A hybrid cladding technology was developed by combining direct energy deposition (DED) and ultrasonic nanocrystal surface modification (UNSM). This is an effective process to control the mechanical properties inside the metal-clad layer, but the scope to improve the internal properties is low. Therefore, in this study, the UNSM process was applied while heating at 300 and 600℃ to increase the effectiveness of this hybrid additive process. To validate the characteristics of this method, a study on the cross-sectional properties upon application of heating was conducted. Hybrid cladding at 300 degrees produced improvements- over a 40% larger area than the results at room temperature. At 600 degrees, the hybrid cladding improved mechanical properties over a larger area by nearly 2 times. In this study, the characteristics of the roomtemperature and the high-temperature hybrid cladding process were analyzed. The proposed method shows a high improvement effect and is a promising method to improve the internal mechanical properties of the cladded layer.

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.