Page 33 - MSAM-1-4

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

Table 5. Columnar grain width, hardness, and mechanical properties at room temperature of Inconel 718 fabricated by selective
electron beam melting with the parameters listed in Table 4.
Sample Columnar grain width (μm) Hardness (Hv) Yield strength (MPa) Tensile strength (MPa) Elongation (%)
P1 20.71±(16.51) 427.70±(32.59) 908.0±(5.0) 1009±(50) 7.6±(1.3)
P2 22.36±(16.85) 440.15±(15.81) 954.5±(14.5) 1270±(9) 34.0±(1.7)
P3 19.11±(15.30) 424.09±(17.34) 914.5±(10.5) 1215±(22) 22.1±(6.5)
P4 14.36±(10.30) 410.82±(12.88) 876.0±(4.0) 1216±(30) 17.5±(3.9)

during SEBM process resulting in precipitation of γ′
and γ′′ phase. However, the deviation indicates that the
microstructures and the precipitates were not uniform.
The tensile strength, yield strength, and elongation at
room temperature of the as-built samples are shown in
Figure 12, and the detailed values are shown in Table 5.
Although the four samples fabricated by optimized
processing parameters obtained high relative density, the
mechanical properties were different. Sample P1 had the
worst mechanical properties, while sample P2 had the
best, although both P1 and P2 were fabricated with the
same energy density. Samples P1 and P3 had relatively
higher relative density and similar hardness values, but
the mechanical properties of sample P3 were significantly
higher than that of sample P1, especially the elongation. Figure 12. Tensile tests at room temperature of as-built Inconel 718
Thereby, as for the SEBM-fabricated fully dense Inconel samples with the parameters listed in Table 4.
718 specimens, it is far from adequate to take energy
density or relative density into consideration to optimize thickness of the powder layer, and the input energy (beam
the mechanical properties. current < 15 mA) is not enough to melt the thicker powder
4. Discussion layer, resulting in a huge lack-of-fusion defects, as shown
in Figure 4B. With the further increase of input energy
4.1. Relationships between surface morphology and (15 mA < beam current < 25 mA), the molten pool is
internal defects enough to pass through the thick powder layer and prevent
The pre-alloyed powder in this study was prepared by lack-of-fusion defects, and the relative density increases
plasma rotating electrode process; hence, the gas pore correspondingly. When the input energy (beam current >
inside the powder and gas pore induced defects can be 25 mA) is too high, the splashing of metal liquid or metal
ignored. As shown in Figure 4, different combinations of vapor makes it impossible to form. Figure 13B shows the
parameters will give rise to different surface morphology in effect of scan speed on relative density, and input energy
SEBM process. Different surface morphology is related to decreases with the increase of scan speed by fixing beam
various types of internal defects. Uneven surface included current. When the input energy gradually decreases, the
lack-of-fusion defect or no defects, even surface included surface morphology changes from even to porous, and
shrinkage porosity defect, while porous surface included the relative density decreases. As shown in Figure 14, the
lack-of-fusion defect. sample with porous surface had a large number of lack-of-
Figure 13A shows the effect of beam current on relative fusion defects, which is the main reason for the low relative
density, and input energy increases with the increase of density. Small and shallow molten pool was generated
current by fixing scanning speed. When the input energy due to the low input energy. The molten pool cannot
gradually increases, the surface morphology changes from effectively penetrate the new layer of powder and combine
even to uneven, and the relative density first decreases with the previous solidified layer. Balling effect causes the
and then increases. The combined effect of the Marangoni partially melted powder to form an isolated molten liquid.
effect, vapor recoil pressure, and electron-beam agitation Due to surface tension and rapid solidification, molten
results in an uneven surface . The convex and concave liquid could not flow into pores and thus combine with
[52]
of the uneven surface led to a large difference in the nearby powder . Therefore, the lack-of-fusion defects
[53]
Volume 1 Issue 4 (2022) 10 https://doi.org/10.18063/msam.v1i4.23