Page 98 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 87 Wednesday, October 18, 2006 7:36 AM
Welding and Cutting Processes used in Shipbuilding 87
To prevent a rapid rise of current and ‘blast off’ of the end of the wire
when it short circuits on the plate, variable inductance is introduced in
the electrical circuit. Smaller wire diameters, 0.8 mm and 1.2 mm, are used
where the dip transfer method is employed on lighter plate at low currents.
The process is suitable for welding light mild steel plate in all positions.
It may be used in shipbuilding as a semi-automatic process, particularly for
welding deckhouses and other light steel assemblies.
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The pulsed MIG/argon process, developed for positional welding of light
aluminium plate, may be used for positional welding of light steel plate but
is likely to prove more expensive.
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Use of the MIG semi-automatic processes can considerably increase
weld output, and lower costs.
This form of welding may also be collectively referred to as MAGS welding,
i.e. metal arc gas-shielded welding.

Plasma Welding This is very similar to TIG welding as the arc is formed
between a pointed tungsten electrode and the plate. But, with the tungsten
electrode positioned within the body of the torch, the plasma arc is sepa-
rated from the shielding gas envelope (see Figure 9.6). Plasma is forced
through a fine-bore copper nozzle which constricts the arc. By varying the
ึ
bore diameter and plasma gas flow rate three different operating modes
can be achieved.

(a) Microplasma – the arc being operated at very low welding currents
(0.1 to 15 amps) and used for welding thin sheets (down to 0.1mm
thickness).
(b) Medium current – the arc being operated at currents from 15 to
200 amps, plasma welding is an alternative to conventional TIG
welding. but with advantage of achieving deeper penetration and
having greater tolerance to surface contamination. Because of the
bulkiness of the torch it is more suited to mechanised welding than
hand welding.
(c) Keyhole plasma – with the arc being operated at currents above
100 amps and by increasing the plasma flow a very powerful plasma
beam is created. This can penetrate thicknesses up to 10mm but
when using a single pass technique is normally limited to a thickness
of 6mm. This operating mode is normally used for welding sheet
metal (over 3mm) in the downhand position.

Other Welding Processes

There are some welding processes which cannot strictly be classified as gas
or arc welding processes and these are considered separately.