24
POWER SUPPLY
Most applications of gas metal arc welding use
a constant voltage power supply. As a result,
any change in arc length (which is directly re-
lated to voltage) results in a large change in
heat input and current. A shorter arc length
will cause a much greater heat input, which
will make the wire electrode melt more quickly
and thereby restore the original arc length.
This helps operators keep the arc length consis-
tent even when manually welding with hand-
held welding guns. To achieve a similar effect,
sometimes a constant current power source
is used in combination with an arc voltage-
controlled wire feed unit. In this case, a change
in arc length makes the wire feed rate adjust
in order to maintain a relatively constant arc
length. In rare circumstances, a constant cur-
rent power source and a constant wire feed
rate unit might be coupled, especially for the
welding of metals with high thermal conduc-
tivities, such as aluminum. This grants the op-
erator additional control over the heat input
into the weld, but requires significant skill to
perform successfully. Alternating current is
rarely used with GMAW; instead, direct cur-
rent is employed and the electrode is generally
positively charged. Since the anode tends to
have a greater heat concentration, this result
in faster melting of the feed wire, which in-
creases weld penetration and welding speed.
The polarity can be reversed only when spe-
cial emissive-coated electrode wires are used,
but since these are not popular, a negatively
charged electrode is rarely employed
TECHNIQUE
The basic technique for GMAW is quite sim-
ple, since the electrode is fed automatically
through the torch. By contrast, in gas tungsten
arc welding, the welder must handle a welding
torch in one hand and a separate filler wire in
the other, and in shielded metal arc welding,
the operator must frequently chip off slag and
change welding electrodes. GMAW requires
only that the operator guide the welding gun
with proper position and orientation along
the area being welded. Keeping a consistent
contact tip-to-work distance (the stick out
distance) is important, because a long stickout
distance can cause the electrode to overheat
and will also waste shielding gas. Stickout
distance varies for different GMAW weld
processes and applications. For short-circuit
transfer, the stickout is generally 1/4 inch to 1/2
inch, for spray transfer the stickout is generally
1/2 inch. The position of the end of the contact
tip to the gas nozzle are related to the stickout
distance and also varies with transfer type and
application. The orientation of the gun is also
important—it should be held so as to bisect
the angle between the workpieces; that is, at
45 degrees for a fillet weld and 90 degrees for
welding a flat surface. The travel angle, or lead
angle, is the angle of the torch with respect to
the direction of travel, and it should generally
remain approximately vertical. However, the
desirable angle changes somewhat depending
on the type of shielding gas used—with pure
inert gases; the bottom of the torch is often
slightly in front of the upper section, while the
opposite is true when the welding atmosphere
is carbon dioxide.
WHEN FINISHED
a. Release the Torch handle trigger and lift the
Torch handle from the workpiece.
b. Press the Power Switch to the Off (O) posi-
tion.
c. Set the Torch handle down on the metal
workbench.
d. Turn the air supply off.
e. Unplug the line cord from the electrical
outlet.
PENETRATION
1. Excess or Burn-
through : Weld droops
on top and underneath,
or falls through entirely,
making a hole.
a. Excessive material at weld; Reduce wire feed
speed
b. Overheating; Increase welding speed and
ensure that welding speed is kept steady.
2. Proper: Weld is visible
underneath and bulges
slightly on top.
SOLM1020 manual.indd 24
26/11/15 2:53 p.m.
