PSI - Issue 68

Mohamed Elsayed et al. / Procedia Structural Integrity 68 (2025) 1003–1009 Elsayed et al./ Structural Integrity Procedia 00 (2025) 000–000

1006

4

1.3.

Microstructure and mechanical properties

1.1.3. Microstructure analysis Microstructure specimens were prepared using a CNC wire cut EDM machine (model FH-300C) with cuts perpendicular to the welding line. After mechanical grinding and polishing with alumina suspension, electrochemical etching was performed at 6 V and 1.5 A for 30-40 seconds in oxalic acid solution. The microstructure images were evaluated using a laser confocal scanning microscope (LCSM), VK-X200 (KEYENCE, Corporation, Osaka, Japan) 1.1.4. Microhardness measurements Microhardness measurements were carried out using a Berkovich indenter on a CSM Instruments H IT tester (Needham, MA, USA), with a load of up to 2 N applied for 15 seconds across the BM, HAZ, and FZ. Microstructure of GTAW joints Fig. 2 shows microstructure images of FZ produced by GTAW technique, captured using LCSM. The images revealed an epitaxial growth mechanism occurred during solidification process, where existing BM grains acted as nucleation sites. The microstructure consists of austenite matrix and 2% delta ferrite phase, as obtained by the ferrite scope (Khedr et al., 2024). Dendritic equiaxed grains grew in the centre of the FZ as a result of heterogeneous nucleation mechanism occurred during solidification process, as mentioned by (Kou, 2020). The average grain size of the equiaxed grains was 6±3 µm. Furthermore, dendritic columnar grains epitaxially form on both sides of the BM, creating a distinct columnar to equiaxed transition (CET) zone, highlighted by red dashed lines. This observed grain structure is consistent with findings reported in the literature (Feng et al., 2023). The dimensional analysis of these columnar grains, including dendrite length and interdendritic spacing, significantly affects the indentation resistance of the FZ. As the dendrite dimensions increase, the indentation resistance decreases as reported in the literature (Khedr et al., 2024). The average width of the FZ was 5.9±0.5 mm. The magnified view (Fig. 2 (b)) revealed the presence of solidification cracks that occurred during the solidification process. These cracks are attributed to the low volume fraction of the present delta ferrite content in the FZ. This content of the δ-ferrite is being lower than 3%, which induces solidification cracks as previously reported by (Kadoi et al., 2023) 3. Results and discussion 1.4.

Fig. 2. LCSM images of Microstructure of the FZ produced by GTAW technique: (a) a general view, and (b) a magnified view of the highlighted area by yellow dashed lines in (a). 1.5. Microstructure of LW joints Fig. 3 shows microstructure images of the FZ produced by LW, captured using LCSM. The microstructure includes cellular, columnar, and equiaxed dendrite structure. Within the columnar dendrites, substructure grains with dendritic patterns are observed due to their growth directions. The columnar grains grow perpendicular to the fusion pool boundary, aligning with the heat flow direction, consistent with the literature (Guo et al., 2015). The high-