Instructions / Assembly

60
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GMAW
S
hielding Gas for Aluminum GMAW
The shielding gas section of this document provides a more
e
xpansive presentation of shielding gases for aluminum and
other filler alloys (see Shielding Gas section on page 12).
T
he recommended shielding gas for welding aluminum up to
approximately 1/2” (12 mm) in thickness is 100% argon. Above
this thickness, where additional energy is needed to melt the
material, it is common to use gas mixtures of 75% argon + 25%
h
elium or 75% helium + 25% argon. The use of helium in the arc
provides additional energy used to accommodate heavier section
thickness welding. It also expands the cross sectional shape of
the finished weld giving it a more rounded appearance.
Shielding gas flow rates range from 30 to 100 cubic feet/hour
(
cfh), (14 to 47 L/min). Higher flow rates are employed for wider
diameter gas nozzles and when using higher helium two-part
blends.
Shielding gas components such as oxygen, hydrogen, or CO
2
should never be employed for aluminum GMAW, even in trace
amounts these gases will adversely affect the weld.
Filler Alloys for Aluminum GMA
W
It is critical to aluminum GMAW that the filler alloy is of high
quality. It provides not only the material that forms the finished
weld, but it also conducts the electrical current necessary to
form the welding arc. The surface of the wire must be smooth
and free from scratches, metal laps, excessive surface oxides,
and contaminants that might add hydrogen to the weld.
Filler Alloy Electrode Surface Contaminants
All aluminum wire must be lubricated during the process of its
manufacture. It is the job of the wire manufacturer to assure
that the lubricants used in wire drawing manufacture are
removed before the electrode is packaged. Usually, this is not a
problem, but occasionally, some spools of wire are found that
have unacceptable levels of residual lubricant on them.
Excessive residual lubricant may result in an erratic arc
performance or produce hydrogen porosity within the weld.
Storage of Aluminum Electrode
Care must be taken in the storage of spooled filler wire. It is
best to store it between uses in its original packing in a low
humidity environment. Some users assure low humidity in the
storage area by installing a 100 watt light bulb in a closed
cabinet. A heated cabinet or an air conditioned environment
that removes humidity from the vicinity of the electrode is critical.
Wire stored in this manner can be stored for several years with
-
out deterioration.
Storage of Aluminum Electrode Mounted on the Wire Drive
Some wire feeders incorporate a cover for the wire spool while
on the wire drive. If the feeder has no such cover, the wire
should be removed and stored when it is between uses,
otherwise shop dust, dirt, and airborne oil can contaminate the
electrode.
A
luminum GMAW Welding Technique
Keywords:
Aluminum Oxide
H
ydrated Aluminum Oxide
Hydrogen Porosity
S
mut (Soot)
Contact Tip to Work Distance (CTWD)
H
ot Start
Push Angle
Drag Angle
Arc Decay
Feathering Technique
Aluminum Oxides and Base Material Contaminants
Before the onset of welding any weld joint configuration it is
important to understand that the surface of aluminum, in all cases,
is comprised of a hard, thin, tightly bonded layer of aluminum
oxide (Al
2
O
3
). The oxide film, in high humidity environments,
because it is porous will absorb moisture. Aluminum oxide can
have an affect on the finished weld appearance, and the hydrated
aluminum oxide, under the heat of the arc, will contribute to the
formation of hydrogen porosity within the finished weld. The
characteristics of aluminum oxide are as follows:
• The melting point for aluminum oxide is higher, 3725°F,
(2042°C), than the base aluminum alloy. The melting point of
aluminum is 1220°F (660°C).
• Aluminum oxide normally continues to grow in thickness over
time – usually this occurs at a very slow rate, but it accelerates
under higher ambient temperatures coupled with higher
humidity.
• Aluminum oxide forms immediately following cleaning, and
over time, it will nearly regain its former density. It is advisable
to weld aluminum soon after it is brushed.
It is for those reasons that aluminum oxide should be removed
from the weld joint prior to welding. Stainless steel power brushes
or hand brushes will remove the heavier oxides, but care should
be taken not to burnish the surfaces to be welded — burnishing
will drive the oxide into the base material. Prior cleaning is not
always required, but the removal of the oxide will contribute to
improved finished weld quality.
The 5XXX base alloys usually have heavier aluminum oxide films
than other base materials. Generally, the thicker oxide layer is
made up of finely divided oxides of both magnesium and
aluminum. Because of the heavy oxide layer the surface of the
5XXX series base alloy is more prone to hydration, and care
must be taken with respect to storing it in a low humidity
environment.
Welding through residual oils, those that remain on the surface
of aluminum parts after shearing, stamping, or machining
operations, will also contribute to hydrogen porosity. It is
important that the surfaces of the aluminum base materials are
clean: free of oil, shop dust, airborne oils, and moisture.
Preparing the weld joint prior to welding should include the
following: