Page 146 - MSAM-4-2

Materials Science in Additive Manufacturing Bi-modal powder spreading behavior of ceramics

10 µm powder shows the most significant improvement in volumes, three in representative x positions and three
flowability (17%) when exposed to elevated temperatures, in representative y positions, were set up. The powder
with its flowability at elevated temperatures comparable sizes and their count were recorded from these sampling
to that of the 20 µm powder at room temperature. The volumes. Using Equation III, the average particle size,
elevated temperature generally improved the Hausner akin to D(50), was calculated for each of the sampling
ratio for smaller powder, indicating better flowability. volumes. The simulation was performed for 5 µm, 20 µm,
and bimodal powders. The simulations were repeated three
Similarly, Carr’s index for 1 µm and 5 µm powders
indicates they have poor flowability, 67,68 which agrees times to capture the random generation of the particles
with the Hausner ratio. 67,68 Except for the 1 µm powder, and the spreading influence on the particles. For the 5 µm
all the other powders showed notable changes in Carr’s and 20 µm powders, the average particle size in both
index, indicating the noteworthy influence of elevated the spreading direction and the direction perpendicular
temperature. While measuring the angle of repose to the spreading direction remains constant. One-way
(Figures 3B and 4), the 10 µm and 20 µm powders showed analysis of variance (ANOVA) at a significance level of 5%
a dramatic increase (66.2% for 10 µm and 90.3% for 20 µm (α = 0.05) revealed that there is no statistically significant
powder) in the angle of repose from the drop in powder variation (p>0.75) in either the spreading direction or the
temperature. On the other hand, 5 µm powder increased perpendicular to the spreading direction.
by around 16% when the temperature dropped to room However, for the bimodal powder, the smaller particles
temperature. Furthermore, the 1 µm powder did not even were predominantly deposited at the beginning of the
flow at room temperature. A close inspection showed high spreading process along the spreading direction, whereas
agglomeration in 1 µm powder, which hindered the flow. larger particles accumulated toward the end. The results
In addition, the relative density was calculated for the are shown in Figure 5. A one-way ANOVA at a significance
powders, which showed that elevated temperature leads level of 5% (α = 0.05) showed that the variation in the
to higher relative density for all the powders. However, average particle size is statistically significant (p=0.001) in
the smaller powders showed significant improvement in the spreading direction. However, the variation in powder
relative density, with the 1 µm showing a 25% increase size perpendicular to the spreading direction was not
with elevated temperature. While larger powder had higher statistically significant (p=0.777). This indicates that, for
relative density, 20 µm powder showed a large amount of bimodal powders, the powder bed does not have a uniform
variation at room temperature. Finally, a loss on ignition test powder size distribution, and there is a directionality in
revealed that the smaller size powders lost more moisture the change along the spreading directions. The results are
content compared to the larger powder, which indicates the shown in Figure 5A.
presence of higher water content in the smaller powders. Following the simulations, spreading experiments
Moisture increases the cohesion between the powder on the machine were conducted for 5 µm, 20 µm, and
particles, which can be attributed to higher agglomeration bimodal powders to collect powder from the bed. The
and poor flowability of the smaller powder size. 65,69 Overall, powders collected from different X locations (start, middle,
flowability and the deposition of smaller powders can be and end) were analyzed for the PSD. The result showed a
improved at elevated temperatures, resulting in better trend of depositing smaller particles at the beginning of the
flowability and higher powder bed density. powder bed and larger particles at the end of the powder
bed. This phenomenon was more prominent for the 20 µm
3.2. Powder bed generation and powder powder than the 5 µm powder. The bimodal powder also
segregation showed a similar trend. Figure 5 shows the trend for D(50)
The DEM simulation was performed following the study of the powder size distribution. In addition, a one-way
conducted by Shahed and Manogharan. Six sampling ANOVA was performed to see if the difference in powder
6
A B C D

Figure 4. Change in angle of repose in 10 µm powder at (A) elevated temperature (120°C), (B) room temperature, and for 20 µm powder at (C) elevated
temperature (120°C) and (D) room temperature

Volume 4 Issue 2 (2025) 7 doi: 10.36922/MSAM02510016