Additive manufacturing(AM) processes using metals are classified into powder bed fusion (PBF) and direct energy deposition (DED), and their components are built in a similar layer-by-layer manner. PBF components are manufactured by selectively melting or sintering powders pre-placed on the build plate, thereby depositing the 2D sections of a 3D shape in the height direction. In DED, the metal surface is melted by irradiating with a laser and supplying metal powder to form a melt pool. Then, the 2D sections of a 3D shape are manufactured layer-by-layer.

AM is more economical than conventional subtractive manufacturing processes due to reduced amount of discarded materials. Moreover, because it can also produce shapes difficult to manufacture by subtractive manufacturing, AM is considered an innovative technology and is being widely applied in various industries for the manufacture of automotive, aerospace, and medical components.

Directed Energy Deposition (DED) is one of the representative metal additive manufacturing techniques used to increase the surface hardness and abrasion resistance of metal parts to achieve desired mechanical properties for the part surface. This method is also utilized in the remanufacturing industry (e.g., remodeling and repair). However, since the surface of the part manufactured through DED is rough, post-processing is required. Surface laser remelting (SLR), in which a laser beam remelts the deposited surface, can improve the surface quality of DED samples.

The SLR can modify the As-built surface without additional equipment. After the deposition process is over the powder supply is stopped, and the laser is scanning the As-built surface. it removes the unmelted powder and spatter formed in the unstable melting pool remaining on the surface . In addition, hardness of SLR treated surface can be increased through rapid cooling of the surface layer.

Ultrasonic nanocrystal surface modification (UNSM) is a technology that uses ultrasonic vibration energy to impact the surface of a material with an amplitude of several μm and tens of thousands of times per second to modify the surface. It makes micro dimples at regular intervals on the surface. In addition, surface severe plastic deformation (S2PD) is induced in UNSM applied area. And the microstructure of the material is refined to a specific depth from the surface.

As can be seen from the schematic diagram of the device, the oscillator generates ultrasonic waves. And it is passed through transducer and booster. When a load is applied to the ball tip located at the end of the device, a compressive stress of up to 30 MPa is applied to the surface at 1.2 million cycles per minute. Through the UNSM process, It can reduce surface roughness and modulus friction. In addition, the surface hardness is improved by forming a severe plastic deformation. Surface that UNSM treated is formed large compressive residual stress.