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Even for those experienced in welding steels, welding aluminum alloys can present quite a challenge. Higher thermal conductivity and low melting point of aluminum alloys can easily lead to burnthrough unless welders follow prescribed procedures. Also, feeding aluminum welding wire during gas-metal-arc-welding (GMAW) presents a challenge because the wire is softer than steel, has a lower column strength, and tends to tangle at the drive roll.
To overcome these challenges, operators need to follow the rules of thumb and equipment-selection guidelines offered here...
Gas-metal-arc-welding: Base-metal preparation: To weld aluminum, operators must take care to clean the base material and remove any aluminum oxide and hydrocarbon contamination from oils or cutting solvents. Aluminum oxide on the surface of the material melts at 3,700 F while the base-material aluminum underneath will melt at 1,200 F. Therefore, leaving any oxide on the surface of the base material will inhibit penetration of the filler metal into the workpiece.
To remove aluminum oxides, use a stainless-steel bristle wire brush or solvents and etching solutions. When using a stainless-steel brush, brush only in one direction. Take care to not brush too roughly: rough brushing can further imbed the oxides in the work piece. Also, use the brush only on aluminum work-don't clean aluminum with a brush that's been used on stainless or carbon steel. When using chemical etching solutions, make sure to remove them from the work before welding.
To minimize the risk of hydrocarbons from oils or cutting solvents entering the weld, remove them with a degreaser. Check that the degreaser does not contain any hydrocarbons.
Preheating: Preheating the aluminum workpiece can help avoid weld cracking. Preheating temperature should not exceed 230 F-use a temperature indicator to prevent overheating. In addition, placing tack welds at the beginning and end of the area to be welded will aid in the preheating effort. Welders should also preheat a thick piece of aluminum when welding it to a thin piece; if cold lapping occurs, try using run-on and run-off tabs.
The push technique: With aluminum, pushing the gun away from the weld puddle rather than pulling it will result in better cleaning action, reduced weld contamination, and improved shielding-gas coverage.
Travel speed: Aluminum welding needs to be performed "hot and fast." Unlike steel, the high thermal conductivity of aluminum dictates use of hotter amperage and voltage settings and higher weld-travel speeds. If travel speed is too slow, the welder risks excessive burnthrough, particularly on thin-gage aluminum sheet.
Shielding Gas: Argon, due to its good cleaning action and penetration profile, is the most common shielding gas used when welding aluminum. Welding 5XXX-series aluminum alloys, a shielding-gas mixture combining argon with helium - 75 percent helium maximum - will minimize the formation of magnesium oxide.
Welding wire: Select an aluminum filler wire that has a melting temperature similar to the base material. The more the operator can narrow-down the melting range of the metal, the easier it will be to weld the alloy. Obtain wire that is 3/64- or 1/16- inch diameter. The larger the wire diameter, the easier it feeds. To weld thin-gage material, an 0.035-inch diameter wire combined with a pulsed-welding procedure at a low wire-feed speed - 100 to 300 in./min - works well.
Convex-shaped welds: In aluminum welding, crater cracking causes most failures. Cracking results from the high rate of thermal expansion of aluminum and the considerable contractions that occur as welds cool. The risk of cracking is greatest with concave craters, since the surface of the crater contracts and tears as it cools. Therefore, welders should build-up craters to form a convex or mound shape. As the weld cools, the convex shape of the crater will compensate for contraction forces.
Power-source selection: When selecting a power source for GMAW of aluminum, first consider the method of transfer -spray-arc or pulse.
Constant-current (cc) and constant-voltage (cv) machines can be used for spray-arc welding. Spray-arc takes a tiny stream of molten metal and sprays it across the arc from the electrode wire to the base material. For thick aluminum that requires welding current in excess of 350 A, cc produces optimum results.
Pulse transfer is usually performed with an inverter power supply. Newer power supplies contain built-in pulsing procedures based on and filler-wire type and diameter. During pulsed GMAW, a droplet of filler metal transfers from the electrode to the workpiece during each pulse of current. This process produces positive droplet transfer and results in less spatter and faster follow speeds than does spray-transfer welding. Using the pulsed GMAW process on aluminum also better-controls heat input, easing out-of-position welding and allowing the operator to weld on thin-gage material at low wire-feed speeds and currents.
Wire feeder: The preferred method for feeding soft aluminum wire long distances is the push-pull method, which employs an enclosed wire-feed cabinet to protect the wire from the environment. A constant-torque variable-speed motor in the wire-feed cabinet helps push and guide the wire through the gun at a constant force and speed. A high-torque motor in the welding gun pulls the wire through and keeps wire-feed speed and arc length consistent.
In some shops, welders use the same wire feeders to deliver steel and aluminum wire. In this case, the use of plastic or Teflon liners will help ensure smooth, consistent aluminum-wire feeding. For guide tubes, use chisel-type outgoing and plastic incoming tubes to support the wire as close to the drive rolls as possible to prevent the wire from tangling. When welding, keep the gun cable as straight as possible to minimize wire-feed resistance. Check for proper alignment between drive rolls and guide tubes to prevent aluminum shaving.
Use drive rolls designed for aluminum. Set drive-roll tension to deliver an even wire-feed rate. Excessive tension will deform the wire and cause rough and erratic feeding; too-little tension results in uneven feeding. Both conditions can lead to an unstable arc and weld porosity.
Welding guns: Use a separate gun liner for welding aluminum. To prevent wire chaffing, try to restrain both ends of the liner to eliminate gaps between the liner and the gas diffuser on the gun.
Change liners often to minimize the potential for the abrasive aluminum oxide to cause wire-feeding problems.
Use a contact tip approximately 0.015 inch larger than the diameter of the filler metal being used - as the tip heats, it will expand into an oval shape and possibly restrict wire feeding. Generally, when a welding current exceeds 200 A use a water-cooled gun to minimize heat buildup and reduce wire-feeding difficulties.
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.
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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