# Investigation of Ti65 Powder Spreading Behavior in Multi-Layer Laser Powder Bed Fusion
## 4 Results and Discussion
### Mechanism Analysis  
The height profiles of the powder beds across Zones 1, 2, and 3 under different spreading velocities are shown in Figure 15. Overall, the height of the powder bed generally exhibits a decreasing trend along the spreading direction, with Zone 3 exhibiting a noticeably lower height compared to the other two zones. Additionally, as the spreading velocity increases, the height profiles of the three zones exhibit distinct variations, with a general decreasing trend along the spreading direction. To explore the underlying mechanisms of this phenomenon, particle movements are analyzed in these three zones. As illustrated in Figure 16a, three local zones (denoted as Zones A1-A3) are defined for detailed analysis. Zone A1 and Zone A3 have identical dimensions (500 \(\upmu\)m \(\times\) 1000 \(\upmu\)m \(\times\) 300 \(\upmu\)m), while Zone 2 (printed region) spans 1000 \(\upmu\)m \(\times\) 1000 \(\upmu\)m \(\times\) 300 \(\upmu\)m. The temporal distribution of particle velocity is shown in Figure 16b-d, where arrows represent the velocity vector and color denotes the velocity magnitude. At \(t\) = 0.079 s, particles in Zone A1 accumulate along the spreading direction due to the obstruction of the  
Figure 14: CN (**a**) and pore size distribution of different zones (**b**) in powder layer 6 when \(V\) = 0.075 m/s. Figure 13: Packing density (**a**), \(Ra\) (**b**), coefficient of variation (**c**), and \(H_{\text{real}}\) (**d**) of different zones from layer 1 to layer 6 when \(V\) = 0.075 m/s. printed region, which induced the formation of a low-velocity area at the interface between Zones A1 and A2. With the advancement of the blade, more particles are accumulated at the interface, which results in an increase in the powder height and packing density. Meanwhile, the particles located in the low-velocity area move upward and then fall to form the dense local structure. By \(t=0.091\) s, the blade has passed through Zone A1; although the particles settle slightly, the previously accumulated particles maintain higher packing densities in both the interface and the leading edge of Zone A2. For Zone A2, particles are directed onto the surface of the printed region by the advancing blade. Their motion is further restricted by the geometry of the underlying printed region, promoting the formation of a densely packed region. At the rear of Zone A2, some particles overflow into Zone A3, contributing to the formation of a pit in the powder bed. In Zone A3, the particles near the A2-A3 interface are displaced outward from the printed region. This leads to the formation of voids due to insufficient particle supply. Even though some particles do migrate to partially fill the voids, a large portion is transported downstream, ultimately resulting in a loosely packed, low-density structure in Zone A3. This issue can be mitigated by reducing the spreading velocity as the blade approaches Zone A3, thereby allowing more time for particle deposition. To further reveal the particle motion behavior, Zone A4 was set in front of the blade (300 \(\upmu\)m \(\times\) 1000 \(\upmu\)m \(\times\) 300 \(\upmu\)m). Figure 17a gives the evolution of average contact force (\(\overline{F}\)) of particles in front of the blade, and Figure 17b,c illustrate the particle motion in the low-velocity area and around the voids, as identified in Figure 16d. As shown, in Zone A1, \(\overline{F}\) gradually increases and reaches its peak at the interface between Zones A1 and A2, implying that the particles at this area experience confinement by the strong contact  
Figure 16: (**a**) Morphology of powder bed at layer 6 across three subzones when \(V=0.075\) m/s; (**b–d**) evolutions of particle velocity in Zones A1, A2 and A3, respectively. Figure 15: Height distributions of different powder zones obtained from different \(V\). forces. Figure 17b further confirms that particle displacements within the low-velocity area are minimal, resulting in a highly compact powder bed due to the confinement.