Page 139 - Welding of Aluminium and its Alloys

124 The welding of aluminium and its alloys

If you are contemplating purchasing new or replacement MIG equip-
ment it is recommended that pulsed MIG power sources are purchased,
even though they are more expensive than conventional equipment. This
will give the fabrication shop a more flexible facility with a wider range of
options than with the straight DC units.

7.2.1.2 Fine wire MIG
As the name suggests the fine wire MIG process uses a fine, small
diameter wire, less than 1.2mm and as small as 0.4mm in diameter, although
wires of 0.4 and 0.6mm in diameter need to be specially ordered from the
wire drawer. Small diameter wires are notoriously difficult to feed and to
eliminate feeding problems a small wire reel and a set of drive rolls are
mounted directly on the welding torch. Welding parameters are in the
ranges 50–140 A and 17–22 V, resulting in a short-circuiting mode of metal
transfer. Travel speeds are generally around 320mm/min, giving low heat
input and enabling thin sheets, around 1mm in thickness, to be welded
without burn-through, excessive penetration or excessive cap height. The
fine wire process, although successful, has now largely been replaced by
pulsed MIG welding.

7.2.1.3 Twin wire MIG

A relatively recent development has been the twin-wire process. The
current that can be used is limited in the single wire process by the forma-
tion of a strong plasma jet at high welding currents. This jet may cause an
irregular bead shape, porosity or excess penetration. The twin wire process
overcomes these difficulties with two independent arcs operating in the
same weld pool, enabling major improvements in productivity to be
achieved. The basis of this is the use of two inverter-based pulsed MIG
power sources coupled in series, each complete with its own microproces-
sor control unit and wire feeder (Fig. 7.9). The two units are linked by a
controller that synchronises the pulses from each unit such that when one
unit is welding on the peak of a current pulse the other unit is on back-
ground current. By this means a stable welding condition is created with
the two arcs operating independently of each other. The wires are fed to a
single torch carrying two contact tips insulated from each other. The wires
may be positioned in tandem, side by side or at any angle in between
enabling the bead width and joint filling to be precisely controlled.
The limitation of twin wire MIG is that the process can only be used in
a mechanised or robotic application. With suitable manipulators, however,
it is capable of very high welding speeds, a 3mm leg length fillet weld, for