# Fluctuations of Tracks and Layers during Aluminium Laser Powder-Bed Fusion
## 4 Discussion
### Fluctuations between Layers  
In theory, the machine movement providing a constant stepwise movement of a defined distance before applying the next powder layer provides always the same powder layer thickness. However, in reality, the powder layer thicknesses vary in different layers. Only the first layer sees the programmed value, while due to denonidation effects and densification, the subsequent powder layer thicknesses show larger heights (Figure 6). This might be a reason why LPBF machines use quite conservative parameters to ensure the melting of the tracks to the structure even when higher amounts of powder must be processed [27]. The knowledge about the process behavior can help to reduce the energy input where, e.g., less powder is available and the energy input would just mainly remelt the previous structure. The observed layer fluctuations (Figure 5) show higher track additions in the uneven layers compared to the even-numbered layers, which was seen in the simulation and experimental results. Possible reasons can be the varying powder amount above the previous layer or surface tension effects that locally deform the structure compared to the expected structure. A comparison between simulated and experimentally measured structure heights in different layers (Figure 14) shows that the model predictions of the layer material additions are quite close to the experimental values. At large hatch distances (h = 100 \(\upmu\)m and h = 120 \(\upmu\)m), the simulation predicts slightly larger structure heights, which can be explained by the previous discussion about additional effects that occur in reality and were not considered in the model. Considering those effects would make the model more precise. However, the quantitative values of some of these effects are not yet known, e.g., the amount of material loss due to spattering. An implementation of those effects requires additional assumptions, which might reduce the possibility of finding possible explanations for the observed effects when the single impacts of too many parameters are present. The large variations of the structure height are reflected as large error bars in Figure 14. When building higher layers, the surface tension effect leads to an accumulation of material around the edges after the beginning and before the end of the layers. This leads to larger heights and a minimum in the center. However, also the very first track (Figure 15a) sees a large amount of powder, which is available for incorporation into the structure, which increases the already extensively grown structure size. Therefore, this geometrical effect can alter the powder availability for the tracks and layers in addition to the denonidation effects discussed before. Figure 15b shows that for the second (even) numbered tracks, less material is available and mainly the previous structure is remelted, which can support the formation of the surface tension-driven round structure, while only a minimum of material is available. Less powder will be available at the high-structure regions and more powder will be available in the structure center. There, the risk of incomplete melting can occur since the laser energy might not be sufficient anymore to melt the available powder to the structure. Interestingly, however, the lower surface in the center of the structure indicates that there was not much powder material available for building those tracks. In Figure 15c, slightly more powder is available compared to Figure 15b. However, the denonidation effects reduce the amount of powder already before processing. Therefore, the powder availability is limited and the structure will remain low in the center region even though the material was available in the beginning of the layer processing. An additional problem to consider is that the laser beam is not focused as expected when hitting the actual powder surface.