Instructions / Assembly

16
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GMAW
C
urrent Density
Keywords:
C
urrent Density
Cross-Sectional Area
Saturated
C
urrent density is defined as the current employed with a
particular electrode diameter divided by its current carrying
cross-sectional area. If the wire feed speed is low, then the cur-
rent density will be low, and vice versa. From this you can deter-
mine that:
• Lower current density applied to a given electrode is associated
with the short-circuit mode of metal transfer.
• Higher current density is associated with the higher energy
modes of metal transfer: globular, axial spray transfer or the
more advanced pulsed spray metal transfer.
The current for a given GMAW solid or metal-cored electrode
will reach a maximum density level. Once this level of current
density is attained, no additional current can be carried by the
electrode. In other words, the electrode has reached its maxi-
mum current density. In particular, Figure 9 demonstrates this
phenomenon for 0.035” (0.9 mm) diameter solid wire. It can be
seen that the current is relatively linear to approximately 200
ampere, but as the current reaches just beyond 210 ampere,
the rise in current becomes exponential. At approximately 280
ampere [720 ipm (18.3 M/min.) wire feed speed], the electrode
reaches its maximum current density. The electrode at this
point becomes saturated with current and no more current can
be added to the electrode. Therefore, the maximum current
density for a given electrode diameter is synonymous with the
concept of current saturation. So it can be speculated that this
phenomenon occurs for all diameters and material types of
electrodes used for GMAW.
It is important to note that once the electrode reaches its
maximum current density, the saturation point, any added wire
feed speed will provide a higher deposition rate with no increase
in current.
E
lectrode Efficiencies
Electrode efficiency is a term that is applied to the percentage of
e
lectrode that actually ends up in the weld deposit. Spatter
levels, smoke, and slag formers affect the electrode efficiency in
GMAW. The electrode efficiency is a numeric value that is
assigned to the particular mode of metal transfer:
• GMAW-S, short-circuit transfer, shielded with an argon + CO
2
gas blend, will typically operate with an electrode efficiency
e
qual to or greater than 93%. Shielded by 100% CO
2
,
the
electrode efficiency will range from 90 to 93%. Typically, CO
2
increases spatter levels to some extent, and argon blends are
typically useful in reducing, but not completely eliminating,
spatter.
• STT™, a dynamically controlled form of GMAW-S, will attain
electrode efficiencies of 98% .
• Globular transfer is associated with higher spatter levels that
profoundly impact electrode efficiency. The efficiency of
globular transfer can vary from 85 to 88%, when shielded with
100% CO
2
. Under argon blends the efficiency may vary from
88 to 90%.
• Axial spray has a higher electrode efficiency. This higher ener-
gy mode of metal transfer is associated with electrode effi-
ciencies of 98%.
• The electrode efficiency for GMAW-P varies depending upon
the welding application and the sophistication of the power
source. Generally, the efficiency factor applied for GMAW-P is
98%, like that for axial spray, but there may be the need for a
higher travel speed application that requires shorter arc
lengths. High speed pulsed spray transfer types of applica-
tions generally introduce higher spatter levels. This necessarily
reduces the electrode efficiency to some lower value.
All of this is related to the amount of electrode that actually ends
up in the weld. If 100 lbs. (45 kg) of 0.035” (0.9 mm) diameter
electrode is purchased for use on a particular project, and the
project calls for the use of GMAW-S, then the effective amount
of electrode that will be expected to end up in the welds will be:
EE x (lbs. Electrode)
= 0.93 x 100 lbs.
= 93 lbs.
NOTE: The calculation assumes no loss of material due to wire clipping.
Deposition Rate
Keywords:
Deposition Rate
Melt-off Rate
The melt-off rate for a particular electrode does not include con-
sideration for the efficiency of the mode of metal transfer or the
process. Its interest is in how much electrode is being melted.
Deposition rate is applied to the amount of electrode, measured
in wire feed speed per unit of time, that is fed into the molten
puddle. Importantly, its value reflects the use of the factor for
electrode efficiency.
800
700
600
500
400
300
200
100
00
5
20
15
10
0 50 100 150 200 250 300 350 400
Welding current, A (DCEP)
Wire feed speed, inches per minute
Wire feed speed, meters per minute
0.030 in. (0.8 mm)
0.035 in. (0.9 mm)
0.045 in. (1.2 mm)
0.052 in. (1.3 mm)
0.062 in. (1.6 mm)
450
FIGURE 9: Typical Welding Currents vs. Wire Feed Speeds
Effects of Variables