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Materials Science in Additive Manufacturing Process optimization of SEBM IN718 via ML

the reliability of processing window. Although the sample center, and there were no significant differences between
manufactured by the optimized processing parameters still the size and depth of dimples. This is consistent with high
contain some defects, post-treatment such as HIP could elongation of P2, while P1 showed low elongation. Brittle
heal those defects, and the original grain structure can fracture was observed at the boundary in Figure 15G, and
be maintained as much as possible . The formation of complete and broken powders in P1 were observed, as
[21]
shrinkage porosity defects in Figure 9A may be ascribed shown in Figure 15H and I, respectively. The size of complete
to the existence of some imperfect spherical powders in powder particle is about 88 μm, which is relatively large
the pre-alloyed powders, which resulted in low-energy among pre-alloyed powder. These unmelted powders did
absorption rate and thermal conductivity, and it prevented not reduce the relative density of the sample, but retained
the liquid flowing into the inter-dendritic region to the original structure of the pre-alloy powder. This, further,
compensate voids . suggests that the equiaxed grains in Figure 10 may stem from
[49]
the unmelted powder. The unmelted powder also resulted
4.3. Mechanical properties and fracture mechanisms in the decrease of plasticity of Inconel 718 sample, because
Inconel 718 samples, especially samples P1 and P2, with the smooth surface reduced the intergranular bonding force,
high relative density (>99.5%) were fabricated by SEBM after and Laves phase inside powder perpendicular to the tensile
process optimization, which displayed different strength direction also decreased the strength. The unmelted powder
and elongation. Samples P1 and P2 had the same energy in each part of the sample became the breach of fracture.
density. Although P1 had the highest density in this study, its Although P3 and P4 had finer columnar grains, the increase
mechanical properties were the worst. As shown in Figure 10, in the number of mixed-equiaxed grain reduced its strength
the difference in mechanical properties may be caused and plasticity. The large area of mixed-equiaxed grain that
by different microstructures. According to the analysis of appeared randomly is the reason for the large variance
[51]
section 3.4, the unmelted powder resulted in equiaxed grains. of the elongation in P3 and P4. Sun et al. fabricated the
The fracture surfaces of P1 and P2 were analyzed to explore Inconel 718 by SEBM with similar microstructure to P1,
the reason, as shown in Figure 15A-F. The samples exhibited whose strength and plasticity were also poor. Therefore, it is
dimple fracture, which indicates the ductile fracture mode. necessary to improve energy input to fully melt the original
Both of them showed fine and equiaxed dimples in the pre-alloyed powder. Reducing the particle size distribution is

A B C

D E F

G H I

Figure 15. SEM images of fracture surface: (A–C) P2; (D–F) P1; (G), (H) and (I) are the corresponding regions in (D).

Volume 1 Issue 4 (2022) 12 https://doi.org/10.18063/msam.v1i4.23