Welding light alloy

Using Pulsed MIG can result in general improvements in weld quality compared to conventional MIG and welding speed compared to TIG.

Although metals are divided into ferrous and non-ferrous metals, the non-ferrous metals are unique and have very different properties that necessitate individual welding techniques.

When talking about welding non-ferrous metals most people think aluminum. The most important thing about welding aluminum is knowing what you’re welding.

“Identifying the base alloy is important,” said Tony Anderson, the Director of Aluminum Welding Technology for ITW North America, the parent company of Miller Electric Mfg. Co.

“There are hundreds of different types of aluminum some are weldable and others are not.”

There are eight different alloys families and each of these families has a family designator.

“Alloy selection is also critical as some aluminum alloys do not mix well with others such as combining 4XXX and 5XXX series alloys,” said Thom Burns, Technical Services Director for AlcoTec Wire Corporation.

“Often there are several choices for welding aluminum where the application dictates the proper filler needed."

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Some of these characteristics of fillers for applications include; Weld Strength, Ductility, Freedom from Hot Cracking, High Service Temperature (above 150 F), and whether the material is to be anodized.

Choosing the right filler material for welding is critical according to Frank Armao, the Director of Aluminum Welding Technology at Lincoln Electric.

“You need different filler material. It is a chemistry thing.”

The compounds in the filler material alloys must complement the compounds in the base material. Using the wrong filler material with the wrong base material will lead to problems of hot cracking, because of the mismatch of the chemistries.

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Clean it well:

Cleaning the aluminum is critical to obtaining a high-quality weld. Unlike the simple wire brushing that suffices as prep for mild steel, aluminum must be degreased first.

“The cleaning is always done in two steps,” said Anderson.

“Degreasing is done with a solvent. You can use any degreasing agent to remove any trace of hydrocarbon from the surface, like oil, grease or paint.”

The second step is wire brushing with a stainless steel brush. “The reason you don’t brush first is that if there was grease on the surface you would just work it into the base material,” said Anderson.

One of the primary reasons to clean is to remove the thin layer of aluminum oxide that coats all aluminum pieces. “All aluminum has a thin oxide coating on it that appears immediately on it once it is exposed to air. You can’t prevent it,” said Armao.

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“If you take it off it will reform again. Aluminum oxide is a thermal insulator, an electrical insulator and an abrasive.”

Aluminum oxide can absorb moisture and become hydrated and this produces porosity in the weld. If you have aluminum oxide and try to weld it you will have many problems. Aluminum oxide melts at three times the temperature of the base material. Pure aluminum melts at about 660 degrees Celsius and the oxide will melt at 1,980 C. Alloys will have different melting points but the oxide will always melt at a temperature three times higher than the oxide.

“Aluminum oxide acts as a barrier but at room temperature it grows exceedingly slowly,” said Armao.

“Lets say you take a sanding disk to an oxidized plate. You have days until you have to weld on it unless it gets wet. In some aerospace applications you will have specifications that if you clean the material and do not weld within 8 hours you must reclean the material. For most applications this is not necessary.”

Equipment:

The two main processes used for welding aluminum are GMAW or MIG, and GTAW or TIG. “In terms of the MIG equipment. It is all to do with feeding the alum- inum wire,” said Anderson.

“The feeding system of the equipment has to be very different that it is for steel because aluminum solid wire is far more difficult to effectively feed through a MIG system without encountering problems associated with burn backs. Because of the aluminum’s physical characteristics it is much softer and it can be deformed very easily in the feeding system. Great care needs to be taken in using only parts that are designed for feeding aluminum and not steel.”

AC is used because of the properties of the alternating current. “Inverters have the ability to use a greater percentage of electrode negative (EN) in the AC cycle, which is where the current flows from the electrode to the workpiece. Increasing the amount of EN also helps achieve good penetration,” said Tom Wermert, Senior Brand Manager for Victor Technologies.

“Most fabricators working with aluminum favour greater amounts of EN because it directs more heat into the workpiece, improving penetration. However, if they notice black, pepper-like flakes in the weld puddle, it means that they need to increase the amount of EP, or 'cleaning action', to remove more of the aluminum oxide. Conversely, if an operator notices that the tungsten starts to ball excessively, it means that there is too much heat on the tungsten (too much EP). Excessively wide etching along the sides of the weld bead may also indicate too much EP. That said, the arc must be allowed to remove the oxide layer.”

Since aluminum has a high thermal conductivity, when you weld this material it tends to dissipate the heat very quickly. “So that means you have to pour a lot of the heat in and do that fast,” said Armao. “The old folklore in aluminum when you weld is hot and fast.”

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To preheat, or not:

Although some weld operators insist on preheating the material before welding most experts in the field say that this practice is unnecessary.

“There is not good reason to preheat aluminum if the proper equipment is sufficient to perform that level of welding,” said Burns.

“Often, aluminum is preheated due to power supplies that can not supply the amount of amperage needed. There are guides within aluminum codes such as in the AWS D1.2 that specifies the amount of preheating that is allowed. Also the use of Oxy-Fuel systems can deposit water vapor and unspent fuel on the surface that can contaminate the weld. Lastly, excessive preheating makes the conductivity of aluminum drop requiring more power to perform the weld.”

Preheating can destroy the properties of heat treatable alloys. “If you have the right equipment then you never need to preheat. Usually the welders that like to preheat are usually the ones that have equipment that is to light duty to do the job,” said Armao.

“I’ve TIG welded aluminum that was seven inches thick without preheating. Now I had a really big powerful weld machine. A little bit of preheat is OK. If you use 100 C preheat to drive off moisture. Too much is bad.”

Gases:

Unlike with filler material, there is not much choice for shielding gases when welding aluminum. This is true for both MIG and TIG welding.

“The most common one is pure argon,” said Armao. “Now if you are welding heavier materials, about 12 mm or so then people start adding helium to the argon, 25 percent to 50 percent helium. That’s about it ; there are no other choices. Helium makes the arc hotter and gives you more penetration.”

Helium is becoming rarer and therefore more expensive. Although researchers are working on a substitute none have been found yet.

For a proper weld bead gas flow control is important. “When TIG welding aluminum, good shielding gas post-flow helps prevent porosity in the weld bead and extends tungsten life,” said Wermert.

“First, set post-flow following weld procedure specifications or using your machine’s set-up guide. Note, that for materials 1/8-inch and thicker, post-flow duration will likely be 11 to 13 seconds or longer. After breaking arc, operators must remind themselves to hold the torch in position over the weld bead so that the shielding gas can do its job.”