*5.1. Structure and Surface Quality*

In the process of the laser powder bed fusion method in the steels under study, as a result of high cooling rates, a finely dispersed structure with supersaturated solid solutions is formed, which provides higher strength characteristics of steels after additive manufacturing than after traditional processing methods.

The maximum effect on the change in surface quality was achieved by ultrasonic cavitation-abrasive finishing based on the combination of cavitation, acoustic flows, and abrasive particles that were evaluated quantitatively. A twofold decrease in the parameters of micro- and submicroroughness was achieved (roughness parameter *Ra* was reduced from 7.24 to 3.04 μm), and no erosion caverns were detected on the researched surfaces. The use of a vibratory grinder reduces the roughness parameter *Ra* of the sample walls made of 12kH18N9T (AISI 321) powder from 14.1 to 5 μm. However, it should be borne in mind that using a vibratory grinder usually reflects the service life of parts negatively.

The quantitative evaluation showed that the di fference in the chosen material's mechanical and physical properties did not reflect the maximal amplitude of 2.0·10−<sup>6</sup> m when δ of the samples was about 6·10−<sup>12</sup> m. The chosen frequency ratio to the natural frequency was about 0.5 and can be improved to improve processing e ffectiveness and stability without adding abrasive. The frequency of the forced oscillations should be in the interval of 29–54 kHz for 20kH13 (AISI 420) steel and 28–52 kHz for 12kH18N9T (AISI 321) steel.
