5 aluminum GMAW FAQs
March 7, 2006
While using aluminum can present challenges in any welding application, knowing the right questions to ask beforehand will help aid success. Issues include knowing what aluminum filler metal to use, how to store it, and what options are available to help eliminate feeding problems.
Aluminum, especially thin-gauge, presents unique welding obstacles. Gas metal arc welding (GMAW) in particular presents challenges in wire feeding and selecting the right type of filler metal and equipment. However, several options can aid in effective aluminum welding.
In its pure form, aluminum is a relatively soft metal that has many uses but requires adding one or more alloys to increase its strength and add qualities that make it suitable for different applications. Common alloys are copper, magnesium, silicon, manganese, and zinc. They are identified by their series numbers:
|1xxx||99 percent minimum aluminum purity|
The following are five questions welders frequently ask when deciding how best to weld aluminum.
While filler metal for steel typically is chosen by matching the tensile strengths, strength is only one consideration in choosing an aluminum filler metal. Usually several different aluminum-alloy filler metals can be used with any of the aluminum-alloy base metals or when welding dissimilar aluminum alloys.
In choosing filler metal, consider the following:
Different filler metals address these considerations to varying degrees. In general, if strength is the primary concern, the filler metal should match the base metal closely in tensile, yield, and ductility.
Most consumables manufacturers, as well as the American Welding Society (AWS), offer information listing the relative values of these qualities of their filler metals for each base alloy.
While no aluminum filler metal fits all needs, 4043 and 5356 are the two most common and make up the majority of aluminum filler metal sales. They can be used with most widely used aluminum-alloy base metals.
The 4043 filler metal often is a favorite among welders because its silicon alloy can increase welding ease and offer good puddle control. It can be used with a variety of base alloys with relatively high marks in all categories. It's forgiving in terms of weld parameters, is clean, and can provide a nice appearance.
Generally, 5356 is used most widely. Another general-purpose filler metal, 5356 gets slightly lower marks for welding ease, but usually offers higher tensile strength than 4043. Its higher columnar strength means it can feed more easily than 4043. It also has a faster melt-off rate, so it requires a faster wire feed speed for the same wire diameter.
Although these two wires comprise the majority of uses, it's important to check a metal wire manufacturer's data sheets to ensure their suitability for your application. More information can be found in the AWS book Specification for Bare Aluminum and Aluminum Alloy Welding Rods and Bare Electrodes, AWS A5.10.
Yes. As a filler metal, aluminum has the same oxidation problems as all aluminum. When left open, either on the shelf or installed in a welding machine, aluminum filler wire will oxidize, which can lead to an erratic arc. Oxidation adds resistance, can produce soot, and can change the wire's ability to feed smoothly. Many operators spend a lot of time adjusting tension settings, changing contact tips, or checking the shielding gas trying to fix the problem when the oxidized wire typically is at fault.
Because aluminum wire has low columnar strength, feeding it has been likened to pushing a wet noodle through a straw. Birdnesting, or the tangling of the wire between the drive roll and the liner, is a common, time-consuming, and costly problem. Clearing it requires the operator to stop welding, cut the wire, discard the wire in the gun, and re-feed new wire through the liner. It also may require cleaning or changing the contact tip because of the burnback caused when the wire stops feeding.
Several types of systems can feed aluminum wire:
Push Only. Feeding aluminum wire through a push-only system can be difficult, but it can be done on a limited basis. It requires U-groove drive rolls to provide more surface contact with the wire, a Teflon® liner, adequate drive-roll pressure, the ability to keep the gun cable straight, and a high tolerance for pain. Any resistance in the line likely will cause the wire to misfeed. Thicker wire, such as 1/16 inch, can be fed consistently in a push-feed system. However, push feeding isn't very dependable for thinner gauges, such as 0.030-in. wire.
Spool Gun. A spool gun helps eliminate birdnesting by putting a 4-in. (1-pound) spool on the gun so the wire feeds only a few inches. Spool guns can accommodate aluminum wire diameters from 0.023 to 1/16 in. and allow the operator to use longer cables, generally from 15 feet to 50 ft.
The roll in a spool gun needs to be changed after every pound of wire is used. In tight spaces, the spool may limit access, requiring the operator to use a longer stick-out. If the operator uses several pounds of aluminum per day, the few minutes needed to change spools can add up. Also, burnback is a possibility when the end of a spool is reached, so many operators stop even though a few turns are left on the spool.
Push-Pull Gun. On a push-pull gun, a motor in the gun pulls the wire through the liner, while the motor in the welding machine or feeder control becomes an assist motor. By maintaining consistent tension on the wire, the push-pull system helps eliminate birdnesting. Because the weight of the spool isn't in the operator's hands, a push-pull gun can offer ergonomic benefits.
With a push-pull gun, the spool needs to be changed less often than with a spool gun, and larger spools can be used. Cables up to 50 ft. long can be used with a push-pull gun. A push-pull system can be more expensive than other types of systems, but it can offer increased productivity and the ability to buy larger spools.
Continuous-feed Push System. This system is relatively new. Its drive system maintains continuous contact with the wire and helps eliminate birdnesting by removing the gap between the drive rolls and the liner. It's limited to pushing wire 15 ft., but the gun is lighter than a spool gun or a push-pull gun and requires no additional maintenance.
Short-circuit transfer mode isn't recommended for aluminum. It's almost impossible to obtain good fusion, and the weld will be prone to breaking or cracking. It shouldn't be used when good appearance or high strength is a requirement.
In spray transfer mode, molten droplets transfer smoothly from the electrode to the puddle. The arc is smooth and stable and produces a nice appearance with good fusion at the sides. Since it involves high heat, burn-through can be a problem on thin (1/8-in. or thinner) material, so it requires a faster travel speed and a thin-gauge (0.030-in.) filler wire to keep heat input down. It's not suitable for out-of-position welds. For that, pulsed welding is recommended.
With pulsed welding or pulsed GMAW, the operator always is in spray transfer mode. The wire transfers across the arc and then drops to a lower amperage, which allows the puddle to cool while maintaining the arc. This allows for out-of-position welding. Also, the pulse agitates the weld puddle, aiding the cleaning action. The heat input can be controlled more than in other modes, so it's possible to weld thin-gauge material and use a large-diameter wire (up to 3/64 in.) with a decreased chance of burn-through and increased deposition rates.
Because you can use 3/64-in. filler wire to weld thin-gauge material, deposition is increased and wire feeding is aided through the use of a stiffer wire.
Chris Roehl is product manager, Miller Electric Mfg. Co., 1635 Spencer St., P.O. Box 1079, Appleton, WI 54912-1079, 920-734-9821, www.millerwelds.com.