• 한국분말재료학회지
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• 제28권2호
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• pp.110-119
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• 2021

In this study, the high-temperature oxidation properties of austenitic 316L stainless steel manufactured by laser powder bed fusion (LPBF) is investigated and compared with conventional 316L manufactured by hot rolling (HR). The initial microstructure of LPBF-SS316L exhibits a molten pool ~100 ㎛ in size and grains grown along the building direction. Isotropic grains (~35 ㎛) are detected in the HR-SS316L. In high-temperature oxidation tests performed at 700℃ and 900℃, LPBF-SS316L demonstrates slightly superior high-temperature oxidation resistance compared to HR-SS316L. After the initial oxidation at 700℃, shown as an increase in weight, almost no further oxidation is observed for both materials. At 900℃, the oxidation weight displays a parabolic trend and both materials exhibit similar behavior. However, at 1100℃, LPBF-SS316L oxidizes in a parabolic manner, but HR-SS316L shows a breakaway oxidation behavior. The oxide layers of LPBF-SS316L and HR-SS316L are mainly composed of Cr₂O₃, Fe-based oxides, and spinel phases. In LPBF-SS316L, a uniform Cr depletion region is observed, whereas a Cr depletion region appears at the grain boundary in HR-SS316L. It is evident from the results that the microstructure and the high-temperature oxidation characteristics and behavior are related.

• 한국분말재료학회지
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• 제31권1호
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• pp.8-15
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• 2024

The emergence of ferrous-medium entropy alloys (FeMEAs) with excellent tensile properties represents a potential direction for designing alloys based on metastable engineering. In this study, an FeMEA is successfully fabricated using laser powder bed fusion (LPBF), a metal additive manufacturing technology. Tensile tests are conducted on the LPBF-processed FeMEA at room temperature and cryogenic temperatures (77 K). At 77 K, the LPBF-processed FeMEA exhibits high yield strength and excellent ultimate tensile strength through active deformation-induced martensitic transformation. Furthermore, due to the low stability of the face-centered cubic (FCC) phase of the LPBF-processed FeMEA based on nano-scale solute heterogeneity, stress-induced martensitic transformation occurs, accompanied by the appearance of a yield point phenomenon during cryogenic tensile deformation. This study elucidates the origin of the yield point phenomenon and deformation behavior of the FeMEA at 77 K.

• 한국분말재료학회지
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• 제31권2호
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• pp.137-145
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• 2024

In order to predict the process window of laser powder bed fusion (LPBF) for printing metallic components, the calculation of volumetric energy density (VED) has been widely calculated for controlling process parameters. However, because it is assumed that the process parameters contribute equally to heat input, the VED still has limitation for predicting the process window of LPBF-processed materials. In this study, an explainable machine learning (xML) approach was adopted to predict and understand the contribution of each process parameter to defect evolution in Ti alloys in the LPBF process. Various ML models were trained, and the Shapley additive explanation method was adopted to quantify the importance of each process parameter. This study can offer effective guidelines for fine-tuning process parameters to fabricate high-quality products using LPBF.

• 한국분말재료학회지
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• 제31권4호
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• pp.308-317
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• 2024

Maraging steel has excellent mechanical properties resulting from the formation of precipitates within the matrix through aging treatment. Maraging steel fabricated by the laser powder bed fusion (LPBF) process is suitable for applications including precise components and optimized design. The anisotropic characteristic, which depends on the stacking direction, affects the mechanical properties. This study aimed to analyze the influence of anisotropy on the wear behavior of maraging steel after aging treatment. The features of additive manufacturing tended to disappear after heat treatment. However, some residual cellular and dendrite structures were observed. In the wear tests, a high wear rate was observed on the building direction plane for all counter materials. This is believed to be because the oxides formed on the wear track positively affected the wear characteristics; meanwhile, the bead shape in the stacking direction surface was vulnerable to wear, leading to significant wear.

• 소성∙가공
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• 제31권3호
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• pp.136-142
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• 2022

The purpose of this study was to investigate the effects of sample thickness and support structure on the residual stress of Hastelloy X superalloy samples fabricated by laser powder bed fusion (LPBF), which is an additive manufacturing process. The residual stresses of LPBF samples with different thicknesses and support structures were measured using X-ray diffraction. The results revealed that as the thickness of sample increased from 2.5 mm to 20 mm, its tensile residual stress gradually decreased from 443.5 MPa to 182.2 MPa. Additionally, the residual stress in the bottom region of sample was higher than that in the top region, and the residual stress difference in the bottom and top regions became more pronounced as the sample thickness decreased. The residual stress of LPBF sample also varied depending on the structure of support. The residual stress of sample decreased with increasing contract area between the sample and the support, because the larger contract area led to smaller temperature gradient throughout the sample.

• Tribology and Lubricants
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• 제39권6호
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• pp.244-249
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• 2023

This paper presents a comprehensive analysis of the impact of surface roughness on the friction and wear properties of SUS 321, an austenitic stainless steel variant produced using the laser powder bed fusion (LPBF) technique, which is a prevalent additive manufacturing method. After the LPBF fabrication, the specimens go a heat treatment process aimed at alleviating residual stress. Subsequently, they are polished extensively to achieve a refined and smooth surface. To deliberately introduce controlled variations in surface roughness, an etching process is employed. This multi-step method encompassed primary etching in a 3M hydrochloric acid solution, followed by secondary etching in a 35 wt% ferric chloride solution, with varying durations applied to different specimens. A comprehensive evaluation of the surface characteristics ensued, employing precise techniques such as surface roughness measurements and meticulous assessments of water droplet contact angles. Following the surface treatment procedures, a series of friction tests are performed to explore the tribological behavior of the etched specimens. This in-depth investigation reached its peak by revealing valuable insights. It clarified a strong correlation between intentionally altered surface roughness, achieved through etching processes, and the resulting tribological performance of LPBF-fabricated SUS 321 stainless steel. This significantly advances our grasp of material behavior in tribological applications.

• Composites Research
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• 제37권2호
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• pp.94-100
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• 2024

In order to evade the premature failure of super duplex stainless steels (SDSSs) in some harsh environments, the increase of their mechanical properties is a promising approach. In this study, based on the laser powder bed fusion (LPBF) technique, SDSS matrix composites without post heat treatment were fabricated by using the powder mixture of SDSSs, super austenitic stainless steels (SASSs) and micron-sized TiC particles. Many in-situ TiC_xN_y nanoparticles were found to be formed by using micron-sized TiC particles in as-built composites, and both fine ferrite and austenite grains were generated. The as-built composites exhibited an excellent combination of high ultimate tensile strength (UTS) (~1066 MPa) and good uniform elongation (UE) (~15.6%), showing a better mechanical property compared with other reported LPBF-fabricated SDSSs, which was mainly attributed to the fine grain, Orowan and dislocation strengthening mechanisms. In particular, the successful fabrication of SDSS matrix composites can set the stage for producing high-performance metallic parts via LPBF technique.

• 한국분말재료학회지
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• 제28권5호
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• pp.410-416
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• 2021

Ti-6Al-4V alloy has a wide range of applications, ranging from turbine blades that require smooth surfaces for aerodynamic purposes to biomedical implants, where a certain surface roughness promotes biomedical compatibility. Therefore, it would be advantageous if the high volumetric density is maintained while controlling the surface roughness during the LPBF of Ti-6Al-4V. In this study, the volumetric energy density is varied by independently changing the laser power and scan speed to document the changes in the relative sample density and surface roughness. The results where the energy density is similar but the process parameters are different are compared. For comparable energy density but higher laser power and scan speed, the relative density remained similar at approximately 99%. However, the surface roughness varies, and the maximum increase rate is approximately 172%. To investigate the cause of the increased surface roughness, a nonlinear finite element heat transfer analysis is performed to compare the maximum temperature, cooling rate, and lifetime of the melt pool with different process parameters.

• 한국분말재료학회지
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• 제31권5호
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• pp.365-373
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• 2024

This review examines the microstructural and mechanical properties of a Ti-6Al-4V alloy produced by wrought processing and powder metallurgy (PM), specifically laser powder bed fusion (LPBF) and hot isostatic pressing. Wrought methods, such as forging and rolling, create equiaxed alpha (α) and beta (β) grain structures with balanced properties, which are ideal for fatigue resistance. In contrast, PM methods, particularly LPBF, often yield a martensitic α' structure with high microhardness, enabling complex geometries but requiring post-processing to improve its properties and reduce stress. The study evaluated the effects of processing parameters on grain size, phase distribution, and material characteristics, guiding the choice of fabrication techniques for optimizing Ti-6Al-4V performance in aerospace, biomedical, and automotive applications. The analysis emphasizes tailored processing to meet advanced engineering demands.

• 한국분말재료학회지
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• 제30권2호
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• pp.146-155
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• 2023

The Ti-6Al-4V lattice structure is widely used in the aerospace industry owing to its high specific strength, specific stiffness, and energy absorption. The quality, performance, and surface roughness of the additively manufactured parts are significantly dependent on various process parameters. Therefore, it is important to study process parameter optimization for relative density and surface roughness control. Here, the part density and surface roughness are examined according to the hatching space, laser power, and scan rotation during laser-powder bed fusion (LPBF), and the optimal process parameters for LPBF are investigated. It has high density and low surface roughness in the specific process parameter ranges of hatching space (0.06-0.12 mm), laser power (225-325 W), and scan rotation (15°). In addition, to investigate the compressive behavior of the lattice structure, a finite element analysis is performed based on the homogenization method. Finite element analysis using the homogenization method indicates that the number of elements decreases from 437,710 to 27 and the analysis time decreases from 3,360 to 9 s. In addition, to verify the reliability of this method, stress-strain data from the compression test and analysis are compared.