Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
detachment force due to its mass overcomes the surface tension force which
acts to prevent droplet separation. Although electromagnetic forces exist
they are not sufficiently developed to influence the droplet detachment at
low currents.
A low mean current is used but the process has very limited positional
capabilities with solid wire GMAW because of the dominant nature of
gravitational forces.
7.2.2 Globular repelled transfer
In some circumstances, a droplet may form at the end of the electrode and be
deflected to one side or even expelled from the arc. This behaviour is commonly
found when electrode negative polarity is used with a solid wire and is
illustrated in Fig. 7.3. The dominant transfer force is gravitational but repulsion
is caused by electromagnetically induced plasma forces or vapour jets which
act on the base of the droplet, at the arc root, to lift the molten material. Once
the droplet has been lifted in this way an asymmetrical magnetic field is
created and the droplet may be rotated or expelled under the influence of the
resultant forces as discussed below. This mode of transfer is usually undesirable
due to the poor stability and high spatter levels which result.
7.2.3 Projected spray transfer
As the current is increased, the size of the droplet usually decreases and the
frequency of transfer increases. In addition, it is found that the droplets are
Repelled
drop
Arc
7.3 Repelled transfer.
Droplet