Thermal management in grinding of superalloys – A critical review Open Access

In view of future developments, increasing demand for materials for high-temperature applications, and performance of aero-engines, advanced superalloys (e.g. single crystal or equiaxed cast superalloys) and their components will be widely used (Perrut et al., 2018; Wang et al., 2022). The grinding process is an important method for precision machining of these difficult-to-machine materials and components. With the continuous application of advanced superalloys in the aviation field and the continuous promotion of technological progress in this field, the dependence on grinding technology is also increasing (Klocke et al., 2015a; Xiao et al., 2021a, b). Therefore, grinding technology research has received widespread attention. Through basic research on the interaction between the grinding wheel and workpiece in the grinding process, innovative grinding process methods, grinding wheel manufacturing technology and enhanced cooling and lubrication methods, can be developed to improve grinding efficiency, improve surface quality.

With the development of high efficiency grinding technology for superalloys towards high grinding speed and large depth of cut, the material removal rate is greatly improved, while the grinding heat generation also significantly rises, and the grinding heat flux can be increased from 1 × 10⁶ W/m² to 1 × 10⁸ W/m² (Miao et al., 2020b; Yang et al., 2023). Besides, with the increase of depth of cut, the grinding zone length (the length of the contact area between the grinding wheel and the workpiece) is several times or tens of times larger than that of shallow grinding. And the air barrier formed by the high-speed rotation of the grinding wheel makes it difficult for the conventional flood coolant to effectively enter the grinding zone, and the cooling and heat dissipation effect is insufficient (Qiu et al., 2020a). At the same time, the complex structure represented by the fir tree blade root further hinders the uniform distribution of coolant in the grinding zone, and on the other hand, the uneven transfer of grinding heat in the complex structure easily causes uneven temperature distribution and local high temperature. It sets back the continuous improvement of grinding quality and efficiency (Chen et al., 2023; Zhu et al., 2022a, b).

Therefore, the thermal management in the grinding of superalloys is important, many researches have been conducted to deeply understand the heat partition or heat transfer paths in the grinding process. Based on that, the novel methods and technology of the grinding process, grinding wheels, coolant delivery, lubrication and enhanced heat transfer by passive thermal devices are investigated. To obtain effective thermal management in the grinding of superalloys is a multidimensional challenge, that needs a sophisticated approach. This paper is supposed to provide a comprehensive review of the current research process and future trends of thermal management in the grinding of superalloys. It covers the research on the current situation of grinding superalloys (Section 3), grinding heat partition analysis (Section 4), thermal management methods (Section 5) and future directions and research trends (Section 6). Through understanding the current research progress of thermal management, researchers and manufacturers are working towards grinding superalloys with high quality and efficiency to promote a more responsible and efficient manufacturing industry.

The scope of this review is to summarize the research progress related to the thermal management in the grinding of superalloys. The current situation of grinding superalloys is studied. Based on that, the heat partition analysis of grinding superalloys is discussed. The methods and technologies to achieve thermal management in grinding are explored, including novel grinding technologies, sharp grinding wheels, highly efficient coolant delivery, improved lubrication and enhanced heat transfer in grinding by passive thermal devices. The core idea of these novel methods is to reduce heat generation by increasing the dynamic sharpness of the process and the sharpness of the grinding wheel, to improve the lubrication conditions to decrease the generation of forces and heat and to transfer heat as much as possible by augment of coolant delivery efficiency or passive thermal device with high heat transport capacity.

This paper aims to provide a holistic review of thermal management in the grinding of superalloys by comprehensively addressing the above aspects. This paper is supposed to provide bits of knowledge and understanding of the opportunities and challenges related to thermal management in grinding. And it can provide ideas for further improving the grinding quality and efficiency of superalloy parts. This paper firstly reviews the current situation of grinding superalloys to show the bottlenecks and requirements of thermal management in grinding processes in Section 3. The heat partition analysis in grinding is summarized in Section 4. Based on that, methods to decrease the heat generation and improve the heat transfer in grinding are evaluated in Section 5. Finally, the future direction and challenges are studied in Section 6. The logic of this paper is shown in Figure 1.

Superalloys are kinds of alloys developed back from the 1940s, which can withstand complex stress and work reliably for a long time under the conditions of oxidation and gas corrosion at elevated temperatures. Superalloys are broadly used in the field of engines, including energy, automobile, space rocket engines and various industrial gas turbine engines (Li et al., 2018; Liu et al., 2022a, b; Mouritz, 2012).

Superalloys have been added with alloying elements such as Ti, Mo, Al, La, Re, Ru, etc. to their matrix, which have the effects of solid solution strengthening, precipitation strengthening or/and dispersion strengthening, thus making them possess excellent high-temperature strength, good oxidation resistance, thermal corrosion resistance, fatigue resistance, fracture toughness and other comprehensive properties (Kong et al., 2021; Shahwaz et al., 2022; Shang et al., 2020).

The excellent performance of superalloys makes their grinding energy larger, which leads to a large amount of grinding heat during the removal process. Due to the low thermal conductivity, the grinding heat is difficult to be timely evacuated and accumulates on the contact zone between the workpiece and grinding wheel, causing high temperatures (Grimmert et al., 2023; Gupta and Chak, 2022; Liu et al., 2022a, b). When grinding superalloys, it is easy for the workpiece material to adhere to the top of the grinding wheel, wrapping the cutting edge of the abrasives and blocking the chip-storage spaces between abrasives. During grinding, these adhering materials come into contact with the workpiece surface, increasing friction and causing a sharp increase in grinding force and grinding temperature (Qian et al., 2022b; Ruzzi et al., 2020; Zhu et al., 2022a, b). It can easily lead to grinding burns and rapid grinding wheel wear (Li et al., 2022; Liang et al., 2023).

It is very easy to have sudden burn-out in grinding of superalloys, resulting in part-scrapping (see Figure 2) (Miao et al., 2020a, c). In order to avoid grinding burns, the grinding parameters are selected conservatively, which limits the improvement of material removal rate (Ding et al., 2010a; Zhao et al., 2020).

The rapid wear of grinding wheels is another major issue that limits the improvement of material removal rate in grinding of superalloys. When using conventional grinding wheels (e.g. vitrified corundum or SiC wheels) to grind superalloys, the grinding wheels are easy to adhesion, clogging and wear, resulting in a generally lower grinding ratio (Cao et al., 2021a; Li et al., 2020; Miao et al., 2019). When using super abrasive grinding wheels (in particular, CBN wheels), the wear of the grinding wheels is still relatively severe, mainly in the form of abrasive wear, damage and adhesion (see Figure 3) (Li et al., 2021b; Naik et al., 2019; Wang et al., 2020).

Grinding burns and rapid wear of grinding wheels during grinding superalloys are easy to occur, which limit the grinding efficiency. Although in many research literature the material removal rate of superalloy grinding is relatively high (Miao et al., 2021; Zhao et al., 2022a, b, c), in order to avoid grinding burns and ensure the grinding quality, the grinding parameters used in the industries are still relatively conservative and the grinding efficiency is still low.

The deeper reason for restricting the further improvement of grinding efficiency of superalloys is the thermal issues. The high mechanical properties and low thermal conductivity of superalloys result in high grinding heat generation. At the same time, the superalloys have a poor heat dissipation capability, in this case, a great amount of grinding heat fails to dissipate in time, which can cause a large temperature rise in the grinding zone. So the grinding burn problems caused by grinding heat are prominent (Qian et al., 2022e, 2023b). In addition, higher temperatures also lead to rapid wear of grinding wheels. To ensure the grinding quality, only relatively conservative grinding parameters can be used, which limits the removal rate of grinding superalloys. To solve the problem of grinding burn, scholars have conducted in-depth research on heat distribution and thermal management in grinding of superalloys.

Due to the low thermal conductivity characteristics of superalloys, grinding burns can easily occur during the grinding process, particularly sudden-burn is easy to occur. The film boiling theory explains the reasons for sudden-burn during creep feed deep grinding, and points out that there is an unstable gas-liquid two-phase heat transfer mechanism in the grinding zone (Guo and Malkin, 1994; OHISHI and FURUKAWA, 1985; Pu et al., 1988). In the creep feed deep grinding, there exists a critical heat flux. When the grinding heat flux is below the critical heat flux, the cooling fluid in the grinding zone stays in a nucleate boiling state and the grinding heat energy is effectively transferred out of the grinding zone through the phase-change of the cooling fluid (Bell et al., 2011; Jin et al., 2002). Nevertheless, when the grinding heat flux exceeds the critical heat flux, the coolant fluid will undergo film boiling, forming a gas film at the interface between the grinding wheel, workpiece and coolant fluid, which hinders effective heat transfer. Most of the heat gathers on the surface of the workpiece, causing high grinding temperatures and burns (Andrew et al., 1985; Bell, 2009; Rowe et al., 2003; Wang and Kou, 2006). Researchers have elaborated the heat transfer paths from the perspective of grinding heat partition. Grinding heat is transmitted to the workpiece, grinding wheel, grinding chips and cooling fluid, respectively (see Figure 4).

In terms of the thermal damage issues in the grinding, the heat transferring to the workpiece attracts lot of attentions. Many researchers have comprehensively analyzed the generation and transfer paths of grinding heat from both macroscopic (Guo and Malkin, 1995; Hahn, 1962) and microscopic (Hahn, 1956; Li and Axinte, 2017) perspectives, taking into account the properties of the workpiece (Malkin and Guo, 2007, 2008), the conditions of the grinding wheel working layer (Nee and Tay, 1981; Outwater and Shaw, 1952), the nature of the coolant (Rowe et al., 1991, 1995) and the influence of grinding parameters (Shaw, 1994, 1996) on the grinding heat partition.

The research on grinding heat partition is first started by (Shaw (1994, 1996) from the simplest dry grinding process. During dry grinding, grinding heat is transferred to the workpiece, the grinding wheel and the grinding chips. In shallow grinding, the thickness of the grinding chip is small and the heat transfer is limited, so the heat carried away by the grinding chips can be ignored. Therefore, the grinding heat involves only two parts, the heat transferred to the workpiece and grinding wheel. By simplifying the grinding heat source as an evenly distributed heat source in the grinding zone, the proportion of heat transferred to the workpiece can be calculated from Eq. (1),

where ε is the heat partition coefficient to the workpiece, (kρc)_s and (kρc)_w are the thermal conduction coefficient of the grinding wheel and workpiece, respectively.

Subsequently, Outerwater and Shaw analyzed the generation of grinding heat from a microscopic perspective (Outwater and Shaw, 1952). The grinding heat is generated from the friction between the grinding abrasives and workpiece interface, between the grinding abrasives and chips interface, and in the shear of chips. Therefore, the grinding heat should also be transferred from these surfaces. Hahn then modifies Outerwater’s model and derives the grinding heat partition coefficient from the heat transfer between the grinding abrasives and workpiece interface as Eq. (2) (Hahn, 1956, 1962).

where k_g is the thermal conductivity of the abrasive grain, r₀ is a parameter related to the contact radius between the abrasive grain and workpiece, v_s is the grinding speed. Based on Eq. (2), Rowe et al. (1996), Rowe and Jin (2001) further consider the influence of grinding wheels on the heat partition, and a more accurate model is then brought up as Eq. (3).

Guo and Malkin point out that the Rowe’s model is more suitable for dry grindings, and the influence of cooling fluid on the heat partition is further taken into account (Guo and Malkin, 1994). The heat partition model which involves the effects of cooling fluid, infeed speed and depth of cut is then built up as Eq. (4).

where γ is the parameter related to the shape of abrasive grain, a_g is the thermal conduction of the abrasive, l_c is the length of grinding zone, C_a is the number of active grains on the working layer of a grinding wheel, A is the parameter reflects the wear condition of abrasive grains, v_w is the infeed speed. The model is validated by the measured and calculated heat partition coefficient from experiments via inverse temperature-matching methods (Malkin and Guo, 2007, 2008).

Lavin and Malkin (Lavine, 1988; Lavine et al., 1989) then hypothesize that the influence of the grinding wheel on heat transfer includes the effects of the cooling fluid and abrasive grains. And the heat transferring to the grinding wheel is also assumed to contain the heat transferring from the grinding wheel to the cooling fluid and to the abrasive grains. In this case, the grinding heat flux transferring to the grinding wheel can be modeled as Eq. (5) (Lavine et al., 1989), and the grinding heat flux transferring to the workpiece can be derived as Eq. (6) (Lavine, 1988).

where T_m is the melting temperature of the workpiece, A_r is the contact area of abrasive grains, A_n is the contact area of the grinding wheel. And in dry grindings, A_r/A_n << 1, the grinding heat partition coefficient can be calculated in Eq. (7).

And in the wet grinding, the grinding heat partition coefficient can be calculated in Eq. (8).

where (kρc)_f is the thermal parameter of the cooling fluid.

Li and Axinte (Shaw, 1996) mentioned that as the temperature of the workpiece increases, the grinding heat partition to the workpiece will change accordingly, and a more accurate model for the grinding heat flux transferring to the workpiece is built as Eq. (9).

where d_s is the diameter of the grinding wheel, k_g is the thermal conductivity of the abrasive grain, E_s,w are the Young’s modulus of the grinding wheel and workpiece, λ_s,w is the Poisson’s ratio of the grinding wheel and workpiece, ρ_mp is the density of the workpiece at the melting temperature. And T_ce,s are the temperature of cooling fluid at the moment that the grinding starts and ends, respectively.

Through theoretical and experimental analysis, it can be concluded that more than 90% of the heat in shallow grinding is transferred to the workpiece, while in high-efficiency grinding, less than 30% of the grinding heat is transferred to the workpiece, with the rest of the grinding heat being carried out of the grinding zone by the coolant. For superalloys the grinding temperature of shallow grinding is between 200 and 600°C, while the grinding temperature during high-efficiency grinding is below 120°C (Li and Axinte, 2017; Maksoud, 2005; Malkin and Guo, 2008; Rowe et al., 2003). In addition, with the increase in material removal rate, more and more grinding heat needs to be transferred to the grinding wheel or cooling fluid to avoid thermal damage to the workpiece, which means that the requirement for enhanced heat transfer in the grinding zone is becoming increasingly high.

Thermal management methods including reducing the heat generation and enhancing heat transfer have been investigated, new techniques and process optimizations have been introduced to control the temperature of grinding superalloys with increased efficiency.

From the perspective of grinding process parameters, increasing the grinding speed can increase the active number of abrasive grains participating in material removal, resulting in a reduction in the average chip thickness of individual grains, which is equivalent to improving the dynamic sharpness of the grinding wheel (Ding et al., 2017a; Ren et al., 2018; Yui and LEE, 1996). And at an increased grinding speed, the specific grinding energy (the consumed energy to remove unit volume of material) decreases which reduces the generation of grinding heat (Dai et al., 2017; Tian et al., 2015). In the late 1960s, research on high-speed grinding technology began. At that time, laboratory grinding speeds reached up to 210–230 m/s (Klocke et al., 1997; Yui and LEE, 1996). Currently, grinding speed at the laboratory can reach 350–500 m/s, and the ultra-high-speed grinding process with a speed of 200–250 m/s has begun to be promoted in industrial applications (Eda et al., 1981; Yang et al., 2015a, b).

The high-speed grinding tests show that with the increment of grinding speed, the grinding force significantly decreases. This is because as the grinding speed increases, the plastic deformation area of the material in front of the abrasive grains becomes smaller and smaller (Shimizu et al., 2002), and during the material removal process the thermal softening effect gradually becomes stronger than the strain rate strengthening effect, resulting in improved material grindability (Figure 5) (Dai et al., 2015; Qiu et al., 2020b). In addition, since the material is easier to grind at an increased grinding speed, the specific grinding energy and the grinding power decrease up to 30%, as a result, the heat generation in high-speed grinding can reduce (Ichida et al., 2006; Linke et al., 2011).

Apart from high-speed grinding, ultrasonic vibration-assisted grinding (UVAG) is lately developed to grind superalloys. Ultrasonic vibration-assisted grinding is the application of acoustic energy in grinding processes, which improves the material removal conditions through high-frequency micro-amplitude vibration between the workpiece or tool (Figure 6) (Abdullah et al., 2012; Cao et al., 2020). It is an important hybrid machining method for various difficult-to-process materials, including superalloys (Singh and Sharma, 2022).

In ultrasonic vibration grinding, the addition of ultrasonic frequency micro-amplitude vibration changes the relative motion trajectory between the abrasive grains and the workpiece (Cao et al., 2023). Through the relative reciprocal vibration between the grains and the workpiece, and the superimposition of the original micro-cutting motion, the abrasive grain trajectory interference is generated, which in turn changes the chip thickness of a single grain (Figure 7) (Qiu et al., 2022; Yue et al., 2022), resulting in changes in the relative proportions of sliding, plowing and cutting process (Cao et al., 2021b, 2022b). As a result, the grinding force and temperature decrease and the specific energy in turn can reduce (Cao et al., 2022a; Zhao et al., 2023). Besides, the grinding efficiency and surface quality improve (Huang et al., 2023; Wu et al., 2021). In particular, when grinding Inconel 718 superalloy with optimized UVAG parameters, the grinding force can reduce 29% and the specific grinding energy can decrease 27% (Abdullah et al., 2012; Bhaduri et al., 2012).

Superabrasives, i.e. diamond and cubic boron nitride (cBN), have excellent physical and thermophysical properties such as high hardness, sharpness and thermal conductivity. Owing to their excellent properties, the superabrasive grinding wheels have a great potential to reduce grinding force, and specific energy and enhance heat transfer in the grinding processes compared with conventional grinding wheels (Nere et al., 2016). In general, superabrasive grinding wheels include electroplated and brazed superabrasive grinding wheels in terms of fabrication methods (Ding et al., 2017c). As for grinding of superalloy, the diamond abrasive grains may have some chemical reactions with Fe or Ni elements, which can accelerate the wheel wear. While the cBN abrasive grains are more stable when grinding superalloys, therefore it is widely used (Kishore et al., 2022; Klocke et al., 2015b).

For most electroplated superabrasive grinding wheels, nickel is commonly used as a bonding material between the superabrasive grains and the metal substrate of the grinding wheel. Traditional electroplating technology is based on the deposition behavior of cathodic metals from aqueous electrolytes (Ismail et al., 2011; Yu et al., 2017). The fabrication process of cBN electroplated wheels involves steps as follows: Pre-treatment of the metal grinding wheel substrate and cBN grains before electroplating; Preparing the electroplating solution; Laying the CBN grains on the wheel working surface, and then thickening the bonding material (Jackson, 2018). The electroplated cBN grinding wheel can significantly decrease the grinding forces and specific grinding energy. The wrought superalloy (e.g. Inconel 718), directionally solidified superalloy (e.g. DZ125), powder metallurgy superalloy (e.g. FGH96) and their profile structures are successfully ground by the electroplated cBN wheels (Gift and Misiolek, 2004; Li et al., 2021a; Zhao et al., 2016a). Compared with conventional grinding wheels, the electroplated cBN wheel can control the specific grinding energy between 50 and 250 J/(mm³), and the material removal rate can reach up to 25 mm³/(mm·s) (Li et al., 2021a; Zhao et al., 2016a). The electroplated cBN grinding wheels are also used to abrasive-machining the difficult-to-cut structures or profile (e.g. fir tree root), the grinding forces and energy can be reduced (Figure 8) (Aspinwall et al., 2007; Li et al., 2021).

The electroplated bonding between cBN grains and wheel matrix is mechanical anchorage, which is relatively weak, and the grinding performance of the electroplated cBN wheel is restricted (Gift et al., 2004). In addition, the protrusion height of cBN grains is small, normally less than 30% grain height protrudes. In this case, the storage for chips limited, and when the wheel is loaded, it is difficult to dress, and the sharpness can decrease (Ding et al., 2017b, c).

In light of drawbacks of electroplated superabrasive grinding wheels, another superabrasive grinding wheel, brazed grinding wheel, is developed since the 1900s. Firstly, the concept of brazing diamond on the surface of grinding wheel was proposed by Chattopadhyay et al. (1991a, b, c) and Ghosh and Chattopadhyay (2007). The brazed superabrasive (diamond or cubic boron nitride) grinding wheel has gradually developed and been applied to the high-efficiency grinding of difficult-to-machining materials (Ding et al., 2010b; Ding et al., 2010). Fu and Chen et al. developed a single-layer brazed diamond grinding wheel (Figure 9) (Chen et al., 2014a, b; Fu et al., 2004). And Su et al. proposed a method of using chemical mechanical effect critical interference to dress the single-layer brazed diamond grinding wheel to maintain its sharpness (Su et al., 2010, 2011, 2016; Zhang et al., 2014). In order to continue increase the sharpness of the brazed superabrasive grinding wheel, and to extend the lifetime, the aggregated superabrasive grains are introduced and fabricated (Zhao et al., 2022a, b, c). The porous brazed aggregated superabrasive grinding wheel is further developed (Figure 10) (Zhao et al., 2018, 2019, 2021). The novel porous brazed grinding wheel has an excellent grindability, the pores in the working layer of the wheel increase the chip storage, and can also promote the cooling fluid to enter the grinding zone (Chen et al., 2014a, b, 2021). In addition, the solid lubricant is also introduced in the porous brazed superabrasive grinding wheel to improve the lubrication condition and to reduce the grinding forces and heat generation (Xiao et al., 2021a, b).

Thermal management is an important aspect in the grinding of superalloys due to their sensitivity to high temperatures. A number of factors can be taken into account to address thermal management, such as opting for lower cutting speeds to minimize the heat generated, using sharp and in good-condition tools for grinding, employing intermittent grinding instead of continuous grinding, preheating the workpiece to reduce thermal shock, choosing the right type of grinding wheel with appropriate specifications and finally the most important consideration among all these factors is the use of effective coolant or lubricant to dissipate heat generated with proper coolant or lubricant flow and pressure (Dambatta et al., 2023; Gupta et al., 2021). At first, the burning issues in the grinding process were eliminated by flood cooling (Rabiei et al., 2017). Subsequently, a novel method known as minimum quantity lubrication (MQL) was created. This method has demonstrated the ability to overcome issues related to high-temperature sensitivity and is a recognized means of addressing environmental concerns. This section reviews some of the common as well as advanced MQL systems specifically used in grinding operations. The grinding operation involves high temperatures; hence an effective lubri-cooling technique is needed. Lopes et al. (2021) conducted a comparison of various oil and water ratios used during grinding with and without a wheel cleaning jet system. Using a compressed air jet, a portion of the blocked chip on the wheel’s cutting surface was removed using this procedure. In addition to increasing the MQL method’s efficiency, the wheel cleaning jet (WCJ) prolonged the wheel’s lifespan. Furthermore, the addition of water to MQL significantly reduced wheel wear but had a negative effect on cutting force and specific energy. Similar type of results was also reported by Daniel et al. (2023). It was found that clogging was higher in methods that did not use the auxiliary cleaning system. Mao et al. (2013) studied the influence of spraying parameters on grinding performance. According to their research, MQL nozzle spraying direction should be angularly positioned toward the grinding wheel. As the air pressure increased and spraying distance decreased, the grinding performance and lubrication was improved. To utilize the superior lubrication performance of castor oil and biodegradability of vegetable oils, Guo et al. (2017) evaluated the performance of the two combined base fluids in MQL grinding of Ni-based alloy. Results indicated that the combination of castor oil with soybean oil among different vegetable oils exhibited improved lubrication performance.

The MQL system has undergone numerous modifications and enhancements, including the addition of ionic fluids, cryogenic air, hybrid nanofluids, electrostatic atomization and the use of ultrasonic vibration assisted atomization (Gong et al., 2021). Heat transfer distribution, debris forming process and thermal induced stresses during grinding operation are shown in Figure 11. The use of cooled compressed air (CA) in conjunction with the MQL method was investigated by Saberi et al. (2016). In order to lower the high temperatures during the grinding process, the air flow was separated into two secondary flows of differing temperatures, passing the cooled flow to the MQL spray nozzle, and releasing the hot air flow that resulted from this split into the surrounding air. A vortex tube was used to get the cooled air. Shabgard et al. (2017) synthesized the nanofluids by submerged electric discharge process exposed to ultrasonic agitation in the presence of Tween 20 as dispersant. The results showed that the synthesized nanofluids are effective in reducing the temperatures and improving lubrication especially in extreme machining conditions. Wang et al. (2016) researched on assessing the tribological interactions between various nanofluids at the wheel/workpiece interface while the Ni-based alloy GH4169 was being MQL ground. It was seen that during the MQL grinding operations, the nanofluid spray formed a thin film from the lubricant along the contact zone. It was discovered that the formation of a thin tribofilm in the grinding zone enhanced the anti-wear and lubrication properties. It was also determined that high-viscosity nanofluids and nanoparticles with spherical or sphere-like molecular structures perform better at lubricating. The order of lubricating properties of six nanoparticles was described as ZrO₂<CNTs < ND < MoS₂<SiO₂<Al₂O₃. By employing vegetable oil (palm and groundnut oil) as the base fluids, Virdi et al. (2021) examined the performance of the NFMQL approach for grinding Inconel-718 alloy. According to the results, the high viscosity of palm oil as the base fluid allowed to grind more effectively than groundnut oil. Also, vegetable oil’s high viscosity allowed the nanoparticles to adhere better to the workpiece surface, allowing them to endure the high temperature of the grinding process. Collectively, better lubricating and heat transmission capabilities were exhibited by hybrid nanoparticles. The performance of MoS₂/CNT nanofluid during the grinding of Ni-based alloy with different types of vegetable oils was reported by Zhang et al. (2015). It was noted that the tribological behavior and heat-carrying capacity of the base fluids were significantly enhanced when palm oil was utilized as the base fluid of the MoS₂/CNT nanofluid. The physical encapsulation of MoS₂/CNT nanofluid is shown in Figure 12. Similarly, the combination of Al₂O₃/SiC nanoparticles showed best lubrication performance compared with pure SiC NPs and pure Al₂O₃ NPs (Zhang et al., 2016). According to the research carried out by Huang et al. (2022) during the grinding of Inconel 718, the combination of MWCNTs (excellent thermophysical properties) and MoS₂ (excellent lubricating properties) with ultrasonic atomization resulted in optimal grinding temperature, surface roughness and better lubrication, when compared with base fluid ultrasonic atomization MQL and nanofluid MQL. Rabiei et al. (2017) evaluated the performance of different nanoparticles to find out the optimal nanoparticle and then combining it with ultrasonic vibration. When compared to dry grinding, the combination caused the grinding temperature to drop by up to 48% (from 254 to 132 °C). Consequently, instead of the black and burned surface obtained via dry grinding, shiny surfaces free from heat damage were obtained.

The cooling in grinding processes is the most important method to dissipate the generated heat and avoid grinding burn-out. It is important to improve the delivery of cooling fluid at a certain speed and flow rate to make it more efficient and sufficient to enter the grinding zone (Zhao et al., 2016b; Zhao et al., 2022b). Webster et al. optimized the geometry of the coolant nozzle, and also designed a new circular nozzle that produces an accumulative jet effect that can more efficiently deliver the cooling fluid into the grinding zone, thereby reducing the grinding temperature (Webster et al., 1995, 2002). They also experimentally confirmed that the closer the nozzle position is to the grinding zone, the better the cooling effect (Webster, 2007; Webster and Gruen, 2008). Zhong and He et al. achieved good cooling effect on the grinding zone by using a multi-nozzle cooling system to cool the grinding process from different angles (He et al., 2013; Zhong et al., 2014, 2015). Ramesh et al. developed a new shoe-shaped nozzle that is closely spaced from the circumference of the grinding wheel, usually only a few millimeters away. As a result, even with low-pressure supply and low velocity of the cooling fluid, the large contact area between the grinding wheel and the delivered coolant results in good wetting effect, thereby improving the cooling effect in the grinding zone (Ramesh et al., 2001).

Considering that the “air barrier” hinders the cooling fluid from entering the grinding zone when the grinding wheel rotates at a high speed, Mohan et al. increase the delivery speed of the cooling fluid to more than 300 m/s by using high pressure, so that the cooling fluid itself can break through the “air barrier” and enter the grinding zone, thus ensuring the cooling effect of the grinding arc zone during high-speed or ultra-high-speed grinding (Mohan, 1996). In addition, a set of air baffle device was used around the grinding wheel to cut off the airflow, which greatly weakened the “air barrier” effect around the grinding wheel, thus achieving the effect that the cooling fluid can smoothly enter the grinding zone for heat dissipation at a low jet speed (Ebbrell et al., 2000; Gong et al., 2007; Mandal et al., 2011). Li and Cai et al. developed a “Y” nozzle for high-speed grinding by combining the air baffle effect and using a higher jet velocity of the coolant at the same time, which achieved low grinding temperature and high material removal rate (Li et al., 2015; Li and Han, 2013; Xiu et al., 2007).

The efficiency of using external jet impingement cooling to deliver coolant into the grinding zone is relatively low. To address this issue, internal grinding wheel cooling methods have been proposed. Peng and Fu et al. have developed slotted grinding wheels and radial water jet systems inside the grinding wheel, which have achieved ideal results in grinding tests (Fu et al., 1999; Peng et al., 2021, 2022a, b).

The essence of the above methods, whether it is external jet cooling or internal cooling by the grinding wheel, is to deliver as much cooling fluid as possible into the grinding zone for enhanced heat transfer, in order to avoid the cooling fluid entering the film boiling state. Nevertheless, the effective utilization efficiency of the cooling fluid is not considered. If only blindly increasing the amount of cooling fluid and the jet flow rate, although it may achieve the cooling effect on the grinding zone, it undoubtedly contradicts the current development trend of sustainable manufacturing, and brings a series of negative impacts, such as a significant increase in energy consumption and production costs, serious environmental pollution and harm to workers' health. In addition, these methods all increase the proportion of heat transferring to the cooling liquid, using the cooling liquid to bring the grinding heat out of the grinding zone. The enhanced heat transfer process of the cooling liquid is related to the grinding process, the wear state of the grinding wheel, the material of the workpiece and the conditions of cooling fluid itself, therefore the cooling effect is difficult to effectively control and can be unstable.

Heat pipe is a type of passive heat transfer device that requires no external power source to operate. It is primarily used to transfer heat from one location to another (Faghri, 2014). The heat pipes are filled with working fluid (e.g. water, R134a, nanofluid, etc.) (Qian et al., 2022d) transfers heat through the evaporation and condensation of the working fluid (Figure 13) (Faghri, 2012; Jouhara et al., 2017). The heat pipe has different types, including gravity heat pipe, oscillating heat pipe, etc. Both gravity heat pipe and oscillating heat pipe (OHP) have a simple and wickless structure, and excellent heat transport capacity, therefore, they have a great potential to apply in grinding of superalloys for enhanced heat transfer (Ma, 2016; Mochizuki et al., 2011). Given that, the gravity heat pipe and oscillating heat pipe are proposed to be combined with the grinding wheel, that is the heat pipe structure is designed and fabricated in the grinding wheel, and a certain working fluid is filled in (J. Chen et al., 2021a). In this case, the effective thermal conductivity of the grinding wheel can be significantly improved, the heat in the grinding zone will be directly transferred out through the heat pipe grinding wheel to avoid accumulating and causing high grinding temperature (Qian et al., 2019).

Based on the core idea of enhanced heat transfer by the heat pipe grinding wheel, the heat pipe grinding wheels are designed for high-speed grinding and high-efficiency grinding. The heat pipe grinding wheel shows an excellent heat transport capacity and temperature-control ability (Figure 14) (He et al., 2014, 2018). In particular, the heat pipe grinding wheel can avoid sudden grinding burn-out. As for Inconel 718 superalloy, the grinding temperature can be stably controlled under 100ºC, under the condition that grinding speed of 160 m/s, feed rate of 120 mm/min and depth of cut of 2 mm. And under the same condition, there occurs a sudden burn-out in the grinding by the grinding wheel without heat pipe, the temperature suddenly reaches up to 900ºC (Chen et al., 2017b).

Furthermore, the heat pipe is designed for dry-grinding of superalloys to eliminate the usage of hazard cooling fluid, which caters to the sustainable machining requirement (He et al., 2016). Under dry grinding, the heat pipe grinding wheel shows a more effective heat transfer performance and more stable temperature-controlling ability, compared with the grinding wheel without heat pipe. When dry grinding Inconel 718 superalloy with the heat pipe grinding wheel, the grinding temperature can be stably controlled under 200ºC under the condition that a grinding speed of 45 m/s, feed rate of 90 mm/min and depth of cut of 1 mm. While under the same condition, the grinding temperature by the grinding wheel without a heat pipe reaches up to 700ºC. Moreover, with the same grinding parameters, in wet grinding by the grinding wheel without a heat pipe, there occurs a sudden burn-out (Chen et al., 2016). In addition, for complex structures, the profile heat pipe grinding wheel is designed and fabricated. The stir tree blade root is successfully ground without thermal damage by the heat pipe grinding wheel, the temperature difference on the profile surface can be controlled below 60ºC and the heat pipe grinding wheel shows an excellent temperature uniformity performance (Chen et al., 2017a; Chen et al., 2021b; Qian et al., 2020a).

The heat transfer mechanism of the heat pipe grinding wheel during the grinding process is also deeply investigated. It is found that with the increment of grinding speed, the internal evaporation of working fluid is gradually suppressed (Chen et al., 2017a). The nucleate boiling heat transfer gradually turns into convection heat transfer, and consequently, the heat transport capacity deteriorates (Chen et al., 2021a; Zhang et al., 2023). In this case, the oscillating heat pipe is proposed to design and fabricate in the grinding wheel (Qian et al., 2019, 2020d). The heat transfer mechanism of an oscillating heat pipe includes the heat transfer by phase change (latent heat transfer) and the heat transfer by the motion of a working fluid (sensible heat transfer), and sensible heat transfer accounts for approximately 90% of the total heat transfer (Nikolayev, 2021; Nikolayev and Nekrashevych, 2018).

The oscillating heat pipe grinding wheels are designed for abrasive-machining and high-efficiency grinding of superalloys (Figure 15) (Qian et al., 2020c, 2022a). The heat transfer mechanism and startup performance of the oscillating heat pipe grinding wheel are deeply investigated, the phase change of the work fluid and circular motion of the trains of liquid plugs and vapor slugs contribute to the highly efficient heat transport and fast startup performance (Qian et al., 2020b, 2022e, 2023a). Owing to the good thermal performance of the oscillating heat pipe grinding wheel, the heat in the grinding zone can be dissipated in time and the grinding temperature can be effectively controlled under the burn-out temperature (Qian et al., 2020e, 2023b). Compared with the conventional wheel without OHPs, due to the enhanced heat transfer, the wear of the oscillating heat pipe grinding wheel is slower, the lifetime is 58% longer (Qian et al., 2022b). In addition, the material removal rate can increase by up to 2.4 times, the hazard cooling fluid can also be eliminated, as a result the energy consumption and carbon emission of grinding Inconel 718 superalloy reduce up to 42% and 56%, respectively (Qian et al., 2022c).

The heat pipe and oscillating heat pipe show an excellent enhanced heat transfer for grinding of superalloys. While, the design of heat pipe/oscillating heat pipe grinding wheel for the certain grinding process is rather complicated. Therefore, a comprehensive and sophisticated design model should be further studied and built. In addition, the heat pipe/oscillating heat pipe grinding wheel will further be studied and applied in the high efficiency deep grinding of superalloys with increased grinding speed and material removal rate.

The future directions and research challenges of thermal management in the grinding of superalloys mainly include the following aspects.

1. New heat transfer mechanisms: The grinding process is a complex process involving multiple physical phenomena, and the heat generated during the process is difficult to effectively dissipated. Therefore, in order to improve the heat transfer efficiency, it is necessary to further explore the mechanism of heat transfer in the grinding process, and develop new heat transfer mechanisms that can effectively dissipate heat in the process.

2. Integration of heat transfer and material removal: In the grinding process, heat transfer and material removal are closely related. Improving the heat transfer efficiency can promote the material removal rate. Therefore, it is necessary to study the integration of heat transfer and material removal in the grinding process and explore how to effectively control the heat transfer and material removal process to achieve high-efficiency grinding of superalloys.

3. Energy-saving and environment-friendly grinding: The grinding process is an important energy-consuming and environment-polluting process. In order to achieve energy-saving, environment-friendly grinding, it is necessary to improve the heat transfer efficiency of the grinding process while ensuring the quality and efficiency of the grinding process, so as to reduce the energy consumption and environmental pollution of the grinding process.

4. The use of coolant and environmental pollution: Currently, the method of enhancing heat dissipation by improving the efficiency of coolant delivery is difficult to meet the demanding cooling requirements of high-efficiency deep grinding of superalloys, and it can cause environmental pollution and harm the health of operators. In the context of green manufacturing under the requirements of “carbon peak and carbon neutrality”, it is of great significance to strengthen the heat transfer of high-temperature alloy grinding and achieve green and high-efficiency grinding.

5. Development of new abrasives and grinding wheels: With the development of science and technology, the development of new abrasives and grinding wheels will bring more possibilities to grinding of superalloys. Future research needs to further explore the development of new abrasives and grinding tools to meet the higher and higher requirements of surface integrity and efficiency for grinding complex superalloy parts.

6. Intelligent control of the grinding process: With the development of intelligent manufacturing, intelligent control of the grinding process has become a development direction. It is necessary to combine heat transfer theory with advanced sensing, monitoring and control technologies to achieve real-time monitoring and control of the grinding process, so as to improve the efficiency and quality of grinding superalloy parts.

7. Multi-physics coupling simulation: The grinding process involves multiple physical phenomena, including mechanics, thermology, fluid mechanics, etc. In order to accurately simulate the grinding process, it is necessary to establish a multi-physics coupling simulation model of the grinding process, which can help researchers understand the mechanism of heat transfer in the grinding process better and optimize the grinding process parameters.

This paper reviews the thermal management in the grinding of superalloys. The current situation of grinding superalloys is studied, the problems which hinder the improvement of grinding quality and efficiency are investigated. By deeply understanding the current research and future directions, thermal management can be more effectively conducted in grinding, therefore the grinding quality and efficiency can be significantly improved. The main conclusions are drawn as follows.

Superalloys have excellent mechanical and physical properties and low thermal conductivity. In the grinding of superalloys, the grinding forces and specific energy are large, and massive heat is generated during the process. It is easy to cause sudden grinding burn-out, and the grinding wheel is also easy to wear. Consequently, the grinding quality and efficiency are difficult to effectively improve.

The grinding heat transfer to the chips, coolant, grinding wheel and workpiece. And the heat transfer to the workpiece can cause a great temperature to rise and thermal damage. The sophisticated heat partition model is derived. In shallow grinding, more than 90% of the heat is transferred to the workpiece. In high-efficiency grinding, less than 30% of the grinding heat is transferred to the workpiece, with the rest of the grinding heat being carried out of the grinding zone by the coolant. And with the increase of grinding efficiency, more and more heat need to be transferred to the grinding wheel or cooling fluid to avoid thermal damage to the workpiece.

Novel methods and technologies are proposed. High-speed grinding and ultrasonic vibration-assisted grinding are developed. The material removal behavior is changed, the grinding forces and specific grinding energy are reduced and the generated heat decreases. The electroplated or brazed superabrasive grinding wheels are invented to significantly increase the sharpness of the wheel, in this case, the grinding forces and heat can be reduced. Recent progress in conventional coolant delivery includes minimum quantity lubrication (MQL) system that combines the compressed air and coolant to make a minuscule mist that penetrates the wheel-workpiece contact zone to reduce the temperature. This effective lubrication reduces temperature, improves the surface quality and results in efficient grinding process.

The cooling fluid delivery methods are investigated and developed to increase the coolant delivery efficiency to dissipate the grinding heat with high efficiency. Therefore, the grinding temperature can be controlled, and the grinding efficiency can be further improved. In addition, passive thermal devices, heat pipes or oscillating heat pipes, are designed and fabricated in the grinding wheels, so the heat transport capacity of the grinding wheel can be increased. The grinding heat can be transferred out directly through the wheel to avoid heat accumulation in the grinding zone and high grinding temperature. As a result, the grinding temperature can be controlled, the grinding quality and efficiency can be increased and the use of hazard cooling fluid can be reduced. By the novel methods, the generation of heat during the grinding of superalloys can be decreased, and at the same time, the transfer of the heat by cooling fluid or passive thermal devices can be enhanced, and thermal management can be achieved.

In the future, the research direction of thermal management in the grinding of superalloys is to combine advanced technologies with traditional grinding technology, explore new heat transfer mechanisms, improve the efficiency and quality of the grinding process, reduce energy consumption and environmental pollution and promote the development of intelligent manufacturing. At the same time, in order to meet the challenges of traditional grinding technology, relevant researchers need to further study the mechanism of the grinding process and provide theoretical support for practical application.

The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 52205476), the Youth Talent Support Project of Jiangsu Provincial Association of Science and Technology (Grant No. TJ-2023-070), the Fund of Prospective Layout of Scientific Research for Nanjing University of Aeronautics and Astronautics (Grant No. 1005-ILB23025-1A), Fundamental Research Funds for the Central Universities (Grant No. NG2024008), Science Center for Gas Turbine Project (Grant No. P2023-B-IV-003-001) and the Fund of Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology (Grant No. 1005-ZAA20003-14).