Instructions / Assembly

6
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GMAW
T
he history of GMAW, gas metal arc welding, had its industrial
i
ntroduction in the late 1940’s. The site was the Battelle
Memorial Institute, and it was there that Hobart and Devers,
sponsored by the Air Reduction Company, researched and
developed the first use of a continuously fed aluminum wire
e
lectrode, shielded with 100% argon gas.
Axial spray transfer for aluminum was the earliest metal transfer
m
ode for the process. This eventually led to the use of argon
plus small additions of oxygen. The oxygen improved arc stability
and finally permitted the use of axial spray transfer on ferrous
materials. The process was limited because of the high energy
level of axial spray transfer to plate thickness material.
In the early 1950’s, the work of Lyubavshkii and Novoshilov
initiated the development of the GMAW process to include the
use of large diameters of steel electrode shielded with carbon
dioxide, a reactive gas. The process development at this stage
was high in weld spatter, and the level of heat generated by the
arc made the process uninviting to welders.
In the late 1950’s improvements in power source technology
and the interface of small diameter electrodes, in the 0.035" -
0.062" (0.9 - 1.6 mm) diameter range, permitted the implemen-
tation of the discrete mode known as short-circuiting transfer.
This development permitted the use of lower heat input welding
on thin sections of base material, and it provided the opportunity
for all-position welding.
In the early 1960’s, power source research and development led
to the introduction of pulsed spray in the GMAW mode. The
idea for pulsed spray transfer, GMAW-P, occurred in the 1950’s
and it conceptually involved the use of a high-speed transition
between a high-energy peak current to a low background
current. The motivation behind the idea was the need to
decrease spatter and eliminate incomplete fusion defects. The
pulsed arc process incorporated the benefits of axial spray
transfer — clean, spatter-free welds having excellent fusion,
with lower heat input. The lower average current provided by
GMAW-P allowed for out-of-position welding capability with
improved weld quality, when compared with short-circuit
transfer.
The 1970’s introduced power source technology, which further
enhanced the development of the GMAW process and GMAW-P
in particular. This period saw the incorporation of the earliest
thyristor power sources for pulsed GMAW. The Welding
Institute of the United Kingdom is largely responsible for
determining the linear relationship between pulsed frequency
and wire feed speed. The algorithm for this mathematical relation
-
ship permitted a fundamental base for subsequent synergic
transistor controlled power sources. The new high speed
electronic controls improved the interface between welding
sophistication and the welding shop floor. The new descriptor
for this development was the word "Synergic." As it relates,
synergy means: one knob control – as the welder increases or
decreases wire feed speed, a predetermined pulsed energy is
automatically applied to the arc. Synergic power sources made
it easier to use GMAW-P.
I
n the 1990’s, research and development in welding power
s
ource technology continued to evolve. The Lincoln Electric
Company took the lead in developing a wide range of power
source platforms designed with the optimized arc in mind.
Widely recognized as Waveform Control Technology™ the
L
incoln Electric welding systems incorporate an inverter based
transformer design with a high speed, computerized control
circuit. Software developed programs provide an expansive
array of synergic and non-synergic optimized arc welding
p
rograms for the following welding processes:
GMAW — Gas Metal Arc Welding
FCAW — Flux-Cored Arc Welding
GTAW — Gas Tungsten Arc Welding
SMAW — Shielded Metal Arc Welding
CAC-A — Carbon Arc Cutting Process
Among the newer advanced Waveform Control Technology™
processes is Surface Tension Transfer™, or STT™. STT is a
low heat input mode of weld metal transfer, which incorporates
a high-speed reactive power source to meet the instantaneous
needs of the arc. The power source is a waveform generator,
which is therefore neither a constant current nor constant
voltage power source.
Unique to STT, is the application of applying welding current
independent of the wire feed speed. This feature has the benefit
of increasing or decreasing the welding current to increase or
decrease heat input. Fundamentally, STT provides an answer
for controlling the welding conditions, that can produce
incomplete fusion. The STT welding mode has the dual benefit
of increasing productivity, and improving overall weld quality.
See
Advanced Welding Processes for GMAW on page 18.
The GMAW process is flexible in its ability to provide sound
welds for a very wide base material type and thickness range.
Central to the application of GMAW is a basic understanding of
the interplay between several essential variables:
• The thickness range of the base material to be welded
will dictate the electrode diameter, and the useable current
range.
• The shielding gas selection will influence the selection of the
mode of metal transfer, and will have a definite effect on the
finished weld profile.
History of Gas Metal Arc Welding