| Summary: | Ultrasonic nanocrystal surface modification (UNSM) technology can be used for the surface treatment of specimens produced by additive manufacturing using directed energy deposition. We investigated the change in the surface quality characteristics of the treated specimens with respect to the UNSM process variables. The roughness and waviness of the deposited surface were adopted as the objective functions and the optimal process conditions were determined by the response surface method (RSM). The surface waviness and roughness were determined to be most affected by the static load, with the scanning speed having the least effect. It was observed that a smaller inter-path interval decreased the surface waviness and roughness, and an excessively large or small static load deteriorated the surface modification results. The finally optimized conditions consisted of a static load of 45 N, inter-path interval of 10 μm, and scanning speed of 2600 mm/min. These conditions produced a surface waviness of 1.8097 μm and roughness of 0.3297 μm, which represent improvements of 80% and 72%, respectively, compared with the untreated specimen. It was further observed that UNSM significantly refined the material grains and considerably increased the martensite. This resulted in a surface microhardness increase of up to 71.2%.
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