Page 85 - MSAM-1-3

Materials Science in Additive Manufacturing Increasing density and strength in binder jetting

The Ryan-Joiner test also indicated that the UFS However, when both the truncated and full datasets
data were not normally distributed. The Kruskal–Wallis were compared, as shown in Figure 8 and Figure S1
(α = 0.05) non-parametric test was performed. It was (in Supplementary File), a similar trend can be seen but
found that both bimodal groups were statistically higher with different statistical significance.
than the 30 µm and 40 µm group, as shown in Figure 8. According to our previously reported MD simulations,
Due to the size limitations of the tube furnaces used for the fine powders filling the interparticle voids can
sintering, only three bars were sintered at a time. It was contribute extra bonds to connecting the coarse particles
suspected that variation in sintering conditions both after sintering . As a result, the rupture strengths are
[26]
within and across particle groups could be present, as enhanced in the bimodal particle mixture than in the
shown in Table 4. This may be an influencing factor on unimodal particle size mixture. It should be noted that the
why statistical differences between some of the unimodal rupture strength observed in the ReaxFF-MD simulations
and bimodal groups were not observed, despite the 168.3% was improved by 40.9%, which was much lower than
and 173.4% increase in UFS from the 10 µm to the 30 + 10 the >170% measured improvements demonstrated in the
and 40 + 10 µm groups, respectively. Results gathered from 3-point bending tests. This could be attributed to the lower
samples that were sintered in the same batch were used to number of fine particles in the MD simulation.
see the impact different sintered runs had on the UFS. As
shown in Table S1 (in Supplementary File), analysis of UFS 5. Discussion
of particle size groups sintered in the same batch resulted
in lower relative standard deviation (RSD). This indicates Based on the findings from this study, it can be observed
significant variations in strength between sintered groups. that bimodal distribution feedstock achieves higher density
and mechanical properties for SS316L. Our work showed
a statistically significant increase in sintered density with
the introduction of bimodal groups when compared to
unimodal distributions, which is in agreement with prior
reports [4,19,20] . The improvement in density after sintering
conditions could be attributed to better filling effects
that could result in more contact and necking, which is
beneficial for sintering and shrinkage reduction. To the
best of our knowledge, this is the first study that reports
a statistically significant increase of more than 20% in
sintered density in the bimodal particle size distribution.
The increase in density observed with our bimodal
results aligns with Du et al.’s findings when studying
silicon carbide in which a 5% increase in bimodal green
Figure 7. Relationship and trend between all particle groups with respect density was observed when compared with the unimodal
[25]
to X-ray computed tomography, caliper measured (individual) density, distributions . Du et al. analyzed different coarse powder
and caliper measured (average) density. fractions (%) for bimodal mixing and found the highest
density values of 70 – 75% coarse particles in the mixture,
which is comparable to the 73% used in this study. The
appropriate selection of a coarse-to-fine ratio has been
shown to impact part density. The ratio used in this study
Table 4. Summary results across particle groups
Group Sintered density (%) Ultimate flexural strength (MPa)
Mean Std. RSD Mean Std. RSD
10 µm 59.77 1.98 3.31% 117.50 74.20 63.15%
20 µm 54.61 2.12 3.88% 96.35 9.81 10.18%
30 µm 53.76 1.43 2.66% 62.80 5.78 9.20%
40 µm 57.40 2.33 4.06% 36.80 30.80 83.70%
30+10 µm 71.83 2.47 3.44% 315.30 59.05 18.74%
Figure 8. Ultimate flexural strength (UFS) shows that bimodal particles
have higher flexural strength. *P = 0.01 – 0.05 and **P < 0.01. 40+10 µm 71.57 2.36 3.30% 321.30 61.80 19.02%

Volume 1 Issue 3 (2022) 7 https://doi.org/10.18063/msam.v1i3.20