Page 160 - Welding of Aluminium and its Alloys

MIG welding 143

Table 7.6 High current mechanised MIG parameters

Thickness Joint type Backing Current Voltage Travel speed
(mm) (A) (V) (mm/min)
12 Square edge Temporary 400 26.5 380
12 Square edge Permanent 450 29 350
19 Square edge Temporary 540 33 275
19 Square edge Two sided 465 29.5 380
25 Square edge Two sided 540 33 275
32 Square edge Two sided 530 33 275
(6mm sight V)

To illustrate the cost benefits of mechanisation take as an example a
12mm thick butt weld. Made using manual MIG this would require four
passes to fill at a travel speed of around 175mm/min, a total weld time
of over 20 minutes per metre. A machine weld using argon as the shield
gas could be made in a single pass at around 480mm/min travel speed, a
total weld time of just over 2 minutes. Using helium as the shielding gas
would reduce this time even further. A set of typical parameters is given in
Table 7.6.
Because of the higher duty cycle achievable with mechanised or auto-
mated welding the power source,wire feeder and torch must be more robust
and rated higher than those required for manual welding.Welding currents
of 600 A or more may be used and this must also be borne in mind when
purchasing a power source. The torch manipulator, whether this is a robot,
a dedicated machine or simply a tractor carriage, must have sufficient power
to give steady and accurate motion at a uniform speed with repeatable,
precise positioning of the filler wire.Although at low welding currents con-
ventional manual equipment may be adapted for mechanisation by attach-
ing the torch to a manipulator, it is advisable to use water-cooled guns and
shielding gas shrouds designed to provide improved gas coverage.

7.6 Mechanised electro-gas welding
A technique described as electro-gas welding was developed by the Alcan
Company in the late 1960s but seemed to drop out of favour in the late
1990s, which is surprising when the advantages of the process are consid-
ered. The weld may only be carried out in the vertical-up (PF) position but
is capable of welding both square edge butt joints and fillet welds with
throats of up to 20mm in a single pass.
To operate successfully the process uses a long arc directed to the back
of the penetration cavity. This provides a deeply penetrating arc that