Directed energy deposition (DED) is one of the additive manufacturing(AM) technology that produce three-dimensional shapes by melting metal powder with a high-power laser beam. DED process can selective deposition because laser and powder are irradiated at the same time. It has been applied to various industrial fields such as product repair and surface reinforcement. However, the surface of DED has traces of the spatter, unmelted powder, and deposited tracks. In this study, we used ultrasonic nanocrystal surface modification (UNSM) to improve DED surface. UNSM, which is attached to the NC/CNC can precise modify the surface. In addition, it is excellent for improving mechanical properties as it can apply a compressive stress of up to 30 MPa 1.6 million times per minute.

This study aimed to broaden the range of UNSM application. So, surface laser remelting (SLR) is applied before UNSM process, to maximize the UNSM effect. In addition, the high-temperature characteristics of UNSM were analyzed by observing of UNSM-treated materials in a high-temperature environment. First, the surface and mechanical properties were improved using the SLR and UNSM process. The continuous application of SLR and UNSM improved significantly the surface roughness and waviness. In addition, the plastically deformed depth treated by UNSM could be further increased by applying SLR before UNSM treatment. As a result, it can improve the mechanical properties inside the material. Next, as a result of comparing the properties of the untreated material and the UNSM treated material in a high-temperature environment, the hardness improvement effect was maintained, but the abrasion properties decreased. In particular, it was confirmed that the amount of abrasion increased significantly at 500 ℃ because the dynamic recrystallization of the UNSM treated material occurred first. Because, UNSM treated specimen has high internal energy. In the future, mechanical property test in a high-temperature environment will be enforced. Through this, basic research on UNSM treated materials in a high temperature environment is to be prepared. These experimental results are expected to help the repair and post-processing industries using AM technology.