PSI - Issue 53

Omid Emadinia et al. / Procedia Structural Integrity 53 (2024) 278–284 Author name / Structural Integrity Procedia 00 (2019) 000–000

280

3

(DM) Leica DM4000 M and a scanning electron microscope (SEM) FEI QUANTA 400 FEG ESEM equipment using Backscattered Electron image mode (BEI). Detailed microscopic analysis was executed using energy-dispersive X ray spectroscopy (EDS) (EDAX Genesis X4M, Oxford Instrument, Oxfordshire, UK), and electron backscatter diffraction (EBSD) (EDAX-TSL OIM EBSD, Mahwah, NJ, USA) technique using TSL OIM Analysis 5.2 software. Grain analysis was conducted by using inverse pole figure (IPF) and boundary maps. Regarding EBSD, a clean-up routine was performed with a grain tolerance angle of 15. ZwickRoell microhardness tester was employed for microhardness measurements using a Vickers indenter and a 0.3 kgf load.

Figure 1 SEM images of IN625 powder

Table 2 L-DED process parameters

Carrier gas flow rate

Shielding Gas flow rate

Spot Size

Beam Inclination

Scanning Speed

Feeding Rate

Laser Power

Layer thickness

Hatch Spacing

Gas

2.5 mm

Argon (99.9%)

2.5 l/min

26 l/min

5º

6 mm/s

12 g/min

1.8 kW

1.7 mm

1.8 mm

Results and discussion

The L-DED processing resulted in a visually defect free structure. The etched cross-section macrograph presented a fish-scale-like pattern, characteristic of AM products (Figure 2a). A distinct difference was observed among the microstructure of the bottom and top most region of the L-DED prints. Figure 2b, illustrates the dendrites constituting the microstructure. In the bottom zone, it seems dominantly constituted by cell-like morphology whereas the top most region primarily comprised of columnar dendrites with secondary arms. This observation was also confirmed by SEM/BEI technique as shown in Figure 2c. During solidification, the thermal gradient (G) and the growth rate (R) dictate the microstructure of solidifying structure. The ratio G/R determines the morphology, and the product GxR determines the size of the solidification structure (Kou 2003). After successive heating and cooling cycles during L-DED process, in the top region, a relatively lower G/R ratio has resulted in the evolution of columnar dendritic morphology as shown in Figure 2b,c-top. Whereas in the bottom region of the printed sample (i.e., closer to the substrate), the microstructure comprised of cellular dendritic structure, Figure 2b,c-bottom.