# Dense Pure Tungsten Fabricated by Selective Laser Melting
## 3 Results
### Parameters Optimization and Surface Morphologies Evolution  
The surface morphologies of specimens fabricated with different laser powers and exposure times are depicted in Figure 5. Three main parameter windows can be defined according to different surface finishing; namely, the dense zone, the continuous zone, and the balling zone. The dense zone is defined as the parameter window which yields parts with a density greater than 90% TD. From Figure 5, it is seen that a laser power of at least 350 W is required to attain dense tungsten parts. The V-shape of the dense zone indicates that this processing window would become even wider with higher-powered lasers. Smooth tracks can also form in the continuous zone, but as the layers are not overlapped enough, this parameter window yields parts with lower densities than the dense zone. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Power Type** & **C** & **O** & **N** \\ \hline Polyhedral powder & 0.0066 & 0.021 & 0.0052 \\ Spherical powder & 0.0083 & 0.010 & 0.0018 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main impurity elements content in two types of powders (wt %). Figure 4: Size distribution of polyhedral and spherical powders. The balling zone is divided into two regions (i.e., Balling Zones I and II), which indicate where balling occurs for either excessive, or insufficient, exposure times (i.e., the scanning speed is too slow/fast). The parameter window defining Balling Zone I is caused by insufficient laser energy and the additive manufacturing process can be remedied by increasing the laser power. Indeed, no balling is observed, even for very short exposure times, if the laser power is above 300 W. However, balling can also appear if the laser's energy density is too high, which defines the Balling Zone II parameter window. As reported in [23], for excessive exposure durations, the large molten pool can \"swallow\" the surrounding powders, which results in larger ball defects forming than those in Balling Zone I (i.e., gaps may appear if powders lying ahead of the advancing melt are sucked into it). Figure 6 reveals the effect of the point distance (pd) and the hatch distance (hd) on the surface morphologies. In SLM, the width of the molten track should be larger than the hd in order to achieve enough overlapping of adjacent layers. Considering that the pd influences the width of the molten track, a careful choice of both the pd and hd must be made to ensure sufficient overlapping of the neighboring tracks. For example, as shown in Figure 6, when the hd is set to 125 \(\upmu\)m, the pd needs to be below 70 \(\upmu\)m to achieve densification. When the hd is set to 100 \(\upmu\)m, the measured molten track widths were 123, 109, 77, 79, and 81 \(\upmu\)m, respectively, and in this case the pd just needs to be below 90 \(\upmu\)m to produce overlapped tracks. It should be noted here that the molten tracks are mostly continuous, indicating the contribution of the powder spheroidization process. Figure 5: The SEM images showing surface morphologies of tungsten parts under different laser powers and exposure times; the point distance and hatch distance are set as 75 \(\upmu\)m and 100 \(\upmu\)m, respectively. The relation between the relative density (RD) and different combinations of point and hatch distances is shown in Figure 7. In general, RD decreases with increasing pd, and it is also seen that the slope of RD versus pd increases with larger hd, indicative of the expanded influence of pd on the densification process. For a smaller hd of 75 \(\upmu\)m, the established molten track will be excessively remolten by subsequent laser scanning, which may cause balling effects, as shown in Figure 6. Consequently, the resulting densities are not high at all scanning speeds (or point distances) and, in fact, show little change with different pd.