Page 150 - Welding of Aluminium and its Alloys

MIG welding 135

7.4 Welding procedures and techniques

A set of outline welding procedures are given in Tables 7.2 and 7.3 for butt
welding using either argon or helium as the shielding gas, and guidance on
parameters for fillet welding is illustrated in Fig. 7.17. The parameters
quoted form a starting point from which to develop a procedure specifically
designed for the application. They are not to be regarded as hard and fast
rules. Also included as Table 7.4 are suggested weld preparations for MIG
welding of a range of plate thicknesses.

7.4.1 Arc starting

Because the wire is fed into the arc immediately that the arc is started
there can be no preheating of the joint as possible with TIG. This results
in shallow penetration and a humped weld bead on starting. Lack of
fusion defects are often encountered – a ‘cold start’ – and weld bead
shape may not be acceptable. To avoid these defects the welder should
strike the arc some 25mm ahead of the desired start point and then
move back to the weld start before beginning to weld forward at a normal
speed.
Arc starting may be achieved using a scratch start where the wire is
allowed to protrude from the contact tip by 10mm and brought to within
20mm of the surface. The trigger is operated and at the same time the
welding torch is moved to scrape the wire tip over the work surface. As
soon as the arc is established the power source senses the change in voltage
and starts the wire feed, the weld pool forms and welding can commence.
A ‘running’ start is one where the wire begins to feed as soon as the trigger
is operated and is short-circuited when it touches the workpiece, establish-
ing the arc. The current surge on short-circuiting may cause arcing within
the contact tip and spatter to adhere to the shroud and contact tip. These
can lead to wire feeding problems.
As mentioned earlier, the new inverter power sources have a facility for
a highly controlled arc start sequence.When the trigger is operated the wire
is fed at a slow and controlled rate until the wire tip touches the workpiece.
It is then retracted slightly and a pilot arc is ignited. Once this is stable the
current is increased at a controlled rate, the wire speed increased to the
desired feed rate and welding commences (Fig. 7.8). This gives a spatter-
free start and a low risk of lack of fusion defects, a major improvement over
the capabilities of older equipment.