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Joining Aluminum with TIG/GTAW

Advice for the novice

Aluminum: beautiful, lightweight, strong, versatile...and a real challenge to weld, especially for beginning welders.

Fortunately, some newer gas tungsten arc welding (GTAW) systems have been designed specifically for aluminum welding. This article describes some of the new equipment available and its benefits, accessories required, points to consider before welding, and the techniques required to make a good weld bead.

GTAW Power Sources

In general, GTAW power sources with an AC/DC output come in four categories, which are listed here in order of lowest to highest price:

1. Light fabrication. Machines designed for light fabrication usually have an AC output from 20 to 165 amps. While they do not incorporate a square wave output or balance control technology, they do produce an arc suitable for a variety of work, including applications for the home hobbyist.

2. Light industrial, maintenance/repair, metal fabrication. This newer class of light industrial machine provides roughly a 15 to 180 AC output and a professional-quality arc. Key features include: a square wave output, a fixed balance control set for more penetration than cleaning (a 60/40 electrode negative (EN) to electrode positive (EP) ratio works best for most applications), built-in high-frequency starting for positive starts without arc wandering, and a built-in stabilizer for a more consistent arc while welding.

3. Industrial production, fabrication, aerospace, repair. Industrial production GTAW power sources have a square wave output with an adjustable balance control. Greater amounts of EN create a deeper, narrower weld bead and better joint penetration. Greater EP values remove more oxide and create a shallower, wider bead. Transformer-rectifier GTAW machines can adjust EN values from 45 to 68 percent.

Machines are available with a variety of outputs, typically rated at 250, 350, and 500 amps with a 40 or 60 percent duty cycle. The low-end amperage range listed for these machines is usually 5, 3 or 25 amps, respectively. These power sources have created millions of code-quality GTA welds.

4. Inverter-based AC GTAW machines. Also considered industrial power sources, an inverter gives the professional welder more capability to tailor the width, depth and appearance of the weld bead for an application.

Inverters can adjust EN duration from 50 to 90 percent. Adding more EN to the cycle may: increase travel speed by up to 20 percent, narrow the weld bead, achieve greater penetration, permit using a smaller-diameter tungsten (to more precisely direct the heat or to make a narrower weld bead), and reduce the size of the etched zone for improved cosmetics.

Operators can adjust the welding output frequency in the range of 20 to 250 hertz. Increasing frequency produces a tight, focused arc cone. This narrows the weld bead, which helps when welding in corners, on root passes, and fillet welds; it also permits faster travel speed on some joints. Decreasing output frequency produces a broader arc cone, which widens the weld bead profile and provides greater cleaning action.

GTAW inverters accept single- or three-phase, 50- or 60-hertz, 230- or 460-volt input power. This provides flexibility when moving the machine between jobs sites or around a large facility. Using three-phase power and welding at 300 amps (460 volts primary), an AC/DC GTAW inverter requires only 18 amps of primary current. A 5- to 300-amp AC/DC GTAW machine weighs about 90 pounds.

GTAW Accessories

If most welding is done at 200 amps or less, an air-cooled torch works well. For welding above 200 amps, a water-cooled torch should be considered. For portability, water coolers can be mounted on a wheeled cart that also carries the power source and gas bottles.

Remote control capabilities usually include current (amperage) and contactor control (the contactor keeps the torch electrically cold until energized and starts and stops the gas flow to the torch). The most popular remote control is a foot pedal that operates much like an auto's gas pedal; the more it is depressed, the more amperage flows. Another type of control - - one that affords greater mobility but is more difficult to learn - - is a finger tip control, which is mounted on the torch.

If most work is done on a bench or around structures that permit mobility, the foot pedal remote control is probably a better option because it's easier to use. Conversely, if most work is done in awkward positions, a finger tip control may be the better choice.

Before Welding Starts

The following suggestions address the basic areas of GTAW setup. However, they are no substitute for carefully reading the operator's manual, watching instructional videos, and following safety precautions, such as wearing protective gloves and glasses.

1. Determine amperage requirements. Each 0.001 inch of metal to be melted requires about 1 amp of welding power. For example, welding 1/8-inch aluminum requires about 125 amps.

2. Select the correct current. AC should be used for aluminum, magnesium, and zinc die cast. When exposed to air, these metals form an oxide layer that melts at a much higher temperature than the base metal. If not removed, this oxide causes incomplete weld fusion.

Fortunately, AC inherently provides a cleaning action. While the EN portion of the AC cycle directs heat into the work and melts the base metal, the EP portion - - where current flows from the work to the electrode - - blasts off the surface oxides.

3. Use the right gas. Usually, pure argon is employed, although thicker weldments may require an argon/helium or other specialty mix. If the wrong gas is used, such as the 75 percent argon/25 percent CO(2) mix commonly used for GMAW, the tungsten immediately will begin to be consumed or deposited in the weld puddle.

4. Set the proper gas flow rate. More is not better, so 15 to 20 cubic feet per hour (CFH) should suffice. Argon is about 1-1/3 heavier than air. When welding in a flat position, the gas naturally flows out of the torch and covers the weld pool. For overhead welding, the gas flow rate should begin at 20 CFH, and small increments of 5 CFH can be made, if necessary.

In any position, if the gas flows out at too high a velocity, it can bounce off the workpiece and start a swirling motion parallel to the torch cup called a venturi. A venturi can pull air into the gas flow, bring in contaminating oxygen and nitrogen, and create pinholes in the weld. Unfortunately, some operators automatically increase the gas flow when they see a pinhole, worsening the problem they tried to fix.

5. Select the right type of tungsten. For AC welding, the traditional practice calls for selecting a pure tungsten electrode and forming a ball at the end of the electrode. This still holds true for most applications and welding with a conventional power source. However, for making critical welds on materials thinner than 0.09 in., or when using a TIG power source with an adjustable frequency output, new recommendations call for treating the tungsten almost as if the weld were being made in the DC mode. Select a 2-percent-type tungsten (thorium, cerium, etc.) and grind it to a point in the long direction, making the point roughly two times as long as the diameter. A 0.010- to 0.030-inch flat should be made on the end to prevent balling and the tungsten from being transferred across the arc.

With a pointed electrode, a skilled operator can place a 1/8-inch bead on a fillet weld made from 1/8-inch aluminum plates. Without this technology, the ball on the end of the electrode would have forced the operator to make a larger weld bead and then grind the bead down to final size.

6. Select the right diameter of tungsten. The current-carrying capacity of a tungsten is directly proportional to the area of its cross section. For example, a 2 percent thoriated, 3/32-inch (0.093-inch) tungsten has a current-carrying capacity of 150 to 250 amps, whereas a 2 percent thoriated, 0.040-inch tungsten has a current-carrying capacity of 15 to 80 amps.

There is no such thing as an all-purpose electrode, despite the reputation of the 3/32-inch electrode. Attempting to weld at 18 amps with a 3/32-inch electrode will create arc starting and arc stability problems; the current is insufficient to drive through the electrode. Conversely, attempting to use a 3/32-inch tungsten to weld at 300 amps creates tungsten "spitting" - - the excess current causes the tungsten to migrate to the workpiece.

7. Avoid tungsten contamination. If the tungsten electrode becomes contaminated by accidentally touching the weld pool, welding must be stopped, because a contaminated electrode can produce an unstable arc. To break off the contaminated portion, the tungsten should be removed from the torch, placed on a table with the contaminated end hanging over the edge, and the contaminated portion struck firmly. The tungsten should then be resharpened.

8. Set the proper tungsten extension. While electrode extension may vary from flush with the gas cup to a distance equal to the cup diameter. A general rule is to start with one electrode diameter, or about 1/8 inch. Joints that make the root of the weld hard to reach require additional extension, although extensions farther than 1/2 inch may result in poor gas coverage and may require a special gas cup.

9. Select the filler metal. The filler rod needs to match the base metal in both type and hardness of metal. The filler rod should be the same diameter as the tungsten electrode.

10. Select a high frequency (HF) mode. For AC welding with transformer-rectifier type machines, continuous HF is required to start and maintain the arc, which has a tendency to go out when the AC square wave travels through the zero amperage point. HF bridges the gap between the electrode and the work, forming a path for the current to follow.

Inverters require HF for arc starting only, as they drive the arc through the zero point so quickly that the arc does not have a chance to go out. For this same reason, inverters produce much less arc flutter. Inverters also offer a lift arc starting method that avoids the use of HF altogether.

11. Control HF emissions. High frequency interferes with computers, printed circuit boards, televisions, and other electronic equipment but is a necessary evil. It can be minimized by hooking the work clamp as close to the weldment as possible, keeping the welding torch and work clamp cables close together (spreading them apart is like creating a big broadcast dish), and keeping the cables repaired to prevent current leaks.

12. Set the balance control. There are no hard rules about setting balance control, but the typical error involves over-balancing the cycle.

Too much cleaning action (EP duration) causes excess heat buildup on the tungsten, which creates a large ball on the end. Subsequently, the arc loses stability, and the operator loses the ability to control the arc's direction and the weld puddle. Arc starts begin to degrade as well.

Too much penetration (or, more precisely, insufficient EP current) results in a scummy weld puddle. If the puddle looks like it has black pepper flakes floating on it, adding more cleaning action will remove these impurities.

13. Set the frequency (inverters only). Decreasing frequency produces a broader arc cone, which widens the weld bead profile and better removes impurities from the surface of the metal. It also transfers the maximum amount of energy to the workpiece, which speeds up applications requiring heavy metal deposition (such as building up a worn part or making a fill pass).

Increasing frequency produces a tight, focused arc cone; this narrows the weld bead, which helps when welding in corners, on root passes, and fillet welds. The operator can direct the arc precisely at the joint and not have the arc dance from plate to plate. Increasing frequency also may increase travel speed up to 40 percent in certain applications.

A good starting point for general welding is 80 to 120 hertz. These frequencies are comfortable to work with, increase control of the arc direction, and boost travel speed. For a fillet weld application with full penetration in the weld without putting too much amperage in the metal, the frequency should be increased to 200 hertz or more. For buildup work, frequency should start at 60 hertz and be adjusted lower from there.

Making a Good GTA Weld

1. Get in a comfortable position and brace yourself. Maneuvering a GTAW torch properly is like trying to write neatly in a small space. It requires a braced arm, slow movements, and a mental focus on the end of the tungsten. In fact, many people grip the torch like a pencil.

2. Hold the torch at the proper angle. Travel angle is defined as the angle relative to the torch in a perpendicular position. Normal welding conditions in all positions call for a travel angle of 15 to 20 degrees. Travel angles beyond this lead to less penetration, poor direction of the weld metal, poor shielding gas coverage, and general arc instability.

For travel direction during GTAW, the push technique should always be used, which involves pushing the torch away from (ahead of) the weld puddle. Pushing ensures good gas coverage of the weld puddle and that oxides have been removed, and it offers the welder a better view of the weld puddle.

3. Practice welding on scrap. When beginning, welders should practice on a flat piece of metal rather than welding a joint or adding filler. They should also practice identifying the weld pool from the base metal, play with the amperage control to find the right amount of heat, and learn how to control the size and shape of the weld puddle.

GTAW is like welding with an acetylene torch in that, if a weld is not satisfactory, the welder can go back over it, remelt it, and repair it. However, aluminum acts as a heat sink, so if too much time is taken to get a molten puddle, a small part easily can be overheated.

4. Starting the arc. To start an arc using HF, the electrode should be held about 1/8 inch from the work and the foot pedal depressed. The electrode should never touch the work during a HF start. Many people tilt the torch and rest the gas cup against the work, establish the arc, and then shift the torch into the proper welding position. With lift arc starting, the welder touches the tungsten to the workpiece, lifts it off the workpiece, and the full welding current begins flowing.

5. Maintain consistent arc length. Arc length is usually one electrode diameter from the work. Varying this arc length produces inconsistencies. One common error that new GTA welders commit is picking up the torch, or tilting the torch too much, to get a better view of the electrode and weld puddle. For a better view, a welder should shift the position of his or her face, typically down and to the side or reposition the workpiece so that the torch is pulled toward the body.

6. Maintain a travel speed consistent with the desired bead shape. Moving the torch too quickly creates a bead that is too narrow, while moving the torch too slowly produces an excessively wide bead. With GTAW, the torch does not need to be moved forward until the weld puddle reaches the desired size. However, holding the torch too long in one spot, especially on thin metal, can result in the arc burning through the base metal.

7. Add filler metal. Once the arc is started and a weld puddle of the desired size is established, filler metal can be added. The filler rod should be held at a 15- to 20-degree angle up from the workpiece, creating a 90-degree angle between the filler rod and the tungsten.

The torch and filler rod should be moved progressively so that the weld pool, hot filler rod end, and solidifying weld are not exposed to air. The hot end of the filler rod should not be moved from the protection of the shielding gas.

Learning GTAW

There is good news and bad news about GTAW of aluminum. The good news is that it is very difficult make a bad GTAW weld. If the welder melts the base metal and gets the filler rod into the weld puddle, a sound weld is most likely to result.

The bad news is that learning to make pretty weld beads, as well as coordinating the hands, feet and eyes, takes patience and practice. However, when a welder becomes proficient, the results are very satisfying.

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