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Welding Parameters and Techniques
Their Effects On The Weld
After having selected the wire and gas
for a weld, operating conditions must be chosen. The four important
parameters are the welding current,
wire electrode extension, welding voltage and arc travel speed. These
parameters will affect the weld characteristics
to a great extent. Because these factors can be varied over a
large range, they are considered the
primary adjustments in any welding operation. Their values should be
recorded for every different type of
weld to permit reproducibility. WELDING
CURRENT The welding
current is the electrical amperage in the power system as the weld is being made.
It is usually read
from the power source meter, but a separate ammeter is often used.
In the mig process, welding current
is directly related to wire- feed speed (if the wire extension beyond the
guide tip is constant). As the wire-feed
speed is varied, the welding current will vary in the same direction. In
other words, an increase (or decrease)
in the wire-feed speed will cause an increase (or decrease) of the
Figure 7-1 shows the typical wire-feed speed vs. welding current relationship
for various diameter E70S-3
wires. This relationship is commonly called the burn-off characteristic.
The graph also shows that when
the diameter of the wire electrode is increased (or decreased) at any wire-feed
speed, the welding current
is higher (or lower). Each type of wire (steel, aluminum, etc.) has a different
burn-off characteristic. One
important fact that should be noticed in Figure 7-1
is the shape of each burn-off curve. In the lower current
range for each wire size, the curve is nearly linear. In other words, for every
addition to the current, there
is a proportional (and constant) increase in the melt off. However, at higher
welding currents, particularly
with small diameter wires, the burn-off curve becomes non-linear. In this region,
higher welding currents
cause larger increases in the burn-off. This is due to resistance heating of the
wire extension beyond
the guide tube. This resistance heating is known at PR heat where I = welding
current and R = resistance.
The greater the welding current, the greater the PR heating.