OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology Such changes are associated with the effects that occur in a liquid medium when ultrasonic frequency vibrations are introduced into it. First of all, these are cavitation and acoustic flows. Cavitation bubbles cause shock waves and cumulative jets upon collapse, which is accompanied by instantaneous pressures up to hundreds of megapascals and temperatures up to several thousand degrees [40–44]. Acoustic flows of various scales mix the treated liquid medium and transfer cavitation bubbles through it [45–47]. Under the influence of cavitation and flows, a number of secondary effects occur, among which the greatest influence on the change in viscosity is played by heating, which occurs when acoustic energy is absorbed by a liquid medium. To assess the effect of heating, the temperature of the adhesive at this moment is indicated on the graphs (Fig. 7) at the beginning and end of each stage of viscosity change. It has also been previously found that the critical heating temperature of the EDP adhesive, after which the polymerization reaction is sharply accelerated, is 45–50 °C. As a result, the processing can be described as follows: after the onset of ultrasonic vibrations under the influence of cavitation and acoustic flows, macromolecules and polymer chains are destroyed and the adhesive composition is uniformly mixed, which is accompanied by slight heating. These processes occur until a certain limit state is reached, at which an almost minimum viscosity is achieved. Next, the second stage begins, in which the work of cavitation bubbles and acoustic flows is spent on additional heating of the adhesive composition. At this point, two opposite processes occur, associated with an increase in temperature – a decrease in viscosity and an acceleration of polymerization. When the acceleration of polymerization begins to prevail, the third stage begins, characterized by a sharp increase in viscosity and heat release during the exothermic reaction. The different dynamics of dependencies are related to processing conditions. With an increase in the amplitude of vibrations, the number of cavitation bubbles increases, and acoustic flows increase, so that the three stages of viscosity change become faster. The optimal conditions are those in which the greatest decrease in viscosity is achieved with the least heating, which corresponds to the end points of the first (beginning of the second) stage. Further, specimens were prepared in the selected conditions to determine microhardness, analyze the submicrostructure, and test the adhesive joint for tensile strength. The results of the studies are presented in Table 2. Fig. 7. Change in adhesive viscosity depending on the ultrasonic treatment mode
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