Experimental Investigation of Cr12 Steel Under Electrostatic Minimum Quantity Lubrication During Grinding

Abstract

In this work, electrostatic minimum quantity lubrication (EMQL) has been applied in grinding. When the droplets were charged, it could promote penetrability in the processing area. The electric field formed between the charged droplets and the surface of Cr12 die steel could affect the hardness of the workpiece surface. The grinding mechanism of EMQL has been revealed under different charging voltage by analyzing the wetting angle of droplets and the hardness of Cr12 surface. The reduction of grinding force (11.5% to 49%), surface roughness (10% to 22.1%), and the increase in grinding ratio (1.9% to 27.3%) and surface quality of EMQL under various charging voltages were studied. The results showed that the wetting angle decreased when the droplets were charged. Compared to MQL, the charged lubricant droplets with better penetrability are easier to penetrate and spread on the contact surface between the grinding wheel and the workpiece, thereby improving the lubrication of the friction interface and obtaining better grinding performance. Moreover, we also found that the positively charged EMQL not only effectively improves the penetrability of droplets but reduces the hardness of the Cr12 surface. Thus, the grinding performances under positively charged EMQL are always better than these under negatively charged when grinding Cr12.

1. Introduction

Grinding is an important operation extensively used in manufacturing industries which depends on the interaction between grinding wheel particles and workpiece surface to remove material. The friction, elastic/plastic deformation, and shear action between the abrasive and the workpiece consume substantial energy. Most of this energy is converted into grinding heat. High heat fluxes during grinding may lead to surface burns and redeposition of chips on the ground surface [1,2,3,4]. As a result, processing difficulty is increased and the surface quality of the workpiece becomes worse, especially in the processing of die steel with high hardness [5,6]. Different cooling and lubrication technologies have been applied to the grinding process to reduce the grinding heat of the grinding area, improve the surface quality of the workpiece, and increase the life of the grinding wheels [7,8,9]. Flood cooling technology is currently the most commonly used cooling lubrication method that effectively reduces the grinding heat and removes debris. But this method requires about 60 L/h of grinding fluid flow [10]. The heavy use of grinding fluid poses severe challenges to resource consumption, manufacturing cost, and environmental protection. Therefore, developing low-consumption, low-cost, pollution-free and high-performance lubrication cooling technology is an eternal pursuit for researchers [11,12].

Dry grinding is an environmentally-friendly lubrication process that seemed to be an alternative to producers [13]. However, the cooling and lubrication of dry grinding in the grinding zone are insufficient as it would lead to short service life for the grinding wheels, poor surface quality, and thermal burn of the workpiece. As a typical representative of environmentally-friendly lubrication technology, minimum quantity lubrication (MQL) has been widely applied in the processing field. MQL uses compressed air to atomize the lubricating fluid into micron-sized droplets that are then sprayed into the cutting area under aerodynamic force to lubricate and cool the workpiece [14,15,16,17]. Banerjee and Sharma [18] applied biodegradable lubricant to MQL turning and found that it could significantly reduce the harm caused to the environment and humans, and that the cost of the fluid was reduced by 65% compared to the traditional method. When MQL was used to grind hard materials, Rabiei et al. [19] and Stephenson et al. [20] observed that it could effectively improve the surface quality of the workpiece, and reduce the grinding force, surface roughness and friction coefficient. Hadad et al. [21] used MQL to grind 100Cr6 bearing steel. It was found that MQL reduced energy consumption by 7% to 10% compared to dry grinding. However, Oliveira et al. [22] and Barbosa et al. [1] pointed out that MQL has several disadvantages, including blockage of chip removal, excessive grinding heat, and so on. Sadeghi et al. [23] noted that, in the process of oil-based MQL grinding, compressed air and droplets did not play an essential role in heat transfer, resulting in a large amount of heat accumulation in the grinding wheel/workpiece interface, finally leading to burns on the surface of the workpiece, low machining accuracy, and reduced life of the grinding wheels.

Electrostatic minimum quantity lubrication (EMQL) is proposed as a combination of MQL and electrostatic spray (ES) technology. It has characteristics such as droplet charging, reducing atomization particle size, and decreasing wetting angle and surface tension of charged droplets. Compared to ordinary droplets, charged droplets significantly improve adsorption and penetration in the processing area, as well as increase the lubrication and cooling capacity [24]. This is expected to improve the performance of traditional MQL in the grinding process when the technical characteristics of lubricating fluid consumption (<100 mL/h), excellent penetrability, and lubrication cooling performance are introduced into the grinding process.

When grinding with EMQL, an electric field is formed between the charged droplets and the workpiece surface. The electric field helps reduce the hardness of the Cr12 surface, which can further improve the grinding performance. Yao et al. [25] applied an electric pulse during the drawing process for austenitic stainless steel. The experimental results showed that the hardness of austenitic stainless steel decreased up to 44% under the action of the electric field, and the ductility and surface quality of the finished product was improved. Zhu and Tang [26] observed that the hardness and elongation of a Ni–Ti alloy rolled with electric pulse assistant equipment was 11% lower and 39.6% higher than for cold rolling. Bao et al. [27] processed AZ31B alloy with an electric pulse auxiliary molding equipment and showed that the application of an electric pulse could improve the formability of this material. In addition, they pointed out that the electric field generated by the electric pulse could increase the kinetic energy of the dislocation and promote the movement of the dislocation. In a study on electrical auxiliary processing of 7475Al, Stephen and Conrad [28] pointed out that the electric field could increase the dislocation density, and that drift electrons could promote the dislocation movement under the action of the electric field. In previous EMQL studies, the machining conditions were limited to turning and milling. The cutting depth range of these conditions was 0.5–2 mm, and their machining performance could not be affected by the microstructure change on the workpiece surface obviously. However, in this study, the cutting depth of EMQL grinding was 10 μm, and the microstructure evolutions could affect its grinding quality more effectively. So, it is meaningful to study the characteristics of EMQL grinding.

Molds have a very important use in modern industry. The main material for making a mold is die steel, which is generally of high hardness and strength. Molds require high surface quality of the inner surface. Therefore, it is meaningful to study the machinability of die steel in modern industry [29,30]. On the basis of the construction of an experimental system for the grinding of EMQL, the electric field strength of charged droplets on workpiece surface under different charging voltages, wetting angle, and surface tension of grinding fluid on Cr12 steel surface were carried out. Moreover, the grinding performance of EMQL was investigated. The effect of different charging voltages on the grinding force and the surface quality of Cr12 in the grinding process were explored, and the mechanism of the synergistic effect of EMQL in the grinding process was revealed.

2. Materials and Methods

The working principle of the EMQL system is shown in Figure 1a. Grinding fluid and compressed air are supplied through the MQL device and transported by a gas–liquid double-layer pipe to the insulated nozzle. The electrode was installed in the upper part of the nozzle through the charging device. The electrode was connected with the output of the high-voltage electrostatic generator (EST802A, Beijing Hua Jinghui Technology Co., Ltd., Beijing, China), and the grinding fluid was charged through contact with the electrode. Finally, the grinding fluid was atomized into charged droplets and sprayed from the nozzle.

A vibratory capacitance electrometer (EST102, Beijing Hua Jinghui Technology Co., Ltd.) was used to detect the potential of the charged droplets near the nozzle at different charging voltages, as displayed in Figure 1b. Micro-hardness of the specimen surfaces ground with MQL and EMQL was measured using a Vickers hardness tester (HV-1000, Shanghai Lianer Experimental Equipment Ltd., Shanghai, China). For each ground specimen, hardness values were measured at five different locations. The material used in the experiment was Cr12 die steel (quenching at 950 °C and tempering at 450 °C). The wetting angle of the charged droplets on the workpiece surface was measured using an electron microscope (VW-6000, Keyence, Osaka, Japan) under different charging voltages.

The test experiments on grinding force based on MQL and EMQL were carried out on a precision plane grinder (MM7120A, Hangzhou Machine Tool Group Co., Ltd., Hangzhou, China). The grinding wheel used in the experiment was corundum grinding wheels (1-250X16X75 WA/F60L 5V, Nanjing Hongteng Abrasive Tools Co., Ltd., Nanjing, China). Before the experiment, a single point dresser was used to dress the grinding wheel to ensure that the parameters of the experiment were consistent. The grinding conditions are summarized in

Table 1. Test conditions of grinding performance.

Experimental Contents	Experimental Conditions
Grinding machine	MM7120A type plane grinder
Grinding wheel	Corundum grinding wheel: grain size. 60#; maximum linear speed. 35 m/s; size: 250 mm × 16 mm × 75 mm
Grinding parameters	The linear speed of the grinding wheel was 20 m/s; the workpiece speed was 150 mm/s; the grinding depth was 10 μm; and the total grindingdepth was 100 μm
Workpiece material	Cr12 die steel; workpiece size: 65 mm × 55 mm × 48 mm
Lubricating fluid	Calteche SYN 40 total synthetic liquid: water = 1:9 (mass ratio)
Charging voltages	0 kV. ±1 kV. ±2 kV. ±3 kV. ±4 kV. ±5 kV
Air pressure	0.4 MPa
Flow rate	50 mL/h
Dressing condition	Using a single point diamond trimmer; the dressing depth was 100 μm; the dressing speed was 20 m/s

.

The grinding normal force (Fn) and tangential force (Ft) were measured with a dynamometer (Kistler9129A, Switzerland Kistler Instrument Co., Ltd., Winterthur, Switzerland), as illustrated in Figure 1c. After testing, the ground specimens and wheel were ultrasonically cleaned in DI water for 10 min. Surface roughness of the specimens was measured perpendicular to grinding direction using a Surftest (SJ-210, Mitutoyo, Kanagawa, Japan). The measurement was performed at five special points with a sampling length of 0.8 mm. Radial wear of the grinding wheel was also measured using a clock gauge in order to calculate grinding ratios (the ratio of volume loss of the material to that of the wheel wear). Three sets of data were measured, and the average values were taken under different conditions. Scanning electron microscopy (EVO18, Zeiss, Baden-Württemberg, Germany) was used to observe the micromorphology of the workpiece under different lubrication conditions.

3. Results and Discussion

When the charged mist was sprayed from the nozzle to the workpiece, an electric field formed between the nozzle and the workpiece. The electric field intensity is shown in Figure 2a. With an increase of charging voltage, the electric field intensity on the workpiece surface also increased. When the charging voltage was positive, the direction of the electric field was from the nozzle to the workpiece. When the charging voltage was negative, the direction was opposite.

To analyze the effect of the electric field on Cr12 surface, the hardness of workpiece surface was investigated. It can be obviously seen from Figure 2b that under a positive charge, the surface hardness of Cr12 decreases with the increase of charging voltage; whereas, under a negative charge, the surface hardness of Cr12 does not change significantly.

Vacancies in metals are negatively charged, and the application of an external electric field to the Cr12 surface promotes vacancies to move along the positive direction of the electric field. Directional movement and accumulation of vacancies promoted the migration of dislocations in Cr12 toward vacancy accumulation areas; the migration and deposition of dislocations reduced the hardness of Cr12 surface and increased its plastic deformation ability [28,31]. At the same time, there were chemical and electrostatic interactions between the impurity atoms in Cr12 surface and the dislocation core region, leading to the motion of impurity atoms toward the dislocation core region [32]. Because of the effect of the electric field and grinding heat, chromium and carbon elements under the surface of Cr12 migrated to the surface with dislocation, as shown in Figure 3. The iron element in cementite (Fe₃C) on the Cr12 surface was replaced by chromium, and finally forming alloy cementite ((Cr,Fe)₇C₃ and (Cr,Fe)₂₃C₆) [32,33,34,35]. With the increase in the chromium content, the transformation process was as follows:

Measurement results for the surface tension of the grinding fluid and the wetting angle of charged droplets on Cr12 surface under different charging voltages are presented in

Table 2. Effect of charging voltages on the wetting angle and surface tension of droplets.

Absolute Voltage (kV)		0	1	2	3	4	5	6	7
Wetting angle on the Cr12 surface (°)	Positive electricity	51	47	45	43	39	38	34	29
	Negative electricity		48	46	43	41	39	35	30
Surface tension (N·m−1)	Positive electricity	0.047	0.044	0.043	0.042	0.040	0.037	0.033	0.028
	Negative electricity		0.045	0.044	0.042	0.041	0.038	0.033	0.029

. With an increase in the absolute value of charging voltage, the wetting angle of charged droplets on Cr12 surface was smaller, which could improve the penetrability of the droplets and promote the grinding fluid to have a better performance in lubrication and cooling in the grinding zone [15].

In the grinding process, the grinding force is the key parameter that affects the grinding performance, workpiece surface quality, and wheel life. The grinding force is produced by elastic deformation, plowing, and cutting when the grinding wheel comes in contact with the workpiece surface [36].

Figure 4 shows the results of different charging voltages on the grinding force, in which 0 kV refers to the MQL mode. Under the same experimental conditions, it was easily found that the normal and tangential grinding forces first decreased and then increased with an increase in the absolute value of the charging voltage. Among these forces, the normal force, Fn, at –3, –4, and –5 kV were 14.2%, 16.8%, and 11.5% lower than those of MQL, respectively. In addition, at +3, +4, and +5 kV, they were 14.9%, 17.6%, and 11.7% lower than those of MQL, respectively (Figure 4a). The tangential force, Ft, at –3, –4, and –5 kV were 28.3%, 36.5%, and 22.8% lower than those of MQL, respectively. Moreover, at +3, +4, and +5 kV, they were 39.6%, 49.0%, and 34.2% lower than those of MQL, respectively (Figure 4b). This indicates the charged droplets were able to provide better cooling and lubrication performance compared to traditional MQL, as the wetting angle of charged droplets on the workpiece surface were decreased. It also helps in retention of grit sharpness. Additionally, because of the fine cutting fluid droplets obtained during EMQL, the cutting fluid can even reach the microgrooves of abrasive grains, keep them sharp and help to reduce the grinding force further. In addition, it can be obviously seen from Figure 4 that the grinding force under positive conditions was less than that under negative conditions. This can be attributed to the fact that the hardness of the workpiece surface was changed owing to the effect of the electric field. Moreover, the excessive charging voltage will also lead to a larger spray cone angle [37]. The overflow of the lubricating fluid during the spray process increases, leading to a reduction in the number of droplets involved in lubrication cooling.

The roughness value is directly related to the lubrication condition, grinding wheel diameter, and material removal rate. The results of different charging voltages on the surface roughness as shown in Figure 5a. It was easily found that the surface roughness of the workpiece first decreased and then increased with an increase in the absolute value of the charging voltage. The Ra values at +3, +4, and +5 kV were 18.8%, 22.1%, and 19.5% lower, and at −3, −4, and −5 kV, they were 10.0%, 18.0% and 14.7% lower than those of MQL, respectively. Charged droplets perform smaller wetting angle and better cooling and lubrication performance. The fine droplets obtained using EMQL facilitate the abrasive grits to retain their sharpness better than those using MQL. The sharp grits and debris slip better on the surface of the workpiece, reduce the scratch on the workpiece and ultimately reduce the surface roughness value of the workpiece. It can also be obviously seen from Figure 5a that the surface roughness values under a positive charge are always less than those under a negative charge at the same absolute value of the charging voltage. This is because the surface hardness of the workpiece was obviously lower than that of other conditions when the droplets were charged positively. The decrease of the hardness of the workpiece surface was conducive to the removal of debris and hence the value of surface roughness during positively charged conditions was decreased.

G ratio is expressed by the ratio of volume of removal of work material to the volume of the wheel, which means “higher is better”. In the present experimental study, G ratio has been calculated under different grinding conditions considered herein and are presented in Figure 5b. It could be seen that the grinding ratio first increases and then decreases with an increase in the absolute value of the charging voltage. Charged droplets exhibit excellent lubrication and cooling performance, thus indicating smooth sliding and cooling effects that prevent grit dislodgement and, consequently, help to achieve an improved G ratio.

Figure 6a shows the surface microtopography of the workpiece under MQL (0 kV). Deep furrows were left on the surface of the workpiece. At the same time, both sides of the groove produced strip protrusion due to the shearing and sliding of the metal, leading to poor surface quality. Figure 6b presents the micrograph under a charging voltage of −4 kV. The charged droplets performed a smaller wetting angle, and they could experience better lubrication and heat dissipation function in the contact area resulting in the surface groove becoming shallow and the plow surface becoming relatively smooth and uniform. It can be obviously seen from Figure 6c that when the charging voltage was +4 kV, the hardness of the surface was lower than that of the other two lubrication modes. The reduction of the hardness of the Cr12 surface allows the grinding wheel to remove materials more easily; the surface roughness of the workpiece will also be reduced significantly. Thus, the scratches on the surface were more uniform, smooth, with almost no protrusion, and the furrow depth was shallowest.

4. Conclusions

To improve the lubrication performance of traditional MQL in the grinding process, an EMQL combined with MQL and ES technology was proposed. The electric field formed between droplets and workpiece surface and the wetting angle of grinding fluid droplets under EMQL were studied. The grinding characteristics of EMQL were compared and discussed. The research results show that:

1. EMQL with positive charging voltage can form an electric field on the surface of the workpiece and the direction is from the nozzle to the surface. Under the action of the electric field, the negatively charged vacancies move from grain boundary to the Cr12 surface, form a vacancy flow, and gather on the surface of Cr12. The dislocations of Cr12 will move toward the Cr12 surface and deposit on it finally under the effect of both the accumulation of vacancies and the grinding heat produced on the Cr12 surface. The increase of the deposition of dislocations reduced the hardness of the Cr12 surface, which is beneficial to the cutting process for abrasive grains.

2. With the increase of the absolute value of the charging voltage, the wetting angle of charged droplet was decreased. Under this condition, charged droplets were easier to spread on the surface of the grinding wheel and the workpiece in the contact area, increasing the contact area and enhancing its lubrication and heat transfer capability.

3. Compared to traditional MQL, EMQL effectively reduces the grinding force and the roughness value of the Cr12 surface during the grinding processing; this also extends the service life of the grinding wheel. The grinding performance was best when the charging voltage was +4 kV. Under this condition, the surface hardness of the workpiece was low, and the lubrication and penetrability of charged droplets were improved. However, The spray angle was larger if the charging voltage was too high. The overflow of the lubricating fluid during the spray process increases, leading to a reduction in the number of droplets involved in lubrication cooling, resulting in bad grinding performances.