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Preparing for aluminum GTAWProper equipment setup and workplace preparation reduce defects

THE FABRICATOR® APRIL 2001 (/publication/fab/issue/13 )APRIL 24, 2001BY: FRANK ARMAO (/AUTHOR/FRANK-ARMAO)

There are right ways and wrong ways to set up welding equipment to accommodate the aluminumGTAW process. Welders should know how to troubleshoot this commonly used process and befamiliar with how to solve some common welding problems.

Even if you are experienced in welding steels, aluminumwelding can present quite a challenge. Aluminum's oxidecoating, higher thermal conductivity, and lower meltingpoint easily can lead to welding problems unless youknow how to prepare for welding and set up theequipment.

Though many processes can be used to join aluminum,the most applicable process for lighter gauges andcosmetically appealing welds is gas tungsten arc welding(GTAW). This article explores the correct ways to set upwelding equipment to accommodate the aluminum GTAWprocess and also discusses techniques fortroubleshooting and solving common welding problems.

Advantages and Disadvantages of GTAW onAluminum

There are advantages and disadvantages of using GTAW on aluminum. On the plus side, GTAW provides the highest-quality welds and offers a great deal of versatility, meaning that you can weld many material thicknesses and jointgeometries. It also is easy to control and weld out of position. If an application has intricate welding requirements or usesthinner materials, GTAW usually is the process of choice.

On the downside, GTAW is a relatively slow process with a low deposition rate. It generally is not used for high-volumeapplications. In addition, the continuous high frequency associated with GTAW can interfere with robots, computers, andother sensitive equipment.

GTAW often is used for such applications as aluminum bicycle frames, architectural components, piping, wheelchairs, andaerospace work. It also is used to weld commercial airline engine casings, seat frames, and ductwork, as well as to repairexisting components, such as cylinder heads.

Polarity and Power Sources

Most materials welded by GTAW require direct current with the tungsten electrode negatively charged and the piece to bewelded positively charged. This is called straight polarity. In this mode, most of the arc's energy goes into the workpiece,which results in the best weld penetration. Aluminum can be welded this way, but it is very difficult.

More often with GTAW, aluminum is welded using alternating current (AC). In AC welding, the arc's action when theelectrode is positive and the workpiece is negative, called reverse polarity, breaks up the oxide on the surface of thealuminum, making welding much easier.Unfortunately, reverse polarity doesn't provide good weld penetration. While inreverse polarity, much of the arc's energy goes into the tungsten electrode and the welding torch. For this reason, larger-diameter tungsten electrodes and heavier-duty torches, often water-cooled, are needed for AC welding.

The cleaning action created by the reverse polarity arc is important, but you do not want to clean more than is necessary,so equipment manufacturers have created a balance control. Found on the front of the machine, the balance knob allowsyou to adjust the amount of penetration (electrode negative) versus the amount of cleaning (electrode positive).

As a rule of thumb, not as much cleaning is needed with high currents as with low currents. Some machines even offer anautobalance feature that has preprogrammed balance based on the current being used.

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Figure 1Object

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Figure 2Object

While changing to AC sounds easy, it creates an unstable arc. When you look at a sine wave, note that as the currentmoves from positive to negative and then back again, it must pass through the zero point. When the voltage goes throughthe zero point, the arc can go out or become unstable.

To correct this unstable arc, welding manufacturers havesuperimposed a low-current, high-voltage radiofrequency on top of the welding current. For most typesof welding, this high frequency is used only to initiate thearc, but for aluminum GTAW, the high frequency ispresent continuously and acts like a pilot arc.

When selecting a power source, it usually is best toselect a newer-technology square wave model, whichalters the characteristics of the sine wave to create amore stable arc (see Figure 1). In addition, look for abalance feature, be it a manual or autobalanceadjustment.

Workpiece Cleaning

Aluminum workpieces must be cleaner than steelworkpieces to be welded effectively. The first step is toremove oils and greases from the workpiece by wipingthe area with a solvent or washing it with a mild alkalinesolution. The second step is to remove all oxides, which can be done with a stainless steel wire brush. Never use thesame brush on aluminum and steel or it will become contaminated.

Not only do you need to clean before beginning to weld, but you also should remove the smut or soot between weldpasses. This soot is fine particles of aluminum oxide and magnesium oxide. Proper welding techniques will minimize theamount of soot produced.

Shielding Gas

Usually, 100 percent argon gas is preferred for aluminum GTAW, but when working with thicker materials, such as 1/2 inchor greater, add helium in the range of 25 to 50 percent. Helium makes the arc hotter and provides for more penetration.

Electrodes

Most operators who weld with DC polarity are accustomed to using a 2 percent thoriated electrode with a sharp tip. But forAC aluminum welding, more heat is put into the tungsten, and the tungsten tip quickly becomes blunt. Once the tip is blunt,using 2 percent thoriated electrodes results in a fluttering arc, which wanders around on the blunt tip.

Instead, start with a blunt tip and use a pure tungsten or zirconiated tungsten electrode. (A zirconiated electrode carriesmore current than pure tungsten.) For AC welding, using a larger-diameter tungsten is advisable (see Figure 2) becauseone that is too small spits tungsten into the weld, creating a defect. Consider using a ceriated or lanthanated electrode.Both types of electrode are versatile and can be used for AC or DC welding.

Torches

If the welding application is light-duty fabrication requiring up to 150 amps, an air-cooled torch may suffice. Higher-amperage jobs require a water-cooled torch used with a water recirculator. One feature to look for in a good torch is aflexible head, which offers maneuverability and reaches tight areas. Because more heat input is used for AC aluminumwelding than for DC welding, a torch that normally is used for DC GTAW may not have sufficient capacity.

Filler Metal

As a rule, filler metal is added to welds made by GTAW. However, most nonheat-treatable aluminum alloys can be weldedwithout adding filler. This is referred to as an autogenous weld. Be careful, though, because if you try to weld any of theheat-treatable alloys, such as 6061, without adding filler metal, they will crack during the welding process (see Figure 3).

As a caution, not all aluminum alloys are weldable. Be sure you know which type of aluminum alloy you are welding, andthen consult a filler metal chart. When handling filler metal, keep it dry in an airtight box to avoid contamination and abuildup of hydrated oxide.

When welding is completed, the weld bead should bebright and shiny without soot (see Figure 4).Approximately 1/16 to 1/8 inch on each side of the weldbead should have a bright strip, meaning that the oxidehas been removed from this area by the welding arc. Ifthe weld is black, an error has been made in the weldingprocess. In most cases, this results when the arc is toolong or the torch angle is wrong.

The weld also should have a distinct, uniform pattern ofripples on the surface and should be smooth, blending inwell with the surrounding area. Remember that in steel,the weld is as strong as the parent; in aluminum, this isnot always the case.

Conclusion

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The Lincoln Electric Company Frank ArmaoContributing WriterThe Lincoln Electric Company22801 St. Clair Ave.Cleveland, OH 44117Phone: 216-481-8100Fax: 216-486-1751

www.lincolnelectric.com(http://www.lincolnelectric.com)Contact via email(mailto:[email protected])

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Even if you are new to aluminum GTAW, you can reduce the number of errors you make by brushing up on how to set thecurrent and prepare the workpiece; selecting the proper gases, electrodes, and torches; and heeding advice from moreexperienced welders. In time, you will feel as confident welding aluminum as you do welding steel.

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He is a member of the AWS D1 Committee, chairman of the AWS D1 Aluminum Subcommittee, and vice chairman ofthe AWS D8G Automotive Aluminum Arc Welding Committee.

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