# Dense Pure Tungsten Fabricated by Selective Laser Melting
## 3 Results  
### Polyhedral and Spherical Tungsten Powders  
As shown in Figure 3a, the initial tungsten powders exhibited polyhedron morphologies with tetragonal and hexagonal faces, which we defined as polyhedral powders in this study. The irregular shape and agglomeration of these polyhedral powders resulted in relatively poor fluidity, with a measured flow rate of 15 s/50 g, and a packing density of 5.39 g/cm\({}^{3}\), which is only 28% of dense tungsten. The spherical tungsten powders obtained after the spheroidization process are depicted in Figure 3b. As pointed out by the white arrows, some of the powders have pore defects which were caused by the incomplete melting during the spheroidization process. It should be noted that there is no agglomeration in spherical powders, and as a result, the packing density increases to 11.0 g/cm\({}^{3}\), double that of the polyhedral powders. Furthermore, the fluidity increases to 6 s/50 g, which leads to a smooth and dense powder layer during the SLM process. As shown in Figure 4, the median diameter of powders decreases from 36 \(\upmu\)m to 28 \(\upmu\)m after the spheroidization process because of the surface shrinkage. According to the larger slope of the red line in Figure 4, it can be seen that the spherical powders have more intensive size distribution, which can be attributed to the vaporizing of fine particles in the spheroidization process. The content of the main impurities of the two powders are listed in Table 1 from which it can be seen that the spherical powders have the same impurity level as the raw powders.  
Figure 3: Scanning electron microscopy (SEM) images showing the morphologies of (**a**) polyhedral and (**b**) spherical tungsten powders; and SEM images showing the top surface of tungsten parts fabricated using (**c**) polyhedral and (**d**) spherical powders.  
These two powders demonstrate different forming properties after selective laser melting. After optimization of the SLM process, the best exposure time was determined to be 350 us for the polyhedral powder. However, as the molten tracks broke easily for this exposure time, balling still occurred and resulted in large defects, with many pores visible in the top surface of the tungsten parts (see Figure 3c). As a result, when the next powder layer is deposited, powders trapped in these pores cause the rolled powder layer to be uneven, leading to a poor forming quality of subsequent layers. The best density achieved with optimized parameters for polyhedral powders was 84% TD. In contrast, when a laser of the same power (200 W) was applied to spherical powders, continuous tracks could be formed (as seen in Figure 3d), and an 88% TD was achieved. The best exposure time for spherical powders was determined to be 250 us, which is shorter than that of polyhedral powders, and taken as further evidence of their better forming properties. The surface arc-shape ripple clearly indicates the laser scanning direction of every molten track. However, as may be seen in Figure 3d, the molten pool tail is shorter than those of common metal materials fabricated by SLM [12], which is caused by the relatively high viscosity of the molten pool. This high viscosity implies that the melt pool temperature after the laser exposure was insufficiently high, which contributes to balling effects. It is also observed in the same figure that the molten tracks did not overlap well, leaving some pores between adjacent tracks. The above results indicate that a 200 W laser is insufficient to achieve dense tungsten.