Getting up to speed with aluminum welding
Welding aluminum is considered difficult by many welders. There are porosity problems and frequent burn throughs when starting to weld it.
“Aluminum is only different to weld, not necessarily difficult,” says Mikael Carriere, manager of technical services for Lincoln Electric. There are two main reasons why aluminum welding differs greatly from welding steel.
The first is aluminum’s inherit ability to absorb and transfer heat. In fact, many heatsinks are made from aluminum for this reason. “Aluminum transfers heat so quickly that a lot of people run into problems because they don’t put enough heat into the weld, so they don’t get good fusion because they’re used to welding steel where the heat doesn’t dissipate as quickly,” says Rob Krause, technical services manager at AlcoTec Wire Corp. “Once you’ve learned how to attack aluminum, it’s really not that difficult.”
Maintaining the proper heat and speed of the weld takes skill and practice.
“Generally speaking you're going to need more heat input and you're going to need to travel faster,” says Krause. When teaching students to weld aluminum, he says the most common correction is to “use more wire and go faster.”
The other characteristic of aluminum welding is the oxidization. All bare aluminum has a thin layer of oxidization on it. “That’s the reason that we use AC to weld all aluminum,” says Mark Kadlec, weld technician for Miller Electric Mfg. Co. “In the AC process, you have a straight and reverse portion of an AC sine wave. The reverse portion of the AC sine wave actually lifts the oxides off the piece of aluminum and allows you to get at the aluminum.”
The aluminum oxides melt at 3,200°F (1760°C) while the aluminum itself melts at 1,200°F (649°C). “For that reason, once we lift the oxides off, then the polarity switches to straight polarity. That’s what’s used to melt the aluminum,” says Kadlec.
Some newer welding power sources have balance control to allow the operator to adjust the sine wave specifying how much time is spent on each side of the sine wave. Welding aluminum is typically always done in AC. “However, there is a process that you can use DC,” explains Kadlec. “You must use 100 per cent helium shielding gas to do that. In MIG, you would use DC reverse polarity. In TIG, you could use DC straight polarity. That’s the way aluminum was welded before AC was really invented and put in welding machines.”
If you have ever tried stick welding aluminum, you would not forget the experience. Achieving a successful weld with this process is very difficult and usually futile. “Stick welding with aluminum is not suggested for any type of structural repair,” says Carriere. “The amount of porosity that is present in aluminum welds made by stick is enormous. The frightening part about aluminum stick is a lot of the time all of the porosity is internal, so many people never even know that it’s there.”
The first step in making a proper weld in aluminum is preparing the joint. This applies to all welds, but it is more crucial in aluminum than in mild steel.
“Keeping aluminum clean is critical to welding success, says Patrick Berube, product manager, aluminum for Hobart. “It is common for lubricants or cutting fluids to be present on aluminum. It is important to degrease the base material with a clean cloth and use a solvent for the purpose, like acetone or a product called Zero Tri.” After degreasing, use a dedicated stainless steel brush to remove the oxide layer and use it only when welding aluminum to avoid cross contamination and weakening of the joint. The removal of the oxidizing layer should be done just prior to welding as the oxidization occurs quickly on bare aluminum when it is left out in the air. The stainless steel brush should be used after the degreasing, so no oils get on the brush contaminating it and the weld joint.
There are many different alloys of aluminum and it is rarely found in its pure natural form. Most share the same weld properties and there is no different approach to welding them. However, not all aluminum alloys are considered arc weldable. “High strength aerospace aluminum alloys (7075) are not considered arc weldable, as they form low melting intermetallic compounds (made of copper, magnesium and zinc). These particles melt at a much lower temperature than the rest of the aluminum matrix, becoming weakened structures that cannot handle the thermal stresses associated with the welding process. 2024 aluminum (Al-Cu-Mg) is also not considered arc weldable,” says Berube. “There are a variety of aluminum alloys, however, that are arc weldable, even in the 2xxx and 7xxx series. 6xxx series and 5xxx series are the most common; they account for the largest portion of aluminum applications and most are weldable. Most of these applications require the use of a filler metal during welding. There are a variety of 5xxx series and 4xxx series (silicon based) filler metals that work well, but selection has to be made carefully based on the requirements of the application.”
Welding two different alloys together is not necessarily problematic if the two alloys are in themselves weldable. “Typically, the only time you run into problems is if the chemistries are not compatible,” says Krause. “But from a weldability standpoint, it’s usually just a matter of the heat transfer.”
MIG welding aluminum
MIG welding is the choice for most welding operations in a manufacturing setting. It has high productivity solid welds and is straightforward to use. In the past, MIG welds on aluminum did not have a good aesthetic look but that has changed with welding power sources now being able to pulse.
“The reason most people choose MIG is because it’s extremely fast,” says Kadlec. For large projects, MIG welding is the typical choice. This has been greatly aided in the past few years by dedicated welding machines for aluminum welding. “There are power sources designed for aluminum welding that help simplify and improve MIG welding on this material,” says Berube. “They feature a push-pull or spool gun. Since MIG welding can be a faster process, with the right aluminum system, a welding operator can expect consistent productivity. The process also involves less training, so welding operators of varying skill sets can be successful.”
Since the aluminum alloy in the filler metal is soft, there have been instances of the filler wire getting caught in the MIG torch cable. The industry has come up with several solutions such as a spool gun where a small spool of aluminum wire is in the torch, and a push pull arrangement where the wire feeds from the machine and a small motor in the torch assists in the feeding of the aluminum filler wire.
“You can use a standard push only depending on the alloy and diameter of your filler wire,” says Krause. “Some alloys and diameters have better feedability than others. Basically, the smaller the diameter and the softer the alloy, the more feeding issues you could run into.”
A spool gun is a good solution if you are not going to be using a lot of wire, as you’re only dealing with one-pound spools at a time, and a push-pull gun lets you run the larger spool. “Now you can run 16 or 20-pound spools or even from a bulk drum container. The wire can run through a 25 or 30 push-pull gun,” explains Krause.
TIG welding aluminum
The TIG process is slow, but more esthetically appealing, says Kadlec.
“You can make it look great. It’s a finished product. You don’t have to grind it. You don’t have to clean it up. There isn’t any spatter. The other part is that we can weld any thickness of material with the TIG process. Your control of heat is much better in the TIG processes than it is in the MIG process.”
TIG is mostly used in smaller, shorter welds. “TIG is used in critical applications with strict acceptance criteria with regards to porosity, such as very small parts with tight corners and radii,” says Carriere.
Choosing a weld process for aluminum means knowing what you are looking for.
“Productivity and quality are the biggest factors to consider,” adds Carriere. “It all depends on the parts to be welded and which specification the weldments must conform to. Usually GMAW is better because of it’s productivity gains and because most acceptance criteria allow a range of porosity which can be met using GMAW.”
Another factor to consider is training. “More training and coordination is required for TIG than MIG welding,” says Berube. “MIG offers faster welding. For applications with short welds or limited joint access, TIG might be the better option. If the welds are long or the aluminum is very thick, MIG would be the better option.” SMT