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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jetting, PBF, Material extrusion, Sheet lamination, and Direct energy deposition. These methods primarily differ in the material deposition stage of the manufacturing process. The process of AM generally involves the following steps (Soni et al., 2021): (i) Preparing a 3D CAD model, (ii) Slicing, (iii) Generating a STL file to define cross-sectional layers, (iv) Depositing material layer-by-layer, and (vi) and post-processing. In laser PBF (LPBF), a thin layer of metal powder is distributed on a build platform, and a high-power laser selectively melts the powder according to the design. Critical parameters influencing the process include laser power (P), layer thickness (T), scanning speed (S), and hatch spacing (H), which collectively define the laser energy density. This energy density, defined as = /( ∙ ∙ ) , plays a key role in determining the melting behaviour, part quality, and microstructural properties of the final product (Yildiz et al., 2024). Optimal selection of these parameters is crucial to minimizing defects like cracks, porosity, and distortion (Soni et al., 2021). Stainless steel 316L (SS 316L), an austenitic alloy, is widely recognized for its superior corrosion resistance and durability, attributed to its low carbon content (≤0.03%) and high molybdenum content. These characteristics make SS 316L ideal for applications requiring high corrosion resistance, particularly in post-welding scenarios where annealing is not feasible. While it can withstand continuous exposure to temperatures ranging from 427°C to 857°C, offering better resistance to carbide precipitation than standard SS 316, its mechanical properties are comparatively lower. The face-centered cubic (FCC) crystal structure of SS 316L imparts significant mechanical strength, enduring elongations up to 40% (Byun et al., 2021). The microstructure of SS 316L can be altered by different manufacturing techniques, such as selective laser melting, which results in refined grain sizes and high dislocation densities (Odnobokova et al., 2023). Inconel 625 (IN625), a nickel-based superalloy, exhibits remarkable strength, high-temperature resistance, and excellent corrosion and oxidation resistance. Developed initially as a replacement for SS 316 in supercritical steam power plants, IN625 has found extensive use in industries that demand high-performance materials. Its composition, which includes nickel, chromium, molybdenum, and niobium, contributes to its strength, particularly in harsh environments. The alloy's FCC structure, characterized by low stacking fault energy, enhances its strength and creep resistance at elevated temperatures (Dutkiewicz et al., 2020, Horke et al, 2019). Solid solution strengthening and the formation of γ" phase precipitates further bolster its mechanical properties. IN625's weldability, machinability, and non-magnetic nature make it suitable for applications such as aircraft ducting, chemical processing equipment, and turbine shroud rings. Its high tensile, fatigue, creep, and rupture strength are crucial for components operating under extreme conditions, underscoring its importance in modern engineering (Mignanelli et al., 2017). Annealing is a crucial heat treatment process used to enhance the ductility and toughness of steels and alloys, while simultaneously reducing hardness. This study focuses on full annealing, the most common industrial annealing method, which consists of three key stages: heating, soaking, and cooling. During the heating stage, specimens are heated above their recrystallization temperature but below their melting point to dissolve carbides and achieve a uniform austenite phase. As temperature gradually increases, dislocations in the crystal lattice decrease, allowing atom migration and relieving internal stresses within the material. The soaking stage follows, where the material is maintained at the target temperature for a specified time, allowing recrystallization to occur. In the cooling stage, grain growth continues, with the final microstructure being determined by both the material composition and the heating conditions. The post-annealing mechanical properties are influenced by the duration of the soaking and cooling stages. The full annealing was applied in this work to SS 316L and IN625 at temperatures ranging from 700°C to 1100°C to assess their effects on the obtained mechanical properties. The goal was to evaluate the effectiveness of annealing in enhancing material performance, tensile and hardness behaviors of these two alloys. 2. Experimental procedure Specimens were fabricated using LPBF technique with a laser power of 350 W in a vertical orientation to fabricate samples according to the ASTM E8/E8M standard. The layer thickness and scanning speed were varied between 30 80 µ m and 1400-2000 mm/s, respectively. Each uniquely labeled specimen underwent heat treatment, a process shown in Fig. 1, at three different temperatures, and mechanical testing was performed on three specimens per batch, yielding a total of 81 specimens for each material. Mechanical properties were assessed through tensile tests on a Zwick/Roell 10kN tensile tester at a speed of 5 mm/min, providing precise measurements of strength and ductility. Hardness testing was conducted using an INNOVATEST Fenix 300RS machine on the Rockwell B scale (HRB) with a 1.59 mm ball