Experimental Research on Pollution-Free Alcohol Cutting Fluid in Scratching of Single-Crystal Copper Material

Abstract

Cutting fluid can improve the heat dissipation and lubrication in the cutting process and thus increase the machining quality. In this work, a pollution-free alcohol solution was proposed as the cutting fluid in an ultra-precision cutting process to explore green cutting fluids. The scratching experiments were conducted with the alcohol cutting fluid to study its effect on the cutting process. It is found that the use of an alcohol cutting fluid, on average, reduces the tangential and normal force about 27–53%, but exhibits few effects on the friction coefficient in the cutting process. Compared to dry cutting, the alcohol cutting fluid reduces the exposed shear slip steps on the outside surface of the chip, which implies the decreased chip deformation degree of workpiece material in the cutting process. The alcohol cutting fluid can reduce microburrs and decrease the machined surface roughness Ra from 21 nm to 9.9 nm in the ultra-precision turning application on single-crystal copper material.

1. Introduction

Single-crystal copper presents excellent electrical and thermal conductivity and is widely used in various industrial fields, such as integrated circuits, communication network devices, and high-performance wires [1,2]. Simultaneously, it is also applied as various substrates for material synthesis due to its good chemical catalytic properties, such as graphene materials [3,4]. Among these application fields, the ultra-smooth surface quality usually is required for single-crystal copper parts, such as various substrates and reflector mirror components [5,6]. Due to the excellent plasticity and low hardness of single-crystal copper, using ultra-precision machining based on the abrasive process makes it difficult to obtain a better machining performance [7,8]. The ultra-precision cutting process with diamond tools is usually applied in the machining of ultra-precision parts made of single-crystal copper and obtains smooth surface quality of nanoscale [9,10]. Wu et al. [11], Wang et al. [12], and Zhang et al. [13] investigated the influence of crystal orientation in the ultra-precision cutting process of single-crystal copper material, such as the cutting force, shear angle, and material dislocation slip, and reported the crystal plane with the best machinability. Liu et al. [14] reported the evolution of the microcrystalline structure, whereas Chen et al. [15] and Zhang et al. [16] studied the chip formation mechanisms in the nano-machining of single-crystal copper material. Dai et al. [17] and Demiral et al. [18] investigated the influence of tool geometry in the machining of single-crystal copper and reported the optimal tool parameters. Wu et al. [19] proposed a magnetic-assisted cutting method and found that it can decrease the friction coefficient and improve the surface quality. These studies mainly focus on the cutting mechanisms to achieve good machined surface quality.

The cutting fluid is used in cutting process to improve the machining quality in four ways, including the increase in heat dissipation to reduce the cutting temperature, the increase in lubrication to reduce the cutting force, the cleaning of chips, and the rust inhibition [20,21]. In metal cutting fields, 3% of the total manufacturing cost is consumed by the cutting fluid; in the automotive industry, this ratio is up to 16~18% [22]. The industry standard cutting fluid can be divided into two main types: water- and oil-based cutting fluids [23,24]. Water-based cutting fluid demonstrates a good cooling effect and is widely used in the case of a large amount of cutting heat. Oil-based cutting fluid exhibits an excellent lubrication effect and is suggested for use in fine machining. However, cutting fluids also bring some adverse issues, such as the environmental pollution, device corrosion, and human health hazards [25,26,27]. Oil-based cutting fluids even generates severe oil mist in high-velocity cutting and induces a fire risk. Hence, a pollution-free and harmless cutting fluid is urgently needed in metal cutting fields. Based on this goal, various green cutting fluids, such as non-edible oils and vegetable-based and coconut oil-based cutting fluids, etc., are being developed in metal cutting process [28,29,30,31]. Alcohol is a pollution-free, renewable raw material that can be extracted from biomass. In the prior research, alcohol has been used as an additive for the organic grinding fluid to enhance the engineering ceramic grinding efficiency; it was found that the ceramic grinding efficiency was 2.4 times higher than common grinding fluids [32].

To explore more green cutting fluids, a pollution-free alcohol solution is proposed as a cutting fluid in the cutting process in this work. The alcohol cutting fluid was first used in a scratching experiment on single-crystal copper material. The cutting force, chip morphologies, and surface quality were deeply investigated to study the effect of the alcohol cutting fluid on the cutting process. Finally, the ultra-precision turning experiment was carried out to verify whether the alcohol cutting fluid improved surface quality.

2. Experimental Procedures

Scratching experiments are usually performed to study and analyze the ultra-precision cutting mechanisms due to their approximate cutting process. In the prior research, scratch test techniques have also been demonstrated to provide a quick and cost-effective evaluation of cutting fluids [33]. In this paper, the used workpiece in scratching experiments was single-crystal copper material with the (111) crystal face prepared by the single-crystal continuous casting method. Before the scratching experiments, the workpiece was machined into a circular shape with an outer ring with the size of Φ148 × 24 mm to perform the scratching experiments along the radial direction, and the workpiece surface was altered so that the surface roughness was less than 10 nm prior to ultra-precision cutting, as depicted in Figure 1. The polycrystalline diamond (PCD) tool was used in scratching experiments, which is usually applied in the machining of non-ferrous metal materials, such as copper and aluminum, as depicted in Figure 1. The PCD tool presents a tool nose radius of 1.0 mm, edge radius of 5 μm, and rake and flank angles of 0 and 5°, respectively.

First, the scratching experiment was performed with a self-developed ultra-precision machine tool, which was composed of a machine body that was made by iron casting, linear motor, precision spindle, and numerical control system, as shown in Figure 2a. The machine tool has a maximum revolution velocity of 12,000 rpm and a repetitive positioning accuracy of about ±2 μm. Importantly, this machine tool offers a spindle lateral runout smaller than 1 μm, which can ensure the high accuracy of the scratching depth in the scratching experiments. This self-developed machine tool is an open system and can be used for different functions, such as scratching experiments and ultra-precision turning operations.

In the preliminary experiments, it was found that many microscratches are usually generated on the machined surface during the ultra-precision turning experiments on single-crystal copper material with dry cutting conditions, which greatly deteriorate the surface quality. The cutting fluid can increase the heat dissipation to reduce the cutting temperature, improve the lubrication to decrease the cutting force, and increase the cutting performance during the metal cutting process. This means that the heat dissipation and lubrication effect brought by the cutting fluid affects the cutting force and chip formation, which are closely related with the surface quality. Hence, it is suggested to avoid the built-up edge and eliminate these surface defects during the ultra-precision cutting process on single-crystal copper material. The type of cutting fluid is significant for the cutting process. However, commonly used cutting fluids in ultra-precision cutting, such as kerosene and oil- and water-based cutting fluids, usually cause environmental pollution and are also difficult to recycle in the self-developed open-style machining system, as depicted in Figure 2. Especially, the kerosene cutting fluid is highly ignitable and dangerous in the cutting process. Hence, industrial alcohol was proposed as the cutting fluid in ultra-precision cutting, which is pollution-free and has no need to be recycled with natural volatilization. The alcohol cutting fluid is renewable and does not cause corrosion to the machine tool. Hence, the effect of alcohol cutting fluid on the cutting process was tested in the scratching experiments. The used cutting fluid is the industrial alcohol had a purity of 95% and was added to the scratching zone by a hose nozzle with the flow rate of 5 mL/min.

The scratching process is depicted in Figure 2c; the designed cutting direction followed the radial direction of a circular workpiece, which is locked by the precision spindle. When the scratching experiment was finished, the spindle was rotated to a certain angle to carry out the next scratching experiment, so all the scratching experiments were finished on the same one workpiece to avoid the effects of material factors. In the single-factor experiments, the scratching depth was altered in a range from 4 μm to 10 μm, with the fixed scratching velocity of 2 mm/s, including the case with a smaller and larger edge radius of the used PCD tool. The scratching velocity was altered in a range from 2 mm/s to 8 mm/s with the fixed scratching depth of 6 μm, which approaches the actual cutting velocity during the ultra-precision turning of the single-crystal copper material, as depicted in

Table 1. Scratching experiment parameters.

Parameters	Value
Scratching depth d (μm)	4, 6, 8, 10
Scratching velocity v (mm/s)	2, 4, 6, 8

. In the scratching process, the force signal was recorded with a scratching force measurement system (9119AA2, Kistler, Winterthur, Switzerland), which includes the dynamometer, charge amplifier, and data acquisition card. After the scratching experiments, the chip morphologies were analyzed by utilizing a scanning electron microscope; surface roughness and scratching morphologies were inspected as well.

3. Experimental Results and Discussions

3.1. Effect of Alcohol Cutting Fluid on the Scratching Force

The typical force signal that is measured in the scratching processes is shown in Figure 3. From the scratching force signal, it is clearly observed that the cut-in and cut-out stages with the force signal rapidly increase and decrease in phenomena. The scratching stage exhibits relatively stable force signal results. The force signal mainly includes the tangential force F_t and normal force F_n, that, respectively, corresponds to the signal F_x and F_y in the force signal results. In this work, the averaged values of the force signal were adopted for the scratching force results.

The comparison of the measured scratching force with and without alcohol cutting fluid by the various scratching parameters is exhibited in Figure 4. It is found that the whole trend in the scratching force with and without alcohol cutting fluid is almost the same, which gradually increases with both the increase in the scratching depth and scratching velocity. Also, the tangential force, which is actually parallel to the scratching velocity and responsible for material removal, is obviously much larger than the normal force. With the larger scratching depth, more material is removed in the scratching process and a larger scratching force is generated. Due to the excellent plastic properties of the single-crystal material with a higher scratching velocity, some plastically deformed materials may adhere on the tool surface and increase the scratching force.

In comparison, it is found that the scratching process with alcohol cutting fluid, both the tangential force and normal force are obviously smaller than that of dry scratching. Under different scratching depths, the use of alcohol cutting fluid, on average, decreases the tangential and normal force by about 0.9 N and 0.4 N, and the decreased ratios, on average, are 27.9% and 27.7%, respectively. Under a different scratching velocity, the tangential force and normal force with the alcohol cutting fluid averagely decrease by about 2.2 N and 0.9 N more than the dry scratching conditions, and their decreased ratios are, on average, 52.9% and 53.6%. It is confirmed that the use of pollution-free alcohol cutting fluid is effective to reduce the cutting force.

3.2. Effect of Alcohol Cutting Fluid on the Friction Coefficient

From the metal cutting mechanisms, the cutting force mainly originates from the metal elastic–plastic deformation process and friction behavior between the workpiece and tool surface. Among them, the friction behavior between the chip and tool surface is the dominant friction action. Hence, the corresponding friction coefficient on the tool rake surface directly affects the friction force ratio in the resultant cutting force; it also affects the chip shape and the machining quality. In scratching experiments, the cutting process is equivalent to the two-dimensional orthogonal cutting operation. As shown in Figure 5, according to the Merchant metal cutting mechanisms [34], the following formula can be obtained:

$${\displaystyle \frac{F_y}{F_z}}=\tan (\beta -\gamma )$$

(1)

where F_z and F_y represent normal and tangential forces F_n and F_t; β represents the angle of friction on the rake surface; and γ represents the rake angle of cutting tool. From the formula, the friction coefficient on the rake surface tanβ can be directly calculated. In this work, the actual rake angle γ of PCD tool is 0°, so the friction coefficient on the rake surface can be approximately calculated as μ = F_n/F_t.

The calculated friction coefficient comparison with and without alcohol cutting fluid by the various scratching parameters is exhibited in Figure 6. The results indicate that the friction coefficient exhibits a slightly decreasing trend with both the increase in scratching depth and velocity. The calculated friction coefficient slightly decreases from 0.47 to 0.4 when the scratching depth increases from 4 μm to 10 μm and marginally decreases from 0.45 to 0.37 when the scratching velocity increases from 4 mm/s to 10 mm/s. It is indicated that the scratching parameters exhibit few effects on the friction coefficient in the cutting process. In comparison, it is found that, although the use of alcohol cutting fluid can significantly reduce the scratching force, the calculated friction coefficient on the tool rake face is almost the same and does not decrease. This indicates that the friction force ratio in the resultant cutting force of the scratching process is approximate. It is known that the resultant cutting force in the cutting process is mainly consumed by the elastic–plastic deformation of the workpiece material and friction action; hence, it is inferred that the use of the alcohol cutting fluid in the scratching process may decrease the force for material elastic–plastic deformation.

3.3. Effect of Alcohol Cutting Fluid on Chip Morphology and Surface Quality

The chips are continuously generated alongside the metal elastic–plastic deformation of the workpiece material during the cutting process. The chip morphology can not only reflect the elastic–plastic deformation situation of the metal material, but is also directly related to the surface morphology during metal cutting. The SEM images of the common chip morphologies are depicted in Figure 7. The formed chip is flat in shape due to the small scratching depth, which corresponds to the chip thickness. The side surface of the chip exhibits a microsawtooth shape due to the extrusion and tension action during metal cutting. From the results, the surface morphology of the chip’s internal side is very smooth, and only some microscratches are observed on the internal surface. The internal surface demonstrates severe friction with the tool rake face and encounters serious tension action as it flows out the cutting region. The outside surface morphology is very rough with many uneven microsteps, which are generated by the shearing slip deformation of the plastic/metal material in the cutting process. The thickness of these shearing slip steps is not fixed and varies from serval micrometers to over ten micrometers, with the maximum thickness measured to be 19.1 μm. In the interior of the shearing slip step, many shearing slip layers that are smaller than the shearing slip step can be observed. When the tool contacts and cuts the workpiece material along the cutting direction, the stress on the metal material gradually increases. A shearing slip step is generated since the stress exceeds the material yield strength.

The chip morphology comparison with and without alcohol cutting fluid under different scratching parameters is depicted in Figure 8. From the results, the outside morphology of the chip becomes flatter with the increase in scratching depth. With a scratching depth of 4 μm, it is easy to observe that the large piece of the shear slip steps is highly exposed on the chip surface. When the scratching depth increases from 4 to 6 μm, the chip surface becomes flatter with the decreased height and thickness of the exposed shear slip steps. It is implied that the plastic deformation degree of the workpiece material reduces with the increase in the cutting depth. With the increase in cutting velocity, these shear slip steps become obviously rougher, which indicates the higher degree of plastic deformation of the workpiece material in the scratching process. This explains and is consistent with the observation that the scratching force increases with the scratching velocity. By comparison, the chip surface with the alcohol cutting fluid is obviously flatter than that of the dry scratching process without cutting fluid. Both the exposed height and thickness of the shear slip steps decrease after the use of the alcohol cutting fluid. This result is consistent with the finding that the alcohol cutting fluid can decrease the degree of plastic shear deformation in the workpiece material in the cutting process, and it validates the previous inference drawn from the scratching force and friction coefficient results.

The SEM images of the groove bottom with and without alcohol cutting fluid is depicted in Figure 9. From the results, the surface morphology becomes rougher with the increase in the scratching depth. Except the copying of the cutting edge texture of the PCD tool, there are many microburrs that appeared on the scratched groove surface when the scratching depth increases. These microburrs are the main surface defects on the scratched surface. The microburrs that are located at the bottom of the texture are generated by the built-up edge in the scratching process, and these microburrs that are located on the side position are generated by the plastic material side flow behavior [11]. The surface morphology of the scratched groove exhibits few changes despite the scratching velocity increasing from 2 mm/s to 8 mm/s. By comparison, with the use of alcohol cutting fluid, the surface morphology of the scratched groove obviously becomes smoother than that of the dry scratching condition. The scratch texture on the scratched groove surface exhibits no change, but these microburrs located on both the bottom and side of the texture significantly decrease. This result indicates that the alcohol cutting fluid can reduce surface microburrs and improve surface quality in the scratching of plastic/metal materials.

From the scratching results, the alcohol cutting fluid is subsequently applied in the ultra-precision turning of the single-crystal copper material with a cutting depth of 6 μm, cutting velocity of 2 mm/s, and feed rate of 0.75 μm/r, as depicted in Figure 10. The machined surface quality comparison with and without alcohol cutting fluid are also depicted to verify the effect of the alcohol cutting fluid. It is found that microscratches and burrs are clearly observed on the machined surface without alcohol cutting fluid. These microscratches and burrs on the machined surface disappeared when the alcohol cutting fluid is used during the ultra-precision turning process. According to the measurement results, the surface roughness Ra reduces from 21 nm to 9.9 nm after the use of the alcohol cutting fluid. This result also confirms the effectiveness of the alcohol cutting fluid in improving the surface quality in ultra-precision turning.

4. Summary and Conclusions

This work presents an experimental investigation on the effect of alcohol cutting fluid on the cutting process of single-crystal copper with scratching experiments. Based on the investigation’s results, the following conclusions were obtained:

1. An alcohol solution was proposed as the cutting fluid in the scratching of single-crystal copper. Compared to the dry scratching of single-crystal copper, the use of alcohol cutting fluid in the scratching process can, on average, reduce the tangential and normal forces by about 27–53%. However, the use of the alcohol cutting fluid in the scratching process exhibits few effects on the friction coefficient in the cutting process.

2. The chips that formed in the scratching of single-crystal copper exhibits a rough side surface, microserrated shape, smooth internal surface, and rough outside surface with many exposed steps that were caused by the shear slipping of the metal material. The use of the alcohol cutting fluid in the scratching process can reduce the height and thickness of the exposed slip steps and creates a flat outside surface on chips. It is indicated that the alcohol cutting fluid can decrease the degree of plastic shear deformation of the metal material in the cutting process.

3. The copied tool texture and microburrs are the main surface defects found on the bottom surface of the scratched groove. The use of the alcohol cutting fluid can reduce microburrs on the scratched surface by lowering plastic deformation in the cutting process. With the application verification, the alcohol cutting fluid can decrease the surface roughness Ra from 21 nm to 9.9 nm in the ultra-precision turning of single-crystal copper material.