February 19, 2001
Aluminum is a real challenge to weld, especially for beginners. A knowledge of the gas tungsten arc welding equipment that is available to do the job as well as required accessories, preparation tips, and proper techniques is a good thing to have before jumping in.
Aluminum: beautiful, lightweight, strong, versatile—and a real challenge to weld, especially for beginners. This article describes some of the new gas tungsten arc welding (GTAW) equipment available and its benefits, accessories required, points to consider before welding, and the techniques required to make a good weld bead.
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 don't 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 about 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 can 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 usually is 5, 3, or 25 amps, respectively.
4. Inverter-based AC. Also considered an industrial power source, 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 as much as 20 percent, narrow the weld bead, achieve greater penetration, allow use of a smaller-diameter tungsten to direct the heat more precisely 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 job 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.
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 wheeled carts that also carry 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 fingertip 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 probably is a better option because it's easier to use. Conversely, if most work is done in awkward positions, a fingertip control may be the better choice.
The following steps and 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, the tungsten immediately will 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 used to weld 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 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.
5. Select the right type of tungsten. For AC welding, traditional practice calls for selecting a pure tungsten electrode and forming a ball at the end of it. 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 inch, or when using a GTAW 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. A 2 percent-type tungsten (thorium, cerium, etc.) should be selected and ground 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. It also is a function of the amount of AC unbalance and the composition of the electrode. 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 15- to 80-amp capacity.
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 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 then should be resharpened.
8. Set the proper tungsten extension. 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 require a special gas cup.
9. Select the correct filler metal. The filler rod needs to be appropriate for the base metal in terms of type and hardness. It should be the same diameter as the tungsten electrode. The welder should refer to charts published by filler metal manufacturers detailing what filler to use for what base metal.
10. Select a high-frequency (HF) mode. For AC welding with transformer-rectifier-type machines, continuous HF typically 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 because 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. They 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 clamp cables close together (spreading them apart is like creating a big broadcast dish), and keeping the cables in good condition to prevent current leaks.
12. Set the balance control. There are no hard rules about setting balance control, but the typical error involves overbalancing 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.
The often-overlooked storage tank area and the lines between the storage tank and the mixer are also important areas to consider.
In most applications, the gas supplier's equipment and piping responsibilities end at the final pressure regulator. For this reason, large portions of piping between the vaporizers and the mixer are a no-man's land regarding inspections. Quite often, malfunctioning relief mechanisms exist around storage tanks, such as tank fill valves that may not be completely closed off and/or may have packing and bonnet leaks. Even though maintaining these areas is not the customer's responsibility, the customer will pay for any gas losses occurring there.
Argon suppliers usually develop a good feel for their customers' consumption rates and develop a trend regarding the amount of liquid argon required to fill a tank to capacity based on the number of loads delivered over time. Any time a customer anticipates a significant drop in consumption, the supplier should be notified so that deliveries can be adjusted accordingly. The supplier also should be notified of any plant shutdowns that will last more that a few days because liquid argon must be stored at supercold temperatures to remain in its liquid state.
If a tank remains at or close to full for a long time, the liquid will absorb some heat, causing it to flash into a vapor. This causes pressure to build in the internal tank, resulting in the release of safety devices such as rupture disks or pressure relief valves. Should this occur, large amounts of gas can be released in a very short period of time. By matching storage tank levels to efficient consumption levels, the chances of pressure-related, inadvertent releases are greatly diminished.
Most suppliers of argon and other gases are concerned about the efficient and safe use of their product. Some provide applications programs, ongoing technical support, and training. While these suppliers perhaps understand the effects of various system inefficiencies more than most, their primary responsibility is to keep the customer supplied with a high-quality product. Because leaks and other system losses are the primary sources for contamination after delivery, suppliers often inform their customers of various types of system deficiencies that may exist, especially if the customer has lodged complaints about quality.
However, it usually is up to the user to correct system inefficiencies. Although some suppliers will provide limited or abbreviated services using their own personnel and resources, for a more thorough and cost-effective analysis, expert, professional, and nonbiased individuals in the field of piping and distribution system analysis should be consulted.
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