PSI - Issue 68

Yasith H. Rajashilpage et al. / Procedia Structural Integrity 68 (2025) 981–987 Y.H. Rajashilpage, R.A. Yildiz, M. Malekan / Structural Integrity Procedia 00 (2025) 000–000

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Hall-Petch relationship, which states that yield strength decreases as grain size increases (Wang et al, 2023, Dubiel et al, 2023). Higher annealing temperatures promote grain growth, leading to a corresponding reduction in yield strength. A reduction in dislocation density during annealing also contributes to the decrease in UTS, as materials produced via AM typically have high dislocation densities that resist plastic deformation. As dislocation density decreases, resistance to applied stress diminishes, lowering UTS values (Lee et al, 2022, Liu et al, 2023). Additionally, phase transformations, such as the dissolution of precipitated carbides or delta phases at grain boundaries, contribute to UTS reduction by weakening the material (Chen et al, 2019). In summary, the observed increase in ductility and decrease in UTS after annealing are directly related to the microstructural changes induced during the process, where grain growth and reduced dislocation density enhance ductility while simultaneously reducing UTS. The test results revealed that increasing layer thickness and scanning speed negatively affected the mechanical properties of the materials, particularly at a layer thickness of 80 μm and a scanning speed of 2000 mm/s. In contrast, optimal performance was observed at a 30 μm layer thickness and a 1400 mm/s scanning speed. The degradation of material properties with increased layer thickness can be attributed to changes in grain size and dislocation density. Larger layer thicknesses result in slower cooling rates and larger melt pools, leading to the formation of larger grains, which reduces the material's strength due to fewer grain boundaries. Moreover, the increased layer thickness can cause a lack of fusion, arising from incomplete melting and insufficient overlap, further diminishing mechanical properties and material density (Wang et al, 2017, Tian et al, 2020). Elevated scanning speeds further degrade material properties, primarily due to increased porosity and structural defects. Higher scanning speeds reduce the laser energy input per unit volume, leading to incomplete melting and fusion of powder particles. This incomplete fusion results in defects such as pores and voids, which serve as stress concentrators, thereby lowering UTS and ductility (Soni et al, 2021, Huang et al, 2022, Yildiz et al, 2024). Additionally, rapid heating and cooling cycles associated with high scanning speeds introduce residual stresses into the material, further degrading mechanical performance. 3.2. Hardness test The hardness test results show a notable decrease in hardness with increasing annealing temperatures in most cases as presented in Table 4, with as-built samples typically exhibiting higher HRB values compared to post-heat-treated specimens. However, exceptions to this trend were observed, such as SS 316L at specific layer thickness and scanning speed combinations (30 μm/2000 mm/s, 55 μm/1400 mm/s, and 80 μm/1700 mm/s), where hardness slightly increased at 1100°C. Similar anomalies were noted for IN625 at 80 μm/1700 mm/s and SS 316L at 80 μm/1400 mm/s. Despite these deviations, the overall trend indicates a decrease in hardness with rising annealing temperatures, with a rapid drop at higher temperatures. The decrease in hardness with increased annealing temperatures can be explained by several microstructural changes, including grain growth, recrystallization, and the dissolution of strengthening phases. At temperatures above 700°C, recrystallization occurs, reducing dislocation density, which is a primary factor contributing to material hardness. The reduction in dislocations leads to a decrease in hardness (Kong et al, 2022, Zhou et al, 2024). Additionally, prolonged annealing promotes grain growth, resulting in fewer grain boundaries, which reduces resistance to dislocation motion and further lowers hardness (Sukumaran et al, 2017). The dissolution of strengthening phases during high temperature annealing also contributes to the reduction in hardness. For SS 316L, the dissolution of carbide and sigma phases, which help maintain hardness, results in softening. Similarly, the stabilization of austenitic phase grains, which are inherently softer and more ductile, contributes to the hardness reduction. In InN625, the γ'' and delta phases dissolve during annealing, leading to a similar decrease in hardness (Kong et al, 2022, Zhou et al, 2024). Overall, the reduction in hardness after heat treatment is primarily attributed to recrystallization, grain growth, and the dissolution of strengthening phases at elevated temperatures, reflecting the microstructural changes induced by the annealing process. Similar to the trends observed in the tensile tests, increasing layer thickness and scanning speed led to a reduction in material hardness. Larger layer thicknesses resulted in slower cooling rates and larger melt pools, as the heat from each deposited layer dissipates more slowly. This slower cooling promotes the development of coarser microstructures, which negatively impact hardness values (Diaz et al, 2022). Furthermore, lower cooling rates enable the formation of larger grains, reducing the number of grain boundaries. This reduction in grain boundaries decreases