Instructions / Assembly

8
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GMAW
Inductance Control
K
eywords:
Rate of Current Rise
Henries
Variable Inductance
F
ixed Inductance
The application of an inductance control feature is typical for
most GMAW power sources. Inductance has effects
only in the
short-circuit transfer mode. Usually, inductance is either fixed or
variable; and this depends upon the design of the power source.
A fixed inductance power source indicates that an optimum level
o
f inductance is built into the power source, and variable
inductance indicates that the amount of inductance applied to
the arc is adjustable. Inductance controls the rate of current rise
following the short-circuit condition. Consequently, its use is
beneficial because its adjustment facilitates adding or decreasing
energy to the short-circuit condition. Inductance plays a role in
the frequency of droplet transfer per unit of time: as the
inductance increases, the frequency of short-circuit metal
transfer decreases. Each droplet contains more energy and toe
wetting improves. As the inductance decreases, the short-
circuit events increase, and the size of the molten droplet
decreases. The objective for the variable inductance control
feature, on any given power source, is to transfer the smallest
molten droplet possible with the least amount of spatter, and
with sufficient energy to ensure good fusion. Additions of
inductance will provide the essential energy to improve toe wetting.
Inductance is measured in Henries, and in a variable inductance
power source it is the resulting arc performance characteristic
that results from the interplay of a combination of electrical
components. These components typically include the choke
filter, capacitors, and power resistors.
Globular Transfer
Globular metal transfer is a GMAW mode of metal transfer,
whereby a continuously fed solid or metal-cored wire electrode
is deposited in a combination of short-circuits and gravity-assisted
large drops. The larger droplets are irregularly shaped.
During the use of all metal-cored or solid wire electrodes for
GMAW, there is a transition where short-circuiting transfer ends
and globular transfer begins. Globular transfer characteristically
gives the appearance of large irregularly shaped molten droplets
that are larger than the diameter of the electrode. The irregularly
shaped molten droplets do not follow an axial detachment from
the electrode, instead they can fall out of the path of the weld or
m
ove towards the contact tip. Cathode jet forces, that move
u
pwards from the work-piece, are responsible for the irregular
shape and the upward spinning motion of the molten droplets.
The process at this current level is difficult to control, and spatter
is severe. Gravity is instrumental in the transfer of the large
molten droplets, with occasional short-circuits.
During the 1960’s and 1970’s, globular transfer was a popular
mode of metal transfer for high production sheet metal fabrica-
tion. The transfer mode is associated with the use of 100% CO
2
s
hielding, but it has also seen heavy use with argon/CO
2
b
lends.
For general fabrication on carbon steel, it provides a mode of
transfer, just below the transition to axial spray transfer, which
has lent itself to higher speed welding.
The use of globular transfer in high production settings is being
replaced with advanced forms of GMAW. The change is being
made to GMAW-P, which results in lower fume levels, lower or
absent spatter levels, and elimination of incomplete fusion
defects.
Advantages of Globular Transfer
• Uses inexpensive CO
2
shielding gas, but is frequently used
with argon/CO
2
blends.
• Is capable of making welds at very high travel speeds.
• Inexpensive solid or metal-cored electrodes.
• Welding equipment is inexpensive.
Limitations of Globular Transfer:
• Higher spatter levels result in costly cleanup.
• Reduced operator appeal.
• Prone to cold lap or cold shut incomplete fusion defects,
which results in costly repairs.
• Weld bead shape is convex, and welds exhibit poor wetting at
the toes.
• High spatter level reduces electrode efficiency to a range of
87 – 93%.
FIGURE
3: Globular Weld Metal Transfer Characteristics
Globular Transfer