## 4 Discussion
### WIP Effect on Mechanical Properties  
From the results of the tensile and hardness tests reported in Section 3, it appears that the WIP process improves tensile strength and ductility but has little effect on hardness. [PERSON] et al. reported that WIP improved the mechanical properties by reducing the pores that occur in the polymer-based ME process and increasing the bonding area between layers or between the extruded strands of the polymers in a plane. In addition, it has been reported that the increase in the bonding force between layers improves the anisotropy that commonly occurs in parts produced by AM [41]. The effect of WIP on the mechanical properties is due to the influence of the temperature and pressure applied during the process on the flow of the polymer. The polymer flow is the reason that the authors first became interested in the WIP process. It seemed possible that, if WIP were introduced into metal ME, the polymer flow by WIP would reduce the distance between metal powders included in the filament, and a dense final part with higher tensile strength could be obtained after debinding and sintering. Figure 15 shows the fractured surface of the tensile specimen. In Figure 15a, which shows the cross-section of a tensile specimen treated with WIP, it can be seen that many pores were irregularly distributed in the interior, whereas there were relatively few pores near the outer wall. By contrast, in the fractured surface of the non-WIP specimen (Figure 15b), a large number of pores were generated along the extrusion path. The pores continuously observed along the extrusion path reached to near the outer wall. This continuous distribution of pores made it easy for cracks from the outside wall to propagate to the interior; the number and the distribution pattern of pores in the part seemed to affect the direction and speed of crack propagation during tensile testing. The distribution of pores could be affected by the WIP process. This may also explain the large difference in strain. The measured tensile strength (which was 2.5 times greater with WIP treatment) strongly aligns with this idea. As shown in Figure 15, if the pore generation along the extrusion path affects the mechanical properties, the mechanical properties can be improved by changing the infill pattern. In the case of a concentric pattern with an infill path parallel to the contour path, the pore formations with an extrusion path are perpendicular to the direction of crack propaga  
Figure 15: Sectional images of fractured surfaces (**a**) with and (**b**) without warm isostatic press. tion, which slows down the crack propagation and potentially improves the mechanical properties. [PERSON] et al. reported that the concentric pattern showed the highest strength in tensile tests with various infill patterns [38]. The introduction of the WIP process to the metal filament ME process can be expected to further enhance the effect of the infill pattern. Table 3 shows the distribution of metal powders in green parts for the two cases (with WIP and without WIP) and the area fraction of metal powders. In the \"without WIP\" case, gaps between strands are formed along the extrusion path and non-uniform powder distributions are clearly observed. However, the gaps cannot be found in the WIPed specimens. This means that the WIP process can help to eliminate the gaps between strands. In addition, the distribution uniformity of metal powders was improved as shown in Table 3. To evaluate the uniformity, ImageJ software was used. Original images were converted into 8 images, and then the area fraction of metal powders was measured. The values of the area fraction of metal powders were 26.62 and 17.16%, respectively. Although the measured area fraction could have some deviation depending on the area taken, the area fraction of metal powders clearly increases with the WIP process. The increase in the area fraction of metal powders can be obtained by measuring the density.