Page 86 - MSAM-1-3

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

has been found to achieve high packing density in other addition, as shown in Figure 9A, gas flow lines are seen
reported studies. Bai et al. demonstrated that although in only one of the samples, indicating that the possible
higher sintered density was always achieved in bimodal impact sample position might have on result variability.
distributions, different mixing ratios (1:3 – 1:6) would The truncated data contain samples only sintered at the
result in different values . same time, resulting in much lower standard deviations, as
[30]
As observed in Figure 6, a clear trend of decrease in shown in Table S1 (in Supplementary File). This is believed
density with an increase in particle size within unimodal to be a further evidence that the higher density seen with
groups can be observed. Similar results have been reported the 40 µm cylinders might be caused by the higher amount
in the literature in other metals and ceramics [2,35-37] . Finer of surface oxides. After performing data analysis on all
particles are known to enhance the sintering process due to sintered density values, the results showed no statistical
the initial necking when compared with coarser particles. difference between the 40 µm and any other unimodal
In addition, the increase in surface energy and contact area groups. In summary, the bimodal groups have a higher
associated with finer particles favors sinterability due to a density when compared to the unimodal groups.
higher particle bonding rate. In this study, density decreased In general, a higher density results in higher mechanical
with an increasing particle size up until the 40 µm group, strength because of the detrimental impact of porosity during
where a slight increase in density was observed. We attribute mechanical loading. Jost et al. also found that the ductility and
this possible outlier to variations in sintering conditions strain at ultimate tensile strength for SS316L manufactured
associated with non-uniform inert atmospheres at the tail through powder bed fusion method were negatively impacted
ends of the tube furnace (Figure 9A). Temperature data were by porosity . Our results presented in Figure 8 agree with
[40]
tracked and gathered during each of the sinter runs, which the claim that higher density (found in bimodal groups)
indicated that an identical heating profile was followed for corresponds to higher UFS. As previously mentioned, the
all samples. It has been reported that the density of SS316L 40 µm group saw a slight deviation from the observed density
metal injection molding parts was impacted by different trend within the unimodal group. Conclusions about this
gas atmospheres as well as other sintering factors . Other phenomenon rely on the possibility of surface porosity or
[38]
papers have discussed the effects of the sintering atmosphere open porosity present with that group. Because the sintered
on SS316L . As tube furnaces only have a relatively small density was calculated using physical measurements, the
[39]
hot zone, any minor differences in the precise placement of bulk volume (including open and closed pores) was used.
the samples in the alumina crucible could have caused this Future investigations should rely on more advanced sintered
observation. Future studies will include real-time monitoring density measurements, such as the Archimedes method or He
of gas flow rate and development of a jig to precisely repeat pycnometry, for more precise measurement.
the placement of sintering samples within the crucible.
The surface porosity might have resulted in higher
During the sintering stage of a binder jetting process, calculated density but lower strength. Reports of
off-gassing takes place as the binder is evaporated from surface defects and roughness’s impact on mechanical
the part (debinding). A higher gas flow rate might be performance are highlighted in Masuo et al.’s work .
[41]
beneficial for uniformly carrying these chemicals out of Overall, no statistical difference was found between
the furnace chamber and away from the part. As shown unimodal groups. The increase in UFS of bimodal particle
in Figure 9, the differences between sintered runs of the size group when compared to unimodal groups is highly
same particle group have different colorations, indicating statically significant. Mechanical values presented with
a slight difference in the gas atmosphere or flow rate. In this work can be compared with a literature investigation
A where Shrestha et al. reported a maximum UFS of 90.10
MPa using binder jetting SS316L with similar sintering
profile with only unimodal distributions.
Finally, the relative density (density calculated/
theoretical density) was low (the highest sintered density
B achieved was around 72%) when compared to densities
achieved by other studies. Based on the literature reports,
sintered density in binder jetting has in some cases reached
around 95% depending on the specific material and the
process parameters used. A reduction in density may be a
Figure 9. Different sinter runs for the same particle size group. *Top right consequence of insufficient sintering conditions, such as
bar corresponds to a different group. low sintered temperatures used and possible part expansion

Volume 1 Issue 3 (2022) 8 https://doi.org/10.18063/msam.v1i3.20