Page 46 - MSAM-4-2

Materials Science in Additive Manufacturing Measuring the porosity of AM components

are of -1.71% to 0.44%, which is significantly lower than constant apparent part densities. The deviations from the
those determined by Llanos, for example. However, the reference density are of maximum 0.06 g/cm³ (0.99%),
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measured values are difficult to compare, as Llanos used resulting in open porosity values of 0.72 – 1.05%. The
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a micrograph analysis and therefore only measured in density values determined are therefore always slightly
one sectional plane. Negative porosity is also not possible below the reference value from the material manufacturer,
and results mathematically from the fact that some of the but are in good agreement with the other literature. 28-30
determined densities are higher than the reference density Deviations from the reference value can occur, for example,
of 6.088 g/cm³ determined by the material manufacturer. due to different printing or cleaning settings, but are
The deviations between the density values are relatively generally low. This also applies to deviations between the
small and can occur, for example, due to the use of analyzed component geometries, which are marginal here.
different measurement methods. According to the material Total porosity values of 0.13 – 1.69% (∥) and 0.26 –
manufacturer, the reference density of the sintered sample 0.84% (⊥) were determined by micrograph analyses, which
was determined by means of buoyancy measurement lead back to true part densities of 5.99 – 6.08 g/cm³ (∥)
according to Archimedes method, which generally leads and 6.04–6.07 g/cm³ (⊥). This validates the reference and
to slightly lower density values than with gas pycnometer
measurements. If one looks at the microscopic images of buoyancy measurements very well and also agrees with
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the analyzed samples (Figure 22), one can also clearly see the results of Llanos and Suominen et al. The values
the presence of pores, which lead to porosities of more than determined with the gas pycnometer are slightly higher
0%. The gas pycnometer was therefore used to measure than the values determined by analyzing the micrograph,
the true density of the sintered ZrO components, which but also take the entire component into account. During
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is slightly higher than the reference density measured the micrograph analyses, it was noticed that there were
according to Archimedes method or the density determined isolated cracks inside the components. Crack formation
in the literature using this measurement method. The is a known problem when sintering powder components
measurement results also show a geometry dependency. and is caused by residual stresses that counteract
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However, this is relatively low in absolute terms, probably component shrinkage. The cracks can sometimes affect
attributed to the sintering process. Nevertheless, it can also the optical determination of porosity and falsify individual
be seen that the more complex the component geometry measurements, leading to higher porosities or lower
and the larger the resulting component surface, the higher apparent densities. This can also contribute to the small
the density – in other words, the opposite behavior to differences between apparent and true density or between
that of the green parts made of ZrO . It can therefore be reference, buoyancy, and micrograph measurements and
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assumed that a larger component surface area leads to a the gas pycnometer measurements. In the analyses carried
better sintering process and a higher density. out in this work, corresponding cracks were detected twice
in the investigated areas and excluded from the porosity
Measurements using the Archimedes method itself calculation in the best possible way on the software
on the components analyzed in this study show relatively side. Influences due to the component geometry are not
considered here for procedural reasons.
A B
4.2. Comparison of density and porosity
determination methods used
A comparison of the three analyzed density and porosity
measurement methods – gas pycnometry, the automated
Archimedes method, and micrograph analysis – is initially
made on the basis of published literature and newly
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defined criteria, which are listed in Table 8.
In practice, non-destructive, highly accurate, highly
repeatable, and reproducible measurements are usually
important. In principle, gas pycnometry and Archimedes’
method are non-destructive. In micrograph analysis,
smaller samples have to be removed from the components
to be analyzed, an action that usually destroys the
Figure 22. Micrographs and binary images of the LCM ceramic component. With regard to measurement accuracy,
components made of ZrO . (A) Section parallel to the build direction;
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(B) section perpendicular to the build direction. Scale bar: 500 μm Figure 8 compares the density measurement results of
Abbreviation: LCM: Lithography-based ceramic manufacturing gas pycnometry and the automated Archimedes method
Volume 4 Issue 2 (2025) 20 doi: 10.36922/MSAM025090010