During the welding of metals along with mechanical vibrations, uniform and finer grain structures can be produced. This increases the toughness and hardness of the metals, because of solidification effects at the weld pool surface. As the weld pool solidifies, grains are not only limited in size, but dendrites growing perpendicular to the fusion line are restricted. While the process is going on, dendrites can be broken up before they grow to become large in size. Hence, the microstructure of the weld metal is improved during the solidification process. Grain refinement occurs and hardness increases in the high-amplitude vibrated specimens. The increase in hardness seems related to the orientation of the crystals.
Also, welding processes inevitably induce a state of residual stress into materials and products. This poses a series of problems, in terms of dimensional stability, corrosion cracking, reduced fatigue life and structural integrity. The conventional way to relieve the residual stresses is by post-weld heat treatment (PWHT), which is an effective process, but it suffers from several disadvantages: the cost of treatment in terms of equipment and energy is high; the growth of oxide scale on the surface implies the need for subsequent finishing processes to remove the scale; in many metals, annealing relieves residual stresses at the cost of important mechanical properties and, in some metals, PWHT is unable to relieve the residual stresses. The use of vibration in welding to reduce residual stresses has been observed.
Reference: The effect of vibratory stress on the welding microstructure and residual stress distribution
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