PSI - Issue 71

Ravi Prakash et al. / Procedia Structural Integrity 71 (2025) 325–332

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Fig. 6. Illustration of temperature distribution during layer deposition: (a) temperature distribution at bottom of substrate for case-I and III, (b) at top of substrate for case-I and III, (c) temperature distribution at bottom of substrate for case-II and IV, and (d) at top of substrate for case-II and IV. Fig. 7 shows the three-dimensional residual stress contours in the build-substrate system for all process parameters (different power and speeds) for cases I, II, III, and IV. The results show that for case I (500 W, 5 mm/s) the maximum magnitude of stress is 666.3 MPa, for case II (500 W, 20 mm/s) the maximum magnitude of stress is 733.2 MPa, for case III (2000 W, 5 mm/s) the maximum magnitude of stress is 598.6 MPa, and for case IV (2000 W, 20 mm/s) the maximum magnitude of stress is 789.5 MPa. Based on these results, an increase in laser scan speed can be predicted to result in higher residual stresses. As the scanning speed increases, the laser interacts with each point for a shorter duration, resulting in a smaller melt pool and reduced heat input to the surrounding material. Consequently, adjacent regions remain at lower temperatures, which increases the temperature gradient between the melt pool and the nearby solid material. This elevated temperature gradient leads to higher thermal stresses during cooling. As a result, residual tensile stresses — particularly near the top layers and along the scan direction — become more pronounced. The longitudinal residual stresses are mainly tensile and typically exceed transverse stresses due to directional solidification and constrained thermal contraction (Bian et al., 2025). These stresses, with tensile zones near the surface and compressive zones at depth, contribute to warping and distortion, particularly in thin-walled structures (Carpenter & Tabei, 2020). Cyclic loading further accelerates crack growth and reduces fatigue life (Yuan et al., 2023). Additionally, reheating from subsequent layers modifies stress distributions, with front-layer preheating increasing stress and back-layer expansion providing partial stress relief. The rise in the temperature gradient and cooling rate causes an increase in residual stress. As speed increases, the melt pool size shrinks, leading to less heating of the surrounding tracks and lower temperature peaks. As a result, the adjacent tracks with lower temperatures will create a larger temperature gradient. A high gradient causes sharp contraction of the melt pool during cooling, while the surrounding material resists it, leading to tensile residual stresses. In titanium alloys, the α - phase (HCP) and β -phase (BCC) have different thermal expansion coefficients and elastic moduli (He et al., 2020), causing uneven contraction and phase-specific internal stresses during cooling (Illarionov et al., 2021). Upon cooli ng from above the β -transus, β - phase transforms into α or martensitic α′, depending on the cooling rate. This transformation involves a volume change that induces additional residual stresses. Rapid cooling, typical in additive manufacturing, promotes martensitic α′ formation, generating tensile stresses due to volumetric expansion (Chen et al., 2021). A higher β -phase fraction aids stress relaxation through its ductility, while a high α -phase fraction can increase residual stresses due to limited plasticity (Chen et al., 2023). When the laser power is increased, it reduces residual stress, which aligns with findings from (Balbaa et al., 2019). Increasing the laser power elevates the energy input into the material, resulting in a larger melt pool and an expanded Heat-Affected Zone (HAZ). This broader heating effect raises the temperature of the surrounding material, thereby reducing the temperature gradient between the melt pool and adjacent regions. As a result, the thermal contraction mismatch becomes less severe, leading to a reduction in residual tensile stresses. However, excessively high laser power can introduce adverse effects such as keyholing, material evaporation, and a higher likelihood of defects like porosity or distortion, especially if the processing parameters are not appropriately controlled. The HAZ, which borders the melt pool, experiences intense thermal cycling without undergoing melting. This can lead to microstructural changes and phase transformations that may alter the mechanical properties of the material. Rapid cooling in this region often induces high tensile residual stresses, which can promote crack initiation, lower toughness, and increase the risk of fatigue failure and fracture (Bukovská et al., 2022). Furthermore, steep stress gradients at the interface between the HAZ and the substrate can weaken bonding and contribute to delamination (Yan et al., 2024).