©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II chemical
terminology) are attracted to the negative pole, in this case the work, at high
velocity. Upon
striking the work surface, they dislodge the oxide coating permitting good electrical
conductivity for the maintenance of
the arc, and eliminate the impurities in the weld metal that
could be caused by these oxides.
2.3.2.5 Direct
current electrode positive is rarely used in gas-tungsten arc welding. Despite
the excellent oxide cleaning action,
the lower heat input in the weld area makes it a slow process,
and in metals having higher thermal conductivity, the heat is rapidly conducted
away from the
weld zone. When used, DCEP is restricted to welding thin sections (under
1/8") of magnesium
and aluminum. 2.3.2.6
Alternating
current is
actually a combination of DCEN and DCEP and is widely
used for welding aluminum. In
a sense, the advantages of both DC processes are combined,
and the weld bead produced is a compromise
of the two. Remember that when welding with 60
Hz current, the electron flow from the electrode tip to the work reverses direction
120 times every
second. Thereby, the intense heat alternates from electrode to work piece,
allowing the use
of an intermediate size electrode. The weld bead is a compromise having
medium penetration
and bead width. The gas ions blast the oxides from the surface of aluminum
and magnesium
during the positive half cycle. Figure 8 illustrates the effects of the
different types of
current used in gas-tungsten arc welding. 2.3.2.7
DC
constant current
power sources
- Constant current power sources, used for
shielded metal arc welding, may also
be used for gas-tungsten arc welding. In applications
where weld integrity is not of utmost
importance, these power sources will suffice. With
machines of this type, the arc must
be initiated by touching the tungsten electrode to the work
and quickly withdrawing it to maintain
the proper arc length. This starting method contaminates
the electrode and blunts the point which has been grounded on the electrode
end. These conditions can cause
weld metal inclusions and poor arc direction. Using a
power source designed for gas tungsten
arc welding with a high frequency stabilizer will eliminate
this problem. The electrode need not be touched to the work for arc initiation.
Instead, the high frequency voltage,
at very low current, is superimposed onto the welding current.
When the electrode is brought to within approximately 1/8 inch of the base
metal, the high
frequency ionizes the gas path, making it conductive and a welding arc is established.
The high frequency is automatically
turned off immediately after arc initiation when using direct
current. 2.3.2.8
AC
Constant Current
Power Source
- Designed for gas tungsten arc welding,
always incorporates high frequency,
and it is turned on throughout the weld cycle to maintain a
stable arc. When welding with
AC, the current passes through 0 twice in every cycle and the
