Fan, Yang, Lin, Fan and Liu report on use of ultrasonic waves to assist GMAW welding. Ultrasonic vibration adds an additional force which is able to help control droplet size in welding. Above is shown their pictures for water and weld metal, high speed camera was used with frame eate of 150 frames per second (Hz) and ultrasonic frequency imposed was 20 kHz with amplitude of 30 μm.
On youtube (http://www.youtube.com/watch?v=uENITui5_jU brusspup has produced a video demonstrating some optical illusions which can be produced when combining video camera and vibration on flowing water.
In bowls and large containers the perturbations on the surface caused by vibrating the container are named after Michael Faraday who published a paper describing the effect in 1831 (Philosophical Transactions of the Royal Society (London), vol. 121, pages 299–318.). Faraday waves/ripples are described in the appendix M. Faraday, On the forms and states assumed by fluids in contact with vibrating elastic surfaces.
I think this should be the same phenomenon when ultrasonically vibrating the weld pool, although it may be worthy of a different name I am not aware of. Applying vibration to the liquid can produce a surface instability which can form complex patterns and standing waves. It seems reasonable that applying a vibration will have an effect on droplet size and perhaps allow more reproducible welding results. In their paper Fan et al measured increased frequency of metal transfer (smaller droplet size) at all feed rates when using ultrasonically assisted GMAW. The ultrasonic vibration also adjusted the shape of the droplet, elongating it along the welding axis. Further work will assess the welds formed by this process, and also effect of acoustic environment for process control.