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Face Grinding Surface Quality of High Volume Fraction SiCp/Al Composite Materials

The existing research on SiCp/Al composite machining mainly focuses on the machining parameters or surface morphology. However, the surface quality of SiCp/Al composites with a high volume fraction has not been extensively studied. In this study, 32 SiCp/Al specimens with a high volume fraction were prepared and their machining parameters measured. The surface quality of the specimens was then tested and the effect of the grinding parameters on the surface quality was analyzed. The grinding quality of the composite specimens was comprehensively analyzed taking the grinding force, friction coefficient, and roughness parameters as the evaluation standards. The best grinding parameters were obtained by analyzing the surface morphology. The results show that, a higher spindle speed should be chosen to obtain a better surface quality. The final surface quality is related to the friction coefficient, surface roughness, and fragmentation degree as well as the quantity and distribution of the defects. Lower feeding amount, lower grinding depth and appropriately higher spindle speed should be chosen to obtain better surface quality. Lower feeding amount, higher grinding depth and spindle speed should be chosen to balance grind efficiently and surface quality. This study proposes a systematic evaluation method, which can be used to guide the machining of SiCp/Al composites with a high volume fraction.


Introduction
SiC p /Al is a composite material with high specific stiffness, specific strength, and thermal conductivity, and a low thermal expansion coefficient. Therefore, it is widely used in precision optical instruments, advanced weapons, and the aerospace field [1][2][3][4]. In the aerospace field, SiC p / Al is widely used in optical structure components and electronic packaging systems. To satisfy the application requirements, SiC p /Al composites with a high volume fraction are needed [5,6]; they have excellent mechanical properties. However, SiC particles exhibit high levels of hardness and strength. Consequently, SiC p /Al composites with a high volume fraction are difficult to machine [7][8][9]. To expand the application of SiC p /Al composites with a high volume fraction, many studies have been conducted by researchers worldwide. Additionally, many research results have been obtained [10][11][12][13].
Ciftci et al. [14] conducted a research on the effect of the volume fraction and particle size on cutting tool wear. The results showed that coated carbide tools had a longer service life. To predict the cutting force during the orthogonal cutting of SiC p /Al composite materials, Kishawy et al. [15] proposed a force model based on energy analysis. Some scholars conducted a research on the effect of the machining parameters and cutting tool structure on the cutting process [16][17][18]. Zheng et al. [19] studied the surface roughness of SiC p /Al composite materials after vibration grinding. The results showed that the spindle speed was the most influential machining parameter. The surface fractal dimension was related to the spindle speed and feeding velocity. Additionally, the surface defects had a significant influence on the surface fractal dimension. Du et al. [20] studied the chip formation mechanism during the grinding of SiC p /Al composite materials, and the topography characteristics of the chips were observed by a scanning electron microscope (SEM). The results showed that flake chips were the most common type. Kannan et al. [21] studied the formation mechanism of serrated chips during the machining of SiC p /Al composite materials. The results showed that stress was concentrated in the contact area between the aluminum matrix and SiC particles. Therefore, key holes were formed in the contact area and serrated chips were generated. Besides, a few researchers carried out single grit scratch tests to investigate SiCp/Al composites removal characteristics [22,23].
According to the references, SiC p /Al composite materials can be precisely machined [24][25][26][27][28][29][30]. However, the machining parameters were not comprehensively analyzed, and the surface morphology was not systematically evaluated in these references. Taking the spindle speed, feeding amount, and grinding depth as the machining parameters, 32 SiC p /Al specimens were grinding machined. The specimens were grouped and their surface quality was analyzed by the cutting force, friction coefficient, and roughness parameters. Further, the grinding qualities of the specimens were analyzed.

Testing Process
As shown in Figure 1(a), the testing system was based on an ultrasonic machine tool. The electroplated diamond wheel in Figure 1(b) was used as the grinding tool. The outer diameter and wall thickness of the grinding tool were 12 mm and 2 mm, respectively. The particle size of the diamond grain was D126 (120/140). The matrix and particle-reinforced materials of the workpieces were Al 2024-T6 and SiC ceramic, respectively. The particle volume fraction and average size of SiC were 62.5% and 40 μm, respectively. The rest of the parameters are listed in Table 1.
The specimens are shown in Figure 1(c). The upper surface was selected as the testing surface and the dimensions of the specimens were 3 mm×8 mm×10 mm. The cooling fluid supplied by Blaser was used for the testing system; the processing parameters are listed in Table 2. All the tests were conducted twice and the average experimental values were obtained.

Detection Method
As shown in Figure 1(b), the Kistler 9257B was used to measure the grinding forces, and the data sampling frequency was set at 9 kHz. All the machined surfaces were washed by the ultrasonic cleaning method before Additionally, the parameters of the surface topography can be analyzed systematically.

Evaluation Standard
To evaluate the machining quality, the grinding force (F), friction coefficient (COF), roughness parameters, and surface morphology were chosen as the evaluation parameters. The specimens were machined by the face-grinding method, and the axial grinding force was obtained as an evaluation parameter. The friction coefficient was used to evaluate the friction condition between the machine tool and specimen. The higher the value of the friction coefficient, the worse the surface quality.
The following parameters were chosen as the roughness parameters: surface roughness (Ra), broken area ratio (S dr ), peak height (R p ), lowest valley (R v ), gradient (S sk ) and kurtosis (S ku ). The parameters are defined as follows.
(1) Ra is the arithmetic average roughness, which represents the arithmetical mean deviation of the profile.
(2) S dr is used to evaluate the fragmentation degrees of the surfaces after grinding. The higher the value of S dr , the larger the fragmentation area.
(3)R p and R v are the peak height and lowest valley of the profile, respectively.
(4) S sk is used to evaluate the profile amplitude distribution. It is equal to 0 if the height of the profile surface is symmetrically distributed, less than 0 if low valleys exist below the middle plane, and higher than 0 if peaks exist above the middle plane.
(5) S ku is the kurtosis of the surface height distribution. S ku is less than 3 if the surface profile is flat and more than 3 if the surface profile is steep. Table 3, the experiments were divided into eight groups according to the feeding amount, grinding depth, and spindle speed. The experimental results of the grinding force (F), friction coefficient (COF), and roughness parameters are listed in Table 3.

Surface Quality under Lower Feeding Amount and Cutting Depth
The surface morphologies of groups 1 and 2 are shown in Figure 2(a) and Figure 3(a), respectively. The four photos on the left are the scanning electron microscope (SEM) results, whereas those in the middle show the height distribution of the surface morphology. We can obtain the four photos on the right by enlarging the central areas of the four photos in the middle. The lower areas are shown in black and blue, and the higher areas are in red. The surface quality parameters of groups 1 and 2 are shown in Figure 2(b) and Figure 3(b), respectively. From Figure 2 and Figure 3, it can be observed that the experimental results of T9 and T10 are relatively superior to those of the other specimens. Core areas of the specimens were selected to compare the grinding qualities of T9 and T10; a magnified three-dimensional profile of the core areas is shown in Figure 4. The average roughness (Ra) and best roughness (Rab) of the core areas were determined. All the experimental parameters are listed in Table 4.  From Figure 4, it can be observed that the surface morphology of T9 is relatively flat. Table 4 shows that the experimental parameters of T9 are relatively superior to those of T10. Therefore, the surface quality of T9 is the best in groups 1 and 2. This mechanism can be explained as follows. The feeding amount, grinding depth, and spindle speed of T9 are 50 mm/min, 5 μm and 6000 r/min, respectively. The average diameter of SiC particles is 40 μm. The particles are relatively steady when the grinding depth is 5 μm. However, the particles are pulled out with the increase of grinding depth, causing the appearance of pits on the grinding surface.

Surface Quality under Lower Feeding Amount and Larger Cutting Depth
The surface morphologies of groups 3 and 4 are shown in Figure 5(a) and Figure 6(a), respectively. Moreover, the surface quality parameters of groups 3 and 4 are shown in Figure 5(b) and Figure 6(b), respectively. From Figure 5, it can be observed that many pits exist on the surfaces of T3 and T7, indicating that their flatness is very poor. The S sk and S ku of T11 are −2.53 and 12.01, respectively. This indicates that the surface of T11 was depressed, and had deep pits. Therefore, the surface quality of T15 is the best in group 3.
From Figure 6, the S dr and R v of T4 and T8 are relatively high. This indicates that many surface defects and deep pits exist on the surface. Moreover, the S sk and S ku of T12 are relatively high, which indicates that the surface defects exist and the profile of T12 is very sharp. Therefore, the surface quality of T16 is the best in group 4.
Core areas of the specimens were selected to compare the grinding quality of T15 and T16. The magnified three-dimensional profiles of the core areas are shown in Figure 7. The Ra and Rab of the core areas were determined. All the experimental parameters are listed in Table 5.
As can be observed from Figure 7, the surface morphology of T16 is relatively flat. Table 5 shows that the experimental parameters of T16 are relatively superior to those of T15. Therefore, the surface quality of T16 is the best in groups 3 and 4. The mechanism can be   explained as follows. The feeding amount, grinding depth, and spindle speed of T16 are 50 mm/min, 80 μm and 8000 r/min, respectively. The average diameter of the SiC particles is 40 μm, and 80 μm is twice the size of the average diameter. Theoretically speaking, SiC particles can be pulled out entirely. However, the particles are irregular and differences exist between the protrusion heights of the diamond particles. Therefore, different surface morphologies appear with the change in grinding parameters.

Surface Quality under Larger Feeding Amount and Lower Cutting Depth
The surface morphologies of groups 5 and 6 are shown in Figure 8(a) and Figure 9(a), respectively. Moreover, their surface quality parameters are shown in Figure 8(b) and Figure 9(b), respectively. As can be observed from Figure 8, the surface morphology of T25 is relatively superior to that of T17, T21, and T29. Figure 9 shows that the surface morphology of T26 is relatively superior to that of T18, T22, and T30. Core areas of the specimens were selected to compare the grinding qualities of T25 and T26. The magnified threedimensional profiles of the core areas are shown in Figure 10. The Ra and Rab of the core areas were determined. All the experimental parameters are listed in Table 6.
As can be observed from Figure 10, the surface morphology of T25 is relatively flat. Table 6 shows that the experimental parameters of T25 are relatively superior to those of T26. Therefore, the surface quality of T25 is the best in groups 5 and 6. The feeding amount, grinding depth, and spindle speed of T25 are 100 mm/min, 5 μm and 6000 r/min, respectively.

Surface Quality under Larger Feeding Amount and Larger Cutting Depth
The surface morphologies of groups 7 and 8 are shown in Figure 11(a) and Figure 12(a), respectively.   Moreover, their surface quality parameters are shown in Figure 11(b) and Figure 12(b), respectively. From Figure 11 and Figure 12, it can be observed that the surface qualities of T27 and T28 are relatively superior to those of other specimens by comprehensive considerations of the experimental parameters and surface morphologies.
Core areas of the specimens were selected to compare the grinding qualities of T27 and T28. The magnified three-dimensional profiles of the core areas are shown in Figure 13. The Ra and Rab of the core areas were determined. All the experimental parameters are listed in Table 7.
As can be observed from Figure 13, many pits exist on the surface of T27. However, the pits are not very deep. This may be because of the pulling out of SiC. Table 7 shows that the S dr , Ra, Rab and Rp of T27 is lower than that of T28. On the whole, surface parameters of T27 are  relatively superior to those of T28. Therefore, the surface quality of T27 is the best in groups 7 and 8.

Three-dimensional Profile Comparison
According to the above analysis, T9, T16, T25, and T27 are relatively superior to the other specimens. The threedimensional profiles of these four specimens are shown in Figure 14. As shown in Figure14, surface quality of T9 is best, lower feeding amount, lower cutting depth and appropriately higher spindle speed should be recommended to obtain better surface quality. As shown in Figure14(c) and (d), obvious feeding marks appears on surfaces of T25 and T27, which should be avoided, so lower feeding amount, higher cutting depth and higher spindle speed should be recommended to balance grinding efficiency and surface quality.

Conclusions
(1) In this study, 32 SiCp/Al specimens with a high volume fraction were prepared and their machining parameters were measured. The surface quality of the specimens was tested, and the effect of the grinding parameters on the surface quality of the SiCp/Al composites was analyzed. (2) Taking the grinding force, friction coefficient, and roughness parameters as the evaluation standards, the surface quality of the specimens under different machining parameters was analyzed. To obtain better surface quality, lower feeding amount, lower cutting depth and appropriately higher spindle speed should be chosen. To balance grinding efficiency and surface quality, lower feeding amount, higher cutting depth and spindle speed should be chosen. (3) This study comprehensively analyzed machining parameters and systematically evaluated the surface morphology of SiCp/Al specimens with a high volume fraction. However, more research work should be conducted in the future. For example, the surface morphology from the view of material properties, and the influence of machining parameters should be fully discussed in the future.

Authors' Contributions
XZ and YG was in charge of the whole trial; XZ wrote the manuscript; GL, MC and BH assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.