Optimization of the cooling strategy during cryogenic milling of Ti-6Al-4 V when applying a sub-zero metalworking fluid

Due to an excellent ratio of high strength to low density, as well as a strong corrosion resistance, the titanium alloy Ti-6Al-4 V is widely used in industrial applications. However, Ti-6Al-4 V is also a difficult-to-cut material because of its low thermal conductivity and high chemical reactivity, especially at elevated temperatures. As a result, machining Ti-6Al-4 V is characterized by high thermal loads and a rapidly progressing thermo-chemical induced tool wear. An adequate cooling strategy is essential to reduce the thermal load and therefore tool wear. Sub-zero metalworking fluids (MWF) which are applied at liquid state but at supply temperatures below the ambient temperature, offer great potential to significantly reduce the thermal load when machining Ti-6Al-4 V. Within the presented research, systematically varied sub-zero cooling strategies are applied when milling Ti-6Al-4 V. The influences of the supply temperature, as well as the volume flow and the outlet velocity are investigated aiming at a reduction of the thermal loads that occur during milling. The milling experiments were recorded using high-speed cameras in order to characterize the impact of the cooling strategies and resolve the behavior of the MWF. Additionally, the novel sub-zero cooling approach is compared to a cryogenic CO2 cooling strategy. The results show that the optimized sub-zero cooling strategy led to a sufficient reduction of the thermal loads and does outperform the cryogenic cooling even at elevated CO2 mass flows.


Introduction and state of the art
Titanium alloys, such as Ti-6Al-4 V, are widely used in industrial applications, due to a strong corrosion resistance as well as an excellent ratio of low density to high strength even at elevated temperatures [1,2]. To achieve a favorable application behavior, mechanically and thermally highly stressed components are therefore made of Ti-6Al-4 V [3]. Despite these favorable characteristics, Ti-6Al-4 V has a very low thermal conductivity and a low heat capacity, leading to difficulties in heat dissipation during cutting [4]. Due to a limited heat transfer via the chips and the workpiece [5,6], high cutting temperatures and thus high thermal loads occur in the cutting zone [7]. In addition, the titanium alloy Ti-6Al-4 V is characterized by a very high chemical reactivity, especially at higher temperatures, resulting in adhesive and chemical reactions between the tool and the workpiece [8]. Due to the high thermal loads and the chemical reactivity, a rapidly progressing tool wear is enhanced, reducing tool life and thus productivity [9]. Therefore, Ti-6Al-4 V is classified as a difficult-to-cut material [10].
To reduce the thermal load, metalworking fluids (MWF) are usually applied when cutting Ti-6Al-4 V. The performance of an MWF is driven by a complex combination of physical and chemical effects (e.g. capillary flow, Marangoni effect) [11]. The cooling effect is the result of the convective heat transfer. This heat transfer depends on the temperature gradient between the MWF, the tool and workpiece, as well as on the heat transfer coefficient [12].
An alternative to conventional MWF is the use of cryogenic cooling strategies. Carbon dioxide snow (CO 2 ) is one of the most commonly used cryogenic coolants [13]. The high cooling effect is achieved due to the low coolant temperatures of the CO 2 (− 78.5 °C [14]). As a result, cryogenic cooling strategies can reduce tool wear and therefore increase the tool life in comparison to dry milling [15] or wet milling using a conventional MWF [16]. In addition to the low coolant temperatures, a favorable heat coefficient is desirable for a sufficient cooling effect. When applying CO 2 as a cryogenic coolant, the heat transfer is mainly achieved by a convection of gaseous CO 2 and the sublimation of solid CO 2 particles [17]. The use of a media supplied at liquid state often results in a comparatively higher heat transfer coefficient as well as better lubrication effects [18].
In previous works, sub-zero MWF were examined, that are applied at liquid state and at temperatures well below 0 °C, therefore combining the benefits of cryogenic and conventional flood cooling approaches [19]. Using a mixture of water and polyhydric alcohols as sub-zero MWF enables supply temperatures as low as -50 °C [20,21].
As a result, a high heat transfer coefficient and therefore high cooling effects are favored during cutting. When turning Ti-6Al-4 V while using a sub-zero MWF, a favorable thermo-mechanical load was observed, resulting from the enhanced heat transfer in superposition with an improved lubrication. Consequently, a better surface quality and less tool wear was observed in comparison to cryogenic cooling approaches [21,22]. First studies in sub-zero milling have also shown the potential for better surface integrity, less adhesion at the cutting edges as well as longer tool life compared to dry machining or the use of cryogenic cooling approaches [21,22]. A systematic configuration of the sub-zero MWF application in terms of supply temperature, flow rate and nozzle outlet velocity was not yet conducted for milling, although it offers a high potential for increasing the efficiency of this novel cooling strategy.
In this work, the sub-zero cooling is varied in terms of supply temperature, volume flow and nozzle outlet velocity when milling. In addition to force and temperature measurements, high-speed recordings are also carried out in order to resolve the interactions of the MWF and the cutting process at varied states. For this purpose an image-based test setup including a high-speed camera and an illumination unit are integrated into the machining center. The focus of this study is the characterization of the interactions resulting from the temperature-dependent viscosity, the outlet velocity and the volume flow on the wetting and cooling effect of the subzero MWF.

Characterization of the sub-zero MWF
A mixture of water and ethane-1,2-diol (MEG) was used as sub-zero MWF. Adding MEG to water causes a freezing point depression, therefore the mixture remains in a stable liquid state down to a temperature of − 50 °C. The mixing ratio of water and MEG influences the thermal conductivity , the specific heat capacity c p and the kinematic viscosity . Considering the heat transfer between the sub-zero MWF and the cutting zone, a low kinematic viscosity and a high heat capacity, as well as a high thermal conductivity are favorable [12,23]. At 20 °C, Water ( = 0.55 W/ mK; c p = 4.2 J/gK) inhibits a much higher thermal conductivity and specific heat capacity than MEG ( = 0.29 W/ mK, c p = 2.3 J/gK) [12]. Furthermore, the viscosity of water ( = 1mm 2 /s) is much lower than the viscosity of MEG ( = 27mm 2 /s) [12], which is why a high amount of water is desirable.
Despite that, the surface tension of water ( = 72mN/m) is higher than the surface tension of pure MEG ( = 47mN/m) resulting in a less pronounced wetting capability [24,25].
Thus, a higher amount of MEG is favorable regarding the wetting capability which improves the convective heat transfer [25,26]. The used mixture is highlighted in the phase diagram in Fig. 1 corresponding to a mass ration of ζ MEG,Water = 1.72.
Besides the mixing ratio of the sub-zero MWF, the supply temperature does also influence the fluid properties. The viscosity is highly temperature dependent, as depicted in Fig. 2. Due to an increasing viscosity, the Reynolds number decreases strongly, therefore lowering the heat transfer. Furthermore, there are pressure losses in the supply system, due to an increased viscosity, which leads to a higher power consumption of the pump [24].
The sub-zero MWF performance is furthermore impacted by the additives within the coolant. In the sub-zero MWF used, disodium metasilicate (Na 2 SiO 3 ) was added as a corrosion inhibitor with a concentration of 3.7 g per 1 kg subzero MWF. By adding disodium metasilicate to the subzero MWF, the wetting capability is also improved [28]. Additionally, 14 g acid tri-isobutyl (C 12 H 27 O 4 P) was added per 1 kg of sub-zero MWF as an anti-wear agent. Hence, an improved MWF performance is achievable through advantageous fluid and flow properties, a good wetting ability as well as a high temperature gradient between the MWF and the cutting zone.

Machining experiments
Ti-6Al-4 V workpieces with a length of 100 mm and a crosssection of A = 10 × 10mm 2 were machined via end milling. The experiments were carried out on a 5-axis machining center (Deckel Maho Gildemeister DMU 70 eVolution 1 ). The experimental setup, as well as the tool and the cutting parameters used for all tests conducted are given in Fig. 3a.
In order to supply the sub-zero MWF to the cutting zone during milling operations, a new cooling system was developed and integrated into the machining center which  is schematically depicted in Fig. 3b. A gear pump (Verder M4X66EE8000TB-8 1 ) is used to transport the sub-zero MWF at a maximum flow rate of 8 l/min which is suitable for milling operations. The combination of the cryostat (Huber Unistat630 1 ) with a plate heat exchanger (Hydac HEX S522-40-00 1 ) enables a stationary process with supply temperatures as low as -40 °C. Furthermore, a flexible adaption of the supply temperature during the milling process is possible. The 3/2-way valve is coupled with the NC control of the machining center to promote a controlled application of the coolant during the cutting tests. All components are thermally insulated to counteract thermal bridges and optimize the cooling performance.
Within this study, the influence of systematically varied cooling strategies on the thermo-mechanical load is examined during sub-zero end milling. A detailed specification of the investigated cooling strategies is depicted in Table 1. The supply temperature T supply , the density ρ of the sub-zero MWF and the volume flow V were measured using a Coriolis flow meter (Siemens FCS 400 1 ). The resulting mass flow ṁ , the outlet velocity c and the supply pressure p were calculated according to Eq. 1 -Eq. 3. In a first step, the influence of a decreasing supply temperature is investigated at a constant volume flow. Since the volume flow was kept constant, the mass flow increases at lower supply temperatures due to the increasing density of the MWF (Fig. 2). Then, the influence of the volume flow is characterized at a constant supply temperature. Here, two different rouse nozzle outlet diameters of d = 1 mm and d = 2 mm were used. This was done in order to investigate the influence of the outlet velocity as well as the supply pressure of the sub-zero MWF at constant volume flows.
For reference, additional milling experiments based on cryogenic cooling with CO 2 were carried out. Here, the mass flow of the CO 2 was systematically varied by utilizing different nozzle outlet diameters ( Table 2). The cryogenic coolant was stored in a siphoned gas cylinder at ambient temperature. The CO 2 was supplied to the cutting zone with an outlet temperature of -78.5 °C [14]. Before starting each test, a uniform workpiece temperature of 20 °C was ensured. Three tests were carried out for every cooling strategy investigated.

Measurement technology
During cutting, the occurring force components F x , F y and F z were measured using a rotating dynamometer  (Kistler Type 9123C 1 ) with a sampling frequency of 15 kHz. The respective RMS was calculated to determine the resulting force via vector addition of the three force components. Using a sampling frequency of 1 kHz and three K thermocouples (NiCr-Ni) with a diameter of 1 mm, the temperature was measured inside the workpiece. The distance between the tip of the thermocouple and the final machined surface was 0.3 mm. The thermocouples were positioned inside eroded holes (⌀ = 1.3 mm) alongside the feed path with a respective distance of 15 mm to each other (see Fig. 4a). This results in a total of nine temperature measurements per case investigated. Before inserting each thermocouple, the holes were filled with thermal paste.
Representative temperature profiles are depicted in Fig. 4 for the three thermocouples used. For every temperature profile, a local minimum is observed since the cooling leads to a decreasing workpiece temperature. After the minimum temperature is reached, the temperature rises to a local maximum, as a result of the heat introduced during cutting. The maximum temperature results from the heat introduced by the tool, counteracting the heat removed by the coolant. Therefore, the maximum temperature is used to indicate the thermal load during milling while the minimum temperature is used to evaluate the cooling performance in dependence of the cooling strategy applied.
The milling process was recorded with a high-speed camera system (NX 8, Integrated Design Tools (IDT) Inc. 1 ) in order to precisely resolve the wetting behavior of the subzero MWF on the tool and workpiece. An entocentric lens with a fixed focal length (atx-i 100 mm F2.8 FF Macro, Tokina Co. Ltd. 1 ) enabled a perspectival shot of the test setup and by that a thorough imaging of the wetting phenomena. The illumination unit (miniConstellation™ 120, IDT Inc. 1 ) was positioned with an offset of 50° to the highspeed camera. Metal housings with a 3'' inspection window (Mammut M/L IP65, autoVimation GmbH 1 ) were used for the protection of the camera and the illumination unit from dust and hose water. A ball joint holder (Mounting Kit 16 m, autoVimation GmbH 1 ) enabled a simple and flexible positioning of both housings. A constant flushing of the housings with pressurized air avoided overheating of the equipment. The experiments were recorded with an aperture setting of f/11 and an acquisition frame rate of 5350fps. The exposure time was set to 100 µs. The recordings were captured without gain enhancement and a manual acquisition start was sufficient for the examination of the wetting phenomena. Each experiment consisted of 500 frames and thereby showed a time period of 0.09 s. The test setup including the high-speed camera and illumination unit is depicted in Fig. 5.

Cryogenic milling
In Fig. 6, the thermo-mechanical load is depicted in dependence of the mass flow during cryogenic milling with CO 2 . Both the minimum and maximum temperature decrease at higher mass flows. As the cutting parameters and therefore Image-based test setup including high-speed camera and illumination unit the duration of the milling process were kept constant, more CO 2 is applied at increased mass flows when milling. Consequently, the higher amount of cryogenic coolant is able to remove more heat from the cutting zone resulting in lower temperatures. In case of dry milling, the minimum temperature is close to the ambient temperature since no coolant is applied to remove the heat. As an average maximum temperature T Max = 41 °C ± 2 °C was measured within the workpiece. In general, during dry milling of Ti-6Al-4 V, a much higher thermal load is expected in the cutting zone. The comparably low maximum temperatures measured here can be explained by the distance between the tip of the thermocouple and the actual cutting zone and the low thermal conductivity of the titanium alloy. As a result, comparably low temperatures were measured inside the workpiece, while the thermal loads directly in the cutting zone are expected to be much higher. However, since the process parameters were kept identical for all cooling strategies examined, the temperatures measured here can still be used to interpret the influence of the cooling strategies on the cooling performance and the occurring thermal loads.
With respect to the resulting forces that have occurred during cryogenic milling, no significant influence of the mass flow and the interlinked workpiece temperatures is observed (Fig. 6). This can be explained by the superposition of two opposing effects. A reduction of the temperature causes an increase in the material resistance, which results in increasing cutting forces. But decreasing temperatures also promote a shrinkage of the workpiece material, which reduces the chip cross section and thus the forces that occur. These effects were also observed in previous studies by us [29].

Sub-zero milling
The occurring temperatures and forces during sub-zero milling are depicted in Fig. 7 in dependence of the supply temperature of the sub-zero MWF. While the volume flow and the outlet velocity were kept constant here, the mass flow and consequently the supply pressure slightly increase with decreasing supply temperatures ( Table 1). The minimum temperature decreases when decreasing the supply temperature. This is expected as the increased temperature gradient increases also the heat flux from the surface of the workpiece to the MWF. However, when changing the supply temperature from T supply = − 30 °C to T supply = -40 °C a slightly higher minimum temperature was measured. The effect of the supply temperature of the sub-zero MWF on its kinematic viscosity represents a potential reason for this phenomenon. As the kinematic viscosity increases exponentially with decreasing temperatures (Fig. 3) the highly viscous state of the MWF prevents a sufficient wetting, especially in the cutting zone. Consequently, less MWF reaches the cutting zone so that the heat transfer coefficient and the heat flux between surface of the workpiece and MWF is reduced.
This assumption is supported by the high-speed imaging depicted in Fig. 8. Here, the cutting process is depicted  Thermo-mechanical load in dependence of supply temperature during sub-zero milling via three representative images in dependence of the supply temperature at a constant volume flow and outlet velocity. As the supply temperature decreases, the increasing viscosity of the MWF is clearly visible. A detailed depiction of the cutting process is given in the Online Resource 1.
A similar trend is observed for the maximum temperature. Since the process parameters and therefore the process kinematics were kept identical for all experiments, the heat introduced by the cutting process can be considered to be roughly identical. As a result, the change of the maximum temperature and by that the thermal load has to result from the different cooling performances characterized by the minimum temperature.
As with cryogenic machining, no significant effect of the cooling strategy on the resulting forces is observed. Despite that, the resulting forces are in general slightly lower during sub-zero milling which can be attributed to favourable lubrication effects.
In Fig. 9, the workpiece temperatures are depicted as a function of the volume flow during sub-zero milling at a constant supply temperature of T supply = − 30 °C. In general, a higher volume flow results in a promoted cooling performance and a decreasing thermal load since more coolant is applied to remove the heat from the cutting zone. Additionally, the outlet velocity and the supply pressure are increased when increasing the volume flow at a constant outlet diameter favoring an improved cooling. In order to characterize the impact of the volume flow, the outlet velocity, and the respective supply pressure, two different nozzle outlet diameters of d = 2 mm (Fig. 8a) and d = 1 mm (Fig. 8b) were examined.
At a constant volume flow, lower minimum and maximum temperatures are reached when using the smaller nozzle outlet diameter, since higher outlet velocities and supply pressures result here. The increase of the outlet velocity favors the convective heat transfer and therefore the cooling performance. Furthermore, a higher supply pressure promotes a better application of the sub-zero MWF into the cutting zone. Consequently, increasing the outlet velocity and the respective supply pressure by reducing the nozzle outlet diameter has a more favorable impact than increasing the volume flow.
In Fig. 10, the application of the sub-zero MWF with two different nozzle outlet diameters is shown during the cutting process via three representative pictures. Here, the supply temperature and the resulting viscosity as well as the volume flow of the sub-zero MWF were kept constant. However due to the smaller nozzle outlet diameter (Fig. 10a) a thinner sub-zero MWF jet and a higher outlet velocity were realized. Due to the higher outlet velocity and the respectively higher outlet pressure, the promoted application behavior is clearly visible. A detailed depiction of the process is presented within the Online Resource 2.
As a result, the highest cooling performance and the lowest thermal load were measured when the highest outlet velocity was used at a moderate volume flow of V = 3.6 l/ min and a supply temperature of T supply = − 30 °C (Fig. 9b). Comparing this optimized sub-zero cooling strategy with the best performing referential cryogenic cooling strategy ( ṁ = 2.7 kg/min, Fig. 6), a similar minimum temperature is observed despite the CO 2 having a much lower supply temperature. But due to the higher heat coefficient of the liquid sub-zero MWF, a comparable cooling performance is realized. With respect to the maximum temperatures of the two cooling strategies, sub-zero milling outperforms the cryogenic cooling as lower thermal loads were measured here. Despite having a similar cooling performance, the better lubrication effects of the sub-zero cooling can reduce the friction during cutting, therefore reducing the amount of heat being generated in the first place and the thermal load respectively. This becomes evident when comparing the differences between the minimum and maximum temperatures within this study. For sub-zero milling, these differences are much smaller, indicating a lower amount of heat being introduced by the cutting process. Due to identical process parameters across all tests, the reduced amount of heat can only be explained by the reduction of friction due to lubrication.

Summary
Within this study, a novel cooling system is presented to apply sub-zero MWF under various cooling strategies for milling operations. The presented setup allows a systematic adjustment of the supply temperature, the volume flow as well as the outlet velocity and the respective supply pressure. Aiming at an increased cooling performance and a reduced thermal load within the workpiece, the influences of these parameters were examined during end milling experiments.
The results show that the temperatures inside the workpiece decrease with decreasing supply temperatures of the MWF since the higher thermal gradient promotes the required cooling performance. Reducing the supply temperature also results in an exponential increase of the kinematic viscosity of the MWF. When the MWF exceeds a critical viscous state, a sufficient wetting of the cutting zone is hindered resulting in higher temperatures in the workpiece. Within the frame of these experiments, an ideal supply temperature is found for T supply = − 30 °C.
Increasing the volume flow as well as the outlet velocity also leads to an improved cooling performance and hence to a reduction of the thermal loads. Here, the impact of the outlet velocity and the respective supply pressure is more significant in comparison to the volume flow, due to the beneficial influence regarding the conductive heat transfer and the promoted wetting of the cutting zone.
For reference, additional experiments were conducted using a cryogenic CO 2 cooling strategy with a supply temperature of T supply = − 78.5 °C and a systematical variation of the mass flow. Increasing the mass flow leads to an improved cooling performance as more cryogenic coolant is supplied to the cutting zone. However, despite the higher supply temperature, the optimized sub-zero cooling strategy outperforms the cryogenic cooling even at elevated CO 2 mass flows.
Future work will examine the application of the sub-zero MWF during milling operations in more detail. In particular, the influence of the cooling strategy on the thermo-mechanical load and the resulting surface integrity under variation of the cutting parameters will be analyzed. Furthermore, the performance of the developed sub-zero cooling strategy will be compared with conventional MWF in respect of the occurring loads as well as the resulting tool wear.
Acknowledgements This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) -project number 172116086 -SFB 926. 1 Naming of specific manufacturers is done solely for the sake of completeness and does not necessarily imply an endorsement of the named companies nor that the products are necessarily the best for the purpose.
Funding Open Access funding enabled and organized by Projekt DEAL.
Data availability Data will be made available on request.
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