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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3.2 Influence of heat treatment temperature on microstructure morphology Improvement in the ductility with strength reduction in LPBF-fabricated Ti6Al4V can be achieved by successive heat treatments involving the transformation of the non- equilibrium phase (acicular α’ martensite) into near equilibrium phase (lamellar α+β phases) [15]. Most of the research focused on the microstructure of the as -built samples and adjustment of the process parameters to increase the quality of as-built products by LPBF technique [16]. Ali et. al. [17] utilised a greater bed preheating temperature of 570°C to eliminate the in-built residual stresses and improve ductility. Meanwhile, others have optimized the mechanical properties by applying only one heat treatment stage at different temperatures on the as-built samples [18]. In the present study, the stress relieving was carried out at a temperature of 650°C and holding for three h to optimize the microstructure and mechanical properties. The stress-relieving heat treatment was carried out in an inert atmosphere by purging Argon gas to avoid oxidation. The optical microstructure of welded joint in fabricated and stress relaxed (SR) condition is shown in Fig.3. The microstructure reveals that α phase surrounding the grains are comparatively more in the sample of SR condition than in the as-fabricated condition. The weld region also shows large columnar grains compared to the sample where both built samples are welded. The sample's SR condition microstructure reveals a comparatively brighter α phase surrounding the prior β grains. The length and width of the prior β grains increase in the SR condition compared to the built -in condition. It is now approximately 1.1-1.8 µm wide and roughly 0.9- 1.2 µm for the SR one. The acicular martensite α΄ phase formed in Ti-6Al-4V alloy developed by LPBF is because of the alloy's high transient temperature under the laser's action during the process. Initially, β phase formation is preferred, which then transforms to α΄ phase by non -diffusion shear process during the subsequent rapid cooling.

a) Base in as-printed side

b) HAZ on as -printed side

c) Base in as-printed SR side d) HAZ on as -printed SR side Fig. 3 The optical microstructure of welded joint in as-printed condition and SR condition