Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
through the arc as shown in Fig. 7.5. This mode of transfer is called streaming
and it is caused by a significant increase in electromagnetic forces. It occurs
more readily with high-resistivity, small-diameter wires (e.g. austenitic stainless
steel) operating at currents above 300 A. Weld pool turbulence and gas
entrainment may limit the usefulness of this mode of transfer.
7.2.5 Drop spray transfer
The transition to projected spray transfer occurs over a relatively narrow
current range but it has been found [111] that an important intermediate
transfer mode can occur in this transition range. This mode of transfer is
characterized by the formation of a solid conic neck on the wire tip and
spherical droplets slightly larger in diameter than the diameter of the filler
wire are initially suspended from the tip before being detached (Fig. 7.6).
Detachment occurs very efficiently and high droplet velocities and very low
spatter losses are measured. With a 1.2 mm carbon steel wire this transfer
mode occurs between 250 and 270 A in argon/5% CO 2, drop velocities of
7 m min –1 have been recorded and a slight increase in melting rate is observed.
The drop spray mode is efficient and ‘clean’ with very low spatter and
particulate fume levels, but, under normal steady DC operating conditions,
it requires very close control of the welding parameters and this can only be
achieved with the high-quality electronic power sources described in Chapter
3; in addition, the operating range is very restricted. The process range can,
7.5 Streaming transfer.
Arc
Droplets