The Care and Feeding of Aluminum
Aluminum, especially thin-gauge aluminum, presents some unique welding challenges. For many, GTAW is the preferred process, but GMAW offers some obvious benefits: higher deposition rates, less operator training and higher productivity. With these benefits comes a few challenges, namely wire feeding and selecting the right type of filler metal and equipment. These challenges can be easily met, however, and we’ll explore some of them now.
Aluminum in its pure form is a relatively soft metal which has many uses, but which requires the addition of an alloy or alloys to increase its strength and to add other qualities suitable for different applications. Common alloys are: copper, magnesium, silicon, manganese, and zinc. They are identified by their series numbers:
- Series Alloy group
- 1xxx 99% minimum aluminum purity
- 2xxx copper
- 3xxx manganese
- 4xxx silicon
- 5xxx magnesium
- 6xxx magnesium-silicon
- 7xxx zinc
- 8xxx other
Q: How do I determine which filler metal to use?
A: While filler metal for steel is typically chosen by matching the tensile strengths, strength is only one of the considerations when 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 alloys together. In choosing the filler metal, consideration is given to
- Base metal composition
- Ease of welding
- Joint design
- Dilution (when the filler wire and base metal combine in the weld puddle to create a different chemical make up in the weld)
- Strength of the weld
- Cracking tendencies
- Corrosion in service
- Color matching, if the material is anodized
Different filler metals address these considerations to varying degrees. In general, if strength is the primary consideration, the filler metal should closely match the base metal in tensile, yield and ductility.
Most consumable manufacturers, as well as the AWS, offer information listing the relative values of these considerations of their filler metals for each base alloy.
Q: Are there any general-purpose aluminum filler metals?
A: 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 the most widely used aluminum alloy base metals. Hobart Brothers recently expanded its product lines with the development of its Smootharc™4043 and Smootharc™5356 wires, which provide excellent feedability, good wetting action and rated excellent compared to competitors products.
4043 is a favorite among welders because its silicon alloy increases ease of welding and gives better puddle control. It can be used with a wide variety of base alloys, getting relatively high marks in all categories. It’s more forgiving in terms of weld parameters. It’s clean and provides a nice appearance.
5356 is another general-purpose filler metal and is probably used most widely. It gets slightly lower marks for ease of welding, but typically offers greater tensile strength when compared to 4043. Its higher columnar strength means it feeds easier than 4043 and it also has a faster melt-off rate, so it requires a faster wire-feed speed for the same diameter wire.
Although these two make up the majority of uses, be sure to check your metal wire manufacturer’s data sheets to ensure their suitability for your particular 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.
Q: Are there any special storage considerations for aluminum filler metal wire?
A: There are. Aluminum as a filler metal has the same oxidation problems as all aluminum. When left open, either on the shelf or installed on the welder, aluminum filler wire will oxidize which can lead to an erratic arc. The oxidation adds resistance, can produce soot and can change the wire’s ability to feed smoothly. Many operators have spent a great deal of time adjusting tension settings, changing contact tips or checking the shielding gas trying to fix the problem when it was the oxidized wire that was at fault.
Feeding Aluminum Wire
Q: How do I eliminate “birdnesting” and other feeding problems?
A: Because of its low columnar strength, feeding aluminum wire 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 refeed 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.
There are several ways to feed aluminum wire: Push only, spool gun, push-pull system and continuous feed push only system.
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 will likely cause the wire to misfeed. Thicker wire, such as 1/16-inch, may be fed consistently in a push-feed system. But for thinner gauges, such as .030-in., push feeding is not very dependable.
Spool Gun: A spool gun, such as the Spoolmatic® 15A or 30A, eliminates the possibility of birdnesting by putting a 4-inch (1-lb.) spool on the gun, so the wire only feeds a few inches. Spool guns can accommodate aluminum wire diameters from .023 to 1/16-inch and allow the operator to use longer cables (15’-50’).
A spool gun needs to have the roll changed after every pound of wire is used, compared with the 8- or 15 lb spool on a push-pull system. In tight spaces, the spool may limit access, requiring the operator to use a longer stickout. If the operator is using several pounds of aluminum per day, the few minutes need to change spools can add up. Also, the chance of burnback exists when the end of a spool is reached, so many operators stop when a few turns are left on the spool.
Push-pull gun: With a push-pull gun, a motor in the gun pulls the wire through the liner, while the motor in the welder or feeder control becomes an assist motor. By maintaining consistent tension on the wire, the push-pull system helps eliminate birdnesting. It is more ergonomic than the spool gun, since the weight of the spool is not in the operator’s hands.
Also, the spool needs to be changed less often than on a spool gun and allows the purchase of larger spools. A push-pull gun also allows cables up to 50’ long. The only disadvantage to a push-pull system is their relatively higher price tag, but the increased productivity and the financial advantage of buying larger spools usually provide a quick return on investment. When you compare the cost of 20 1-lb. spools of .030-inch 4043 aluminum filler wire with one 20-lb. spool of the same diameter wire, plus the time to change 20 spools versus one spool, you’ll see that for high volume use, a push-pull system makes financial sense. Push-pull guns, such as the optional XR™-A Python, can only be used with welders built for their use, like the Millermatic® 350P, which automatically recognizes which gun is being used.
A welding distributor, picked at random, lists a price for a 1-lb spool of .030-inch aluminum wire as $5.23. A 20-lb. roll lists for $92, which works out to a difference of $0.63 per pound. Now, add the time for 20 spool changes (and possibly burnbacks) for the spool gun as compared with one change for the larger spool. At 5 minutes to change a spool, that’s 95 minutes of extra time for each 20 lbs. of filler wire used.
Continuous-feed push system: This is a relatively new addition to the field. This type of system uses a special drive system that maintains continuous contact with the wire and eliminates the possibility of birdnesting since there is no gap between the drive rolls and the liner. While it is limited to pushing the wire 15 feet, the gun is lighter than either the spool gun or the push-pull gun and requires no additional maintenance.
Short-Circuit, Spray Transfer and Pulsed MIG Modes
Q: Which mode should I use?
A: The short-circuit transfer mode is not recommended for aluminum. It’s almost impossible to obtain good fusion and the weld will be prone to breaking or cracking. It should certainly not be used when appearance or strength is an issue.
In spray transfer, molten droplets are smoothly transferred from the electrode to the puddle. The arc is very smooth and stable and produces a nice appearance with good fusion at the sides. Since it involves high heat, burn-through can be an issue on thin (1/8-in. or less) material, so it requires a faster travel speed and a thin gauge (.030-in) filler wire to keep the heat input down. It’s not suitable for out-of-position welds, though. For that, pulsed welding is recommended.
With pulsed welding, or pulsed MIG, you’re always in spray transfer mode; the wire will transfer across the arc, then drop to a lower amperage, allowing the puddle to cool while maintaining the arc. This allows for out-of-position welding, and the pulse agitates the weld puddle, aiding the cleaning action. Because the heat input can be better controlled, (on some machines, you can even shape the pulse waves) you can weld thinner gauge material and use a larger diameter wire (up to 3/64”), with a decreased chance of burn-through and increased deposition rates.
Because you can use .045-inch filler wire to weld thin gauge material, you not only increase the deposition rate and aid feeding by using a stiffer wire, you can save money on filler wire. The same distributor who charges $92 for a 20-lb. spool of .030-in aluminum filler wire (see above), charges $74.00 for a 20-lb spool of .045-inch wire—a difference of $0.90 per pound. Add in the decreased time and materials for rejected parts, and pulsed MIG may look like an attractive alternative, depending on your particular application. A new feature on the Millermatic® 350P, an industry leader in aluminum pulsed welding technology, is the Aluminum Pulse Hot Start, which automatically provides more arc power to eliminate the "cold start" that is inherent to aluminum starts.
Pulsed MIG has come a long way in recent years. It has become very easy to use and very easy to train operators to make quality welds—much easier that to train a TIG operator. Plus the appearance of a pulsed MIG weld can rival that of a TIG weld. An all-in-one pulsed MIG unit can cost 30% less than an industrial power source, pulsing controls and bench-top style wire feeder, while offering comparable or superior aluminum performance and easy-to-use controls.