## 3 Results and Discussions
### Morphological Analysis of Untreated and Treated Samples Fabricated with FDM  
Figure 10 shows the morphology of pure PLA, PLA/untreated WSP, and PLA/NaSi treated WSP, taken from the tensile fractured surface of FDM samples. The pure PLA specimen shows a stair-step fractured surface with some cracks, indicating a full brittle fracture with some plastic features (Figure 10a). The fracture surface of PLA/untreated WSP (Figure 10b) is smoother than that of pure PLA, implying a full brittle fracture. The WSP particles in Figure 10b were partially aggregated. The micro voids in Figure 10b resulted from the pull-out of WSP particles during the tensile process. The voids and aggregated particles both revealed that there was small energy consumption during the pulling-out of the particle. The leaks between the two layers were obvious and there was no apparent deformation that happened during the tensile test. This phenomenon showed that the introduction of the WSP particles reduced the flow of polymer and lowered the ductility.  
Figure 10c-f show the effect of NaOH/KH550 on the morphology of the PLA/WSP biocomposite samples. It can be observed that the number and the size of the voids and defects decreased remarkably with the rise of the KH550 content. In Figure 10c, the surface of voids is rougher than those in Figure 10b. The matrix around the voids was extended when the WSP particles were pulled out. There was a small gap between particles and matrix. This situation implied that there was higher energy consumption during the particles pull-out and a more interfacial compatibility between fillers and matrix. A slight deformation of leaks between layers showed enhanced plastic fluidity. With the increasing of KH550 content, the voids became smaller in Figure 10d and gradually disappeared in Figure 10e_f_; the WSP particles dispersed more evenly (Figure 10d) than those in Figure 10c; the extension degree of polymers around voids and WSP particles became stronger (Figure 10d_e_,_g_), until it dispersed on almost the whole surface (Figure 10f,h). When increasing the ratio of treated WSP, the leak in surface was dramatically decreased (Figure 10g) until it completely disappeared (Figure 10h);  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **Tensile Strength (MPa)** & **Tensile Modulus (MPa)** \\ \hline PLA/5 wt.%WSP & 52.5 \(\pm\) 0.7 & 2570.5 \(\pm\) 22.8 \\ PLA/10 wt.%WSP & 56.2 \(\pm\) 0.6 & 2080.5 \(\pm\) 10.8 \\ PLA/15 wt.%WSP & 55.3 \(\pm\) 0.3 & 2177.9 \(\pm\) 19.4 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mechanical properties of Fused Deposition Modeling (FDM) Poly (lactic acid) /Walnut shell powder (PLA/WSP) biocomposite samples.  
the voids due to the particle pull-out became very shallow; and, the WSP particles were integrate as a whole with the PLA matrix in Figure 10h, showing a good adhesion between WSP and PLA matrix. The results showed the effect of NaOH/KH550 on the improvement of the particle dispersion and the surface compatibility, meaning an enhancement of heat-resistance and mechanical properties.  
Figure 10: The scanning electron microscopy (SEM) morphology of FDM samples: (**a**) pure PLA, (**b**) PLA/untreated WSP, (**c**) PLA/2%NaSi treated WSP, (**d**) PLA/4%NaSi treated WSP, (**e**) PLA/6%NaSi treated WSP, (**f**) PLA/8%NaSi treated WSP, (**g**) PLA/10 wt.% treated WSP, and (**h**) PLA/15 wt.% treated WSP.