Like standard aluminum, anodized aluminum also requires a 100 percent argon
shielding gas or an argon/helium mixture
and a 2 percent ceriated (orange stripe) or
thoriated (red stripe) tungsten. [Note:
Thorium is radioactive. Always follow the
manufacturer’s warnings, instructions, and
the Material Safety Data Sheet for its use.]
Pure tungsten (green stripe) also can be
used, but only up to about 70 percent the
amperage of ceriated or thoriated tungsten.
When preparing the tungsten, grind it
to the same type of point that you would
when welding steel or stainless steel. Be sure
to regrind the tip when it rounds off and
causes the arc to become unstable. Repeat
this process as necessary.
A power source capable of alternating
current (AC) is also necessary because the
electrode-positive portion of the current
cycle breaks apart the oxide layer.
Although some traditional transformer-based GTAW power sources are capable of
successfully welding anodized aluminum,
an inverter is highly recommended for its
balance and frequency controls.
Balance control allows you to adjust
how long the current spends in each part of
the AC cycle. For example, a balance control
set to 30/70 means that the current spends
30 percent of its cycle in electrode-positive,
cleaning the oxide layer from the base material, and 70 percent in electrode-negative,
directing the electrical energy into the weld
joint and joining the two pieces of material.
Your experience and skill will determine
how you set the balance control. A more
skilled welder often can use a higher percentage of electrode-negative (80 to 90 percent)
in order to work faster.
The frequency control function available
on inverter units allows you to determine
the length of time that it takes the unit to
complete one full current cycle (the combined time spent in electrode-positive and
electrode-negative.)
Transformer-based power sources produce an output of 60 Hz (50 Hz in
Canada), which is the same frequency that
comes from a standard outlet. Inverters,
however, are able to adjust the frequency
from 20 to 400 Hz. For anodized aluminum, a frequency of about 160 to 200
Hz generally produces the best results. The
This fishing rod holder is being welded using a water-cooled torch.
frequency produces a narrower arc cone
and, consequently, a narrower weld bead
and HAZ. The strength of the weld
increases by reducing the area of the base
material exposed to the heat of the arc. A
narrower weld bead reduces the time and
filler metal needed to make the weld.
An inverter machine also eliminates the
need for a backhanding technique.
Backhanding, also known as backing around
the weld, involves making two passes on
each weld joint—one pass going forward to
clean off the oxide layer and then reversing
direction to add filler metal to the area just
cleaned. In addition to doubling the time it
takes to weld the joint, backhanding requires
the workpiece to be heated twice, and this
can reduce the strength of the weld.
metal with a clean cloth prior to welding
helps prevent weld defects such as porosity,
inclusions, and lack of fusion.
Although the electrode-positive portion of
the AC cycle helps remove the oxide layer that
builds up on aluminum, you may want to use
a dedicated stainless steel brush to manually
clean the material before welding. Using
this brush for other purposes will introduce
foreign contaminants to the weld joint and
compromise the weld’s structural integrity.
An oxide layer begins to form immediately
after brushing, so it is also necessary to
rebrush a piece that sits for an extended
amount of time without being welded.
Material Preparation
Properly preparing the material is critical to
ensuring strong and cosmetically appealing
welds on anodized aluminum.
The material should be at room temperature for at least an hour before welding,
otherwise condensation can form within
the aluminum and react with the argon
shielding gas, creating porosity and a black,
sooty appearance on the weld.
Cleaning any residue, dirt, or other foreign material from the base metal and filler
Technique
Anodized aluminum requires a technique
known as bumping, which is a way to
remove the thick oxide layer without putting
too much heat into the weld pool. Because
it uses a fingertip control switch, a foot
pedal is unnecessary for this technique.
Bumping involves very briefly starting
an arc using the finger control switch,
adding the filler metal, extinguishing the
arc, moving down the weld joint about 1⁄ 16
in., and repeating the process. Turning the
fingertip switch off gives the weld pool a
moment to cool and solidify before you
reintroduce the heat with a new arc.
