PSI - Issue 56

Sapam Ningthemba Singh et al. / Procedia Structural Integrity 56 (2024) 11–18 Sapam Ningthemba Singh et al./ Structural Integrity Procedia 00 (2019) 000–000

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3. Results and discussions The initial microscopic inspection of the fabricated samples revealed surface micro-cracks which are common to AM and welding processes. These cracks were not visible to visual inspections. Upon polishing, these cracks were eliminated and a polished surface without cracks were obtained before the fatigue testing. Isolated and very small voids were observed on the interface between the deposition pattern and the initial boundary deposition. Such voids can act as a crack initiation point. There was a 61.4% increase in fatigue life from sample no. 1 to 2, followed by a 15.6% decrease from sample no. 2 to 3 and 25.7% from sample no. 3 to 4 in LDED-only samples as seen in Fig. 3(a). The increase in the fatigue life is mainly due to an increase in laser power, allowing for more proper melting and re melting of the successive layers. However, further increasing the laser power results in slower cooling of the fabricated parts, resulting in coarser microstructures that are not favorable for fatigue life (Amine, Newkirk, and Liou 2014; Farshidianfar, Khajepour, and Gerlich 2016). Higher laser power encourages more deformation, and a higher heating/cooling cycle induces more unwanted stresses. This is the reason for the reduced fatigue life in sample no. 3&4. Significant improvement in the fatigue life in samples 1 (56.51%) and 4 (39.26%) was observed for the LDED+LSPed as compared to samples without LSP. However, for samples 2 and 3, a slight decrease of 0.99% and 11.40%, respectively were observed for LDED+LSPed samples as compared to the LDED-only samples. The primary reason for the decrease in fatigue life is the gaps observed in samples 2&3 in the LSPed area, as shown in Fig. 3(a). Another reason can be due to internal defects on the LDED+LSPed samples, surface cracks, or due to uneven distribution of LSP on the surfaces. Even though the samples of the same process parameters were prepared in one go, there are still chances that microstructure and void distribution can be different, resulting in different fatigue life. The mode of fatigue failure was prominently ductile mode of failure. Multiple cracks and voids were observed during the fractography of the fractured surfaces. River lines, voids (dimples), striations, and cracks were observed as seen in Fig. 3(b). No significant difference in the fracture pattern was observed between the LDED-only samples and LDED+LSPed samples, as seen in Fig. 4. This shows the effects of LSP are limited to surfaces and near-surface only which is in agreement with the results in the literature (X. Zhang et al. 2022; Hareharen et al. 2023; Bai et al. 2023). The fatigue performance is within the range of existing literature (M. Zhang et al. 2018; Ghouse et al. 2018). This is significant considering the high layer thickness adopted in the current study.

Fig. 3: (a) fatigue life of LDED and LDED+LSPed samples, and (b) SEM image of fractured surface.