A Guide to Aluminum Welding

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.

Visual inspection

• Cold lapping in the first inch of the weld may be caused by failure to completely remove the aluminum-oxide layer.
• When this occurs, the filler material will sit on top of the oxide, failing to penetrate into the base material. Also, since aluminum quickly pulls heat away from the weld, the arc melts the filler metal going in but not the base material.
• An excessively wide weld bead may indicate excessive current or arc voltage (arc length).
• A cold, convex bead might indicate insufficient current or arc voltage.
• Pitting (porosity) in the surface of the weld results from excessive arc length.
• An excessive amount of black soot remaining on the workpiece following GTAW indicates inadequate shielding-gas coverage or aluminum contamination of the tungsten electrode. Brown soot remains on GMA welds if arc length is too short.

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