Instructions / Assembly

61
GMAW
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•
Remove oils from the surface using non-petroleum based
s
olvents first. Then wipe the parts dry using a clean (unused)
shop towel. Acetone is commonly used.
• Remove the oxides from the weld joint using a stainless steel
power or hand brush. If the brush is air driven, be certain that
the compressed air used to clean the material does not
contain oil. Never use stainless steel brushes that have been
p
reviously used on carbon steels, and never use carbon steel
brushes for removing aluminum oxide.
W
elding Techniques
T
he formation of black soot on the surface, or the adjacent
areas of a weld, is referred to as smut. It is made up of finely
divided oxides of aluminum and magnesium. They usually
indicate that adjustments in technique are necessary.
At the onset of learning to weld using aluminum GMAW the
most common mistake made is to hold too long a contact tip to
work distance (CTWD). Shorter CTWDs, 1/2” to 5/8”
(13 - 16 mm), are required when welding aluminum. If the CTWD
is too long, then the gas shielding will be insufficient. In the
absence of adequate shielding gas the weld will become gray,
and in the most severe case, the arc may bore into the work
piece.
At the start of an aluminum weld it is not uncommon to get a
cold looking weld bead for the first 1/2” (13 mm). This is due to
the high thermal conductivity of aluminum. This condition can be
minimized by using a power supply with a hot start. A common
alternative used in the absence of a hot start control is to strike
the arc about 1” (25 mm) ahead of the nominal weld starting
point, and quickly maneuver the arc back to the desired starting
point. This action has the effect of providing preheat to the
aluminum base material, and it provides improved fusion at the
beginning of the weld.
While welding, the torch must be held with a push angle of 5° to
10° (also known as a leading torch angle). If the torch is applied
using a drag angle (also known as a lagging torch angle), then
the gas shielding will be absent from the molten puddle and the
finished weld will appear gray or black.
Care must be taken in extinguishing the arc. Terminating the arc
abruptly will result in a deep weld crater that may contain a
shrinkage crack in the crater. There are a number of ways to
minimize the size and depth of the crater.
•
Weld tabs can be used and the weld may be started and
t
erminated on them.
• A power supply with an arc decay control allows the electrode
and current to tail off for a predetermined wire feed speed per
u
nit of time. This permits a controlled fill of the aluminum weld
crater.
• Near the end of the weld, progressively increase the travel
s
peed. The effect here is to reduce the size of the weld bead
and diminish the overall size of the crater. This is known as a
"feathering” or “back step” technique.
• At the end of the weld, reverse the direction of the torch to
p
lace the crater within the body of the weld bead.
S
ome welders learn aluminum GMAW by keeping a very steady,
constant motion in the travel direction to make a very smooth
weld with a minimum of weld ripples, this is known as a straight
progression type weld bead, see Figure 40. Other welders learn
to weld by using the back step technique, see Figure 41. Each
of these techniques produce a weld with distinct, evenly spaced
ripples. Each technique produces acceptable finished welds.
The finished weld should be bright and free from oxides and
smut. A “frost line” or cleaning stripe approximately 1/16” to 1/8”
(1.6 to 3.2 mm) wide should be visible along each edge of the
weld. These stripes show the area where the reverse polarity
arc has removed the oxide from the aluminum surface. If the
weld metal is black or gray, or if the cleaning stripes are not
present, something is wrong. The most likely causes are either
the arc length is too long or the torch angle is wrong. If some
weld smut is present outside these areas - this is expected.
There will also be some weld smut present at weld starts, stops
and at internal and external corners. More smut will be present
when using 5XXX filler than with 4XXX filler.
Filler Metal Selection
Most common aluminum filler alloys fall into the 4XXX and 5XXX
families, with a few coming from the 1XXX, 2XXX, and the casting
alloys. The chemical composition of the common aluminum filler
alloys is shown in Table 23 on page 64.
There are a number of characteristics which determine the best
filler metal choice for a given base material, or combination of
base materials. Among these are:
•
Freedom from hot cracking.
FIGURE 40: Straight Progression Weld
FIGURE 41: A Back Step Technique Weld