Abstract <p>The results of the study of changes in the microstructure and elemental composition of titanium waveguides made of BT3-1 alloy (Ti—6 Al—2 Sn—4 Zr—2 Mo), which are used for ultrasonic welding of plastic products, are presented. Using a combination of methods, including optical, confocal scanning laser microscopy, raster and transmission electron microscopy, as well as energy-dispersive X-ray microanalysis, the morphology of the surface and the structure of the near-surface layers of the waveguides were studied. The processes of cavitation erosion, which lead to the formation of pores and a regular topography on the contact surface, were identified. In the near-surface layer, the development of the process of grinding the grain structure with the formation of nanosized crystallites (about 50 nm in diameter) with the formation of a surface amorphous layer (about 4 μm thick) enriched with carbon, oxygen, and silicon has been detected. It has been established that these changes are caused by the simultaneous effect of mechanical stress, high-frequency ultrasonic vibrations, local heating, and chemical interaction with the material of the welded parts, including the transfer of plastic components (such as SiO₂) into the waveguide material.</p>