©
COPYRIGHT 1998 THE ESAB GROUP, INC. LESSON
II 2.3.2.2
Direct
current electrode
negative (DCEN)
is produced when the electrode is
connected to the negative terminal
of the power source. Since the electrons flow from the
electrode to the plate, approximately
70% of the heat of the arc is concentrated at the work, and
approximately 30% at the electrode end. This allows the use of smaller tungsten
elec- trodes that
produce a relatively narrow concentrated arc. The weld shape has deep penetra-
tion and is quite narrow. See
Figure 8. Direct current electrode negative is suitable for weld-
ing most metals. Magnesium and
aluminum have a refractory oxide coating on the surface that
must be physically removed immediately
prior to welding if DCSP is to be used. 2.3.2.3
Direct
current electrode
positive (DCEP)
is produced when the electrode is
connected to the positive terminal
of the welding power source. In this condition, the electrons
flow from the work to the electrode
tip, concentrating approximately 70% of the heat of the arc
at the electrode and 30% at the work.
This higher heat at the electrode necessitates using
larger diameter tungsten to prevent
it from melting and contaminating the weld metal. Since
the electrode diameter is larger and
the heat is less concentrated at the work, the resultant
weld bead is relatively wide and shallow.
See Figure 8. 2.3.2.4
Aluminum and magnesium are two metals
that have a heavy oxide coating that acts as
an insulator and must be removed before successful welding can take place. Welding
with electrode
positive provides a good oxide cleaning action in the arc. If we were to
study the physics
of the welding arc, we find that the electric current causes the shielding gas
atoms to lose
some of their electrons. Since electrons are negatively charged, these gas
atoms now are
unbalanced and have an excessive positive charge. As we learned in Lesson
I, unlike charges
attract. These positively charged atoms (or positive ions as they are known
in FIGURE 8
Electrode Oxide
Heat Polarity
Penetration
Cleaning Concentration
Direct Current
Alternating Current
Medium Penetration
Medium Width Bead
Good Cleans
Oxide on Each
Half Cycle
Alternates Between
Electrode and Work
Straight Polarity
Electrode Negative
Deep Penetration
Narrow Bead
Direct Current
Reverse Polarity
Electrode Positive
Shallow Penetration
Wide Bead Maximum
None At
Work At
Electrode GAS
IONS +
_ ELECTRON
FLOW _
_ +
+ EFFECTS
OF CURRENT TYPE - GAS TUNGSTEN ARC WELDING
