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This web site was first established in 1997 by Ed Craig. Contact Ed. firstname.lastname@example.org
& FLUX CORED WELDING AND ARC BLOW."
Written by Ed Craig
welding ferrous steels lack of fusion and weld wandering can be caused through
uncontrolled deflection of the arc, "arc blow". Arc blow can be the
result of distortion of the magnetic fields produced by the welding current.
The reaction of the magnetic field that surrounds the MIG wire tip and the
current flow in the plate to the current return cable can be sufficient to deflect
the arc plasma and weld bead. Reducing the arc current magnetic field deflection
can be reduced by positioning the current return cable so that welding is always
towards or away from the ground clamp.
blow is responsible for many welding issues from erratic welds that wander to
weld undercut and inconsistent weld penetration or weld porosity. Arc blow is
especially prevalent in MIG robot cells influenced by the fixture and its many
conductivity connections to the grounded parts. Too frequently the arc blow is
not diagnosed as the cause of a specific weld problem, especially in robot cell.
Arc blow is most prominent in the high current, open arc MIG spray transfer mode
and to a much lesser degree the pulsed MIG transfer modes. Arc blow is also common
with flux-cored welds and DC, SMAW applications > 230 amps.
MIG and flux cored open arc, weld transfer modes, the weld stream cascades axially
across the arc plasma. The electrons travel from the negative work to the positive
anode . Arc blow will cause deflection of the arc plasma and weld stream causing
the arc to wander. The most common arc blow deflections are forwards and backwards
arc blow typically occurs when welding away from the ground.
Back arc blow occurs when welding towards the end of the
weld joint, into
the corner or towards the ground.
the MIG weld circuit, electrons in contrast to current, travel from the ground,
through the work, across the arc, up through the electrode wire, and back to the
power source. The electron (current) flow causes a magnetic field to surround
the conductor (electrode wire tip). Magnetic arc blow causes an in-balance in
the magnetic field that surrounds the MIG arc plasma.
arc blow is influenced by many factors. To name a few, arc blow is influenced
amount of weld current used,
* the ground cable size, connections and length,
* the location of the ground,
* the arc location,
* the welded component
* the shape and the material type welded,
* the fixture components,
* the proximity of steel none grounded components to the grounded parts,
* weld gaps,
* steels in the fixture have high magnetic permeability.
* nickel alloys.
* weld joint depth.
* arc length.
SIMPLE EXAMPLE OF HOW ARC BLOW FORWARD AND
BACKWARD DEFLECTION CAN OCCUR.
of you will know one of the most common causes of arc blow, welding a MIG spray
fillet weld towards the inside corner, to reduce the arc blow as you head towards
[a] use lower weld current will help reduce the arc blow,
change the ground positions,
[c] or try welding from the inside corner outwards
using back hand instead of fore hand technique.
[d] If you have pulsed MIG
transfer, the lower average pulsed weld current will reduce the magnetic field.
you have ever welded with submerged arc (SAW) and used more than one wire electrode,
you would be aware of how the magnetic field interacts between the wires and agitate
the weld puddles. With this situation you will soon recognize how important the
ground placement is. To overcome the magnetic arc disruptions in Twin Wire MIG
welding, DC positive polarity with pulsed transfer may be used with one wire and
an AC arc for the second wire. Also two pulsed modes may be used with the peak
and back ground set differently through the power source electronics. As we see
more use of electronic AC MIG power sources, two AC power sources may be utilizing
using different phase shifting or a Scott connection to minimize arc blow.
current reduces arc blow. The reversal of the weld current creates eddy currents
in the metal being welded. The fields established by the eddy current minimizes
the strength of the magnetic fields. The magnetic field intensity is also proportional
to the square of the weld current used.
THERMAL INFLUENCE ON ARC BLOW.
In MIG and flux cored welding, we have cathode spots on the work or weld surface.
The cathode spots transfer electrons across the arc to the anode spots on the
electrode wire tip. For a stable MIG arc, the MIG arc spray transfer physics require
that the cathode and anode hot spots maintain a continuous, uninterupted flow
of electrons. The faster the MIG weld speed, the easier it is to see the arc plasma
lagging behind attracted to the hotter weld location rather than the cold area
in front of the wire.
The lagging arc
plasma prefers the hot trailing weld rather than travelling
forward into the
slight plasma lag is typically not a concern in manual MIG welding, however in
automated and robot MIG welds, especially the high weld speed butt welds and <3/16
fillet applications, the plasma lag when added to other magnetic disruptions can
cause both arc and weld instability. This is a reason why a high energy argon
- 20 percent CO2 mix, is better suited for high speed applications than a lower
energy (narrower, less dense, less energy plasma) 10 % CO2 mix, argon oxygen mix
or tri mixes containing < 10 CO2 and oxy.
ARC BLOW REDUCTION. The following are a few things you
can do to reduce the
affects of MIG arc blow.
Use a smaller wire diameter with less weld current, (make the arc more forcefull).
* Reduce MIG or flux weld voltage to reduce the arc length, (increases palsma
* Add more clamps and improve part tolerances to reduce the weld
gaps in the part.
* Add more weld tacks and make tacks larger.
the welding direction.
* Change the weld sequence.
* Add additional ground
cables or wrap cables around part or fixture.
* If using spray transfer and
the magnetic field issues are not resolved try AC.
* Instead of MIG spray
use pulsed MIG.
* If the welded part is in a fixture increase or change the
metal fixture contact spots.
* Remember steel fixtures can become magnetized
* Steel fixtures with conductive metal inserts such as aluminum
or copper can cause magnetic disruptions.
* Create equal mass in the fixture
* If using argon oxygen mixes, or argon with <10 CO2, try argon
15 to 20 CO2.
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