©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II arc
must be reestablished each time it does so. The oxide coating on metals,
such as aluminum
and magnesium, can act much like a rectifier as discussed in Lesson I. The
positive half-cycle
will be eliminated if the arc does not reignite, causing an unstable condition.
Continuous high frequency maintains
an ionized path for the welding arc, and assures arc re-
ignition each time the current changes
direction. AC is extensively used for welding aluminum
and magnesium.
2.3.2.9 AC/DC
Constant Current
Power Sources
- Designed for gas tungsten arc
welding, are available, and can be
used for welding practically all metals. The gas tungsten
arc welding process is usually chosen
because of the high quality welds it can produce. The
metals that are commonly welded with
this process, such as stainless steel, aluminum and some
of the more exotic metals, cost many times the price of mild steel; and therefore,
the power sources
designed for this process have many desirable features to insure high quality
welds. Among these are:
1. Remote
current control, which allows the operator to control welding
amperage with
a hand control on the torch, or a foot control at the welding station.
2. Automatic
soft-start, which prevents a high current surge when the arc
is initiated.
3. Shielding
gas and cooling water solenoid valves, which automatically control
flow before, during and for an adjustable
length of time after the weld is completed. 4.
Spot-weld timers,
which automatically control all elements during each
spot-weld cycle.
Other options and accessories are also
available. 2.3.2.10 Power
sources for automatic welding with complete programmable output are also
available. Such units are used
extensively for the automatic welding of pipe in position. The
welding current is automatically varied
as the torch travels around the pipe. Some units provide
a pulsed welding current where the amperage is automatically varied between a
low and high several
times per second. This produces welds with good penetration and improved
weld bead shape.
2.3.3
Torches
- The torch is actually an electrode holder that supplies
welding current to the
tungsten electrode, and an inert gas shield to the arc zone. The electrode
is held in a collet-like
clamping device that allows adjustment so that the proper length of electrode
pro- trudes beyond
the shielding gas cup. Manual torches are designed to accept electrodes
of 3
