## 4 Results and Discussion  
### Formation Mechanism of Edge Defects  
The main objective of the finite element (FE) analysis of turning 17 vol.% SiC\({}_{\text{p}}\)/2009 Al composites was to study the formation mechanism of edge defects, so as to provide theory and process guidance for the machining of 17 vol.% SiC\({}_{\text{p}}\)/2009 Al composites in order to reduce the edge defects at the exit end as much as possible. In terms of the distribution law of cutting-induced stress and the change of the chip morphology characteristics, the formation process of the edge defects was divided into eight stages, and they can explain the process very well. The first stage is the initial contact between the turning tool and chips as the turning tool cuts into the workpiece (shown in Figure 4a); it demonstrates a continuous cutting process, accompanied with the formation of non-continuous chips through shear deformation according to [PERSON] stress distribution, and the shear deformation zone can be obviously seen, where \(\varPhi\) is the shear angle and \(T_{0}\) the undeformed chip thickness. As the turning tool moves, the elastic deformation zone (less than the yield strength of 17 vol.% SiC\({}_{\text{p}}\)/2009 Al composites) appears at the workpiece edge due to the elastic bending, and meanwhile the plastic deformation zone around the shear zone was found to be propagated towards the edge at the exit end, as shown in Figure 4b,c. The figure shows that the plastic deformation zone extended near the workpiece edge and the plastic deformation zone near the shear zone continued to move forward and it moved towards connecting with the plastic deformation zone around the edge at the exit end. Figure 4d shows the critical transition stage: in the undeformed chip layer there was a stress transition zone where plastic deformation just arrived at the exit end, and at this time the distance between the tool nose point A and the workpiece exit end is thought to be a critical distance \(\omega\). The stress transition zone transferred the maximum equivalent stress to the edge at the exit end under the cutting path and impeded the formation of non-continuous chips, especially for hard brittle materials. Figure 4e shows the flexure deformation stage, and cutting shear action induced flexure deformation at the pivot point \(O\). As the cutting proceeded, the location of pivot point gradually moved toward the cutting path. The stress concentration zone under the cutting path is also defined as the negative shear deformation zone, and the angle \(\psi\) between the cutting path and the connecting line from the point \(O\) to tool nose is called the negative shear angle. Figure 4f shows the stage of end crack initiation: as the cutting-induced stress level exceeded the tensile yield strength of brittle SiC\({}_{\text{p}}\)/2009 Al composites, the cracks initiated near the tool nose, where the internal high-density dislocation occurred, and promoted the propagation of cracks into the negative shear deformation zone and grew towards the pivot point \(O\). Figure 4g shows the stage of crack propagation along the negative shear angle: the crack propagation toward the pivot point \(O\) occurred. Additionally, a great many micro cracks under the mixed tensile and shear modes initiated inside the negative shear deformation zone, and these micro cracks further grew and coalesced under mixed loadings of cutting. At the same time, the edge at the workpiece exit \begin{table}
\begin{tabular}{c c} \hline \hline
**Items** & **Contents** \\ \hline
**Tools** & \\ Insert material & Polycrystalline diamond (PCD CTH025) \\ Grain size, & 25 \(\upmu\)m \\ Tool nose radius & 0.1 mm \\ Rake angle & 0\({}^{\circ}\) \\ Clearance angle & 10\({}^{\circ}\) \\ Inclination angle & 10\({}^{\circ}\) \\ \hline
**Machining conditions** & \\ Depth of cut (mm) & 1, 1.5, 2, 2.5, 3 \\ Feed rate (mm/min) & 97, 171, 230, 295, 389 \\ Rotational speed (r/min) & 160, 240, 350, 500, 720 \\ Machining environment & Dry \\ \hline \hline \end{tabular}
\end{table}