4.1 Grinding force
The grinding force is a critical parameter in the grinding process. In comparison to dry grinding, it can generate an oil film in the grinding region (contact area between abrasive flank and material, and between abrasive flank and debris), lowering the friction coefficient and hence the grinding force. The effect of cooling conditions on the grinding force of DD5 under different grinding parameters was measured by a dynamometer, as shown in Fig. 5, Fig. 6, and Fig. 7.
From Fig. 5, Fig. 6, and Fig. 7, it can be obtained that in the range of experimental parameters, the magnitude of the grinding force is linearly negatively correlated with the grinding speed, and linearly positively correlated with the grinding depth and the feed rate; under different grinding parameters, the grinding force obtained by DD5 under MQL conditions is the smallest, followed by that under conventional flood conditions, and the grinding force under dry grinding conditions is the largest. As can be seen from Fig. 7, the grinding force gradually increases with the increase of feed rate, but its transformation trend is more gentle. Compared with the other two factors, the change of feed rate has less effect on the grinding force.
When the grinding speed increases or feed rate decreases, the number of grinding grains per unit time increases, resulting in a reduction in the maximum undeformed cutting thickness of the single abrasive, thus reducing the grinding force. Compared with dry grinding, the lubricating fluid helps to form an oil film in the grinding area, reducing the friction coefficient during grinding and thus directly reducing the grinding force. Compared with conventional pouring lubrication, MQL can effectively break the air barrier caused by the high-speed rotation of the grinding wheel into the grinding area for lubrication and cooling, which can significantly reduce the grinding force during the grinding process.
4.2 Surface roughness
A LEXT OLS4100 confocal microscope was used to measure the workpiece's surface roughness, and the influence laws of each grinding parameter on the surface roughness of DD5 under different cooling conditions were plotted, as shown in Fig. 8, Fig. 9, and Fig. 10.
From Fig. 8, within the experiment settings, the surface roughness of the workpiece reduces as the grinding speed increases. The reason for this is that as the grinding speed increases, the quantity of abrasive grains involved in grinding per unit time increases, and the maximum undeformed cutting thickness decreases, which reduces the plasticity of the workpiece material and removes the lamellar chips before it can be deformed during grinding, taking away part of the grinding heat, reducing the machined surface roughness and improving the surface finish. From Fig. 9 and Fig. 10, it can be seen that the surface roughness of the workpiece increases with the rise of grinding depth or feed rate within the experimental parameters. For the reason that when the grinding depth or feed speed rises, the maximum undeformed cutting thickness of the single abrasive increases, the surface roughness Ra of the workpiece increases accordingly, and the grinding surface quality decreases.
Under different grinding process parameters, the surface roughness of DD5 obtained under MQL condition is the smallest, followed by the surface roughness obtained under conventional pouring condition, and the surface roughness obtained under dry grinding condition is the largest. This is mainly since the lubricating fluid can penetrate the grinding area to form an oil film, which reduces the friction coefficient and reduces the generation of grinding heat, thus significantly reducing the surface roughness and enhancing the surface quality of DD5. Compared with the traditional pouring lubrication, MQL can effectively break the boundary layer of air in the grinding area and enter the grinding area for lubrication and cooling. The high-pressure airflow constitutes convective heat exchange with the gas environment in the grinding area, reducing the grinding temperature. The high-pressure airflow also carries away the generated grinding chips, preventing material coating or grinding wheel clogging, etc.
4.3 Surface morphology
The surface formation mechanism of grinding process determines its surface microstructure, which will have certain effects on its machining quality, and a tiny number of micro defects may undoubtedly occur during the process, which includes micro grooves, micro protrusions and micro-cracks, etc. A small number of grinding burns will also be generated during the grinding process. The grinding surface morphology of the DD5 material was examined using FEM under various cooling settings, as illustrated in Fig. 11.
As seen in Fig. 11, DD5 is prone to microscopic defects during the grinding process. In particular, DD5 grinding under dry environment has more significant microscopic defects with the lowest surface quality compared to Flood and MQL. To further explore the impact of cooling conditions on the surface morphology of DD5, the VHX-1000E super depth-of-field microscope was utilized to examine its surface morphology under different grinding depths, as illustrated in Fig. 12.
From Fig. 12, it can be seen that the grinding surfaces acquired with conventional flood cooling and MQL cooling are significantly better than those obtained with dry grinding under different cooling conditions and grinding depths. With the gradual rise of the grinding depth, the quality of the grinding surface gradually deteriorates and phenomena such as chip adhesion appear, and even local grinding burns appear when the grinding depth reaches 100 µm under the conditions of dry grinding and using MQL cooling. Although MQL cooling has a good lubricating effect, its cooling effect is limited. In the actual machining process, the grinding depth should be properly reduced to obtain better grinding surface quality.
To further investigate the abrasion marks of the grinding process surface and its distribution characteristics, Micromeasure 3D profiler was used to obtain the 3D profile of the grinding surface of DD5 under MQL cooling conditions, as shown in Fig. 13. Among them, the three-dimensional profile is shown on the left, the X-axis profile fitting curve is shown in the middle, and the height frequency distribution is shown on the right.
From Fig. 13, it can be seen that the variation pattern of the three-dimensional profile of the DD5 grinding surface at different grinding depths is basically the same as that of the two-dimensional grinding surface morphology. When the grinding depth is larger, deeper gullies appear in the grinding surface, the variation range of the height in the 3D profile is larger, and the probability distribution range of the grinding surface height is wider. When the grinding depth is smaller, the grinding marks in the grinding surface are wider and the distribution tends to be uniform, the variation range of the height in the 3D profile is smaller, the probability distribution range of the grinding surface height is narrower, the texture of the grinding surface is clear, and the grinding surface quality is better. The probability distribution of the grinding surface height approximately obeyed the normal distribution.
4.4 Subsurface microstructure
The Ni-base single crystal superalloy is subjected to various compound thermal and mechanical loads during the grinding process, and the material undergoes plastic deformation, resulting in various defects or damages on the surface and subsurface of the workpiece. The machined experimental workpiece was first to cut into subsurface specimens with a width of 1 mm using a CA20 low-speed wire-cut machine. Then, the subsurface specimens were ground and polished after mounting on the mounting machine. Finally, the subsurface specimens were etched. The treated subsurface specimens were observed by FEM, as shown in Fig. 14.
As can be seen in Fig. 14, the internal microstructure of the Ni-base single crystal superalloy DD5 consists of γ phase and γ' phase, γ' equivalently embedded in γ phase, and its volume fraction is about 70%. After the grinding process with different cooling conditions, a certain thickness of the work-hardening layer and plastic deformation layer appeared on the grinding subsurface of the Ni-base single crystal superalloy DD5, where the work-hardening layer was located at the most superficial layer, followed by the plastic deformation layer, the elastic deformation layer, and the matrix. The γ phase and γ' phase in the plastic deformation layer produced distortion and plastic slip phenomena under the action of grinding process. After conventional flood cooling and MQL cooling, the thickness of the work-hardened layer on the grinding subsurface was significantly smaller than that of the work-hardened layer in dry grinding, which indicates that the use of lubricating fluid can effectively suppress the grinding work-hardening phenomenon. This is because the lubricating fluid helps to form an oil film in the grinding area, which reduces the friction coefficient and friction force, thus reducing the degree of grinding hardening.
The grinding surface undergoes violent plastic deformation, the surface lattice of the workpiece is distorted, the surface grains are broken, elongated and distorted, hindering their deformation, improving the strength and hardness of the metal, and the material is strengthened by the deformation. At the same time, the grinding surface also undergoes thermal softening effect in high-temperature environment. Under this dual effect of weakening and strengthening, it may cause the machined surface to work harden and may also reduce the hardness of the machined surface. Microhardness is one of the most important means of studying the microstructural properties of metallic materials. It is obtained by applying a small load on the surface to be measured using a microhardness indenter to produce a micro indentation, which is calculated by determining the diagonal length of the micro indentation. Since the load applied by Vickers hardness is smooth and has no impact on the sample, the microhardness experiment in this paper adopts the Vickers hardness method and uses a diamond orthotropic indenter for loading, with the angle between the two opposite faces at the top of the vertebrae being 136°, and the four faces at the top of the indenter intersecting at a point, with the length of any intersection line between the opposite faces not greater than 1 µm.
A digital display Vickers microhardness tester was used to examine the subsurface microhardness of the Ni-base single crystal superalloy DD5 under three grinding processing methods (grinding speed of 25 m/s, grinding depth of 60 µm, and feed rate of 0.6 m/min) and to obtain micro indentations at different measurement positions. The microscopic indentations at different measurement positions were obtained using the step method, as shown in Fig. 15. With the grinding surface as the starting point, the diamond orthotropic indenter was slowly pressed into the measured sample surface and moved toward the substrate, and the load was applied at an interval of 5 µm, with a load size of 0.245 N and a holding time of 8 s. A total of 30 times were loaded, and the measurement range was from 5 to 150 µm below the grinding surface. after obtaining the two diagonal lengths of the micro indentation, the average value was taken as the length of the micro indentation d. The microhardness HV at different measurement locations was calculated according to Eq. (1).
Where: d is the length of the diagonal of the microscopic indentation (µm); P is the load applied to the experiment (N).
Based on the obtained microhardness values at different measurement locations on the ground subsurface, the curves of DD5 subsurface microhardness variation patterns under different cooling conditions were plotted, as shown in Fig. 16, Fig. 17, and Fig. 18.
As can be seen in Fig. 16, Fig. 17, and Fig. 18, the subsurface microhardness under the three grinding processing methods (dry, flood, and MQL) of the Ni-base single crystal superalloy DD5 has a similar variation pattern. The subsurface microhardness of the three grinding methods decreases sharply with increasing distance from the grinding surface from 5 µm to 15 µm, with the microhardness of dry grinding being the largest, followed by that of using conventional flood, and the microhardness of cooling with MQL being the smallest. At a distance of 20 µm-150 µm from the grinding surface (material substrate position), the grinding process of the three grinding processing methods had less influence on their subsurface microhardness, and the microhardness of all three fluctuated around 540 HV. Based on the above conclusions, it can be concluded that the microhardness of the grinding metamorphic layer is greater than the microhardness of the material matrix, which indicates that the grinding process of the Ni-base single-crystal superalloy DD5 prompted the strengthening of its ground surface organization.
4.6 Chip morphology
The microscopic morphology of grinding chips can represent the removal mechanism of material grinding, while the size and shape of grinding chips can reflect the plastic deformation of material and the form of removal during the grinding process. The formation of grinding chips marks the realization of the grinding process. Studying the formation process of grinding chips of Ni-base single crystal superalloys can help reveal the grinding and removal mechanism of Ni-base single crystal superalloys more deeply and offer a theoretical foundation for analyzing the grinding process and grinding surface quality. As illustrated in Fig. 19, a swarf collecting box was put on the grinding machine table to collect the swarf created during the grinding experiment (under varied cooling settings), and the surface morphology was studied using the Ultra Plus field emission scanning electron microscope.
As shown in Fig. 19, under different cooling conditions, DD5 mainly produces serrated chips, which mainly have two typical surfaces: contact surface and free surface. The side inclined to the rake face of abrasive particles is called the contact surface, which is mainly smooth and flat, and there are traces of abrasive particles sliding on the inner surface of abrasive chips. The opposite side is known as the free surface; unlike the contact surface, the free surface is primarily exhibited as a lamellar structure. The reason for this is that when the stress on the shear surface surpasses the strength limit of the material in the process of shaping and deforming the material under the action of the abrasive grains, the abrasive chips are sheared to form lamellar chips. During the grinding process, the interaction between the abrasive grains and the workpiece generates a large amount of grinding heat. Meanwhile, because the abrasive grains generally have a negative front angle, the grinding will intensify the material deformation and generate high temperature. As a large amount of grinding heat is not released in time, the material is more likely to undergo adiabatic shearing to form serrated chips. Ni-base single crystal superalloy has high strength, thermal strength and low thermal conductivity, and is a typical difficult-to-machine material, so the grinding temperature generated during grinding is difficult to conduct out, and the degree of adiabatic shear is more obvious, and the shear slip phenomenon is easy to occur.
At the same time, a phenomenon can be found that the chip morphology of DD5 in MQL or conventional flood environment is more regular than that in dry environmental conditions. The degree of sawing of the chips in the flood or MQL environment is obvious, and the chip formation frequency fluctuates less and the chip formation is stable. In contrast, the chip morphology in dry environment is less regular, the chip formation frequency fluctuates more, and the chip formation is not stable. This is because the degree of work hardening of DD5 in dry environment is greater, and the microhardness of the material increases, which makes it difficult to form chips. Conventional flood lubrication and MQL help to reduce the microhardness of the material and improve the removal rate of the material.
The individual grinding chips (in dry, flood, and MQL environments) were observed for different grinding wheel linear speeds, and Fig. 20 was obtained.
As shown in Fig. 20, under different cooling conditions, the frequency of serrated unit formation and the degree of serration on the free surface of the grinding chips increased with the increase of grinding speed. When the grinding wheel linear speed was 15 m/s and 25 m/s, the frequency of formation of serrated units on the free surface of the grinding chips and the degree of serration were lower. When the grinding speed is 35m/s, the frequency of formation of serrated units and the degree of serration on the surface of the grinding chip is the highest, and the size of the grinding chip is larger at this time. This is because. With the increasing speed, the number of abrasive grains involved in the grinding process per unit time increases, the temperature in the deformation zone of the material rises sharply, and the transient high temperature generated by the grinding process promotes its thermoplastic instability, making the material more susceptible to adiabatic shear slip, generating more slip elements per unit time and increasing the degree of swarf serration.