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.

This study investigated the influence of cooling rate and Sn addition on the microstructure formation of as-cast GCD700 spheroidal graphite cast irons. Changes in cooling rate manifested as step cast thickness differences. Optical microstructures of as-cast GCD700 alloys revealed α-ferrite and pearlite and dispersed graphite nodules. In all examined thicknesses without Sn, the α-ferrite, rather than the pearlite, surrounding graphite nodules appeared to dominate microstructures, and the graphite looked well rounded, whereas microstructure containing 0.09% Sn had a significantly expanded pearlite area. Image analysis showed numbers of graphite nodules increased only on decreasing cast thickness. However, the phase fractions of ferrite and pearlite were not dependent on thickness. For samples containing Sn, pearlite fractions significantly increased with Sn content. Thermodynamic calculations and scanning electron microscopy-based microstructural analysis confirmed that the Sn contents examined had no significant effect on phase formation, Sn segregation, or the relationships between ferrite and Fe3C orientations in pearlite.

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.

Isothermal low cycle fatigue (LCF) behavior of a crystal nickel-based superalloy CMSX-4, a material for high-pressure turbine first stage rotor blade, was investigated at elevated temperatures. Strain-controlled LCF tests were performed under various test conditions, such as mechanical strain amplitude. Stress response and cyclic deformation were investigated, and equations of LCF life prediction were derived through the Coffin-Manson method. In addition, fatigue-induced fracture mechanism and microstructural evolution were investigated, using scanning electron microscopy (SEM). Results revealed that cyclic behavior of the CMSX-4 superalloy, was characterized by cyclic softening with increasing number of cycles at 800oC and 900oC. LCF of the CMSX-4 superalloy at 800oC and 900oC could be affected mainly by elastic damage in fatigue processing. Fatigue cracks were initiated in the surface oxide layer of the specimen. The plane of fracture surface was tilted toward <001> direction. The fatigue fracture mechanism was quasi-cleavage fracture at 800oC and 900oC. In all broken specimens, the γˊ phase morphology maintained cuboidal shape.