# Fluctuations of Tracks and Layers during Aluminium Laser Powder-Bed Fusion
## 4 Discussion
### Fluctuations within One Layer  
The cross-sectional views of the built structures (Figure 9) confirmed that the single tracks show large depths compared to the added material, which shows that the energy input is large compared to the desired volume to melt, which is a typical trend for LPBF processes to use conservative process parameters to avoid defects [27]. Similar trends were seen in [18], where volume elements were seen to be melted several times, which requires a lot of energy but creates no additional value, since material is not added.  
It was observed that fluctuations occur in track appearance within one layer, as was also documented in [20] or [28], where variations of the track depths were observed. The simulation results showed that variations of available powder occur due to the varying powder available for the single tracks. An alternating characteristic was found (Figure 5) in the observed modelled tracks within one layer. Since the model is based on assumptions about powder availability and powder pick-up from neighboring sections, the varying availability of powder can explain the observed variations. This confirms that the powder availability model predicts the material availability well.  
At higher layers, the simulation overestimates the material addition compared to the experimentally measured ones for all hatch distances (Figure 11), while when building only a few tracks, the simulation prediction and experiments are similar or even the experimental material usage is slightly higher. The deviations are possibly related to effects that were not considered in the simple model: there is, e.g., a potential powder particle loss during the processing by induced gas flows [29] from the vapor plume [22]. Additionally, scattering can happen, which is induced by the melt pool dynamics [30] or resonant melt pool waves [31], denoting material losses. High-speed images could show that non-melted particles and agglomerations of powder particles are ejected from the processing zone [20]. This effect, in combination with the powder pick-up from neighboring sections, leads to the denudation zone and reduces the amount of available powder for the subsequent tracks.  
The amplitudes of the fluctuations vary within one layer. When less space above the previous track was available, the layer became smoother. This observation is another hint that the denudation effect is different for each track. The first layer shows quite different boundary conditions compared to the subsequent layers. In the first layer, the first track sees a comparably high amount of powder, since no denudation zone was created before the processing (Figure 5). However, the track heights within the layer show large fluctuations in the simulations (Figure 5) and experiments (Figure 13) compared to higher layers. The denudation effects seem to have a larger impact on the powder availability and  
Figure 13: (**a**) Cross section of a \(10\times 5\) structure (hat distance h = 80 \(\upmu\)m) and (**b**) derived distances of the lowest point of the single tracks measured from the zero line.  
lead to more variations. In higher layers, the track height variations within one layer were reduced (Figure 5). The environment consisting of powder seems to support the powder to remain closer to the processing zone. Also, the observation that, at higher hatch distances, the powder availability is in general higher (Figure 7), indicates that the effect of particles that are blown away by induced gas flows is the minor effect compared to the powder pick-up from neighboring sections.