1 Introduction

Continuous development of cooling lubricants and cooling lubrication strategies is necessary in order to be able to meet the growing requirements for the machining of high-temperature materials and alloys in the future. In addition, the goal of sustainable design of manufacturing processes must be pursued. Based on these requirements, cryogenic minimum quantity lubrication (CMQL) has been developed in recent years. To investigate the properties of the two media CO2 and oil during the joint feed to the tool cutting edge, selected lubricant experiments are carried out to investigate the influence of temperature-controlled carbon dioxide on sprayability and solubility in CMQL. A conventional cooling is implemented for the CO2 after withdrawal from the riser bundle in order to guarantee a constant liquid state of the medium during the supply.

2 State of the Art

In this chapter, the bio-based CMQL and the physical and chemical properties of CO2 are explained in more detail.

2.1 Cryogenic Minimung Quantity Lubrication with Bio-Based Oils

In CMQL, a minimal amount of oil is added to the cryogenic medium (liquid CO2 (LCO2) or liquid nitrogen (LN2)) to ensure sufficient lubrication in addition to cooling of the contact zone. Thus, the advantages of the two cooling lubrication strategies MQL and cryogenic cooling can be used synergetically. The CMQL-supply can be divided into an external supply to the machining zone or an internal supply through the machine spindle and tool. Furthermore, the two media can be fed separately using a so-called two-channel solution or as a mixture in a one-channel system. The solubility and flow characteristics of a multiphase mixture do not have to be taken into account when the media are supplied separately. In addition bio-based oils can be used to increase the sustainability of the CMQL. A bio-based lubricant can be declared as such, if its proportion of renewable raw materials is at least 25 percent and its degradability is more than 60 percent. Triglycerides can be cited as examples of bio-based lubricants. These are esterified glycerol to which three fatty acids are bound. The configuration of the fatty acids and their state of saturation determine the chemical and physical properties of the lubricant. [1].

A one-channel CMQL system was developed in 2015 at the REP-chair [2]. Extensive research on the use of bio-based oils for CMQL has already been carried out with this system. The influence of fatty alcohols, mineral oils, synthetic and natural esters on the milling of Ti6Al4V compared to conventional flood cooling resulted in the highest tool life when using and natural esters with CMQL [3]. Further investigations on the influence of bio-based base oils and additive oils on the solubility and sprayability and on drilling, milling and turning of stainless steel and roller bearing steel confirm the high potential using natural esters for CMQL compared to conventional flood cooling with regard to longer tool life and reduction of wear and forces [4, 5]. This results in economic and ecological advantages, while increasing the process efficiency.

2.2 Properties of Carbon Dioxide

Carbon dioxide exists in four states: solid (s), liquid (l), gaseous (g) and supercritical (sc), see p-T diagram in Fig. 1a. At the triple point, the three phases (s, l and g) are in thermodynamic equilibrium. At ambient pressure (1.013 bar), the temperature of CO2 is T = −78.5 ℃. In this state, CO2 exists in a solid aggregate and is known as dry ice. In the range of these pressure and temperature regions, solid CO2 changes directly into the gaseous phase during phase transformation and sublimates afterwards. Several scientific studies have already dealt with the thermodynamic processes during cryogenic cooling. Krämer investigated the thermodynamic process of carbon dioxide within the feed and the nozzle and Pursell investigated the exit mechanism of carbon dioxide after the nozzle in more detail, defining the mechanisms that occur [6, 7]. Since the cooling effect of CO2 is primarily generated by the sublimation of solid into gaseous phase, too high proportion of gaseous CO2 can reduce the cooling effect. Another cooling effect occurs immediately after exiting the nozzle. This phenomenon is known as the Joule-Thomson effect. According to this, a throttled pressurized gas expands once it has left the nozzle, losing additional temperature in the process [8].

When assessing the solubility of two substances, the principle “like dissolves in like” often applies. This refers to interactions between molecules, which are described with the aid of the polarity of a substance. In general, a mixture is understood as a substance formed from two pure substances. In a heterogeneous mixture, the two substances are in separate phases (multiphase), and in a homogeneous mixture, the two media have mixed completely at the molecular level and the state corresponds to a single-phase aggregate state. The physical properties of the mixture are determined by the respective proportions of the mixed media.

Fig. 1.
figure 1

a. CO2: p-T diagramm. b. T-x diagramm (right).

Depending on the thermodynamic state the variables pressure, temperature or volume, and the composition of the mixture can change. This is usually represented in a T-x diagram, where the temperature T is plotted against the proportion x of the mixture partners involved. The value range of the diagram is divided into the states heterogeneous and homogeneous mixture, for example, with the aid of a parabolic line. With a T-x diagram, it is now possible to determine, those mixture configurations consisting of temperature T and substance fractions x in which either complete mixing (1) is possible or two separate phases (1′ + 1″) occur. The area enclosed by the segregation curve is therefore also referred to as the mixing gap. Figure 1b shows, for example, a change in state of a mixture from A to B when the temperature is lowered. In this case, state A describes a homogeneous mixture, and if the temperature is lowered from TA to TB, the phases separate when the area enclosed by the segregation curve is reached.

The aim of this investigation is to influence the thermodynamic conditions in order to be able to realize a process-safe and thus low-pulsation feed of the CO2-oil mixture into the process zone which is more independent of the initial state of the CO2, e.g. with regard to fluctuating densities. Furthermore, the influence on sprayability and solubility will be investigated as a process property of CMQL for machining.

3 Experimental Setup

In this chapter, the experimental boundary conditions are considered. The cryogenic mixing system with temperature-controlled carbon dioxide through pre-cooling, the oils investigated and the setup for the sprayability and solubility tests are explained.

3.1 Cryogenic Mixing System with Temperature-controlled Carbon Dioxide

The temperature and pressure conditions during the feed of the two media illustrate the changes of the CO2 aggregate state within the CMQL-system developed at the REP chair [2], see Fig. 2. The system consists auf an HPLC-pump, a coriolis sensor and an oil reservoir. The CO2 taken from the riser bottles is pre-compressed at T = 20 ℃ with a pressure of p = 50.37 bar. The CO2 remains in this state until it enters the nozzle (1). Inside the nozzle (2), the atmospheric pressure outside the nozzle causes a pressure equalization. The pressure drops to p = 5.18 bar, at a temperature of T = −56.6 ℃. At the triple point the CO2 is present in the three aggregate states: gaseous, liquid and solid. At the nozzle outlet (3) prevails directly atmospheric pressure of p = 1.013 bar. The CO2 reaches its minimum temperature of T = −78.5 ℃ and dry ice is formed, which sublimates as it equals the ambient temperature.

Fig. 2.
figure 2

Cryogenic mixing system REP chair [9]

For a safe application of the system with low pulsation, the CO2 must be in a stable liquid state at the system inlet. Changes in ambient conditions, e.g. lower temperature, low CO2 level or defects in the riser bottles, can lead to an increasing gas phase ratio and a lower CO2 density. A pre-study was carried out at the chair to increase the density by cooling the CO2 supply line. Figure 3a describes the experimental setup where cooling is achieved by means of a cooling circuit in which a water tank (V = 50 l) is cooled to a constant temperature of T = 4 ℃ and the CO2 is fed through a pipe with a length of l = 6.5 m. The aim of CO2 cooling is to maximize the liquid ratio of CO2 arriving at the mixing valve of the one-channel CMQL system. Since the cooling effect of the CO2 is primarily generated by sublimation, this can be optimized by subsequent liquification of the CO2 in the feed. The CO2-temperature (with a mass flow rate of \(\dot{m}_{CO2}\) = 5 kg/m) was reduced from T = 21 ℃ to T = 14.84 ℃ and the density was increased from 550 kg/m3 to over 800 kg/m3. The thermodynamic relationships are explained using the T-s diagram, see Fig. 3b. Initial state 1 (T = 21 ℃, p = 57.3 bar, ρ = 500 kg/m3) characterizes the CO2 as it exits the bundle and describes a state within the wet steam region. If a cooling effect is exerted on the CO2 at constant pressure, the density increases as the temperature decreases (state 2). This causes a shift of the CO2 state to the boiling line, and a stable liquid state is achieved.

Fig. 3.
figure 3

a. T-s diagram CO2. b. experimental setup.

3.2 Base Oils

In previous test series, it was shown that the properties of the base oil are decisive for the mixing and spraying behavior in CMQL. The three base are hydrocarbons, natural esters and synthetic esters. The chemical structure an RRM content is shown in Table 1.

Table 1. Examined oils

3.3 Dynamic Solubility and Spray Pattern Tests

The solubility processes during the single-channel feed of the two media oil and CO2 can be investigated with the aid of a high-pressure viewing cell. The cell developed at the REP chair is used in a coupled experimental setup with the free jet nozzle to investigate the spray pattern. This combination of the dynamic solubility test and the spray pattern test results in an increased significance of the results, since the processes in the free jet can be put into a direct relation to the flow behavior in the cell. The two high-speed cameras used each document a partial test. The recording parameters and the experimental setup of the nozzle with lighting and camera are summarized in Fig. 4.

Fig. 4.
figure 4

Experimental setup dynamic solubility and spray pattern tests

4 Results

The video material of the dynamic viewing cell is evaluated with regard to the assessment of the tested oils with the aid of the “laminar boundary flow” criterion. This is the phenomenon of a laminar boundary flow with a sawtooth shape forming in the lower part of the viewing area during the flow process in the case of NE03 base oil, which flows more slowly than the rest of the CO2-oil mixture. This occurrence is marked with a red box in Fig. 5. This event occurs in all additivated variants of this base oil. In this study, as already mentioned, only the pure base oils will be discussed in detail.

When the experiment is carried out with variation of boundary conditions by CO2 cooling, the contents of the viewing cell become much clearer with cooling of the CO2 and are characterized by fewer visible individual particles, see Fig. 5. Using the base oil NE03 as an example, significantly fewer detectable CO2 particles are evident at higher density. Complementary, a lower pulsation in the mass flow of LCO2 was detected with pre-cooled CO2. The density is also characterized by higher stability during the feed. Laminar boundary flow, typical for the NE 03 oil with high viscosity, cannot be prevented by cooling. The density of the CO2 does not seem to have any influence on this phenomenon. In further series of experiments it was shown that the flow velocity of the CO2-oil mixture does have an influence on the laminar boundary flow. With increasing flow velocity in the pipe, the laminar boundary flow decreases or does not occur at all. In this test setup, the influence of pre-cooled CO2 on the spraying behavior was investigated for a nozzle diameter of di = 0.2 mm. It was shown that due to the higher density of the CO2, which was demonstrably achieved with the cooling, the spraying behavior changes. The main jet gives a focused impression which is also superior in length to that without CO2 cooling. Cooling the CO2 increases the atomization of the oils and widens the spray corridor with a greater main jet length and intensity. A finer and wider oil application results. Process stability in terms of pulsation of the CO2-oil mixture at nozzle exit can be significantly improved.

Fig. 5.
figure 5

Results of dynamic solubility and spray pattern tests

Using the VW-9000 motion analyzer program, the videos of the dynamic view cell can be evaluated using particle tracking. The tracking tool must be calibrated to the inner diameter of the line of sight. Subsequently, a CO2 or oil particle is selected and tracked over the entire line of sight. Finally, the tracked distance can be evaluated with respect to the absolute velocity. The velocities of the different oils, depending on nozzle diameter and CO2 cooling, are summarized in Fig. 6.

Fig. 6.
figure 6

Flow velocity depending on nozzle diameter and CO2 cooling

As expected, particles moving with a greater nozzle diameter of di = 0.3 mm without cooling are much faster than those moving with di = 0.2 mm without cooling. Cooling of CO2 reduces the flow velocity within the feed due to higher density. When implementing CO2 cooling in practical applications for machining operations, this influence must be taken into account. The influence of the lower flow velocity with simultaneously stronger atomization of the lubricant on the oil application and cooling of the contact zone of tool and workpiece must be investigated in further investigations.

5 Summary

In summary, the following influences of temperature-controlled carbon dioxide on sprayability and solubility in CMQL with bio-based lubricants can be determined. The cooling of the CO2 increases the atomization of the oils and widens the spray corridor with a greater main spray length and intensity. The process stability in terms of pulsation of the CO2-oil mixture at nozzle exit can be significantly improved with an increase in density from 550 kg/m3 to over 800 kg/m3. By cooling the CO2, the cell content appears transparent due to completely liquid state and flow velocity is slowed down. NE03 provides a slower boundary flow of oil deposited at the outer edge of the viewing cell, even during cooling. This is in agreement with the results of previous investigations of the REP chair which showed that the boundary flow disappears at a higher flow velocity. In addition to the investigation of the influence of lower flow velocity with simultaneously stronger atomization of the lubricant on the machining, further factors of the cooling method must be investigated in detail. These include the influence of cooling at higher flow rates and velocities, but also the components of the cooling system in terms of cooling capacity, cooling length and the materials used.