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

However, crack density decreased at the expense of diameter of 12 mm and 30% having a diameter of 5 mm,
relative density as porosity became an issue with additives. were employed in a corundum jar. Figure 1 displays the
Therefore, crack-free and pore-free fabrication of AA7075 morphology of the AA7075, elemental Zr, and blended
is a very difficult task. Many researchers have placed powders. Figure 1E-H show the close-up view of the
their focus on either porosity or cracking. Therefore, it respective powders, indicating that the ball milling process
is imperative to devise a method to fabricate crack-free maintains the original surface morphology and shape of
samples with low porosity. the AA7075 powders. The green arrows in Figure 1C and D

The present study aims to find the process window mark the irregular Zr particles. As expected, more irregular
for the production of crack-free components of the Zr particles can be found in powders with 1 w.t.% additives.
modified high-strength AA7075 alloy with low porosity. In Figure 1I, the bright white areas indicated by the green
It is known that the maximum solid solubility of Zr arrows in the back-scattered electron image refer to Zr. As
in Al is ~ 0.25 w.t.% under equilibrium solidification seen, Zr particles with size around 1 μm adhered to the
conditions [37] . However, LPBF processing involves rapid aluminum powders. Moreover, elemental particles with
heating and cooling cycles under far-from-equilibrium size larger than 20 μm were rarely found as the original
conditions, which may significantly enhance Zr particles had cracks and could be milled into smaller ones.
solubility. Taking consideration the melting conditions Parts with a 5 × 5 × 6 mm dimension were fabricated
of elemental powders as well, the concentrations of on a substrate using an SLM 125 HL machine (SLM
0.3 w.t.% and 1 w.t.% Zr were investigated to find out Solutions Group AG, Lubeck, Germany), which is
whether a suitable amount of Zr is able to achieve equipped with a fiber laser (1064 nm), with a maximum
sufficient precipitation of Al-Zr intermetallic nucleants. laser power of 400 W and a focus diameter of 80 μm. An
The relationship between the processing parameters and argon atmosphere with <0.1% oxygen was provided to
defects features was further investigated in the modified prevent oxidation and interstitial element contaminations
AA7075. The results of this study would contribute to a during the manufacturing process. To decrease the
better understanding of defect formation in Zr-modified thermal stress, the substrate was preheated to 150°C
AA7075 through LPBF. before fabrication. Sixty-four samples classified in 16
groups were printed in order to obtain the processing
2. Materials and methods window. The laser power ranges from 250 to 325 W,
The materials prepared for the LPBF fabrication were with an interval of 25 W, and the scanning speed ranges
commercial spherical AA7075 powders and irregular from 1000 to 1300 mm/s, with an interval of 100 mm/s.
Zr particles (supplied by Beijing Crigoo Materials Table 2 lists the volumetric energy density (VED) which
Technology Co., Ltd.). The size of the AA7075 powders can be calculated by Equation 1.
ranges between 20 and 63 μm while elemental Zr powders VED = P/vht (1)
were sieved to remove particles with a size over 75 μm.
The chemical composition analysis of the as-received where P is the laser power (W), v is the scanning speed
AA7075 and Zr powders was performed using inductively (mm/s), h is the hatching space (μm), and t is the layer
coupled plasma optical emission spectroscopy (ICP-OES, thickness (μm). The hatching space and layer thickness in
NCS Plasma 3000). The chemical compositions of the the present study were 120 μm and 30 μm, respectively.
as-received AA7075 and Zr powders are listed in Table 1. Figure 2 shows how the 64 parts were fabricated with the
They were blended to prepare modified AA7075 alloys scanning strategy. The planes for further characterization
with different contents, 0.3 w.t.% and 1 w.t.%, respectively. are also illustrated in the upper right inset.
The mixtures were ball milled in an argon atmosphere After fabrication, the samples were removed from
for 16 h at a rotation rate of 300 rpm. The ball-to-powder the substrate via wire-cut electrical discharge machining
weight ratio was 1:1. ZrO milling balls, consisted of (EDM). The parts were then cleaned by sonicating in
2
20% of them having a diameter of 15 mm, 50% having a ethanol and dried with clean compressed dry air. Density

Table 1. Chemical compositions of the AA7075 and Zr powders in w.t. %.

Elements Si Fe Mg Cr Zn Cu Mn Ti Ca Cr H O Zr Al
Alloys
AA7075 0.06 0.06 2.53 0.21 5.9 1.57 <0.02 0.005 / / / 0.046 / Bal.
Zr 0.001 0.098 0.001 0.012 / / / / 0.003 0.012 0.002 0.094 Bal. 0.001

Volume 1 Issue 1 (2022) 3 https://doi.org/10.18063/msam.v1i1.4