PSI - Issue 54
Ivana Zetková et al. / Procedia Structural Integrity 54 (2024) 256–263 Author name / Structural Integrity Procedia 00 (2019) 000 – 000
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and designs more prone to residual stress than others. For example, residual stresses are more pronounced in thin parts and overhangs ( D. Wang at al.) Thermal residual stresses are typical for the SLM (Selective Laser Melting) technology. The principle of the process is to melt the material at high scanning speeds, which naturally creates high temperature gradients in the printed material, accompanied by deformations in the nano, micro, and macro scale. The most important parameters determining the size and shape of residual stress profiles are the material properties, sample and substrate height, laser scanning strategy, and heating conditions. In 3D printing of maraging steel, both the mentioned thermal stresses and so-called transformation stresses, caused during the martensitic transformation, are present. Volume changes occur due to the phase transition from austenite (face-centered cubic lattice, den sest packing) to martensite (supersaturated solid solution of carbon in α -iron with body-centered tetragonal lattice). These changes lead to dimensional changes, deformations, and the formation of predominantly compressive residual stresses (Trojan at al). Residual stresses in additively manufactured parts can pose significant limitations on their practical use, especially if they exceed the allowable deformation limit or if the presence of cracks poses a risk to the reliability of the component. Unwanted tensile residual stresses in the subsurface layer reduce mechanical and corrosion properties, while increasing the crack propagation rate, the risk of stress corrosion cracking, susceptibility to intergranular corrosion, etc. Therefore, there is a need to minimize or convert tensile residual stresses in 3D printed parts to compressive values. The possibilities of influencing residual stresses can be divided into three stages. The first stage is related to the preparation processes, the second stage to the printing process, and the third stage to post-processing. The topic of minimizing tensile stresses in 3D printed parts is addressed in professional publications and research projects, as it is a topic whose consequences limit the expansion of AM for the production of complexly loaded parts. Researchers Cheng and To proposed an optimization method for determining the optimal orientation determining of a printed part to minimize support structures and residual stresses. The method is based on dividing the part into three groups based on the type of overhang, for which a suitable support type is then generated. The authors replaced conventional supports with defined porous structures with a cubic geometry. The effectiveness of the proposed method is demonstrated on three types of this structure. The diagonal design of a cubic structure appears to be the most suitable for minimizing both volume and residual stress. Researchers Mukherjee et al. investigated the effects of input temperature and layer thickness on residual stresses reduction using a combination of thermal, fluid, and mechanical models. The research was conducted on Inconel 718 and Ti-6Al-4V materials. Reduced stresses were achieved by reducing the layer thickness by up to 30%, or by doubling the heat input (RS reduction by 20%). Patcharapit and Shi-Chune investigated the process parameters effects on residual stresses reduction. They focused on input energy, which was evaluated by measuring the heat on the surface of the part. The results show that input energy and scanning length affect the magnitude of residual stress, while scanning length also affects the occurrence of defects. The authors report that the optimal parameters for printing Ti-6Al- 4V alloy are a laser power of 200 W, a scanning speed of 1000 mm/s, and a scanning length of 1 mm. However, these settings have not been verified in terms of mechanical properties. In a study by Kruth et al., the effects of process parameters, including printing strategy, laser power and speed, and geometric orientation of the part in the printing space, on residual stresses were investigated. Smaller scan area and increased beam energy input (VED) led to a reduction in residual stresses. Lu et al. also studied the effects of scanning strategies and concluded that a scanning strategy with a "checkerboard" pattern has the least impact on the results compared to other scanning strategies. They also investigated the effect of checkerboard size on residual stresses. It was found that a checkerboard with a size of 2×2 mm2 led to the smallest residual stresses, but samples with this strategy also showed numerous cracks. A checkerboard size of 5×5 mm2 was considered to be the optimal size for achieving higher material density, mechanical properties, and relatively lower residual stresses. Another way to reduce residual stresses is generally heat treatment. This is also the case for components manufactured by 3D printing. Lin et al. investigated the effects of annealing on a FeCoCrNi alloy. Before annealing, tensile residual stresses were identified on the surface of the part. During annealing, tensile residual stresses decreased with increasing temperature. At a temperature of 773 K, a reduction of 120 MPa was observed. To completely relax the residual stresses, the temperature had to be increased to 1573 K. On the other hand, the final hardness of the part
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