PSI - Issue 69

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

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Fig.9: the morphology of the melt pool and the expansion of the temperature field along the melt track as captured at six different time steps (laser power P=150W, scan speed v=115 cm/s). 1600 Kelvin is not a maximum temperature in the melt-pool. The maximum temperature in the melt-pool reached a value around 5500 K which is written in the text. In this figure the maximum temperature is considered equal to 1600 K to show the melt-pool since the solidus and liquids temperature of the NiTi is around the 1600 K.

Figure 10 presents a temporal sequence illustrating key dynamic phenomena during the LPBF process. The sequence begins with the laser's intense energy creating a localized melt pool on the powder bed, often forming a 'keyhole' or depression zone, as shown in Fig.10(a). This zone is a hotspot of thermal activity, where rapid phase transitions occur as material transitions from solid to liquid and, under high energy input, to vapor. Vaporization in this region generates recoil pressure, which drives molten material from the center towards the bottom and tail of the melt pool, influencing its depth and morphology. However, if the recoil pressure becomes excessive or the keyhole collapses unevenly, it may trap gas within the melt pool, leading to the formation of keyhole porosity—voids that adversely affect the mechanical properties of the final component. As the process continues, the denudation phase begins. Recoil pressure and Marangoni convection jointly sweep powder particles away from the laser interaction zone, creating a 'denuded' area around the depression zone. Finally, as shown in Fig.10(b-f), when the laser ceases, the depression zone is replenished by molten material undergoing cooling and solidification. This critical stage completes the LPBF cycle and prepares the system for the next track, with the solidification process playing a pivotal role in determining the final track's quality and consistency.