Instructions / Assembly

7
GMAW
www.lincolnelectric.com
Short-Circuit Metal Transfer
Short-circuiting metal transfer, known by the acronym GMAW-S,
is a mode of metal transfer, whereby a continuously fed solid or
metal-cored wire electrode is deposited during repeated electrical
short-circuits.
The short-circuiting metal transfer mode is the low heat input
mode of metal transfer for GMAW. All of the metal transfer
occurs when the electrode is electrically shorted (in physical
contact) with the base material or molten puddle. Central to the
successful operation of short-circuiting transfer is the diameter
of electrode, the shielding gas type and the welding procedure
employed. This mode of metal transfer typically supports the use
of 0.025” - 0.045” (0.6 - 1.1 mm) diameter electrodes shielded
with either 100% CO
2
or a mixture of 75-80% argon, plus
25-20% CO
2
. The low heat input attribute makes it ideal for
sheet metal thickness materials. The useable base material
thickness range for short-circuiting transfer is typically considered
to be 0.024” – 0.20” (0.6 – 5.0 mm) material. Other names
commonly applied to short-circuiting transfer include short arc
microwire welding, fine wire welding, and dip transfer.
Advantages of Short-Circuiting Transfer
• All-position capability, including flat, horizontal, vertical-up,
vertical-down and overhead.
• Handles poor fit-up extremely well, and is capable of root
pass work on pipe applications.
• Lower heat input reduces weldment distortion.
•
Higher operator appeal and ease of use.
• Higher electrode efficiencies, 93% or more.
Limitations of Short-Circuiting Transfer
• Restricted to sheet metal thickness range and open roots of
groove joints on heavier sections of base material.
• Poor welding procedure control can result in incomplete
fusion. Cold lap and cold shut are additional terms that serve
to describe incomplete fusion defects.
• Poor procedure control can result in excessive spatter, and
will increase weldment cleanup cost.
• To prevent the loss of shielding gas to the wind, welding out-
doors may require the use of a windscreen(s).
FIGURE 1: Pinch Effect During Short-Circuiting Transfer
Current (A)
Electrode
P ∝ A
2
Pinch effect force, P
Description of Short-Circuiting Transfer
T
he transfer of a single molten droplet of electrode occurs
during the shorting phase of the transfer cycle (See Figure 2).
Physical contact of the electrode occurs with the molten weld
pool, and the number of short-circuiting events can occur up to
2
00 times per second. The current delivered by the welding
power supply rises, and the rise in current accompanies an
increase in the magnetic force applied to the end of the
electrode. The electromagnetic field, which surrounds the
e
lectrode, provides the force, which squeezes (more commonly
k
nown as pinch) the molten droplet from the end of the electrode.
Because of the low-heat input associated with short-circuiting
transfer, it is more commonly applied to sheet metal thickness
material. However, it has frequently found use for welding the
root pass in thicker sections of material in open groove joints.
The short-circuiting mode lends itself to root pass applications
on heavier plate groove welds or pipe.
Solid wire electrodes for short-circuiting transfer range from
0.025” - 0.045” (0.6 –1.1 mm). The shielding gas selection
includes 100% CO
2
, and binary blends of argon + CO
2
or
argon + O
2
. Occasionally ternary blends, (three part mixes), of
argon + CO
2
+ oxygen are sometimes employed to meet the
needs of a particular application.
AB C D
E
Short
Voltage Current
Extinction
Reignition
Arcing Period
Time
Zero
Zero
FIGURE 2: Oscillograms and Sketches of Short
Circuiting Transfer
Time
Short
Zero
Zero
Current
Voltage
Reignition
Extinction
Arcing Period
A
A
B
B
C
C
D
D
E
E
Modes of Metal Transfer
A
A
The solid or metal-cored electrode makes physical contact with the molten puddle.
The arc voltage approaches zero, and the current level increases. The rate of rise to
the peak current is affected by the amount of applied inductance.
B
B
This point demonstrates the effect of electromagnetic forces that are applied
uniformly around the electrode. The application of this force necks or pinches the
electrode. The voltage very slowly begins to climb through the period before
detachment, and the current continues to climb to a peak value.
C
C
This is the point where the molten droplet is forced from the tip of the electrode.
The current reaches its maximum peak at this point. Jet forces are applied to the
molten puddle and their action prevents the molten puddle from rebounding and
reattaching itself to the electrode.
D
D
This is the tail-out region of the short-circuit waveform, and it is during this down-
ward excursion toward the background current when the molten droplet reforms.
E
E
The electrode at this point is, once again, making contact with the molten puddle,
preparing for the transfer of another droplet. The frequency of this varies between
20 and 200 times per second. The frequency of the short-circuit events is
influenced by the amount of inductance and the type of shielding gas. Additions of
argon increase the frequency of short-circuits and it reduces the size of the molten
droplet.