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.

We studied compressive behavior of two types of lattice structures having small-scale struts fabricated by utilizing a metal additive manufacturing process. Generally known, the lattice structure has some advantages such as lightweight and high specific mechanical strength, allowing diverse potential applications in the aerospace and mobility industries. In this work, we proposed two types of lattice such as body-centered truss (BCT) and octahedral truss (OCT) that were designed and fabricated for a compression test. From the experimental results, the OCT has much higher strength than the BCT, and all cases showed several buckling modes during the compressive behavior. Furthermore, ‘restructuring’ occurred with BCT, and the compressive force increased overall but fluctuated due to the restructuring by an increase of compression. Through this work, we found out that the BCT has the interesting compressive behaviors, and a repetitive bucking-restructuring was found. In fact, its strength could be increased continuously by the restructuring during compression. In conclusion, the BCT has key-characteristics of lightweight and re-strengthening, which are applicable to various applications in the industry.

Recently, the use of stainless steels have been increased steadily as a sustainable structural material in infrastructures and thanks to its superior corrosion resistance, fire resistance and ductility compared with those of carbon steels. In this paper, block shear fracture behaviors in base metal of fillet-welded connection fabricated with austenitic stainless steel (STS304L) were investigated through monotonic tensile test. Main variables are weld lengths in the longitudinal and the transverse directions of applied force. Gas tungsten arc welding (GTAW) which is also known as tungsten inert gas (TIG) welding was chosen to join two metals. As a result, test specimens failed by typical block shear fracture (the combination of tensile fracture and shear-out fracture) in base steel. With the increase of two weld lengths, the ultimate strengths of specimens tended to get higher. Block shear fracture strengths predicted by current design specifications and existing proposed equations for welded connections were compared with those of test results. It is found that the discrepancy of strength prediction resulted from the effect of stress triaxiality on welded connections and the difference of material properties with carbon steel. Therefore, modified block shear fracture equation was suggested in this paper.

This paper aims to verify the reliability of the creep-life assessment regarding the STS304H-Type tube for which the hardness method of H. Tanaka is used. For this purpose, the creep-rupture test and the hardness test were conducted with a new tube and used tubes that were exposed to 96,000 hr under a 650oC condition. The hardness value of the used tubes is higher than that of the new tube by approximately 12 Hv to 15 Hv. This test result was applied to the creep-life assessment of the STS304H-Type for which the H. Tanaka hardness method was used, and the life consumption of the used tubes was evaluated as 13%. The rupture times of the new and used tubes are 802 hr and 707 hr, respectively. The use of the test results as a substitution of the results of the Larson Miller Parameter for a life-assessment tool produced a life-consumption calculation of approximately 12%. Similar results can be confirmed between the Larson Miller Parameter method and the hardness method.