Study on removal mechanism and surface quality of high volume fraction SiCp/Al composites based on meso-scale

: The milling process of SiCp/Al composites with high volume fraction and large particle size has been studied in this paper. The stress and strain distribution of SiC reinforced particles and the removal mechanism of the material are analysed. The effects of milling depth and feed per tooth on surface quality were analysed. The effect of feed per tooth on the thickness of subsurface damage layer is revealed. The results show that in the end milling process of high volume fraction SiCp/Al composites, the blade diameter is larger relative to the particle size, which leads to the main removal forms of particle size: extrusion crushing and rolling crushing. The surface defects of the machined workpiece mainly include cavity, crack and delamination caused by extrusion of aluminum matrix. The surface quality of the machined workpiece can be improved by increasing the milling depth appropriately. The increase of the feed rate of each tooth will lead to the increase of the surface defect of the machined workpiece and the deterioration of the surface quality. When the feed rate per tooth increases from 4 to 8 μm, the thickness of subsurface damage increases from 47.7 to 60.5 μm. It is found that the ratio between the minimum cutting thickness of SiCp/Al composites and the radius of the cutting edge should be less than or equal to 4%.


1.introduction
Silicon carbide particle aluminum matrix (SiCp/Al) composites have excellent comprehensive properties such as high specific strength, high specific modulus and high temperature resistance. They are mainly used in satellite bearings, aero-engines and inertial navigation systems. With the increasing demand for material properties in the fields of aerospace, automotive and optical precision instruments. SiCp/Al composites have attracted more and more attention due to their excellent properties.
As the urgent need of SiCp/Al composites in various projects, the processing technology of SiCp/Al composites has been widely concerned by scholars.
Wang et al. [1] established the random distribution model of circular SiC particles and the random distribution model of polygon SiC particles with high volume fraction. The simulation results show that the main causes of defects are the rotation, pull-out, crushing, micro-fracture and cutting of SiC. In the aspect of cutting force, the fluctuation of cutting force of polygon particle model is larger than that of circular particle model. Teng et al. [2] used Abaqus software to simulate the micromachining process considering the cutting edge radius, and established a two-dimensional finite element model of micromachining. The Von Mises stress and strain distribution in the workpiece under the influence of the interaction between the tool and the particle, chip formation process, cutting force and chip thickness were revealed. Niu et al. [3] conducted a processing experiment with polycrystalline diamond (PCD) tool on a high-precision miniature milling machine. The interaction between cutter and particle and the process of chip formation are analysed. The surface roughness, morphology, texture and defect of workpiece were analysed. And the optimum technological parameters are selected. Suresh Kumar Reddy et al. [4] studied the surface quality and subsurface damage degree of SiCp/Al composites and aluminum alloys under different cutting conditions. By comparing the surface integrity (surface roughness, residual stress, microstructure and microhardness), the machinability of the two materials is understood. The research results are helpful to better understand the end milling process. Pramanik et al. [5] analysed the influence of feed rate on surface roughness, surface profile, surface morphology, chip surface, chip ratio, machining force and force signal. The results show that the machining speed has no obvious effect on the machining force at low feed rate, but the machining speed decreases with the increase of feed rate at high feed rate. Dabade et al.[6] studied two kinds of SiCp/Al 10% composites with different particle sizes (220 mesh and 600 mesh). The effect of hot working on the machinability and surface quality of SiCp/Al composites was studied. The results show that Al/SiC/10P/220 and Al/SiC/10P/600 composites have good surface roughness at 60℃. Ghoreishi et al.[7] studied the effect of different cutting parameters on tool wear of low volume fraction SiCp/Al composites during low temperature and high speed cutting. It is found that low temperature cooling can reduce tool wear. Liu et al. [8]  2 Simulation analysis based on particle random distribution model

Establishment of particle random distribution model
The finite element simulation model of SiC aluminum matrix composites with random distribution of particles of different sizes was established. In order to observe the particle removal mechanism in the cutting process more clearly, the 3D micro-milling model was simplified into a two-dimensional orthogonal simulation experiment. The volume ratio of the composite material is 60%, the particle diameter is mainly 20μm and 60μm SiC particles mixed, the average particle size is about 40μm, the matrix material is AL2024. In the simulation, it is assumed that the particle shape is round and the particles are distributed randomly in the matrix without overlap. The particle radius is randomly distributed within 10-30μm, and the particle random distribution and the milling model principle are shown in Fig. 1.

Material characteristics and failure criteria
The matrix of the composite is Al2024, and the reinforcement is SiC particles. The elastic phase of the material is mainly determined by the elastic modulus and Poisson's ratio. The parameters are shown in Table 1. Aluminium is a typical plastic material. The Johnson cook (J-C) model is used to characterize its plastic deformation stage [25], and it is often characterized as follows: where  is the flow stress (MPa);  is the effective plastic strain;  is the effective plastic strain rate; 0  is the reference plastic strain rate; T is the environment temperature (°C); m T is the melting point temperature of the material; A is the yield stress of the material (MPa); B is the work hardening parameters of the material (MPa); C is the strain rate enhancement index; m is the temperature change rate index; and n is the strain hardening index.
The Johnson-Cook fracture criterion and damage parameter D were used to judge the material removal.
D was set to be 1, and the unit was separated and removed. The expression is where f  is the failure strain and   is the effective plastic strain increment under an increase in the load unit.
Using the J-C fracture criterion [26], the calculation formula for equivalent plastic strain is where f  is the equivalent plastic strain; m  is the average value of positive pressure (MPa);  is the effective stress (MPa); and 5 1d d are the material failure parameters. Table 2 shows the J-C model and J-C fracture model parameters of the 2024 aluminium alloy. Table 2 Some parameters of matrix J-C model and J-C fracture model Tr (℃) 20 The PCD tool is set as an analytical rigid body in the simulation. The cutting-edge radius is 10 μm, the rake angle is  7 and the relief angle is  20 . The contact mode is face-to-face contact, and the Coulomb friction model is adopted, with a friction coefficient of 0.35. The bottom of the matrix and the upper right part of the tool are constrained by the boundary.

Criteria for chip separation
The characteristics of the aluminium matrix also adopt the J-C and J-C fracture models [28], adopt brittleness removal criteria for SiC particles, and select a tensile stress standard to judge, and its characterization is as follows:  Table 3. Table 3 Parameters of material brittleness removal [28,29]

The relationship between the experiment and simulation
In the experiment, the maximum milling depth is 50 μm, which is very small compared to the cutter diameter of 1000 μm, as shown in Fig. 3. In this case, the effect of the helix angle on chip formation and cutting force is negligible. Therefore, the machining process of 3D micro-milling is simplified to a 2D micro-orthogonal machining process. It is assumed that the uncut chip thickness in the cutting process is the same as that set in the simulation.

Study on particle removal mechanism and influence of surface defect formation
In the simulation, the cutting speed is set as 2198 mm/s, which is equivalent to the spindle speed of 14000 r/min, and the cutting thickness is 45 um. After the simulation, the removal of particles and surface defects were analysed, and it was found that the removal forms of composite materials were different at different locations of milling particles, as shown in Fig. 4.  It can be seen from Fig. 5 that in the cutting process, the irregular stress distribution and stress concentration area are mainly the contact area between the tool and the particle. It can be concluded that stress concentration and irregular stress distribution are the main reasons for particle breakage, peeling, extrusion and surface defects.
After the particles are peeled off, the surface of the workpiece appears many pits, some SiC particles are pressed by the cutting tool, such as aluminum alloy matrix, some broken particles remain in the pits, the particle cracks are uneven, the surface deformation is serious.
When the cutting thickness increases from 25μm to 65μm, the surface pits become larger, the number of surface burrs decreases, and the width and depth of cracks increase.This is because the matrix size effect decreases when the cutting thickness increases from 25μm to 65μm. The increase of cutting thickness causes the particles to extrude each other, the depth of crack propagation increases, the amount of deformation continues to expand, and the surface of the workpiece cracks and pits.In addition, the volume of particles being cut increases, which leads to the increase of cutting force and the deterioration of surface quality.
With constant cutting speed and different cutting thickness, the equivalent plastic strain distribution is shown in Fig. 5.

4.Experimental scheme
The experiment mainly studies and analyses the surface defects formed by a PCD end mill milling a This experiment was carried out on the 3D micro-milling machine platform as shown in Fig. 7, The surface roughness value of the material cuttings was measured by a real colour scanning microscope. The surface micromorphology of the material cuttings was observed by a Zeiss SIGMA 500 field emission scanning electron microscope, and the experimental equipment is shown in Fig. 8. In the single factor milling test, the processing parameters used are shown in Table 4 below.

Influence of milling depth on particle removal and surface defect formation
The micro-morphology of the machined surface of the workpiece was detected by using a true color confocal microscope. When the spindle speed and feed rate remain unchanged and different milling depths are used, the three-dimensional morphology characteristics of the machined surface of the workpiece are shown in Fig. 9. When the spindle speed and feed rate are constant and the milling depth is different, the changing trend of surface roughness is shown in Fig. 10. With the increase of milling depth, its size and depth decrease obviously, the surface scratches weaken obviously, and the surface consistency is enhanced. As can be seen from Fig. 10, when the milling depth increases from 25 to 45 μm, the surface roughness decreases slowly. When the milling depth increases from 45 μm to 65 μm, the surface roughness decreases sharply.
A large number of small cracks and pits can be observed from Fig. 11(a). Part of the reason is the cracking phenomenon of the cover layer due to the extrusion of the aluminum matrix. Another part of the reason is that the milling particles are broken and peeled off to produce cracks and pits. By observing Fig. 11(b) and (c), it is found that with the increase of milling depth, the material cracks and pits gradually decrease, and the scratches left by SiC particles on the matrix surface also begin to decrease.
At this time, the feed rate per tooth is 4 μm, which is less than the minimum cutting thickness of aluminum matrix. The size effect is significant when the matrix is removed, and the coating effect is strong. But the material contained in the particle is large, and the silicon carbide particle is brittle material, when the cutter milling particle position upper, the particle will be completely broken, so when the milling depth is small, the cutting tool of particle crushing, extrusion effect is more obvious. The crushed particles fall off and produce more pits. In addition, the broken particles move with the milling cutter, scratch the surface of the workpiece, and promote the further increase of surface defects. When the milling depth is between 45 μm and 65 μm, the milling position of the particles is mostly the lower part, the crushed particles are mostly completely separated from the pits, the surface defects are reduced, and the surface consistency is enhanced. The heat generated in the milling process will also increase, and the residual heat will cause the softening of the aluminum matrix and make it wrap on the surface of the particles, thus improving the processing quality.

Study on the influence of feed rate per tooth on particle removal and surface defect formation
The spindle speed and milling depth remain unchanged. When different feed rate of each tooth are selected, the three-dimensional morphology characteristics of the machined surface of the workpiece are shown in Fig. 12. As can be seen from Fig. 12, when the feed rate per tooth increases from 4 to 8 μm, the number of small holes, pits, cracks and burrs on the surface increases significantly, and the surface roughness increases from 1.035 to 1.256 μm. When the feed rate of each tooth is 4 μm, as shown in Fig. 12 (a), the machined surface is relatively smooth. At this time, because the cutting thickness is small, which is smaller than the minimum cutting thickness of aluminum matrix, the size effect of matrix removal is significant, and the coating effect on the surface is strong, and the machined surface is relatively smooth. In addition, the volume of the particles is small, so that the milling force is relatively small, the broken particles are less, the impact on the surface quality is small, so the surface quality is better. Each tooth feeding including increased from 4 μm to 6 μm, between substrate size effect is abate, began to appear particle removal, due to the cutting edge radius is bigger, particles are mainly composed of crush removal, under the extrusion of the cutting tool, as the tool of particles under crush slide scratches on the surface, as shown in Fig. 13 (b), surface can see a few scratches. When the feed rate of each tooth increases from 6 μm to 8 μm, the broken part and volume of particles increase, and the milling force increases, which aggravates the wear on the machined surface and tool, resulting in the increase of surface scratches and the deterioration of machining quality, as shown in Fig. 13 (b) and (c). In addition, with the increase of feed per tooth, the undeformed cutting thickness increases, which leads to the increase of milling force and the increase of surface roughness.
When the spindle speed is 18000 r/min, the milling depth is 65 μm, and the feed rate is different, the surface defects of the processed material are shown in Fig. 13.  When the feed rate of each tooth is greater than 6 μm, the increasing trend of surface roughness becomes slower. This is because with the increase of feed rate of each tooth, the volume of the material to be milled increases, resulting in increased friction, and the effective milling time remains unchanged, most of the heat can not be dissipated, resulting in the softening of the matrix and the reduction of milling force, thus improving the processing quality and weakening the increasing trend.
In addition, the residual chips in the milling process melt on the surface of the workpiece under the action of heat in the system, so that the defects on the surface of the material can be remedied and the surface quality can be improved. According to the detection results in Fig.   16, point a contains 56.03% Si element and 12.25% Al element, and point d contains 48.86% Si element and 29.41% Al element, which can prove that aluminum matrix is attached to SiC particles.
Considering the particle size (25~60μm) and the value of feed per tooth, when the feed per tooth is too large, some small particles will be pulled out, resulting in pits on the surface and affecting the machining quality, as shown in Fig. 12 (b) and (c). This is because the particle is milling volume increases, resulting in the increase of milling force, spindle vibration aggravation, resulting in the particle is extruded and broken generated by the crack and cavity, and the particle is pulled out caused by pits and other defects.
The sub-surface damage thickness of the workpiece, the sub-surface damage graph and the sub-surface damage thickness curve of the workpiece are measured, as shown in Fig. 16 Fig. 16 (b). With the increase of feed rate per tooth, the thickness of damage layer increases, and the subsurface crack and pit become deeper. This is the transition from ductile to brittle removal. As the feed rate of each tooth continues to increase, the thickness of the damaged layer increases again, and cracks and crack pits also increase, as shown in Fig. 16(c). At this time, the material is mainly removed through the brittle mode. The condition of subsurface defects also has a great influence on the surface quality.

Conclusion
In this paper, the removal mechanism of milling SiCp/Al composites and the influence of various cutting parameters on surface roughness are studied by the method of simulation and experiment. The following conclusions are drawn through analysis.
In the micro milling process of high volume fraction SiC based composite materials, the removal forms of particles are pulling out, crushing and pressing, but because the blade diameter of the tool is relatively large to the particles, the removal form of particles is mainly crushing.
There are many pits on the surface of the workpiece when the particles are peeled off the matrix. Some SiC particles are pressed by the cutting tool, such as the aluminum alloy matrix, and some residual broken In the milling process of SiCp/Al composites with meso-scale tool, the surface defects mainly include cavity, crack and delamination caused by extrusion of aluminum matrix.The minimum cutting thickness of SiCp/Al composites is less than or equal to 4μm, and its ratio to the radius of the cutting edge circle should be less than or equal to 4%.

Ethical Approval
This research project has been approved by the Ethics Committee of Liaoning University of Technology.

Consent to Participate
I solemnly declare that the paper "Research on Micro Milling Mechanism and Surface Roughness of High Volume Fraction SiCp/Al Composites" presented by us is the result of our research. This paper does not contain any work published or written by any other individual or group, except for the content specifically noted and cited in the paper. I fully realize that the legal consequences of this statement shall be borne by me

Consent to Publish The Author confirms:
that the work described has not been published before; that it is not under consideration for publication elsewhere; that its publication has been approved by all co-authors, if any; that its publication has been approved by the responsible authorities at the institution where the work is carried out.
The Author agrees to publication in the Journal indicated below and also to publication of the article in English by Springer in Springer's corresponding English-language journal. The copyright to the English-language article is transferred to Springer effective if and when the article is accepted for publication. The author warrants that his/her contribution is original and that he/she has full power to make this grant. The author signs for and accepts responsibility for releasing this material on behalf of anyand all co-authors.

Conflicts of interest/Competing interests
The authors have declared that no conflict of interest exists.

Availability of data and material
The data used to support the findings of this study are available from the corresponding author upon request.