Compressive residual stress on material surfaces leads to the improvement of fatigue resistance. However, cut surfaces generally have weak tensile residual stress, instead of the expected high compressive residual stress. In this study, high compressive residual stress exceeding -1000MPa was generated on the cut surface by appropriately combining ultrasonic vibration and changing the shape of the cutting edge. In addition, the surface hardness increased with increasing compressive residual stress on the cut surface. Fatigue testing showed that the improvement of the surface integrity contributed markedly to improvement of material fatigue resistance.
As powder bed fusion, which is the main technology in Additive Manufacturing, can built complex parts with high strength, it is expected to be useful as a next-generation manufacturing technology. However, this technology has problems associated with large surface roughness of parts. At present, the surfaces of parts are smoothed by post-processing procedures, such as mechanical polishing, but it is not possible to deal with complicated shapes. Therefore, a method for smoothing the surfaces during the build process is required. Factors that cause the surface of parts to be rough include insufficient melting of powder material and supply of the powder material to liquid regions. In this study, the influence of energy supply to the powder material on the surface roughness was investigated by varying the molten state of the processing region by re-exposure to a laser. The results indicated that the surface roughness can be reduced by optimizing the re-exposure energy, which also affects the strength of parts.