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Welding aluminum is different than welding steel, but can be achieved successfully by following prescribed techniques and best practices. Because of aluminum’s high thermal conductivity, heat transfers away from the weld much faster than for steel. This property, combined with the low melting point of aluminum, can quickly lead to burthrough unless special care is taken. 
Equipment with a higher welding current may be required to supply the necessary heat to fully melt and fuse the metal. As it cools, aluminum shrinks significantly more than steel, so special care must be taken to prevent craters and cracking.
Issues with porosity are a greater problem when welding aluminum, as compared with steel. When the base metal and filler material heat up and become liquid, they absorb hydrogen. Then, as the molten material begins to solidify, the hydrogen forms bubbles trapped in the metal. Eventually the bubbles work their way to the surface, leaving behind voids and porosity. To prevent this problem, a helium/argon shielding gas mixture, rather than pure argon, can keep hydrogen at bay.
Choosing the proper base metal grade for welding is important, as some alloys and tempers are much easier than others to weld. Keep in mind that welding can change the material properties of some alloys, degrading tensile strength by up to 1/3 in the welded joint, for example. This reduced strength can be partly increased by heat treatment after welding.
Prepping the Surface
Even small amounts of contaminants can cause problems when welding aluminum. The natural aluminum oxide coating that forms on the base metal can add contaminants and should be removed prior to welding to avoid porosity in the weld. These oxides have a very high melting temperature, which can create issues during the welding process.
First, remove oil, grease and dirt by wiping the surfaces to be welded with a degreaser such as acetone or aluminum cleaner, making sure to allow it to evaporate before welding. Sometimes micro-etching with chemicals is used to remove the oxide coating. Afterwards, use a clean stainless-steel brush that has not been used on any metal other than aluminum to completely remove any remaining oxides. Then perform the weld as soon as possible, to prevent any further buildup of oxidation.
Choosing a Filler Material
A key factor in successfully welding aluminum is to match the filler metal to the base material, and to the application. A selection chart allows you to match the metal to the optimal filler material, depending on characteristics desired.
For example, if the base metal is AL 6061, choose a filler metal of 4043 or 5356 aluminum to avoid the tendency to crack
Welding Equipment and Methods
Aluminum should be welded with either a TIG (GTAW) or MIG (GMAW) process. Use only argon-helium or argon gas to shield the weld.
To prevent cracking caused by the expansion of aluminum when heated, followed by contraction as the weld cools, preheat the area, but don’t overheat, which could cause burn-through. At the end of the weld, don’t leave a crater, which will inevitably lead to cracking as it cools. Instead, back weld for an inch or so and leave a convex-shaped mound.
For MIG (GMAW) welding of aluminum, use a fast travel speed and higher heat inputs to prevent burnthrough, especially on thinner pieces.
Feeding aluminum welding wire through the torch can cause problems because the soft wire has a low column strength and tends to tangle at the drive roll or get caught and shred the aluminum. Use plastic liners and inlet guides so that the aluminum wire is not abraded as it travels through the drive system. U-groove drive rolls designed specifically for aluminum prevent the soft filler wire from being squashed. Finally, use of a water-cooled gun can minimize heat buildup and reduce problems with wire feeding.
Industrial Metal Supply stocks a wide variety of aluminum products, as well as welding equipment and supplies.
