Features of the superposition of ultrasonic vibrations in the welding process

Abstract

Introduction. The main problem in obtaining welded joints is the nonuniform heating of the joint zone, which leads to differences in the structure and properties of the weld metal and the base metal. One of the ways to intensify the welding process is the use of ultrasonic vibrations. As a result of the analysis of methods for introducing ultrasonic vibrations into the melting zone, a method of superimposing vibrations on the elements to be welded was chosen for experimental studies. This method makes it possible to influence the welded elements throughout the entire welding cycle from the melt bath to complete crystallization of the metal. Methods. Experimental studies were carried out on plates made of carbon structural steel St3 (ASTM A568M, AISI 1017, DIN 17100) and aluminum deformable non-hardened alloy AMg4 (EN AW-5086, AW-AL Mg4, 5086). As a source of oscillations, a rod magnetostrictive oscillatory system was used, the end of which was rigidly fixed on one of the welded plates. To determine the places of application of the oscillation source and the welding zone, a calculation method is proposed based on the equality of the resonant frequencies of the used oscillatory system and the natural frequency of bending vibrations of the welding component. It is shown that the optimal places for the application of vibrations and welding will be the antinodes of oscillations, which have the maximum amplitude. Welds were obtained by the method of semi-automatic gas metal arc welding. Results and Discussion. Microstructural study of obtained samples showed a significant decrease in the proportion of dendritic segregation. The changes in the structure are the result of the effects that occur in the liquid melt when ultrasonic vibrations are introduced. The main effects are sound pressure, cavitation and acoustical streaming. The structure change mechanism consists in the dispersion of growing dendrites and crystallization nuclei under the action of shock waves and cumulative jets that occur when cavitation bubbles collapse. The formed fragments of dendrites are new crystallization nuclei that propagate through the melt pool under the action of acoustic currents. Then the process is repeated. The resulting effects affect the kinetics of the crystallization process – the degree of supercooling increases, the number of crystallization nuclei formed per unit time increases, and the rate of its growth decreases. Changes in the structure of the weld metal lead to an increase in the quality of the welded joint, which reduces welding deformations, increases the tensile strength and significantly increases ductility.