Recent Advances in Drilling Tool Temperature: A State-of-the-Art Review

Drilling is regarded as the most complex manufacturing process compared with other conventional machining processes. During the drilling process, most of the energy consumed in metal cutting is converted to heat and increases temperature considerably. The resulting thermal phenomena are important since they influence the mode of deformation, the final metallurgical state of the machined surface, and the rate of tool wear. Hence, understanding the temperature characteristics in the drilling process is crucial for enhancing the drill performance and process efficiency. Extensive efforts have been conducted to measure and control the drilling tool temperature successively. However, very few studies have been conducted from a comprehensive perspective to review all the efforts. To address this gap in the literature, a rigorous review concerning the state-of-the-art results and advances in drilling tool temperature is presented in this paper by referring to the wide comparisons among literature analyses. The multiple aspects of drilling tool temperature are precisely detailed and discussed in terms of theoretical analysis and thermal modeling, methods for temperature measuring, the effect of cutting parameters, tool geometries and hole-making methods on temperature and temperature controlling by different cooling methods. In conclusion, several possible future research directions are discussed to offer potential insights for the drilling community and future researchers.


Introduction
With the rapid development of the aerospace industry, the application of advanced materials such as titanium alloys, carbon fiber reinforced polymer (CFRP), or hybrid CFRP/Ti stacks is widely increasing. For these advanced materials, there are various disadvantages in the machining process due to their inherent properties, which can easily result in excessive cutting force, high cutting temperatures, severe tool wear and poor surface quality. All these present a serious challenge to mechanical processing. As one of the most common and necessary processing technologies, drilling is taking widely used in automotive, aerospace and many other industries to produce holes of various sizes and depths. In addition, it is usually one of the final steps in the fabrication of mechanical components and has considerable economical importance. For example, in order to meet the process conditions such as component connection and assembly, mechanical components generally need to be subjected to secondary machining after forming, and drilling accounts for 50% of the secondary machining [1]. According to statistics, there are about 3 million assembly holes in an aircraft, and the cost of the holes accounts for about 3%-5% of the entire aircraft [2,3]. In the working process of the aircraft, due to the influence of cyclic stress, the assembly holes are prone to generate fatigue damage, and more than 30% of the aircraft failures are caused by the quality of the holes [4]. Therefore, it is of great significance to achieve high-efficiency and highquality drilling processing.
As a semi-closed processing method, drilling is also regarded as the most complex manufacturing process compared with other conventional machining processes

Open Access
Chinese Journal of Mechanical Engineering  35:148 (turning, milling, boring, etc). During the drilling process, most of the mechanical energy required for the metal cutting process is converted into heat which is transferred into the chip, the workpiece and the cutting tool. So drilling can achieve higher temperatures than other processing methods. Related research has shown that drilling temperature plays a significant role in drilling performance and tool wear [5]. This is owing to the fact that the special working environment of drilling makes it difficult for chips to evacuate, which leads to a large amount of heat accumulating, rapid tool wear and greatly shortens the service life of the tool. Furthermore, due to the increase of the drilling temperature, it may cause material softening and thus affect the tool life and hole-making quality. Therefore, strengthening the research on drilling temperature and further understanding the mechanism of the interaction between temperature and various parameters in the drilling process are extremely important for extending tool life, reducing the surface roughness of the hole, improving the quality of the hole, and improving the performance of the workpiece [6]. In recent years, extensive efforts have been conducted to measure and control the tool temperature in material removal processes, including temperature measuring methods, the effect of processing parameters and tool geometries, and different hole-making processes, etc. However, to the best of the authors' knowledge, very few studies have been conducted from a comprehensive perspective. An up-to-date, critical review of the drilling tool temperature is necessary. For this purpose, the authors aim to fulfil this requirement and provide a critical overview of the broad field of drilling tool temperature as well as the future works based on the systematic arrangement and analysis of related research. This article is organized into six sections follows by conclusions and outlooks. Its overall structure is illustrated in Figure 1. It begins with the theoretical analysis and modeling of drilling temperature in Section 2. Section 3 outlines the up-to-date development of methods for measuring drilling temperature and describes their measuring principles. Then it elaborates on the effect of cutting parameters, tool geometries and hole-making methods on the drilling temperature in Sections 4, 5, 6, respectively. Furthermore, a variety of auxiliary cooling methods to control the drilling temperature is summarized in Section 7. Finally, the conclusions and possible future research directions for drilling tool temperature are drawn and discussed in Section 8.

Theoretical Analysis and Modeling of Drilling Temperature
Cutting heat is an important physical phenomenon in the drilling process, and the drilling temperature is an extremely important factor that affects the performance of the tool and workpiece, such as the hole accuracy, surface roughness and tool wear. Therefore, theoretical analysis on drilling temperature is indispensable, which can help researchers to reduce the tool temperature from the source. Agapiou et al. regarded the drill as a semi-infinite body to establish an analysis model of the temperature distribution along the cutting edge, and used transient analysis to partition heat among the tool, chips and workpieces to obtain the drilling temperature [7]. Watson et al. proposed a hypothesis that the cutting edge of the drill was regarded as a series of elementary cutting tools, and each elementary cutting tool (ECT) was performed a simple metal cutting operation during the drilling process [8]. Ke et al. calculated the cutting force of each ECT using the thrust and torque measured during drilling, and estimated the chip speed through the chip thickness and shear angle corresponding to each ECT. So the friction and heat generated during the drilling process can be determined [9]. With the development of computer technology, the finite element method has been widely used in mechanical processing. Bono et al. established a model to predict the heat flow into the workpiece, and used embedded thermocouples to conduct experiments to verify that the model can accurately predict the temperature of the workpiece within a certain drilling speed and feed range [10][11][12]. Fuh et al. also used the finite element method to study the effects of the penetration depth, the web thickness, the cutting speed and the helix angle on drilling temperature and predicted the temperature distribution along the cutting edge [13]. Kalidas et al. established a model to predict the temperature of the workpiece which used the inverse heat transfer method to determine the heat flow and the distribution coefficient of the cutting edge and the chisel edge, and obtained the temperature of the workpiece during the drilling process in both wet and dry conditions with coated and uncoated drills [14,15]. Kuzu et al. also employed the method of inverse heat conduction and considered the heat convection coefficient after using minimum quantity lubrication (MQL). The inverse method was consistent with the experimental temperature measurement results [16]. Wu et al. proposed a finite element model to predict the drilling temperature based on an equivalent model, which took into account the influence of feed rate on the working rake angle and working relief angle. Results showed that drilling temperature of simulations had good agreement with the experimental ones [17]. Camille et al. presented a mixed numerical-experimental approach which relied on the strain and temperature measurements at arbitrary point in the cutting edge of the drill, and then used the measurement results to predict the energy in the material removal process. The results proved that the method can determine the heat generated during the drilling process and the way of heat transfer in the workpiece. The model is shown in Figure 2 [18].
When establishing the temperature model, considering the difference of the workpiece material, the model may be different. Nouari et al. believed that drilling temperature is the most important factor causing tool wear, and utilized experiments and finite element modeling to determine the relationship between tool wear and temperature during aluminum alloy drilling [6].
Because of the anisotropy and heterogeneity of carbon fiber epoxy composites, Zhu et al. established the temperature file model of the unidirectional composites after homogenizing, then conducted experiments with k-type thermocouples and infrared thermometer to measure the drilling temperature at the exit of the hole, and the results showed that the temperature file model was consistent with the experiments [19]. Wang et al. also proposed a fiber orientation-based analytical model to predict the heat partition ratio based on classical hertz contact theory, and the temperature distribution on the workpiece obtained by the infrared thermometer verified the correctness of the model. Furthermore, the fiber orientations had a notable impact on this ratio [20]  for simultaneously estimating the temperature distribution of the tool and the finite element model is shown in Figure 3 [21,22]. Álvarez et al. considered the thermal phenomena involved in the drilling of composite, and proposed a comprehensive thermal analysis approach, which used both an analytical estimation of heat generated during drilling and numerical modeling for heat propagation and the risk of thermal damage can be obtained indirectly through the measurement of thrust and torque [23]. Most of the models described above divided the drill into ECT (see Table 1) which relied on experimental data such as cutting force, chip thickness and shear angle to measure the temperature. Therefore, obtaining the heat partition ratio is relatively limited. In order to correctly express the heat distribution mechanism among tools, workpieces and chips in the drilling process, it is necessary to further study the theoretical model of cutting heat from the perspective of thermal-mechanical coupling so that researchers can conduct more in-depth research on the drilling temperature.

Techniques for Drilling Temperature Measurement
The temperature generated during the drilling process will cause tool wear and shorten tool life. What's more, drilling temperature has a non-negligible effect on the hole quality and surface integrity. Therefore, researchers  used various techniques to measure the temperature distribution of the workpiece and tool to minimize the damage caused by the cutting heat, as shown in Figure 4. As early as the 1940s, Schmidt et al. embedded a thermometer in the workpiece in advance to measure the temperature of the tool, chips and workpiece to determine partition ratio of the cutting heat during the drilling process, then summarized that the heat transferred to the chip accounts for 70%-80%, about 10% in the workpiece and the remaining heat was transferred to the tool [24]. With the development of technology, the existing temperature measurement techniques can be divided into contact measurement, non-contact measurement and indirect measurement. The contact measurement has a long time and a wide range of application among the measurement techniques, mainly include tool-work thermocouple, embedded thermocouple method and tool-foil thermocouple system, which is the most mature method for temperature measurement. However, due to the need to install thermocouples and collect signals when applying contact measurement, there may be problems such as limited installation of thermocouples and difficulty in signal extraction in some cases. Non-contact measurement includes pyrometer, infrared radiation camera, and optical fiber two-color pyrometer, which can measure the drilling temperature simply and intuitively. However, it needs to be calibrated before measurement, and unless specially positioned, it is difficult to measure the temperature when the drill is inside the workpiece. The indirect measurement includes metallographic method and scanning electron microscopy method [25][26][27], which observes the metallographic changes of metal materials in different high-temperature environments. However, this technique is only applicable to the high temperature above 600℃, and the tool should be destroyed for sample preparation, which makes this technique unable to be popularized.

Tool-Work Thermocouple
The tool-work thermocouple is the most common type of thermocouple in machining research, mainly because it's easiest to implement [28,29]. As seen in Figure 5, the tool-work thermocouple, made up of a tool and a workpiece of different materials, develops an electric motive force when starting drilling. Then the relationship between temperature and electromotive force is found. Despite its low cost and easy implementation, it is not clear whether the tool-work thermocouple actually measures the average temperature or the lowest temperature. In fact, the electromotive force generated by the toolwork thermocouple does not even match the average temperature of the interface, unless the temperature is uniform or the signal changes linearly with temperature [30,31]. Therefore, using this kind of thermocouple for measuring temperature is not recommended.

Embedded Thermocouple
The embedded thermocouple is to embed the thermocouple in the workpiece or the tool, and fix it by welding or epoxy resin. Additionally, measuring drilling temperature with embedded thermocouples requires fixing the drill and rotating the workpiece as shown in Figures 6 and 7. Compared with tool-work thermocouples, embedded thermocouples provide higher measurement accuracy, respond faster, and measure a wider temperature range [32][33][34].
Devries et al. embedded iron-constantan thermocouples at different points on the flank face of the tool to measure the drilling temperature, and they also found that the workpiece properties, such as the presence of pilot holes and the size, affected the drilling temperature substantially [35]. Battaglia et al. inserted the thermocouple into the oil hole of a carbide double cutting drill to determine temperature distribution and estimated the heat fluxes on the cutting edge [36]. Li et al. embedded thermocouple into carved groove on a fixed drill that was continuously sprayed with coolant to determine its temperature distribution when drilling a rotating titanium workpiece. They also established a finite element model, and analyzed the data collected from the thermocouple using inverse heat transfer analysis, which verified the validity of the finite element model they proposed [37,38].  Also, thermocouples can be embedded into a workpiece to study its temperature distribution. As the drill passed by each thermocouple, the friction between the drill and the workpiece raised the temperature of each thermocouple's immediate surroundings, creating electromotive force that were used to determined temperature distribution. In the research of Pérez et al., four thermocouples were embedded in the workpiece at different distances near the hole to measure the temperature to study the influence of process parameters and workpiece material properties on heat dissipation [39]. Cardoso et al. determined the heat flux and convection coefficient in drilling by using the inverse heat conduction method. What's more, three thermocouples were positioned at 3, 7 and 11 mm from the drill entrance to obtain the temperature gradient along the axis of the hole, as shown in Figure 7 [40].

Tool-Foil Thermocouple System
As shown in Figure 8, tool-foil thermocouple system proposed by Bono et al. can be regarded as a combination of a tool-work thermocouple system and an embedded thermocouple. During drilling process, the drill and foil can form a hot junction as the drill contacts the foil. When the drill and foil are connected to the cold reference junction, the thermoelectric circuit is closed to generate and collect a voltage proportional to the temperature [41]. Bono et al. later also used the tool-foil thermocouple system and combined a finite element model to study the temperature distribution on the cutting edge of the drill, research showed that the finite element simulation results were consistent with the experimental ones [42]. In Zhu's research, a tool-foil thermocouple system was also used to measure the temperature when drilling Al/ Ti stacks. The research pointed out that the feed rate had a more obvious effect on the drilling temperature than the cutting speed, and the drilling temperature decreased from the center of the bit along the cutting edge [3].

Non-Contact Measurement Techniques
The above-mentioned method of using thermocouple measurement usually affects the heat flow and temperature gradient of the tool and the workpiece. On the contrary, the non-contact measurement obviously improves these shortcomings. For this reason, non-contact measurement methods are used because of the ability to

Optical Pyrometer and Optical Fiber Two-Color Pyrometer
The optical pyrometer is the simplest radiation measurement device used in drilling research. It consists of a lens that focuses the infrared radiation on a single photosensitive cell. Then this cell generates a signal, which is amplified, processed, and then output to an LCD screen, computer or data logger. Bhowmick et al. used a pyrometer to measure the temperature when drilling magnesium alloy AM60. The research results showed that: compared with dry drilling, MQL drilling had a significantly lower maximum temperature on the workpiece, which can effectively improve the problem of material softening and reduce magnesium adhesion and built-up edge formation [52]. Taskesen et al. also used an optical pyrometer to measure the temperature of the drill. Due to the different distances from the measured object, the optical pyrometer has different spot diameters. Therefore, the optical pyrometer must be moved with the tool to keep the distance between the tool and the pyrometer and the diameter of the spot constant. For this purpose, as shown in Figure 9, one end of the fastening device was installed on the spindle of the machine tool, and the other end was fixed on the pyrometer [53]. When comparing the cooling effect of MQL and liquid carbon dioxide ( LCO 2 ), Luka et al used a pyrometer to measure the temperature. The device is shown in Figure 10. The results showed that: LCO 2 proved more efficient at lower drilling temperatures and LCO 2 + MQL combination performed better in reducing temperature [54]. There are two photosensitive elements in the two-color pyrometer, and each element is sensitive to different wavelengths of infrared radiation. Consequently, the element only measures two specific wavelengths of infrared radiation, reducing errors in the measurement process. Oezkaya et al. measured the drilling temperature along the cutting edge by using a two-color fiber optic pyrometer (see Figure 11). The results showed that the increase in cutting speed and feed rate will significantly increase the temperature of the cutting edge corners while the temperature at the inner diameter of the tool remains almost unchanged [55].

Infrared Camera
Infrared camera also plays an important role in noncontact measurement. By using special filters, lenses, microbolometer arrays or combinations thereof, infrared camera receive infrared radiation and measure its intensity. After processing the signal generated by the photosensitive cells, the camera generates thermal images of the target object, which are color-coded to represent different temperatures. Dörr et al. studied the performance of different drill coatings by using an infrared  camera. In order to make sure optical access to the drilling site, a 45° deflection mirror was placed under the workpiece which can reflect the infrared radiation to the infrared camera from the drill's exit point, then the tool temperature was obtained. The result showed that it was possible to reduce the thermal stresses by using special coatings, thereby reducing tool wear and extending tool life [56]. However, when the drill is inside the workpiece, the infrared camera cannot detect the temperature of the drill. Therefore, the infrared camera usually needs to be used together with thermocouples in the drilling process. For example, Pérez et al. used thermocouples embedded in the workpiece and an infrared camera to study the effect of material properties and cutting speed on the heat dissipation of drilling CFRP [39]. Khashaba et al. used an infrared camera and two thermocouples installed in the internal coolant holes of the drill to study the heat affected zone (HAZ) and drill point temperature, as shown in Figure 12. The study pointed out that the thickness of the workpiece, the spindle speed and the feed rate all had a significant effect on the drilling temperature [57]. Bonnet et al. divided the drill into four parts, and calculated their heat flow according to the different cutting speeds and different working rake angles of the four parts during the drilling process, then measured the drilling temperature using an infrared camera, as shown in Figure 13 [17].

Effect of Process Parameters on Drilling Temperature
The process parameters, such as spindle speed and feed rate, directly participate in the entire machining process of the workpiece, which affect the temperature of the drilling process, and in turn affect the quality of the drilling [58][59][60][61]. And researchers have done a lot of research in this direction, which shows the importance of process parameters to control the drilling temperature.

Effect of Process Parameters During Drilling Metallic Materials
In Bağci's study of the temperature distribution of the tool, drilling temperature was measured by embedded thermocouples through the oil hole of the drill. Experiments were conducted by using two different workpiece materials, Al 7075-T651 alloy and AISI 1040 steel. It was observed that the temperature decreased with the increase of the feed speed for the same drilling depth and spindle speed. Additionally, the temperature increased on the drill with the increase of the spindle speed for the same feed rate during drilling AISI 1040 steel, while the temperature decreased with the increase of the spindle speed for Al 7075-T651 materials [62]. In the study of drilling AISI 1045, the conclusion presented that as the spindle speed and feed rate increased, the drilling temperature also increased [63,64]. Parida et al. observed that torque and thrust increased with the increase of the cutting speed, which in turn increased the drilling temperature when drilling Ti-6Al-4V at a low cutting speed; However, the opposite results were observed at a high cutting speed because the increase of cutting speed reduced the hardness of workpiece due to thermal softening, as a result, reducing the drilling temperature [65,66]. Although the experimental conditions may be different, the trend of the influence on drilling temperature was similar, that is, as the feed speed and cutting speed increased, the drilling temperature increased when drilling aluminum alloys and magnesium alloys [3,67,68]. Figure 14 shows the effect of process parameters on drilling temperature when drilling different metal materials.

Effect of Process Parameters During Drilling GFRP and CFRP
Ünal and Xu have studied the drilling temperature characteristics of glass-fiber reinforced plastic (GFRP) and CFRP respectively. From their experimental results, no matter which materials they drilled, the drilling temperature increased with the increase of the cutting speed and feed rate. The reason was that as the feed rate increased, the contact time between the tool and the workpiece will be shortened, which led to a reduction in the heat generated by friction and thus lowered the drilling temperature. What's more, as the cutting speed increased, the cutting area per unit time will be longer, so that the drilling temperature will be higher [69,70]. In the research on drilling CFRP, Chen et al. pointed out that low spindle speed and high feed rate can effectively reduce the generation of cutting heat when the spindle speed was in the range of 1500-4500 r/min and the feed rate increased from 0.02 to 0.06 mm/rev. However, low spindle speed and high feed rate will increase the cutting force, which may increase the possibility of delamination defects in CFRP drilling. Therefore, while paying attention to how to suppress the cutting heat, the dialectical relationship between the feed rate and cutting speed, cutting force and cutting heat should be considered, and the processing parameters should be selected reasonably [71]. Zitoune's research showed that the drilling temperature increased with the increase of the feed rate, while the cutting speed had a relatively small effect on the drilling temperature when drilling CFRP [72].

Effect of Process Parameters During Drilling Laminated Materials
An et al. pointed out that the drilling temperature will increase with the increase of feed rate and cutting speed, and the cutting speed had a more significant influence on the drilling temperature than the feed speed whether drilling the stacks from the CFRP layer or Ti-6Al-4V layer when drilling CFRP/Ti stacks. Furthermore, the maximum temperature when drilling the stacks from the CFRP layer was also 2%-14% higher than the maximum temperature when drilling the stacks from the Ti-6Al-4V layer [73]. Figure 17 shows the effect of process parameters on drilling temperature when drilling CFRP/Ti stacks. Weinert et al. found that the cutting speed was positively correlated with the flank surface temperature of the tool when the cutting speed was in the range of 35-200 m/min and Chen also obtained the same conclusion in his research [33,74]. In contrast, Rawat et al. pointed out that as the feed rate increased, the flank surface of the tool showed a decrease in temperature [75]. Sorrentino et al. combined numerical analysis and experiment to predict the trend of temperature with spindle speed and feed rate during the drilling process. The results showed that the maximum temperature of the tool was positively correlated with the spindle speed and negatively correlated with the feed rate. For the corresponding workpiece, when drilling at the low-speed, the temperature decreased with the increase of the feed rate, while at the high-speed, the temperature of the workpiece was almost constant [76]. It can be seen from the above research that, in the drilling process, the process parameters and the drilling temperature are not simply linear, and their impact on the hole quality is not simply a linear superposition. It should be noted that the selected tool material, workpiece material, cooling process, etc. are all important factors that affect the correlation mechanism between process parameters and drilling temperature. Therefore, it needs more in-depth and systematic exploration to accurately grasp the correlation mechanism between process parameters and drilling temperature.

Effect of Tool Geometries on Drilling Temperature
Scholars have done a lot of research on the improvement of the tool geometries [77][78][79][80][81], but there are relatively few studies on the influence of the tool geometries on the drilling temperature. For drilling processing, the optimization and improvement of the tool geometries are conducive to improving the drilling temperature, realizing high-efficiency and high-quality processing, increasing tool life and reducing processing costs. Therefore, the reasonable design of the tool geometries is also very important to reduce the cutting heat.

Effect of Point Angle and Helical Angle on Drilling Temperature
Liang et al. conducted simulation analysis and experiments on the cutting heat of conventional drilling and ultrasonic vibration-assisted drilling under different tool point angles. The results showed that compared with conventional drilling(CD), cutting heat can be reduced significantly in ultrasonic vibration-assisted drilling(UAD). Additionally, as the tool point angle increased, the drilling temperature will also increase, and a proper point angle had a significant effect on reducing burr height and surface roughness, as shown in Figure  18 [82]. Kumar et al. explored the impact of tool geometries on the chip removal ability and tool wear. It was found that the drill with 130° point angle, 30° helix angle and smaller chisel edge thickness had less wear on the flank face and chisel edge, and the chips were easier to discharge. On the contrary, for the drill with 118° point angle, 20° helix angle and a larger chisel edge thickness, the local plastic deformation of the workpiece and the tool wear were more serious. In addition, this drill increased the contact area between the tool and the workpiece, which increased the friction and then the drilling temperature also rose [83]. Yao et al. analyzed the drilling temperature using low-frequency vibration drilling. The experimental results showed that the point angle, helix angle of the drill had little effect on the temperature of low-frequency vibration drilling. Because the vibration can make the tool periodically leave the machining surface of the workpiece, and then the tool was cooled after being in contact with air. Additionally, titanium alloy chips have changed from band-shaped chips in traditional drilling to chip-shaped chips, which can be discharged from the spiral groove with the rotation of the tool during the drilling process and take away a large amount of cutting heat. Therefore, low-frequency vibration drilling can control the temperature at a lower level, and the influence of the geometric parameters of the tool on the drilling temperature can be ignored [84]. Fuh et al. revealed some important thermal phenomena related to the tool geometries, for example, the helix angle was positively related to the drilling temperature and as the web thickness became thicker, the drilling temperature decreased [13]. Chen and Wang et al. pointed out that the increase in the helix angle caused the increase of the rake angle, which led to the reduction of the friction between the chip and the rake surface, and the drilling temperature decreased accordingly [74,85]. It can be seen that they have obtained different results on The reason for this phenomenon may be due to different workpiece materials and different cutting parameters. In addition, as the helix angle changed, the shape of the chips also changed, which affected chip discharge and thus affected the drilling temperature [86,87]. Therefore, in my opinion, the mechanism of the influence of the helix angle on the drilling temperature needs more indepth research.

Effect of Macro/Micro Tool Geometries on Drilling Temperature
Sugita et al. proposed a novel drill design with a thinner cutting edge and chisel edge which will shape the tool compared with a twist drill. Therefore, during the drilling process, the cutting force was lower than that of twist drill, which led to a reduction in cutting heat.
Furthermore, due to its special structure, the drill had a stronger chip removal performance, which helped to dissipate heat [88]. Shu et al. designed a novel drill for drilling CFRP. As shown in Figure 19, the web and lip thinning design will efficiently increase the rake angle and change the material removal mechanism of CFRP from compression and tensile fracture to shearing cutting and compression shear, which had a significant reduction on the drilling temperature and cutting force. In addition, as we know, the elevation in cutting force and temperature have an impact on the exit delamination and microdamage of the hole wall. Therefore, the novel drill with superior thermo-mechanical properties can effectively achieve CFRP damage-free drilling of CFPR [89][90][91][92]. Chen et al. applied the 3D finite element method to analyze two drill bits with different cross-sectional shapes, and obtained the temperature distribution. The results showed that the thick web drill with a curved cutting edge had a more uniform and lower temperature distribution along the edge and flank surface than the thick web drill with a straight cutting edge [93]. Müller et al. designed different groove-shape structures on the first rake face of the drill to improve the cooling efficiency. The results showed that different groove structures can change the heat flow rate to achieve faster heat dissipation, and the cooling efficiency of different groove shapes was also different [94]. Pang et al. used the finite element method to analyze the performance of three different micro-textured drills, namely pit micro-textured, convex micro-textured and grooved micro-textured drill in reducing the drilling temperature. The results showed that the drilling temperature of the micro-textured drill was lower than that of the conventional drill. This was because the micro-textured drill reduced the actual contact area between the tool and the chip which will reduce the cutting heat generated by the friction, and the cooling effect of the groove micro-textured tool was stronger than that of the convex micro-textured tool, as shown in Figure 20 [95]. Wika et al. studied the impact of drills with different helix angles and different numbers of flutes on the temperature when drilling CFRP/Ti stacks. The research showed that the drilling temperature of the double-flute drill with large helix angle was lower than that of the three-flute drill with small helix angle. This was due to the double-flute drill with large helix angle had a strong chip removal ability and a sharper cutting edge, which was conducive to heat dissipation compared with the three-flute drill with small helix angle [96].
Although the existing literatures have conducted a lot of research on different hole making methods, they rarely give a systematic explanation of the effect of hole making methods on drilling temperature. Therefore, the work of this chapter is to link the work of different researchers and conduct a scientific and systematic analysis.

Effect of Low Frequency Vibration-Assisted on Drilling Temperature
LF-VAD can be regarded as superimposing an axial harmonic motion at the vibration frequency below 200 Hz on the basis of conventional drilling, which can effectively reduce the drilling temperature, improve surface integrity and facilitate chip breaking and chip removal (see Figure 21) [109][110][111], while conventional drilling showed a higher chip-rake face friction, higher drilling temperature, and poor chip removal mechanism. Research by Okamura et al. found that LF-VAD with an amplitude of 0.4 mm and a frequency of 30 Hz can effectively control chip formation, greatly reducing the drilling temperature, tool wear and burr height at the hole exit [112]. Okamura et al. established an LF-VAD temperature model to determine the drilling temperature during the drilling and non-drilling periods. Due to the influence of the low-frequency vibration, the drilling tool periodically separated from the workpiece, resulting in the repeated rise and fall of the drilling temperature. The study showed that the simulated drilling temperature was in good agreement with the experimental results at the rising stage. However, in the dropping stage, because the friction heat, that is between the drill side and the inner surface of the hole, and the high-temperature chips accumulated in the drill flute were not considered, the simulated drilling temperature during the non-drilling period was significantly different from the experimental results [113]. When drilling CFRP/Ti stacks, Pecat et al. found that LFVAD can significantly improve borehole quality compared with conventional drilling. Additionally, it was observed that LF-VAD can achieve a sufficient chip extraction which can effectively prevent  The research on drilling temperature with differently micro-textured drilling tools: a Schematic diagram of micro-texture anti-friction and heat conduction mechanism, b-d Micro-texture section shape of drilling tools of grooved micro-texture, convex micro-texture and pit micro-texture, e-h Temperature distribution of drilling process with non-micro-textured drilling tool and different micro-textured drilling tool [95]

Figure 21
Chip morphology in conventional drilling and LFVAD [110,111]  exit [116]. In the experiment comparing conventional drilling and LF-VAD, Yao et al. found that a larger amplitude can lower the drilling temperature, and improve the hole quality and chip removal performance. However, as the amplitude increased, the axial force also increased, which was likely to cause delamination defects. Therefore, various factors should be considered comprehensively to obtain better hole-making quality [117]. Sadek et al. found that LF-VAD can reduce the thermal and mechanical defects associated with CFRP compared with conventional drilling (Figure 22). In this study, the optimized low-frequency vibration conditions can reduce the axial force by 40% and the drilling temperature by 50% without causing delamination defects [118].

Effect of Ultrasonic Vibration-Assisted on Drilling Temperature
The UAD trajectory is composed of the high-speed rotation t, the feed motion and the ultrasonic vibration of the tool at a vibration frequency above 16 kHz, and its trajectory is shown in Figure 23. A large number of studies have proved that UAD can help solve the problem of drilling difficult-to-machine materials, so the technology has been widely used in the field of drilling. Li and Liao et al. found that in UAD, the high amplitude was beneficial to enhance chip removal efficiency, and can effectively reduce the friction of the tool-workpiece interface, thereby reducing thrust and drilling temperature [119,120]. In order to explore the effect of UAD on the temperature of the CFRP-Ti stacks interface, a thermocouple was placed near the stacks interface to measure the temperature, as shown in Figure 24. The experimental results showed that, in Ti6Al4V drilling stage, the drilling temperature of both conventional drilling and UAD rose rapidly and far exceeded the matrix glass transition temperature, which explained that drilling of the Ti6Al4V was the main reason for the thermal damage of CFRP [121,122], and Sanda et al. came to the same conclusion [123]. This was due to the periodic contact and separation between the tool and the workpiece during the drilling process, which can effectively reduce the friction of the tool-workpiece interface, increase the cooling time of the drill, and effectively avoid the accumulation of cutting heat [124]. Li et al. came up with the following conclusions: most of the titanium alloy chips produced by conventional drilling and UAD were continuous banded chips, while the chips of LF-VAD and ultrasonic and low frequency compound vibration drilling were discontinuous sector-shaped chips. However, the chip size of ultrasonic and low frequency compound vibration drilling was smaller compared with LF-VAD which led to better heat dissipation performance [125].
Cong et al. explored the effects of ultrasonic power, cutting speed and feed rate on drilling temperature when ultrasonic vibration-assisted drilling of CFRP. The experimental results were shown in Figure 25, and it could be found that the maximum drilling temperature decreased with the ultrasonic power and feed rate. However, as the cutting speed increased, the maximum drilling temperature first climbed up and then declined [126]. Makhdum et al. conducted a comparative experiment between UAD and conventional drilling on CFRP, and found that the temperature of UAD was much higher than that of conventional drilling. This was mainly due to the repeated impact of UAD which inevitably led to temperature rise in the tool-workpiece interface. In other words, as long as the tool and workpiece separation occurred in each vibratory cycle, the local temperature will be affected by the imposed vibrations [127]. In Pujana's research, it pointed out that although the use of ultrasonic vibration to drill Ti6Al4V can reduce the feed force while the drilling temperature was higher than that of conventional drilling, and Yan also verified this conclusion through the finite element method [128,129]. Moghaddas et al. explored the influence of ultrasonic vibrations on the drilling temperature in the UAD of stainless steel 316, alloy steel 4340 and aluminum 6061. The results illustrated that the increase in vibration amplitude resulted in a significant reduction in force and a higher drilling temperature [130].

Effect of Orbital Drilling on Drilling Temperature
Orbital drilling is also called helical milling. Its tool path is composed of three independent motions, which are the rotary motion of the tool itself, the orbital motion around the axis of hole and the feed motion. The trajectory of the tool is spiral, as shown in Figure 26 [131,132]. Due to the special processing form of orbital drilling, it will not produce continuous chips like conventional drilling. Furthermore, the diameter of the tool is smaller than the hole diameter, so the chip removal ability performs better which leads to better heat dissipation compared with conventional drilling [133].
Comparing orbital drilling with conventional drilling, it is found that orbital drilling can effectively reduce the cutting temperature and axial force, and improve the surface quality. In the conventional drilling process, there are 2-4 cutting edges cutting at the same time while the orbital drilling has only one cutting edge in instantaneous contact with the tool-work interface which can obviously lower the axial force. What's more, the cooling effect of the rotating airflow between the tool and the workpiece and the excellent chip removal ability of the helical milling can effectively control the drilling temperature. Additionally, the reduction in drilling temperature and cutting force have an important impact on improving the accuracy of hole making, reducing machining damage and extending tool life [134][135][136][137][138]. Sakamoto et al. found that the faster the speed, the lower the drilling temperature when orbital drilling CFRP. This was because the high cutting speed increased the chip size and improved the material removal rate which accounted for the reduction of drilling temperature [139]. Liu et al. separately established the heat conduction models to learn about the temperature distribution in the workpiece when orbital drilling Ti6Al4V, CFRP and CFRP-Ti stacks. Based on the characteristics of helical milling, the models simplified the end edge and peripheral edge of the milling cutter into two heat sources, and then used different methods to solve the heat conduction model, such as Green's function, inverse heat conduction and integral transformation [140][141][142]. Figure 27 shows the temperature changes of different materials in orbital drilling.

Effect of Laser Drilling on Temperature
Laser drilling is a process of using a focused laser beam to melt or evaporate the material and remove the material along the depth of the hole (see Figure 28) [143,144]. As a non-contact hole-making technology, laser drilling has great application potential. However, due to its high-temperature and high-radiation, it is easy to produce a heat-affected zone (HAZ), in which the structure and properties of the material change. Therefore, scholars have conducted a lot of research on how to solve the problem of heat-affected zone.
Bharatish et al. conducted laser drilling on ceramics and found that increasing the pulse frequency can effectively reduce the extent of the heat-affected zone [145]. For Ti6Al4V, because of the low thermal conductivity, the influence of laser power and pulse frequency on the heat-affected zone was negligible, but as the thickness of the workpiece increased, the range of the HAZ will also expand [146,147]. What's more, Chatterjee et al. found that, when laser drilling Ti6Al4V, more heat was generated as the laser power increased, which allowed the heat to be effectively transferred to the bottom of the hole and alleviated the taper [148,149]. Mishra's research showed that when laser drilling was performed on aluminum, the range of the heat-affected zone was proportional to the pulse frequency. This was because the larger the pulse frequency, the smaller the time interval between pulses, and thermal diffusion will occur in the radial direction, thus forming a larger heat-affected zone [150,151]. Leone et al. studied the relationship between the HAZ and different process parameters such as average power, cutting speed, pulse frequency and pulse duration when laser drilling CFRP. He found that the range of the heataffected zone between 170-1600 μm occurred at the center of the laminate in correspondence with unidirectional lamina and the heat-affected zone expansion was related to the spot overlap [152]. Weber et al. proposed a perpendicular heat flow model, which can derive the best pulse parameters according to the quality needs when laser drilling CFRP [153]. Ye et al. compared the influence of different laser drilling methods on the heataffected zone, and found that the laser rotary cutting had a smaller heat-affected zone under the same parameters, but the surface flatness of the hole was slightly worse than the parallel filling and the cross filling [154]. Li et al. explored and compared the drilling defects including machined surface micro-defects, dimension error and heat-affected zone under spiral scanning mode and concentric scanning mode. The results showed that, compared with concentric scanning mode, spiral scanning mode reduced the size of HAZ by 33.42% and matrix recession (MR) by 24.83%, but the dimension errors were larger, including the circularity error and taper error, as shown in Figure 29 [155].

Controlling Methods for Drilling Temperature
During the drilling process, because it is in a semi-closed machining environment, the temperature rises sharply, which affects the tool life and the quality of the machined surface. Hence, a lot of efforts have been conducted to control the drilling temperature. In the traditional method, using a large amount of cutting fluid can indeed effectively reduce the drilling temperature and improve the drilling performance. However, large-scale use of cutting fluid is likely to cause environmental pollution and high costs such as material and manpower. In order to reduce the use of cutting fluid and lower the drilling temperature, scholars around the world have developed and applied different green cooling technologies, such as dry cutting, minimum quantity lubrication, cryogenic cooling and air cooling technology [156][157][158][159].

Minimum Quantity Lubrication (MQL)
The mechanism of MQL technology: when the atomized lubricating fluid is sprayed on the tool-chip and toolworkpiece interface, a lubricating oil film will be formed, thereby reducing the cutting heat generated by friction to achieve the cooling and lubrication of the contact interface. Due to the excellent performance of MQL technology in reducing tool wear, lowering drilling temperature and improving drilling quality, a large number of scholars have conducted research on this cooling technology [160][161][162][163][164][165][166][167]. Bhowmick 35:148 equivalent to that of conventional flooded coolant and better than dry cutting. In another study, he used fatty acid-based MQL, which had a better cooling effect than H 2 O-MQL [52,168]. Rahim et al. also evaluated the effects of synthetic esters and palm oil-based MQL in drilling Ti6Al4V titanium alloys. The results showed that both synthetic esters and palm oil-based MQL greatly improved tool life. Furthermore, palm oil-based MQL had a superior lubricating ability which reduced the high friction. Additionally, the cooling capacity of palm oilbased MQL was good enough to reduce the drilling temperature [169]. Mathew's research showed that when drilling under the MQL environment, the drilling temperature was significantly lower compared with dry cutting (see Figure 30). What's more, due to the excess heat generated at the machining area was taken away by the cutting fluid supplied as the mist which can inhibit the built-up edge formation and effectively improve the surface roughness [170]. Murthy et al. evaluated the quality characteristics of dimensional deviation of hole diameter, chip thickness and energy consumption etc. when drilling aluminum alloy by using dry cutting, MQL and cutting fluid. The experimental results showed that compared with dry and wet machining, MQL reduced the dimensional deviation by 60% and 50% respectively. What's more, the chip thickness was also greatly reduced, thereby improving the chip removal and heat dissipation effect. Additionally, cutting energy consumption also reduced by 21.2% and 33% respectively [171].

Figure 26
Moving locus and kinematics of orbital drilling [132]. Kelly et al. found that it can effectively reduce thrust, torque and drilling temperature when using MQL for drilling. Furthermore, the research also pointed out that the alignment of the feed nozzle in relation to the tool, the pressure of the cutting fluid and the cutting fluid flow can be optimized to maximize the tool life [172].

Figure 27
The effect on temperature when orbital drilling different materials [135,137,138,141] Page 30 of 40 Zhu et al. Chinese Journal of Mechanical Engineering (2022) 35:148 Zeilmann et al. found that, for the temperature distribution around the workpiece, the temperature measured using MQL applied internally through the drill (MQL int) was much smaller than the temperature measured by MQL applied with an external nozzle (MQL ext) [173]. Brinksmeier used MQL applied internally through the drill for drilling Aluminum CFPR/Ti6Al4V multi-layer materials and reached the same conclusion [174].
The cooling system based on cold compressed air can effectively reduce the friction and heat in the cutting area, reduce tool wear during high-speed cutting, and can also play a good effect in drilling difficult-to-machine materials.
Domingo et al. found that the air cooling system can effectively reduce the energy consumption of the drilling process, and the air cooling system is suitable for highspeed processing by using vortex tubes to cool compressed air, and drills under different cutting parameters and ambient temperatures (−22 °C, 0 °C, 22 °C) [184]. Compared to the effects of dry cutting and air cooling on the cutting characteristics, Liu et al. found that the air cooling system can effectively reduce the drilling temperature and increase the service life of the tool. In addition, increasing the cutting speed and feed rate will weaken the cooling effect because of the increased heatsource area and heat flux and the maximum airflow was the most effective for reducing the drilling temperature [185].
Wu et al. explored the sustainable and high-throughput drilling of compacted graphite iron using dry cutting, through-the-drill compressed air and MQL respectively. It was found that the tool life under through-the-drill compressed air was longer than that of dry cutting and MQL. Through observation in a high-speed camera, it  35:148 was found that the chip removal speed under air cooling was faster than dry cutting and MQL, so it can effectively reduce tool wear, which was the main reason for the longer tool life under air cooling [186,187]. But in another study, it was found that the tool life of drilling with MQL was twice as long as that under air-cooling conditions [188]. This may be related to the air flow rate in the air cooling system, workpiece materials and cutting fluid used by MQL. Wang et al. proposed a reversed-air cooling technology for both enhancing the hole-exit support and reducing the drilling temperature. Its schematic diagram is shown in Figure 31, with the help of a vacuum cleaner, the air pressure in the cavity will reduce. Due to the pressure difference between the inside and outside of the cavity, an airflow will form from the outside to the inside which was opposite to the feed direction. The experimental results showed that when drilling CFRP, the reverse airflow can effectively reduce the drilling temperature and improve the hole-exit quality [189]. Fu et al. also proposed a reversed-air cooling system for purpose of achieving lowdamage drilling of CFRP, as shown in Figure 32. Research showed that this system can effectively reduce the drilling temperature. In addition, due to the reverse air flow, an axial force opposite to the feed direction was generated, which strengthened the support of the material near the hole exit to a certain extent, and effectively suppressed the generation of burrs at the hole exit [190].

Cryogenic Cooling
Cryogenic cooling technology usually uses liquid nitrogen ( LN 2 ) and liquid carbon dioxide ( LCO 2 ) as coolants to lower the temperature, which is clean and environmentally friendly [191][192][193][194][195]. But different from other methods. It greatly reduces the temperature in the cutting zone and locally changes the mechanical properties of the workpiece surface and subsurface. A large number of studies have shown that cryogenic cooling technology can increase the hardness of the tool, reduce the coefficient of friction, and thereby extend the service life of the tool [196][197][198][199][200][201][202]. Perçin et al. presented a series of experimental investigations of the effects of cooling methods, such as dry cutting, flooded, MQL and cryogenic cooling when drilling Ti6Al4V. Research showed that increasing the spindle speed and reducing the feed rate can reduce the burr height. Among different cooling strategies, cryogenic cooling was the most effective method to reduce the burr height, and it performed better in terms of improving tool life [203]. Ahmed et al. performed a series of experiments when drilling Ti6Al4V in different cooling environments: cryogenic cooling and wet cooling environment. Based on TOPSIS method, they concluded that the use of liquid nitrogen for cryogenic cooling had longer tool life and better chip performance than the use of cutting fluids [204]. In another study of theirs, LN 2 cryogenic cooling and cutting fluid were used as coolants for drilling. The results proved that the use of LN 2 cryogenic cooling greatly reduced the drilling temperature, cutting force and torque, improved the surface roughness, and fully validated the superiority of LN 2 cryogenic cooling [205]. Barnes et al. observed that the tool cooled with liquid nitrogen had less exit delamination and overall internal damage when drilling CFRP, and the tool life was longer compared with dry cutting. However, the application of LN 2 cryogenic cooling didn't significantly improve the drilling performance with respect to tool wear and cutting force [206]. When drilling Inconel 718, Uçak et al. found that the use of LN 2 cryogenic cooling can significantly reduce the drilling temperature (see Figure 33), while the hole quality and surface integrity were improved to a certain extent [207]. Liquid carbon dioxide ( LCO 2 ) is also often applied in cryogenic cooling drilling. Under high pressure and low temperature conditions, carbon dioxide changes from gas to liquid, and it will absorb a lot of heat to reduce the drilling temperature when it evaporates. Nelson et al. drilled CFRP-Ti stacks under the cooling conditions of flooded and LCO 2 cryogenic cooling respectively. It was observed that the drilling temperature using LCO 2 cryogenic cooling was reduced by about 27% and energy consumption was reduced by 17%, what's more, a better surface finish and longer tool life can be obtained [208]. Sadik et al. conducted LCO 2 applied with the external nozzle and internally through the drill respectively. The research showed that LCO 2 applied internally through the drill significantly increased the tool life, and the effect of internal supply of CO 2 was expressed in two ways. On the one hand, it can effectively prevent the entry of external impurities. On the other hand, the internal supply of CO 2 can reduce built-up edge on the periphery insert [209].

Conclusions and Outlooks
In the past few decades, extensive efforts have been conducted on the drilling process and achieved a better understanding of drilling tool temperature. In this present paper, the recent advancements in drilling temperature have been reviewed with particular attention to theoretical analysis and thermal modeling, methods for temperature measuring, the effect of various factors (cutting parameters, tool geometries, and hole-making methods) on drilling temperature, and temperature controlling by different cooling methods. Based on the comprehensive review, some key conclusions on the current state-of-the-art and several possible prospects for future work can be drawn as follows.
(1) Based on the theoretical analysis, some significant thermal models during the drilling process have been conducted considering cutting parameters, tool geometries and tool wear. They also have been verified by numerical and experimental studies. However, relatively limited publications were found in the open literature dealing with the heat partition ratio among tools, workpieces and chips in the drilling. Further studies concerning thermal-mechanical coupling are necessary to establish the thermal model during the drilling process. (2) The application of temperature measurement methods is relatively mature, but various methods have certain shortcomings. For instance, by using ther- mocouples, they are pre-embedded in the workpiece or tool, so they cannot accurately measure the drilling temperature of the corresponding point; By using non-contact measurement methods, the temperature cannot be measured correctly as the drill penetrates the workpiece. Therefore, in actual experiments, it is more advocated to combine different measurement methods to reflect the temperature correctly during the drilling process. (3) A lot of studies on the influence of cutting speed, feed rate and tool geometries on drilling temperature have been conducted. Judging from the results of various studies, the different workpiece materials, tool materials, and cooling methods used in the experiment may result in different results. The influence of these factors on drilling temperature is not a purely linear relationship. The selected tool material, workpiece material, cooling process, etc. are all important factors that affect the mechanism related to drilling temperature. Therefore, accu-rately grasping the correlation mechanism of cutting speed, feed rate, tool geometries and other factors on drilling temperature requires more in-depth and systematic exploration. (4) In terms of hole-making methods, compared to conventional machining, the performances of LF-CAD and UAD in the field of advanced materials machining are proven to be superior owing to their various advantages. They can effectively improve the chip removal ability and hole quality, extend tool life and reduce the drilling tool temperature by using appropriate machining parameters (ultrasonic frequency, amplitude, etc). In addition, the orbital drilling is verified to be prone to generate discontinuous chips due to its special hole making mechanism, which can reduce the drilling temperature effectively. However, most of the existing studies are concerned with the 1D vibration drilling, the 2D UAD, especially on the drilling temperature during 2D UAD processing, is relatively limited. Consequently, in order to improve the machining efficiency and reduce the drilling tool temperature, novel intermittent drilling processes should be developed in the future. (5) In terms of cooling methods, this present paper systematically outlines the existing cooling methods, which have a beneficial effect on reducing the drilling temperature and improving hole quality.
Although the cooling ability of MQL is not as good as other cooling methods, it is more prominent in decreasing tool wear. Air cooling performs better in chip removal and heat dissipation, while the cryogenic cooling has the strongest cooling effect.
However, more attention should be paid to brittle fracture of the material. In the development of new cooling methods, although a lot of research has been carried out and some beneficial results have been achieved, it is difficult to popularize and promote. The development and popularization of new cooling methods not only need to be effective in controlling drilling temperature and improving hole quality, but also should control manufacturing costs and be environmentally friendly, so there is still a long way to go.