For the tested cutting conditions, LCO₂ proved to be less effective than LN₂ in increasing tool life, despite the fact that LCO₂ generated the smallest cutting forces.

* Adhesion was the main wear mechanism detected. Adherent layers of titanium were observed on the tungsten carbide matrix of the tools for all cutting conditions.

* Further studies will be performed to optimize the delivery system of the cryogenic coolants, especially LCO₂, attempting to increase tool life more significantly.

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