3.1 Tool wear
The helical milling is a semi-closed machining. A large amount of heat generated during the cutting process is not easily taken away by chips and tools, which is easy to cause tool wear. Tool wear not only affects hole quality, but also affects processing efficiency and increases processing cost . After the experiment is completed, the tool wear is observed, and the tool wear forms under the two cutting conditions are shown in Fig. 3.
Figure 3 presents the tool wear forms under two cutting conditions, where the main wear forms are micro-chipping, corner rounding and adhesion. Under jet cold air condition, the corner rounding is relatively serious, and the cutting edge is slightly chipped. Helical milling is an intermittent cutting, the cutting edge is subjected to continuous alternating loads. Due to the brittle nature of PCD material, micro-chipping is prone to occur at the cutting edge, especially when the temperature gradient is large. The corner rounding is mainly from micro chipping and the frictions between the tool and the workpiece. In addition, the chip is easy to bond on the tool surface under the combined thermal-force actions.
In the process of CFRP helical milling, the flank and transverse edge of the tool have different degrees of wear, and the flank wear is the main wear form [19, 20]. Therefore, the flank wear is measured and analyzed. After the experiment is completed, in order to further determine the influence of two cutting conditions on tool wear, the trend of tool wear changing with the holes number is recorded as shown in Fig. 4.
As shown in Fig. 4, with the increase of the holes number, the tools under both cutting conditions appear different degrees of wear, and the wear values show an upward trend. On the one hand, with the increase of the holes number, the scratching effect on the tool flank is continuously increased. In addition, during the helical milling process, there are more carbon powder particles scratching the tool resulting in increased tool wear. On the other hand, due to poor thermal conductivity of the matrix, more and more cutting heat accumulates. This further increases the tool wear.
Compared with dry cutting condition, tool wear is more serious under jet cold air condition. With the increase of the machined holes, the cutting temperature of the tool in helical milling continuously increases resulting in a large change in the temperature gradient of the cutting edge in jet cold air cutting condition. According to Fig. 3, the micro-chipping and corner rounding is relatively serious under jet cold air condition. Therefore, the tool is more occur to wear than the tool under dry cutting condition.
3.2 Cutting force
The magnitude and fluctuation of cutting force directly reflect the machining state and affect the surface quality [21, 22]. In the hole processing on CFRP, the cutting force directly affects the hole quality, which is the main factor causing the material delamination around the hole wall, the tearing of the entrance and exit of the hole, the burr and other major defects . The resultant force is collected as shown in Eq. (1).
The cutting force obtained after processing the data is shown in Fig. 5.
Figure 5 shows the cutting force created under dry cutting condition and jet cold air condition. It can be seen from Fig. 4 that the cutting force under jet cold air condition is larger than that under dry cutting condition. This is because the matrix material of CFRP is a kind of resins, which determines the final performances of CFRP, such as structural strength and structural stiffness. The resin becomes harder after low temperature cooling, so that the strength of CFRP increase with the decrease of temperature. It will lead to the increase of cutting forces.
Among many machining defects, delamination is one that has a fatal effect on the hole quality [24, 25]. It refers to the CFRP interlayer stress or manufacturing defects caused by delamination between the composite layer separation failure phenomenon. It causes a decrease in the tensile strength of the CFRP laminates. Under the working conditions of alternating fatigue load, delamination further expands and it will ultimately lead to early termination of service life of CFRP components. The delamination diagram is shown in Fig. 6.
In the process of helical milling, when the tool begins to contact with the workpiece material and the main cutting edge is not fully cut into the workpiece material, the cutting force pushes the removed material into the helical groove. These materials rise along the helical groove surface before cutting, resulting in an upward peeling force. The peeling force separates the unresected region in the upper layer, namely the peeling delamination. On the other hand, when the tool is about to cut out the material, because the number of remaining uncut layers of the material is less and less, if the cutting force exceeds the interlayer bonding strength of the material, debonding occurs between the layers around the exit, resulting in push-out delamination. The delamination occurs in the interlayer region, so it not only depends on the properties of the fiber, but also depends on the properties of the resin . CFRP laminates are delaminated on both the entrance side and the exit side in helical milling. In this study, the entrance side is focused and studied considering the peeling forces.
The ratio of the maximum stratified diameter Dmax to the nominal diameter Dnorm of the hole Fd is taken as the standard to measure the delamination degree , namely the diameter delamination factor, which is referred to as the delamination factor Fd. The formula is shown in Eq. (2).
After the experiment, the diameter of the hole processing under two cutting conditions is measured. The delamination factor obtained after processing the data is shown in Fig. 7.
Figure 7 is the delamination factor of each group under dry cutting condition and jet cold air condition. It can be seen from Figure 7 that the delamination factor of hole processing under jet cold air condition is smaller than that under dry cutting condition, indicating that the delamination phenomenon at the hole inlet is effectively suppressed under jet cold air condition. Under jet cold air condition, due to the large cutting force, the material can be more completely removed and pushed into the helical groove resulting a smaller peeling force than that under dry cutting condition. In addition, under jet cold air condition, the fiber extrusion in the helical milling process is inhibited due to the low temperature. The extension distance of the fiber crack in the axial and radial directions is reduced. Therefore, the hole surface is smoother and the delamination factor is smaller.
The delamination factor of the sixteenth hole is the largest with the cutting parameters of n = 20000 rpm, fza = 0.08 mm/z, ap = 0.2 mm. The delamination factor is 1.145 and 1.148 under jet cold air condition and under dry cutting condition, respectively. The fifth hole has the smallest delamination factor with the cutting parameters of n = 12000 rpm, fza = 0.02 mm/z, ap = 0.4 mm. The delamination factor is 1.128 and 1.130 under jet cold air condition and under dry cutting condition, respectively.
3.4 Surface morphology
Surface morphology is a key factor affecting the performances and reliability of materials, which can further reflect the processing quality of workpiece . Based on the analysis in chapter 3.3, the delamination factor of the sixteenth and the fifth holes are the largest and the smallest, respectively. The surface morphology of the corresponding processing hole is shown in Fig. 8 and Fig. 9.
After the experiment, no obvious surface fiber burrs and tears occurred on the entrance surface of the processing holes under two cutting conditions. However, it can be observed in Fig. 8a that there is chip dust adsorption on the surface at the outlet of the processing hole under dry cutting condition. Fig. 8 and Fig. 9 show that the processing holes surfaces under jet cold air condition is smoother than that under dry cutting condition. The reason is that low temperature cooling improves the strength of the resin and the brittleness of the fiber, thereby makes the surface of the CFRP material smoother. Therefore, the hole quality under jet cold air condition is better than that under dry cutting condition.