Welding Steel Vs. Welding Aluminum

John Brennan

Steel is a common construction material and most welders start by learning how to weld steel. Depending on your line of work, aluminum welding may be a valuable skill to learn as well.

Weld aluminum using methods similar to those for steel.

Although the process is similar, aluminum welding is a little trickier, and you have to treat the metal a little differently because aluminum has different properties.


Aluminum is much lighter and less dense than steel, but its thermal conductivity -- the rate of heat transfer -- is about six times greater. This high thermal conductivity explains why aluminum welds solidify much faster than a comparable steel weld. Aluminum also has a lower melting point, although aluminum objects are coated in a thin "skin" of aluminum oxide, which melts at a much higher temperature than steel. You have to remove this oxide skin in order to get good results.


Many welders use the color of the heated metal to keep track of their progress when working with steel. This approach is a little problematic when it comes to aluminum, however, because aluminum doesn't show color quite as well, and the molten metal looks silvery but is less obvious. If you're not careful, you can end up with a puddle of liquid metal dripping away leaving a hole in its wake.


High-carbon steel requires heat-treatment after welding and is more difficult to handle. Mild and low-carbon steels, by contrast, are much simpler and do not require special pretreatment. For aluminum, however, pretreatment is much more critical. It can readily combine with hydrogen and must be gradually raised to ambient temperature if it was stored in a cooler space. It should be cleaned with non-chlorinated solvents and cleaned with a stainless steel wire brush to remove surface oxides. For both aluminum and steel, the edges you plan to weld should be cleaned.


MIG or metal-inert gas welding is a common technique for both aluminum and steel. Use argon mixed with carbon dioxide for steel, but for aluminum you need straight argon. Thicker sections of aluminum can be welded with a mixture of helium and argon if desired. Many techniques that work well with steel, however, will not work with aluminum. Oxy-acetylene torch welding, for example, is a poorer approach with aluminum because the metal can absorb hydrogen gas, leading to defects in the weld.