레이저 용융 적층 공정으로 제조된 소재의 초음파 나노표면개질에 관한 연구

Abstract

The laser metal deposition(LMD) technology has garnered much attention for being able to realize complex shapes and producing little waste material, thereby addressing the limits of existing metal product processing. Direct energy deposition (DED), a branch of laser melting deposition technology, can realize precise structures and excellent mechanical properties, and thus, it is used for various applications such as strengthening the surface of products(hardfacing) or remodeling and repairing the damaged components. Howerver, surfaces that have been deposited by DED exhibit poor quality because the deposition bead layer shape and spatter remain on the surfaces. The surface characteristics affect not only the appearance of the product but also its mechanical functions and lifetime, making post-milling processing indispensable for refining the surface after the DED process. Furthermore, In a metal additive manufacturing process, metal powder is melted by a high-power laser before solidification; thus, a rapid temperature change is experienced, which generates tensile residual stress. The tensile residual stress in the DEDed M4 may reduce the fatigue performance and shorten the product life. UNSM technology not only improving surface roughness and refining surface microstructure by applying severe plastic deformation(S2PD) to the material surface, but also induces the compressive residual stress and improves the wear-resistance of the materials. Given that UNSM can improve a material's surface performance, in this study, UNSM technology is applied to DEDed surfaces with the aim of improving the DED product's surface quality. It was found to the UNSM technology has a beneficial effect on reducing the surface roughness and waviness by improving the poor surface of DED products. Also, it was confirmed that the UNSM technology can induce a compressive residual stress to the DED product and improve mechanical performance including wear resistance.