OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 2 2022 Zone II: during further cooling without vibrations, dendrite growth continues and new dendrites are formed. Dendrites start forming along with vibrations. Due to specifi c features of the distribution of cavitation bubbles, last ones are primarily accumulated in places of small irregularities that are the zone of dendrites in this case. Zone III: dendrite growth continues without vibrations and nuclei start forming in the remaining volume of the molten pool. The collapse of cavitation bubbles results in the dispersion of dendrites, the fragments of which are carried by acoustic streams deep into the weld. These pieces are crystallization nuclei and at the same time the areas attracting cavitation bubbles. Zone IV: the growth of dendrites continues without vibrations; wherein non-dendritic nuclei grow and new dendrites are formed. Along with ultrasound, the cavitation activity goes down and the number of bubbles is reduced due to cooling and the associated increase in the molten metal viscosity. Dendrites and its fragments start to grow and new nuclei are formed. Bubble collapse continues having a dispersing effect. When the growth of dendrites and remaining nuclei continues without vibrations, the fi nal weld structure is formed. The effect of ultrasonic effects ceases when the melt reaches a high viscosity, and the dendrites and nuclei formed by this moment grow until complete solidifi cation. Thus, the introduction of ultrasonic vibrations reduces the crystallization start temperature, increases the number of nuclei formed, and decreases its growth rate. This results in a fi ne structure with a signifi cantly reduced proportion of dendritic segregation. Determination of weld joint properties The obtained changes in the microstructure lead to an increased weld joint quality. A weld joint obtained by applying vibrations and having a regular structure with decreased dendritic segregation has a lower shrinkage during cool-down, which reduces weld deformations. This causes a decreased defl ection of the joint (Fig. 9). a b Fig. 9. Geometrical parameters of a welded joint made of AMg4 alloy: a – without vibrations applied; b – with vibrations applied In the case of identical geometric parameters of the weld bead, as a result of shrinkage decreased for the St3 joint from 145ʹ to 21ʹ and from 124ʹ to 10ʹ for AMg4 joint. It means that elements welded with vibrations remain parallel, and without vibrations, the slope of one plate relative to the other will be ≈2.5 mm by 100 mm of length, which is especially critical for elongated welded structures. Tension tests of joints (Fig. 10) also show that the weld characteristics are improved. Applying vibrations results in a 5–10 % increased ultimate strength. The ultrasound had a higher effect on the plasticity of the weld metal, the elongation of which increases by 13–22 %.
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