PSI - Issue 68

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

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magnification image Fig. 3 (b) shows an absence of solidification cracks, which is an advantage of the LW over the GTAW technique, as observed in the Fig. 2 (b). The centre of the FZ consists mainly of equiaxed dendrite grains, formed due to a low G/R ratio (temperature gradient/solidification rate). The average grain size in the FZ was 5±2 µm, larger than that of the BM, which measured 4±2 µm. This difference is attributed to the higher energy input (lower cooling rate) during the LW process compared to the energy density per millimetre length in the LPBF process, the slower cooling rate in after the LW process provides sufficient time for grain coarsening, resulting in the formation of larger grains. This observed grain structure is consistent with findings in the literature (Zhang et al., 2021) Additionally, the average width of FZ was 0.5 mm, which is 10 times lower than that produced by GTAW technique), attributed to higher solidification rate after LW processing.

Fig. 3. LCSM images of the FZ microstructure produced by LW technique: where: (a) a general view, and (b) a magnified view of the yellow dashed lines in (a). 1.6. Indentation hardness results Fig.4 shows the micro-indentation hardness curves for BM, HAZ, and FZs produced by GTAW and LW techniques. Table 1 summarized the results of micro-indentation hardness tests including penetration depth (PD) and corresponding hardness. The results indicated that as the penetration depth increases, referring to a reduction in its resistance to indentation. The average indentation hardness of the BM was measured at 2.26 ± 0.08 GPa, while the fusion FZs exhibited lower hardness values than those of BM. Specifically, the FZ hardness was recorded at 1.98 ± 0.18 GPa for GTAW joints and 2.1 ± 0.14 GPa for LW joints. The higher cooling rate in the LW process, compared to the GTAW technique, resulted in a finer grain structure in the FZ, leading to increased hardness in LW joints, aligned with findings reported in the literature (S. Kumar & Shahi, 2011). Furthermore, the laser beam which is employed during welding utilizes a Yb: YAG disc laser system similar to the printing of the BMs, resulting in formation of fine cellular structure in the FZ.