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.

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.