Feasibility Study on Cryogenic Milling of Carbon Fiber Reinforced Silicon Carbide Composites

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1.Aerospace Research Institute of Materials & Processing Technology，Science and Technology on Advanced Functional Composites Laboratory，Beijing 100076，P.R.China；

2.College of Mechanical and Electrical Engineering，Nanjing University of Aeronautics and Astronautics，Nanjing 210016，P.R.China

（Received 26 March 2020；revised 23 April 2020；accepted 18 May 2020）

Abstract: Carbon fiber reinforced silicon carbide matrix（Cf/SiC）composites have the most potential application for high-temperature components of aerospace high-end equipment. However，high cutting temperature，rapid tool wear and severe surface damages are the main problems in dry cutting Cf/SiC composites process. The feasibility study on cryogenic milling of Cf/SiC composites using liquid nitrogen as coolant is investigated. Influences of milling parameters and coolant on temperature，cutting force，surface quality and tool wear are investigated，which is compared with dry cutting. Experimental results reveal that the cutting temperature in cryogenic milling of Cf/SiC composites is reduced by about 40%—60% compared with dry cutting. The milling force increases gradually with the increase of spindle speed，feed rate，depth and width of milling in cryogenic milling process. In addition，the machined surface quality in cryogenic milling is superior to that in dry cutting process. Fiber fracture，matrix damage and fiber matrix debonding are main material removal mechanisms. Flank face wear is the main wear form of the polycrystalline diamond（PCD）end mills. The tool life is prolonged in the cryogenic milling process because the reduced temperature inhibits the softening of Co binder and phase transition of diamond in the PCD end mills.

Key words：Cf/SiC composites；cryogenic milling；cutting temperature；surface quality；tool wear

0 Introduction

Carbon fiber reinforced silicon carbide matrix（Cf/SiC）composites have many superior mechani?cal and thermal properties，such as low density，high hardness，high specific strength，excellent wear resistance，high thermal stability，and chemi?cal corrosion resistance. These properties make Cf/SiC composites widely used in thermal protection systems of space vehicle，nozzles and flaps of rocket motors，car brake systems，aero engine，et al.［1-3］.Generally，components made of Cf/SiC composites are manufactured in near-the-net shape. However，the secondary machining process，address such as drilling，milling，and grinding，is still necessary in order to obtain the required dimensional tolerance and surface quality. The machinability of Cf/SiC composites is poor because of their high hardness and brittleness，as well as heterogeneous and aniso?tropic characteristics. The machining process is fea?tured by rapid tool wear，low process efficiency，surface defects in terms of cavities，cracks，delami?nation and subsurface damage［4-7］.

Many studies have been conducted on the ma?chining of Cf/SiC composites with traditional and non-traditional processes［8］. Among them，the grind?ing and the ultrasonic vibration assisted machining are widely used. The material removal mechanisms in grinding of Cf/SiC composites were studied via single-abrasive scratch tests［9-10］. The results showed that the grinding parameters had significant influ?ence on the surface quality and subsurface defects.The grinding force decreased with increase of the grinding speed. The damage behaviors of the com?posites consisted of fiber breakage，fiber/matrix in?terfacial debonding and matrix cracking. Tawakoli et al.［11］developed a specially designed segmented grinding wheel to reduce the rubbing and plowing re?gimes in grinding of Cf/SiC composites. The specif?ic grinding energy decreased consequently，and the experimental results showed that the grinding forces reduced significantly using the presented method.Qu et al.［12］carried out an experimental study on minimum quantity lubrication（MQL）in grinding of Cf/SiC composites. The grinding temperature re?duced substantially because a large amount of heat was removed by water vapor.Excellent surface qual?ity and low grinding force were also achieved in the MQL process. Theoretical and experimental studies have presented that ultrasonic vibration assisted ma?chining can reduce the cutting force and obtain high surface integrity［13］. Li et al.［14］carried out conven?tional grinding and ultrasonic assisted grinding on Cf/SiC composites. The results showed that the sur?face roughness of Cf/SiC composites machined by ultrasonic assisted grinding was smaller than that of by conventional grinding，and ultrasonic assisted grinding process had less damage to mechanical properties of the materials. Even though the im?proved surface quality can be achieved via grinding process，the material removal rate is relatively low.In addition，ultrasonic generator and complex ma?chining system are needed in ultrasonic vibration as?sisted machining，which makes it costly.

Cryogenic machining technology refers to a cut?ting technology that uses cold air，liquid nitrogen or liquid carbon dioxide and other cryogenic fluids to cool workpieces，cutting tools or cutting areas in the cutting process. At present，many researchers have conducted research on cryogenic machining technol?ogy. Bermingham et al.［15］used liquid nitrogen as the coolant in turning Ti-6Al-4V and found that the tool life increased under certain cutting parameters and the chip shape was also improved at the same time. Manimaran et al.［16］used liquid nitrogen as the coolant to carry out cryogenic grinding experiments on AISI D3 steel. Compared with dry and wet cool?ing， the cryogenic cooling reduced the surface roughness evidently. Dhar et al.［17］studied the cryo?genic turning of AISI 1040 and E4340C steel with carbide cutters and found a significant reduction in the tool wear rate and the surface roughness through the application of cryogenic cooling. Sadik et al.［18］used physical vapor deposition（PVD）coated tools to mill Ti-6Al-4V under liquid carbon dioxide cool?ing. It demonstrated that with increasing the flow rate of coolant，the tool life was improved. Yang［19］carried out comparative study on dry and cryogenic cutting of 316L stainless steel. Liquid nitrogen was used as the coolant. It was concluded that the cryo?genic cutting could get the improved surface quali?ty，the reduced tool wear and cutting force.

At present，the cryogenic machining process is mainly applied to machine typical difficult-to-ma?chine metallic materials，such as high-temperature alloy，titanium alloy，and high-strength steels. Very few reports on cryogenic machining of Cf/SiC com?posites can be noted. In this paper，the feasibility study on cryogenic milling of Cf/SiC composites us?ing liquid nitrogen as the coolant is carried out. Cut?ting temperature，cutting force，tool wear，and the machined surface quality under various cutting pa?rameters are investigated. The material removal mechanisms and tool wear mechanisms are ana?lyzed. For comparison，dry milling is also conduct?ed under the same cutting parameters.

1 Experimental Setup

Cf/SiC composites are used as the workpiece material in this paper. The surface morphology and schematic diagram of the structure are shown in Fig.1. The carbon fiber yarns include warp yarns，weft yarns，andZyarns. Warp yarns are alternately woven up and down on weft yarns on each layer of plain weave fabrics，whileZyarns are perpendicular to the warp and weft layers. The properties of the composites are provided in Table 1.

Fig.1 Surface morphology and structure of Cf/SiC composites

Table 1 Material properties of Cf/SiC composites

The cryogenic milling experimental setup is shown in Fig.2. The cryogenic cooling system in?cludes a liquid nitrogen container，pressure gauges，valves，a stainless steel pipe covered with sponge，a nozzle with the inner diameter of 4 mm and some clamping devices. The liquid nitrogen with a pres?sure of 1.1 MPa is sprayed into the cutting area. A five-axis machining center （MIKRON-UCP710）with a maximum spindle speed of 18 000 r/min and a maximum feed rate of 20 m/min is used. Down milling is adopted in all the experiments. The pro?cess parameters are listed in Table 2. Before each set of the experiment，the workpiece and the cutting tool are cooled for 5 min. Two fluted polycrystalline diamond（PCD）end mills are used. The diameter，the rake angle，the clearance angle and the inclina?tion angle，of the end mill are 10 mm，3°，10°，and 0°，respectively.

Fig.2 Experimental equipment of cryogenic cooling system and milling process

Table 2 Machining parameters used in this work

The schematic diagram of temperature mea?surement is shown in Fig.3. A micro K-type thermo?couple wire is sandwiched between two pieces of Cf/SiC blocks，and mica sheets are used to insulate them from each other. In the cutting process，ther?mocouple wires generate thermoelectric potential signals due to the change of the temperature. The signals are collected by a dynamic signal acquisition card（NI USB-6218），and then processed by the relevant software on the computer. The cutting force is measured with a dynamic dynamometer（KISTLER 9625B）. The surface morphology and microstructure of the material are observed by an op?tical microscope（VHX-600）and a scanning elec?tron microscope （Hitachi S3400）. The surface roughness is measured by a laser scanning micro?scope（LSM 700）. The tool wear morphology is ob?served by a digital camera microscope（UCMOS 10000KPA CCD）.

Fig.3 Illustration of cutting temperature measurement method

2 Results and Discussion

2.1 Cutting temperature

Fig.4 shows the milling temperature in both dry and cryogenic milling conditions when the spin?dle speed，feed rate，depth and width of milling are 3 000 r/min，60 mm/min，3 mm，and 2 mm，re?spectively. The temperature in cryogenic milling is 286—405 ℃，while that in dry milling is about 732 ℃. The temperature is significantly reduced by using liquid nitrogen as the coolant. This is because cryogenic liquid nitrogen can cool the cutting heat generated in the machining area，which can de?crease the cutting temperature.

Fig.4 Milling temperature under different cool?ing conditions

In addition，the flowing liquid nitrogen has the functions of removing chips，lubricating the cutting area and reducing the heat generated by the friction between the tool and the workpiece. Furthermore，the vaporization of liquid nitrogen can absorb a great quantity of cutting heat，taking away part of the heat from the tool，the workpiece and the cutting ar?ea. Therefore，the generated cutting temperature in cryogenic milling of Cf/SiC composites is signifi?cantly lower than that of in dry milling process.

2.2 Milling forces

Fig.5 shows the change of milling forces in three directions（cutting forceFx，thrust forceFyand axial cutting forceFz）under different cutting pa?rameters. It can be observed from Fig. 5 thatFyandFzare far lower thanFxat the same cutting parame?ters. As shown in Fig.5（a），when the feed rate，the width and depth of milling are fixed，the cutting force ofFxincreases with increase of the spindle speed. With the range of spindle speed increasing from 1 000 r/min to 5 000 r/min，the corresponding cutting forceFxincreases dramatically. However，when the spindle speed increases from 5 000 r/min to 7 000 r/min，the increasing magnitude ofFxis relatively small. When the spindle speed，width and depth of milling are unchanged，milling forces in three directions show an increasing trend with an in?crement of feed rate（as shown in Fig.5（b））. When the feed rate increases，the nominal thickness and cross-sectional area of the cutting layer increase，the volume of the workpiece material that is cut in a unit time increases，and then the extrusion，rebound and friction between the tool and the workpiece material constantly increase correspondingly. Therefore，the cutting resistance and the material deformation ener?gy in the cutting process increase，resulting in an in?crease of the milling force with increasing the feed rate.

As shown in Fig.5（c），with increasing the width of milling from 1 mm to 4 mm，Fxincreases continuously whereas the changes ofFyandFzare relatively small. In addition，Fig.5（d）shows that with increasing the depth of milling from 0.5 mm to 2 mm，the change of the cutting forceFxis extreme?ly obvious and it is increased by a large margin.However，the milling forces inYandZdirections is far lower than that inXdirection，and the changing amplitude is also minor. As the depth or width of cutting increases，the geometry of the cutting layer changes and the area of the cutting layer increases，which promotes material removal rate，thereby in?creasing the friction between the tool and the cutting layer material and the cutting force.

Fig.5 Variations of milling force in three directions under different cutting parameters

2.3 Surface quality and material removal mechanism

In this work，the processed surface of the Cf/SiC composites material is also composed of a SiC matrix between yarn，weft，Zcarbon fiber yarn and carbon fiber，apart from the holes formed by the de?fects of the material itself. Fig.6 presents the remov?al process of carbon fiber yarn in three directions.Fig.7 shows the surface roughness of Cf/SiC com?posites processed under dry milling and cryogenic milling conditions atn=3 000 r/min，vf=60 mm/min，ap=3 mm andae=2 mm. Under the cryogen?ic milling condition，the surface roughnessSaandSqof the machined surface area of warp，weft andZyarns are significantly lower than those under dry milling condition. Thus，the surface quality in liquid nitrogen cooling assisted milling condition is better than that of in dry milling. The reason is that excel?lent cooling effect of liquid nitrogen can reduce the plasticity of the carbon fiber and deformation degree of the workpiece material in the cutting zone，there?by reducing the deformation of the machining sur?face and improving the quality of the machined sur?face.

Fig.6 SEM image of the machined surface

Fig.7 Surface roughness of the machined surface under dry and cryogenic milling conditions

Fig.8 presents the schematic diagram of materi?al removal process while milling Cf/SiC composites along with the warp direction. Due to different me?chanical properties，the removal mechanism of car?bon fiber is different from that of the matrix materi?al. SiC ceramic matrix is a typical brittle and diffi?cult-to-cut material with high strength. During the cutting process，the silicon carbide material has lit?tle bending，tensile and compressive deformation.However，cracks continue to appear and propagate in the matrix，eventually leading to the failure of ma?terials. Different from the silicon carbide，carbon fi?ber has relatively low strength，high toughness and plastic mechanical properties. Therefore，in the mill?ing process，the carbon fiber in Cf/SiC composites can bend and deform with limited deflection. In addi?tion，carbon fibers are prone to brittle fracture under the effects of compressive stress，shear stress and bending stress.

Fig.8 Schematic diagram of material removal process

As shown in Fig.9，this study divides carbon fi?bers in three fiber directions（warp，weft，Z）to dis?cuss the material removal mechanism. The angleαis defined as the rotation angle of the milling cutter in the stage of cutting into the workpiece. The angleβis defined as the counter-clockwise angle between the cutting speed direction and the carbon fiber direc?tion. Fig.9（a）describes the schematic diagram of the removal mechanism of the warp carbon fiber yarn. In this case，the angleβis an obtuse angle. In addition，the shear force on the carbon fiber and the matrix is much higher than the compression force and the bending force. Under the action of stress，the inner of the carbon fiber and matrix have the oc?currence of minor crack，which continues to extend until brittle fracture occurs（shown in the yellow rect?angle in Fig.9（a））. Furthermore，the process of brittle fracture of carbon fiber by the shearing force also generates shearing and compressive forces on the surrounding fibers，which continues to generate long crack，eventually leading to brittle fracture.

In Fig.9（b），the compression and bending forc?es on the carbon fiber and the matrix are greater than the shear forces because the angleβis an acute angle. In addition，the matrix is brittle material and exist minor deformation under the stress，where car?bon fiber has high bending strength and it will de?form to a certain extent after being stressed，thereby leading to the debonding of the interface. Further?more，carbon fiber is under the action of compres?sive and bending forces，and the stress concentra?tion is mainly at the bottom. When the stress is greater than the breaking strength，brittle fracture occurs at the bottom of the carbon fiber. The brittle matrix collapses or fails due to the huge stresses that continue to generate the crack and extend to the en?tire area of Cf/SiC composites. The carbon fiber and substrate in theZdirection are mainly affected by the shearing force of the milling cutter bottom edge and the pressing force of the rake face. The carbon fiber and the matrix have the occurrence of brittle fracture along with the cutting direction under the ac?tion of shearing and compression forces，thereby achieving material removal. The material bonding strength at the yarn crossings is low and more pit?ted. In addition，during the milling process，the car?bon fiber is relatively easy to bend and deform after being stressed，and the crack occurs. After the fiber breaks from the bottom or the head，the carbon fi?ber pulls up along with the surface near the pit.

Fig.9 Illustrations of material removal process for warp, weft and Z yarns

2.4 Tool wear

In this work，tool wear under the milling pa?rameters ofn=3 000 r/min，vf=60 mm/min，ap=3 mm andae=2 mm is studied. The main fail?ure of polycrystalline diamond（PCD）tool is flank face wear. The wear area of tool flank face is charac?terized with a digital camera microscope，and the width of wear landVBis measured. The value of wear criteria in this paper isVB=0.3 mm. Fig.10 shows the width of flank face wearVBunder dry milling and cryogenic milling of Cf/SiC composites.It can be observed that with the increase of the mill?ing length，the width of flank face wear increases rapidly and the tool wear is faster in the initial stage，while laterVBincreases slowly. However，the changes ofVBon the flank face under two condi?tions are obviously different. Compared with the cryogenic milling，the overall growth rate ofVBduring the dry milling is higher，that is，the tool wear in the dry milling is faster than that in the cryo?genic milling. In the cryogenic milling，the increase ofVBis relatively slow andVBreaches the wear criteria until the milling length attaining 4 000 mm.The tool durability is improved by the cryogenic milling compared with the dry cutting.

Fig.10 Width of flank wear land

In addition，in the process of liquid nitrogen as?sisted milling of Cf/SiC composites，the liquid nitro?gen medium can suppress the tool wear and prolong the tool service life. Furthermore，due to the cool?ing effect of liquid nitrogen，the cutting temperature reduces，thus avoiding the tool material phase transi?tion，the binder softening and adhesive wear on the tool during liquid nitrogen assisted milling process.

Fig.11 shows the tool wear morphology of the flank face. In the dry milling experiments，the wear of tool flank face is stepped and the wear land is rela?tively uniform，resulting in the breakage of the cut?ting edge and tool nose. This is due to the high mill?ing temperature during the dry milling， which causes the softening of the tool binder Co phase，re?sulting in a decrease in the bonding strength of the tool material，a reduction in the strength of the cut?ting edge，and the failure of the cutting tool. With the friction and scratch of chips between the work?piece and the cutting tool，and the tool material phase transition caused by high temperature，the tool wear rate aggravates rapidly. During the pro?cess of the cryogenic milling，continuous high-fre?quency scratches and serious frictional shocks take place on the cutting tool，resulting in obvious groove-like wear on the cutting edge and the side zone. Owing to the cooling effect of liquid nitrogen，the cutting temperature remains lower than both of the phase transition temperature of the tool material and the softening temperature of the binder. There?fore，no obvious adhesive wear is observed.

Fig.11 Morphologies of tool flank face wear under two con?ditions

3 Conclusions

The feasibility study on the cryogenic milling of Cf/SiC composites using liquid nitrogen as the coolant is carried out. Under the same cutting pa?rameters，the cutting temperature generated by the cryogenic milling is extremely lower than that in the dry cutting of Cf/SiC composites. In addition，thrust forceFyand axial forceFzare obviously lower than cutting forceFxat the same cutting parame?ters，and cutting forceFxincreases gradually with one of these parameters including spindle speed，feed rate，depth and width of milling in the process of cryogenic milling with the aid of liquid nitrogen.Fiber fracture，matrix damage and fiber matrix debonding are main material removal mechanisms during the cutting process. Furthermore，the surface roughnessSaandSqof the machined surface area of warp，weft andZyarns in cryogenic milling are sig?nificantly lower than that in dry cutting atn=3 000 r/min，vf=60 mm/min，ap=3 mm，andae=2 mm. The main wear mechanism of PCD tool in cryogenic milling is flank face wear，and the wear rate is slower than that in dry cutting process.