PSI - Issue 69

Mohammadjavad Abdollahzadeh et al. / Procedia Structural Integrity 69 (2025) 2–19

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Fig.10: Temperature field and velocity vectors in the melt-pool on the center X-Z plane. From the figure it is clear that because of the laser power, there is a recoil pressure which can lead to depression zone. As a result, it pushes the fluid flow to the back (tail). However, after laser passes through this area, because of the surface tension the system tends to minimize the surface area, by filling out the depression zone.

Figure 11 showcases the chronological development of the melt pool in the Y-Z plane, with temperature distributions plotted at various intervals to provide a time-ordered representation. At 270 µs, the laser has not yet interacted with the material, leaving the powder bed solid, undisturbed, and ready for the thermal event. Upon the laser's initial contact at t = 285 µs, a melt pool begins to form under conduction mode. Here, laser energy is primarily absorbed at the surface and conducted downward, resulting in a shallow yet wide melt pool. As the laser interaction progresses from t = 285 µs to t = 360 µs, the depression zone deepens incrementally, and evaporation occurs due to elevated temperatures. Within this active melt pool, dynamic phenomena unfold. Rayleigh instability, driven by surface tension disparities, induces ripples on the melt pool's surface, while the Marangoni effect, caused by temperature gradients, generates convective flow within the molten material. When the laser ceases at t = 400 µs, the thermal energy dissipates, triggering solidification. Surface tension and capillary forces draw molten material back to the center, gradually filling the depression zone, with solidification completing around t = 800 µs. Analysis of the figure reveals significant porosities in the melt pool's final stages, particularly post-solidification. These void spaces primarily arise from interstitial gaps between unmelted powder particles. Due to the random particle arrangement and varying inter-particle distances, inconsistent heat distribution leads to differential melting. Unmelted regions that fail to reach their melting point form these porosities, which can adversely affect the mechanical properties and performance of the final product. Mitigating these defects requires thorough process control, including adjusting laser power settings or employing pre-heating strategies, to ensure consistent melting and minimize porosity formation. This underscores the importance of optimizing LPBF parameters to improve the reliability and quality of produced components.