In this study, we investigated characteristics and mechanical properties of SKD61 repaired using the direct energy deposition (DED) process. Mechanical properties of the repaired product can vary depending on the base material and powder used in the DED process. To prepare for DED repairing for a damaged part, we conducted experiments using two different powders (H13 and P21). Experimental results showed that both powders were deposited without defects in the surface or interface between the deposited zone and the substrate. Hardness measurements indicated that the repaired region of the Repaired-H13 sample exhibited higher hardness than the base material, while the Repaired-P21 sample showed a sharp increase in hardness in the heat-affected zone (HAZ). Additionally, tensile test results revealed that the Repaired-H13 sample had lower tensile strength and elongation than the base material, whereas the Repaired-P21 sample demonstrated higher tensile strength and yield strength with a higher elongation than the Repaired-H13 sample. In case of Repaired-H13, it was confirmed that interfacial crack occurred due to a high hardness difference between the repaired part and the substrate.
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Microstructure and mechanical properties of P21 tool steel fabricated via laser powder bed fusion A. Rajesh Kannan, V. Rajkumar, S. Maheshwaran, N. Siva Shanmugam, Wonjoo Lee, Jonghun Yoon Materials Letters.2025; 398: 138930. CrossRef
This study aimed to determine effects of ultrasonic nanocrystal surface modification (UNSM) as a surface pre-process on performance and surface characteristics of bolts manufactured through a screw rolling process. Surface roughness, hardness, and microstructural changes after UNSM treatment were examined. Results showed no significant defects such as cracks in all fabricated samples after screw rolling of bolt pre-processed by UNSM treatment. In addition, material flow was continuously maintained without disconnection. After UNSM treatment, surface roughness was improved for both body and screw parts. The surface roughness of the UNSM treated screw part was improved the most at 43%. Hardness test showed the greatest increase in hardness on the surface hit by the UNSM ball tip, with hardness improved to about 500 μm deep from the surface. The hardness at the screw part was the highest at 471 HV, which was attributed to the fact that grains near the surface were deformed and refined by UNSM treatment followed by screw rolling. Near the surface of the screw, refined grains and high dislocation density were clearly observed by EBSD mapping. These results confirm that UNSM treatment before screw rolling is effective in improving mechanical properties of screw rolled bolts.
Hybrid additive manufacturing (AM) refers to a combination of two metal AM techniques: material deposition by powder bed fusion (PBF) and additional building by directed energy deposition (DED). This study focused on different characteristics in accordance with relative deposition directions of PBF and DED during hybrid AM production. Characteristics of the sample fabricated by hybrid AM (i.e., hybrid sample) were compared with those of the sample fabricated by PBF or DED. Ferrite was dominant in the microstructure of PBF deposits with very fine retained austenite observed locally. In contrast, lath martensite and retained austenite were formed uniformly in the microstructure of DED deposits. Different microstructures in the two processes were attributed to differences of cooling rate. In DED deposits, microhardness was significantly decreased owing to a high retained austenite fraction. However, in the hybrid sample, microhardness was rapidly increased in the HAZ owing to aging heat treatment for long-term deposition. Principal wear mechanisms of PBF and DED samples were oxidative wear and plastic deformation, respectively.
Anodic aluminum oxide (AAO) is widely used in various industrial fields to increase the mechanical property or corrosion resistance of the product surface. In this study, mechanical properties were measured according to the thickness of AAO through the nanoindentation test. The maximum indentation load, elastic modulus, and hardness were measured for different thicknesses of AAO. It was confirmed that the majority of the mechanical property values increased with the thickness. Various fracture shapes based on the thickness were analyzed by observing pressure marks on the surface using FE-SEM equipment. Apparently, it is proposed that the optimum AAO thickness with desired mechanical properties can be obtained, which is expected to possess immense economic value as per the optimization of the production time of AAO based products.
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In-situ Wired and Wireless Material Testing System with Nanometer-level Displacement Control Kyoung Seok Park, Pill Ho Kim, Chung-Seog Oh Journal of the Korean Society for Precision Engineering.2024; 41(11): 881. CrossRef
Recently, interest in astronomy has increased internationally, and the technological development of lenses for large space telescopes is progressing. The multi-order diffractive engineered (MODE) lenses can make a large space telescope light and thin. However, because glass lenses are difficult to machine, we have adopted a method of molding at high temperature and high pressure. The STAVAX is commercially available chrome alloy stainless steel, and it is applied as various mold materials. The ultrasonic vibration cutting was adopted for ultra-precision machining because the tool wear is severe when cutting the STAVAX with a diamond tool. To achieve a flat surface for smooth ultrasonic vibration cutting, we performed a precise shape cutting using a CBN tool and confirmed and observed changes in the surface roughness and hardness depending on the cutting conditions. The ultrasonic vibration cutting was performed on the surface of the machine using a CBN tool, and the surface roughness was observed. It was confirmed that the surface roughness was impacted by the surface hardness. The specimens with low surface hardness showed the highest surface roughness at approximately 3 nm.
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Study on Reduction of Pyrolysis Shrinkage in the Carbonization of Furan Precursor by Addition of Vitreous Carbon Powder Young Kyu Kim, Dong-in Hong, Hongmin Kim, Suho Ahn, Seok-Min Kim Journal of the Korean Society for Precision Engineering.2024; 41(2): 139. CrossRef
Fabrication and Characterization of Automotive Aspheric Camera Lens Mold based on Ultra-precision Diamond Turning Process Ji-Young Jeong, Hwan-Jin Choi, Jong Sung Park, Jong-Keun Sim, Young-Jae Kim, Eun-Ji Gwak, Doo-Sun Choi, Tae-Jin Je, Jun Sae Han Journal of the Korean Society for Precision Engineering.2024; 41(2): 101. CrossRef
Analysis of Environmental Factors Affecting the Machining Accuracy Young Bok Kim, Wee Sam Lee, June Park, Yeon Hwang, June Key Lee Journal of the Korean Society of Manufacturing Process Engineers.2021; 20(7): 15. CrossRef
The objective of this study was to perform surface hardening experiments of titanium alloy using laser. The surface hardness value after laser hardening treatment was observed to increase with respect to the inflow of laser energy. However, when the laser energy exceeded the critical value, damage and cracks were observed on the surface of the material. The relationship between surface hardness values and process variables such as laser energy, scan speed, and number of laser scans was quantitatively modeled through the design of experiments and analysis of variance. Using the established mathematical model, the surface hardness value of the material can be predicted accurately with an average of 10% error over various process conditions. Analysis of the surface composition of the material using energy dispersive spectrometry showed that titanium oxide was the main cause of the increasing surface hardness. Further studies will be conducted to improve the accuracy and predictability of the model using nonlinear modeling techniques.
The present research investigates the wear analysis on SCM440 and SUS410 alloys for a high- load roller chain. In this wear analysis test, we concentrate on two wear factors: wear loss and coefficient of friction. For the wear test, reciprocating and block on ling analyses were used to assess the variation of wear characteristics. The applied normal loads were fixed at 10, 30, and 50 N in all tests. The test results showed that the Rockwell hardness average value of the SCM440 alloy is near 29.6 HRC, and the coefficient of friction and the wear loss are 0.62 and 3.8 mg, respectively. In addition, wear behavior was evaluated using lubricating oil.
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Study on Improvement of Catheter Tip Forming Process according to Plating Characteristics in Mold Han Chang Lee, Jinhyuk Jung, Gyu Ik Lee, Woojin Kim, Gyu Man Kim, Bong Gu Lee Journal of the Korean Society for Precision Engineering.2022; 39(9): 711. CrossRef
In this study, a numerical analysis on the impact response of HHA (High Hardness Armor Plate) sequences under a 7.62 mm projectile impact was performed to obtain the fundamental design data for a combat-vehicle platform. Recently, the ballistic-protection levels for combat vehicles have increased, and ballistic-protection designs should now be able to deflect multi-hit projectiles. To study the ballistic-impact characteristics, armor-plate sequences of one or two layers with a gap of 0 mm to 2 mm between the front and rear plate were defined under the same weight and thickness. For the certification of the reliability of the numerical model, ballistic tests and an analysis of the single plate under the 7.62 mm projectile impact were performed and analyzed. On the basis of a valid numerical model, a numerical analysis was performed and analyzed. Lastly, it was proved that the performances of the two-layer sequence with the 2 mm gap regarding the impact-response acceleration, deflection efficiency, and penetration depth are the highest.
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