## 3 Results and Discussion
### Microscopic Analysis  
Microscopic images of the top surface along with 3D height maps of various samples produced with different laser energy densities can be seen in Figure 18 and Figure 19, respectively. These images allow us to explain various aspects that resulted in the observed and measured surface roughness and hardness.  
Figure 16: Effect of annealing on Rockwell hardness HRC on the top surface.  
Figure 17: Effect of annealing on Rockwell hardness HRC on the side surface.  
When looking at the surfaces seen in Figure 18, it is clear that the surface of the samples produced with low LED, as in Figure 18a, exhibits a high surface roughness, as there are a lot of randomized asperities that cause an uneven surface texture, which is responsible for the high average surface roughness. The surface in Figure 18b displays more regularity and therefore a slightly lower measured surface roughness, while Figure 18c is of a surface that displays large wavy topography and large depressions and peaks at the border of scan tracks, which are also responsible for the increased surface roughness. Figure 19 goes on to support these claims; it can be seen that major depressions are formed on the surface at the locations where scan vectors change direction, namely at the border of the scanning cell. From Figure 19a,b, it is clear that that this depression is smaller when printing with a lower LED, as the melt pools formed are smaller and do not have large drilling effects on the surface. However, as can be seen in Figure 19c, when printing with a larger LED, the melt pool, especially at the edge of the scan vectors, can transition into the keyhole mode, which results in larger drilling into the surface, which agrees with what was reported in [21]. This also results in a higher surface roughness and possibly in the incidence of keyhole-induced porosity.  
Figure 20 shows microscopic images of the polished surfaces, which can provide a better idea of the surface integrity that can affect the hardness. Figure 20a shows a surface that manifested with a large number of porosities and a lack of fusion defects that were caused due to insufficient melting energy absorbed from the laser. This increased porosity was responsible for the marginally decreased hardness of the low-LED sample. In Figure 20b, we see the absence of surface defects, which explains the higher hardness of this surface, and Figure 20c also shows the absence of any major porosity, which explains the slightly higher measured hardness of the sample. Minor defects may have manifested in the form of keyhole-induced porosity [21].  
Figure 19: 3D Height maps of (**a**) low-LED test piece (0.23 J/mm), (**b**) reference LED test piece (0.33 J/mm) and (**c**) high-LED test piece (0.50 J/mm).  
Figure 18: Top surface of (**a**) low-LED test piece (0.23 J/mm), (**b**) reference LED test piece (0.33 J/mm), and (**c**) high-LED test piece (0.50 J/mm).  
The observations made above were confirmed using density measurements made by employing the Archimedes principle. Noting that annealed grade AISI 420 has a density of 7.800 g/cm\({}^{3}\), the measured density for the low-LED (0.23 J/mm) sample was 7.668 g/cm\({}^{3}\), which indicates the presence of a higher amount of porosity within the volume of the sample according to the explanations provided above, such as due to a lack of fusion pores. This increased porosity of the lower-LED samples was also responsible for the lower measured hardness.  
The high-LED (50 J/mm) sample exhibited an average density of 7.715 g/cm\({}^{3}\), which points towards the presence of lower porosity in the sample. This is also in accordance with the explanations provided above, where the reduced density may have been caused by the presence of a small number of keyhole pores. The increased density contributed to the higher hardness when compared to the lower-LED samples.