Page 97 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 86 Wednesday, October 18, 2006 7:36 AM
86 Ship Construction
Basically the process is as illustrated in Figure 9.6, a wire feed motor
supplying wire via guide rollers through a contact tube in the torch to the
arc. An inert gas is supplied to the torch to shield the arc, and electrical
connections are made to the contact tube and workpiece. Welding is almost
always done with a DC source and electrode positive for regular metal
transfer, and when welding aluminium to remove the oxide film by the action of
the arc cathode. Although the process may be fully automatic, semi-automatic
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processes as illustrated with hand gun are now in greater use, and are
particularly suitable in many cases for application to shipyard work.
Initially aluminium accounted for most of the MIG welding, with argon
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being used as the inert shielding gas. Much of the welding undertaken on
aluminium deckhouses, and liquid methane gas tanks of specialized carriers,
has made use of this process. Generally larger wire sizes and heavier
currents have been employed in this work, metal transfer in the arc being
by means of a spray transfer, that is metal droplets being projected at high
speed across the arc. At low currents metal transfer in the arc is rather dif-
ficult and very little fusion of the plate results, which has made the welding
of light aluminium plate rather difficult with the MIG/argon process. The
introduction of the ‘pulsed arc’ process has to some extent overcome this
problem and made positional welding easier. Here a low level current is used
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with high level pulses of current which detach the metal from the electrode
and accelerate it across the arc to give good penetration.
Early work on the welding of mild steel with the metal inert gas process
made use of argon as a shielding gas; but as this gas is rather expensive, and
satisfactory welding could only be accomplished in the downhand position,
an alternative shielding gas was sought. Research in this direction was con-
centrated on the use of CO as the shielding gas, and the MIG/CO process
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is now widely used for welding mild steel. Using higher current values with
thicker steel plate a fine spray transfer of the metal from the electrode across
the arc is achieved, with a deep penetration. Wire diameters in excess of
1.6mm are used, and currents above about 350 amps are required to obtain this
form of transfer. Much of the higher current work is undertaken with auto-
matic machines, but some semi-automatic torches are available to operate
in this range in the hands of skilled welders. Welding is downhand only.
On thinner plating where lower currents would be employed a different
mode of transfer of metal in the arc is achieved with the MIG/CO process.
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This form of welding is referred to as the dip transfer (or short circuiting)
process. The sequence of metal transfer is (see Figure 9.6):

1. Establish the arc.
2. Wire fed into arc until it makes contact with plate.
3. Resistance heating of wire in contact with plate.
4. Pinch effect, detaching heated portion of wire as droplet of molten metal.
5. Re-establish the arc.