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Materials Science in Additive Manufacturing Crack-free AA7075 with Zr modification via LPBF

A B

Figure 3. Relative density of the LPBF AA7075 of different chemical compositions under various (A) laser power and (B) scanning speed.

Hillsboro, OR, USA) for detailed microstructure.
Transmission electron microscope (TEM) and scanning
TEM (STEM) were performed at 200 kV using a Tecnai
G2 Spirit TWIN equipped with an energy dispersive X-ray
spectrometer (EDS) to evaluate microstructure evolution,
particularly the distribution of elemental Zr. The TEM
sample was prepared by grinding and polishing a small
piece of the printed block to about 100–120 μm thickness,
and then cutting 3 mm disks using a disk punch. The disks
were ion-beam thinned for microstructural observation.
3. Results and discussion

3.1. Relative density
Relative density, the ratio of the measured density of
LPBF-fabricated parts to the theoretical density of bulk
materials, is a reflection of both cracks and porosity. Figure 4. Relative density of the LPBF AA7075 of different chemical
Figure 3 shows how the relative densities of alloys with compositions under different VED.
different compositions vary with VED. Figure 3A and B
display the influence of laser power and scanning speed,
respectively. It can be seen that the density of AA7075 is the increase of the VED. The phenomenon is similar when
sensitive to laser power over the low VED range. Parts Zr was mixed. It is also interesting to know that when the
3
printed using 0.3 w.t.% Zr-modified AA7075 resemble to VED is as low as 53 J/mm , the relative densities for the
their counterparts fabricated with as-received powders. three alloys were close, which means that Zr additions
The relative density of AA7075 with 1 w.t.% Zr addition show little effect. The relative density of the AA7075 with
has a positive relation with laser power over the full VED 1 w.t.% Zr addition turns out to be the highest over that of
range. On the other hand, the density of AA7075 with low the AA7075 and 0.3 w.t.% Zr-modified alloys. 0.3 w.t.% Zr
Zr addition is sensitive to scanning speed over the high addition only contributed to a slight increment in relative
VED range. More Zr addition leads to higher sensitivity density compared with that of the unmodified bulks.
to scanning speed. Different laser power and scanning 3.2. Crack
speed may lead to the same VED; hence, the relative
densities under the same VED were combined, as shown Figure 5 shows the representative microstructure along
in Figure 4. The relative density of the AA7075 grew with the building direction of the three printed parts. The 0.3

Volume 1 Issue 1 (2022) 5 https://doi.org/10.18063/msam.v1i1.4