# Investigation of Ti65 Powder Spreading Behavior in Multi-Layer Laser Powder Bed Fusion
## 4 Results and Discussion
### Variation in Powder Bed Quality over Successive Layers
#### 4.1.1 Macroscopic Properties  
The packing density, defined as the volume of powder particles divided by the volume of the powder bed [38], is a key metric for evaluating the powder bed quality. Figure 7a presents the evolutions of packing densities of powder beds over printed regions at different layer heights. The packing densities in the first two printed layers are extremely low (below 0.10) but increase rapidly from the third layer onward, eventually stabilizing as the number of printed layers increases. Additionally, for a given printed region, an increase in spreading velocity leads to a reduction in packing density.  
In addition to the packing density, the surface roughness \(Ra\) is another important index to evaluate the powder bed quality [19]. As shown in Figure 7b, the \(Ra\) values for the first and second layers are notably high but gradually decrease and stabilize as more layers are added. A reduction in spreading velocity can significantly reduce surface roughness.  
The trends observed in both packing density and surface roughness are in good agreement with previous studies involving other materials [16]. The poor quality of the first two powder layers is primarily attributed to the low \(H_{\text{set}}\), which limits the amount of powder released for powder bed formation. As the number of layers increases, the height of the previously deposited powder bed rises, mitigating boundary effects and improving packing. This indicates that the powder bed characteristics after the third layer can better represent the powder bed quality during the actual printing process, and can provide more effective guidance and optimization for the real AM operations. Moreover, slower spreading velocities provide more time for particle rearrangement and void filling, thereby improving the powder bed quality.  
In a practical 3D printing process, the uniformity of the powder bed, characterized by the coefficient of variation (\(CV\)), is also a critical quality metric. As shown in Figure 8a, the variation in \(CV\) across successive powder beds reveals that, in addition to the powder beds in the first two layered printed regions, the \(CV\) reaches higher values at the third layer and then gradually decreases. The abnormal low values of \(CV\) in the first two layered powder beds can be ascribed to the insufficient powders involved; thus, using incomplete powder beds caused by boundary effects should be avoided during printing. It can also be observed from Figure 8a that during multi-layer spreading, decreasing spreading velocity can lower the \(CV\) value so that the powder bed quality can be improved.  
Figure 7: Packing density (**a**) and \(Ra\) (**b**) of the powder bed across successive layers at different spreading velocities.  
In addition, the height of the spread bed, \(H_{\text{real}}\), defined as the distance between the bottom of the blade and the average height of the printed region [16], is also analyzed and presented in Figure 8b. It is observed that \(H_{\text{real}}\) gradually increases with layer number and eventually stabilizes. Notably, reducing the spreading velocity results in a lower stable \(H_{\text{real}}\). As the number of printed layers increases, the density of the powder bed gradually improves, causing the height of the printed region to increase (where the limit of the height of the printed region is \(H_{\text{set}}\)). However, the rate of increase in height reduces with each layer. On the other hand, the decrease in spreading velocity increases the packing density of the powder bed so that the height of the printed region increases, which leads to a decrease in \(H_{\text{real}}\).