Figure 8-5.The ac welding cycle.
Figure 8-4.Effects of polarity on the weld.
also turn the main welding current on and off at the same
time. This not only allows the operator to start and stop
without leaving the work but also to adjust the current
Most of these welding machines can produce both
ac and dc current. The choice of ac or dc depends on the
welding characteristics required.
DIRECT CURRENT. As you learned in chapter
7, a direct-current welding circuit maybe either straight
or reverse polarity. When the machine is set on straight
polarity, the electrons flow from the electrode to the
plate, concentrating most of the heat on the work With
reverse polarity, the flow of electrons is from the plate
to the electrode, thus causing a greater concentration of
heat at the electrode. Because of this intense heat, the
electrode tends to melt off; therefore, direct-current
reverse polarity (DCRP) requires a larger diameter elec-
trode than direct-current straight polarity (DCSP).
The effects of polarity on the weld are shown in
figure 8-4. Notice that DCSP produces a narrow, deep
weld. Since the heat is concentrated on the work, the
welding process is more rapid and there is less distortion
of the base metal. Overall, straight polarity is preferred
over reverse polarity because you can achieve better
DCRP forms a wide and shallow weld and is rarely
used in the GTAW process. The exception to this is when
it is used to weld sections of aluminum or magnesium.
DCRP has excellent cleaning power that results from the
Figure 8-6.ACHF combines the desired cleaning action of
DCRP with the good penetration of DCSP.
action of positive-charged gas ions. When these gas ions
strike the metal, they pierce the oxide film and form a
path for the welding current to follow. This same clean-
ing action occurs in the reverse polarity half of an
alternating-current welding cycle.
ALTERNATING CURRENT. AS shown in fig-
ure 8-5, ac welding is actually a combination of DCSP
and DCRP; however, the electrical characteristics of the
oxides on the metal often prevent the current from
flowing smoothly in the reverse polarity half of the
cycle. This partial or complete stoppage of current flow
(rectification) causes the arc to be unstable and some-
times go out. Ac welding machines were developed with
a high-frequency current flow unit to prevent this recti-
fication. The high-frequency current pierces the oxide
film and forms a path for the welding current to follow.
The effects of alternating current high-frequency
(ACHF) are shown in figure 8-6. Notice that ACHF
offers both the advantages of DCRP and DCSP. ACHF
is excellent for welding aluminum.