Welding aluminum makes countless products possible. Like auto pistons, aircraft, marine valves, bikes, structural aluminum beams, and many others. Whether welding aluminum plate, sheet, bar, or tube to make these and other products, knowing the aluminum grade is what’s most critical.
Aluminum is alloyed with various elements which accentuate certain characteristics, like conductivity and strength. Variability in chemical and mechanical properties means that aluminum alloys are not all created equal. They are not interchangeable when it comes to weldability. You should never weld aluminum if you aren’t certain of its specific alloy composition.
For starters, aluminum alloys fall into two categories, heat treatable and non-heat treatable. Non-heat treatable alloys are hardened by cold working and must not be tempered prior to. Heat treatable aluminum alloys, on the other hand, have been heated to about 400°F.
Kloeckner Metals is a full-line aluminum supplier and service center. Download this aluminum spec sheet to view what Kloeckner Metals routinely stocks.
When it comes to welding these aluminum alloys, the alloys that have been previously heated are more susceptible to weakening and cracking.
Welding is a value added service that demands special knowledge, skill, and certification. When compared to steel and other metals, aluminum alloys are susceptible to poor and ineffective welds. This is especially true when the proper factors are not thoughtfully considered.
The Universal Technical Institute outlines 4 challenges to welding all aluminum:
There are two additional factors to consider when welding aluminum:
One of the most important steps in how to weld aluminum is cleaning the aluminum base metal and filler rod thoroughly before welding.
To clean the aluminum use acetone and a stainless steel brush. Do not use the brush for anything else but aluminum, and if you can afford to, use a new brush each time you work with new material grades. Thoroughly dry all aluminum and weld within 8 hours. Clean the aluminum if more than 8 hours passes between the initial cleaning and beginning the welding process.
Oxidation is one of the biggest challenges when it comes to welding aluminum. When the base metal and filler rod are not thoroughly cleaned prior to welding, three times the amount of heat will be required in order to break through the surface oxidation layer.
By the time the oxidation layer has been melted through, you will have a swimming pool of melted aluminum and clotty soot. In other words, the aluminum underneath the oxide layer had been melting the entire time. Cleaning the oxide layer equalizes the melting temperature leading to clean, strong welds.
You will know if the base aluminum and filler rod weren’t cleaned properly if your arc wanders, the filler won’t blend, and an uncomfortable surface tension develops.
There are many different kinds of welding processes. Some are outdated and have been replaced with new technology, and some are better for steel than aluminum.
You’ll notice several of the welding processes listed above include “arc welding” in their name. So, what does it mean?
Arc welding is an extremely common welding process that requires electricity to heat the metal. As electricity flows from the filler metal—also known as the rod, or electrode—onto the base metal for joining you get an arc. The many different types of arc welding can vary in the type of current needed, or whether the welding is semi-automated, fully automated, or manual.
Probably the most popular technique to weld aluminum, is the Gas Tungsten Arc Welding, otherwise called “TIG” (tungsten inert gas) welding.
TIG welding is particularly great when working with lighter gauge aluminum. Plus, it is mechanically strong and visually appealing. For these reasons, it rose to a real golden era as the popularity of aluminum grew in the automotive industry. Most professional welders working in the automotive industry prefer the TIG welding process.
TIG welding is a manual welding process and is sometimes criticized for being slow. However, when an operator is skilled, the difference in weld speed compared to other processes is negligible.
When you light up the arc, you’ll first notice a frosty area. This frosty area is also known as cleaning action. It’s the oxide layer melting.
Filler rod cannot be introduced until you see a shiny wet puddle, which indicates that the oxide layer has been melted through. Your travel speed should match the rate at which the aluminum is melting. As you wire feed manually, you dab along the line being welded. You’re looking for good taper and stable arc.
Sometimes viewed as the quicker way to weld aluminum is the Gas Metal Arc Welding (GMAW) or “MIG” welding process. MIG welding is semi-automated using a spool gun, and is considered better for welding thicker aluminum.
Argon is typically the gas used in MIG welding. You can also get an argon-helium blend. Helium increases the arc and is generally better for thicker metals.
Spool guns can sometimes be less predictable than the manual dabbing process of TIG welding. Welding machines will note a wire speed recommendation, but you can scale that back, depending how well you can control the spool gun at that speed and temperature. Burning through contact tips during MIG welding is common, so plan to have more of them.
Another important consideration during MIG welding is the type of angle. Generally, push angles are preferred when using a spool gun for welding aluminum. The angle of the spool gun impacts the cleaning action and porosity of the weld.
Using a heat sink during MIG welding is also recommended. Heat sinks will make it possible to weld a little slower by distributing heat away from the spool gun. This is great when first learning how to MIG weld, and also helps to prevent against cracking.
Many welding machines include the option to TIG and MIG weld. Depending on your preferred welding processes, select TIG or MIG when it prompts you to select the tool.
Next, you’ll select the working metal. For example: 100% argon positive electrode, if MIG welding.
Then, you’ll type in the diameter, wire speed, and thickness. Note that the amps required depends directly on the thickness of the aluminum being welded.
Compared to welding steel, everything is more sensitive with aluminum welding. This makes aluminum welding a little more challenging.
One main difference when welding aluminum is the required power source. Aluminum can only be welded with alternating current. AC breaks through the oxide film layer, which melts at a higher temperature than the aluminum.
DCEN welding current flows negative to positive. This is used when welding steel, nickel alloys, and titanium. But it will not break through the aluminum oxide. If you attempt to weld with a DCEN current you’ll get a sooty weld, which, after a little polishing, will reveal holes throughout the weld. It doesn’t work.
DCEP welding current flows with reverse-polarity. You get the desired cleaning action as the current flows from the workpiece to the electrode. However, the electrode will overheat, leading to an uneven arc. So, even though cleaning action is good, the electrode will get so hot that your filler rod will form a ball at the tip and become unable to join to the base metal.
With AC current you get a mix of EP and EN. Good heat input and good cleaning action make welding aluminum viable.
Aluminum also has a greater thermal conductivity than steel. The heat created when welding aluminum is dispersed more rapidly than when welding an iron-based alloy. This increases the potential for cold starts, where penetration of the weld is too shallow.
Increased thermal conductivity can also increase the potential for large craters, or concave dips in a weld. This happens because by the time you reach the end of a weld there is more heat than was present at the start, as the heat has been dispersed throughout the metal. Craters should be filled.
We hope this helps you take into account the major steps and considerations when welding aluminum. The most important takeaway is that the composition of the alloy will dictate how you weld, followed by the particular welding process selected. Choose an alloy that is weldable, and that also fits the desired end product’s mechanical requirements. Select a welder who is familiar with aluminum alloys, and a welding process that is comfortable to them, and that is also suitable for the thickness of the aluminum being welded.
Kloeckner Metals is a full-line aluminum supplier and service center. Kloeckner Metals combines a national footprint with the best fabrication and processing technologies, and innovative customer service solutions.
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