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A Guide to Aluminum Welding
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Gas-tungsten-arc-welding
GTAW is frequently the process of choice for aluminum
welding because of its highly concentrated arc that
allows for more control of heat input. It also produces
smooth, high-quality spatter-free welds. Many of
the same rules-of-thumb that apply to GMAW also
apply to GTAW, including those for base-material
preparation, preheating, and deposit of convex-shaped
welds.
Since aluminum conducts heat well, it tends to absorb
heat from the arc. Therefore, before the welder
using GTAW deposits filler metal to the puddle,
he should pause to let the arc clean the base metal
and let it build up sufficient heat. Welders experienced
in GTAW of steel will find this waiting period unusual
since it is not required with steel. In fact, stainless
steel keeps all of the heat in the weld puddle.
Power-source
selection: Aluminum is one of the most demanding
applications for GTAW. It requires low current for
thin materials, 1/16 inch and thinner, and high
currents on thick sections, ¸ inch and up. Aluminum
is said to drive the high amperages of most power
sources on the market today. The best GTAW machines
can operate at a wide range of outputs, from less
than 5A up to 350 A on AAC. This differs greatly
from welding on steel, which typically calls for
low-A DC. For example, a ¹ inch-thick steel can
be welded at 90 A, but ¹-inch aluminum requires
180 A.
The power source must have the capability to start
well on ac polarity with low currents, 10-15 A.
To accomplish this, a high-frequency arc-starting
circuit is recommended. This circuit will help establish
an ac arc between the tungsten and the base material,
even under less-than-perfect conditions such as
poor grounding. A drawback to the high-frequency
circuit: it may interfere and affect the radio frequencies
of shop and office microprocessor-based equipment.
To avoid this interference, ground the power supply
with a 14-gauge or larger wire connected from the
case of the machine to a copper earth ground.
High-amperage AC stabilization:
The high amperage requirements for welding aluminum
call for a power source that produces a stable ac
output at over 200 A. Look for a consistent arc
from the tungsten to the workpiece, one that doesn't
wander, surge, or cause excessive tungsten consumption.
Some state-of the-art power supplies allow welders
to increase the amount of negative current in the
ac sign wave, minimizing tungsten spitting and transfer
across the arc. Further, some power sources carry
a built-in feature called AC wave balance that automatically
provides a stable arc at high amperages but still
allows manual adjustment of the wave to increase
oxide-removal action or weld penetration.
Water-cooled torch: to GTA-weld aluminum, the torch needs to be compact and able to perform at low currents, while also being able to handle current up to 225 A. An air-cooled torch works fine for welding at low currents or when the welder has enough access to the weld joints to reach them using the larger high-amperage air-cooled torches. Otherwise, a water-cooled torch is recommended. The cool water supply for the torch typically comes from a water recirculator to protect it against freezing or overheating.
Torch sleeve: Torch sleeves fit over the water-cooled torch cables to prevent the thermal plastic hoses from melting should they come in contact with hot materials. The sleeves are lightweight, low-friction, and flexible.
Tungsten types: For aluminum GTA-welding, pure tungsten has been the traditional electrode choice because of its low cost and availability in ground condition. But for improved current-carrying capacity and to minimize tungsten transfer across the arc, zirconiated tungstens work best. Ceriated and lanthanated tungsten electrodes perform similar to zirconiated electrodes, with improved DC operation.
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