PSI - Issue 37

Jin Kim et al. / Procedia Structural Integrity 37 (2022) 282–291 Kim et al./ Structural Integrity Procedia 00 (2021) 000 – 000

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lower cutting speed and higher depth-of-cut for tangential direction due to intensified intermittent tool-chip interaction. Due to rapid tool wear, it was impossible to measure accurate cutting forces above 30 m/min. Especially, average tangential forces seemed to be much lower than expected due to high tool wear affecting the depth-of-cut at =60 m/min. 4.2. Cutting force results for Exp. 3 & 4 The two workpieces resulted in different cutting forces as an effect of different volume fractions and particle sizes (Fig. 3). First, the cutting force in tangential direction for 225XE was 42.3% lower than 217XG using the carbide tool, and 34.5% lower for the PCD tool. A similar trend was observed for other forces, except radial forces for 225XE using the carbide tool, which was higher than 217XG. This was due to inherent lower yield in higher particle-sized composite (El-Kady and Fathy 2014), with progressive tool wear increasing the radial cutting force by time. The real-time cutting forces were averaged for comparison. The highest cutting force reduction was in tangential direction for 217XG (60.5%) as an effect of the highest vibration amplitude for 225XE (29.5%). This was due to the larger volume fraction and particle size. 225XE showed more powder-like chips, resulting in less tool-chip interaction than the longer chips formed in UAT of 217XG. The radial and feed forces were also reduced in UAT compared to CT due to the small vibrations that cause separation with the workpiece. In general, CT using the PCD tool resulted in tangential>radial>feed cutting forces. For CT of 225XE, the tangential and feed forces using the carbide tool were lower than using the PCD tool. However, radial forces were higher, indicating more cutting forces were distributed in the radial direction for soft carbide tools that leads to lathe chattering and severe tool wear in the radial direction. 4.3. Macroscopic surface profile and roughness for Exp. 3 & 4 Fig. 4(a) shows raw (left) and contour profiles (right) of the machined surfaces for Exp. 3 and 4. After CT the 217XG specimen showed minor surface tearing compared to CT for 225XE. This is due to an increased chance of surface damage by particle dragging/rolling and pull-outs from smaller and more finely dispersed particles. However, it is found that surface damage from these does not significantly affect the overall surface profile but the evenness of feed grooves. CT using PCD tool seemed to cause more surface damage for both composites due to the hard tool surface. This led to particle fracture and forced ejection that resulted in higher cutting forces with irregular feed grooves. On the other hand, CT using soft carbide tools exhibited particle impingement rather than forced ejection. Hence the tool trajectory was maintained, although regular chattering also affected the surface irregularities. The raw surfaces after UAT for both composites showed evenly spaced vibration grooves (27.5 µm/groove) that were lower than feed grooves, indicating stable tangential vibration with minimal damping. In general, the surface after UAT showed evener feed grooves for both materials. The surface improvement from UAT for 217XG was due to less particle dragging/rolling, less formation of Built Up Edge (BUE) from tangential vibration, and surface polishing and cold working from radial and feed vibration. The UAT surface improvement for 225XE was mainly due to elimination chattering as well as increased dynamic stability.