PSI - Issue 56

S Anand Kumar et al. / Procedia Structural Integrity 56 (2024) 65–70 / Structural Integrity Procedia 00 (2019) 000–000

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and macro examination. The microstructure characterization of welded samples was carried out using an OM Leica DMI3000 at different locations on the specimens.

Fig.1 Welded samples of Ti6Al4V alloy

3. Results and Discussion 3.1 Microstructure of welded joint in as-printed condition

The microstructure characterization was carried out at different locations on the welded specimen. Figure 2 describes the microstructure of the welded specimen of LPBF fabricated specimen with LPBF fabricated specimen. The LPBF samples were characterized by the microstructure forming the acicular α’ martensitic phase [8]. The reason for developing the acicular α’ martensitic phase is that the temperature of the Ti6Al4V alloy is exceptionally high under the laser action. This most likely forms the diffusionless α’ phase due to the subsequent rapid cooling process. The cooling rate within LPBF processing exceeds a crucial cooling rate of 410 K/s, which is suggested as a martensite α’ formation rate [12]. Consequently, acicular α’ martensite forms rather than equilibrium α and β phases in Ti-6Al-4V manufactured by LPBF (Fig. 2a). It is observed from the microstructure that the grains were surrounded by the bright α phase. In the HAZ region shown in Fig. 2b, it is observed that the presence of α phase around the grains increased and there was a clear distinction between the base and weld zone. In the weld zone, columnar grains are observed growing towards the centre of the weld, as evident from Fig. 2c. Zhan - Yong Zhao et. al.[7] had reported the interior of the equiaxed grains mainly consisted of acicular martensite α΄ phase and the β phase was not detected by XRD in the TC4 alloy fabricated by LPBF. Bey et al.[8] also identified the absence of β phase in as-built TC4 alloys. The microstructure in as-built condition usually has low ductility and high strength [13 14]. Thus, significant effort has been exerted to comprehend the process parameters on microstructures and mechanical properties in optimising the as-built conditions.

a) Base

b) HAZ

c) Weld

Fig. 2 The optical microstructure of welded joint in as-printed condition