Why is Aluminum So Difficult to Weld?

Aluminum is quickly becoming a popular choice of metal in fabrication. It’s lightweight, non-corrosive, and visually appealing, making it the perfect material for many projects. However, many of the very same properties that make aluminum a great option for fabrication are also reasons it can be difficult to work with.

While it may be tempting to assume that welding principles can be generalized across a range of metals, this is far from the case. Welding is about chemistry, and each metal has unique chemical properties that require specific processes. Aluminum doesn’t act the way other metals do during welding, so it can be difficult to determine weld progress and quality.

Simply put, aluminum is difficult to weld because it is a soft, highly sensitive metal insulated by a tougher oxidized layer. Aluminum is highly susceptible to impurities in its molten state, increasing the risk of ending up with weak, porous welds. Its low melting point and excellent thermal conductivity create a much smaller window of workability than you’d get with a metal like steel. This small window, coupled with aluminum’s lack of visual indications, can make gauging your weld progress a challenge.

Special tools and techniques are essential to effectively navigate some of aluminum’s more complicated properties. You don’t have to be a master welder to learn how to work with aluminum though. It just takes understanding the material—why it acts the way it does and what you can do to most effectively work with it. With that knowledge, and practice, anyone can learn to weld aluminum like a pro.

Aluminum vs. Steel

Steel and aluminum react very differently to heat, making the transfer of welding principles from one to the other anything but seamless. For starters, the melting point of aluminum is significantly lower than that of steel, 1,221 F (660.3 C) and 2,500 F (1,370 C) respectively.

Steel exhibits tell-tale behaviors during welding that help guide you along the process. As heat is applied, the material changes color, glows, and then goes molten. Steel gives you indications for when to add materials and move on. Aluminum doesn’t. The material just gets shinier as it is heated. This lack of visual indicators can make it difficult to recognize the stages you’re looking for during the heating process. Once you’ve worked with aluminum enough to be familiar with its behavior, picking up on these cues becomes easier. Learning to weld aluminum is really just that—a whole new learning experience.

Most welding tools and techniques are made to work with steel, not aluminum. Later on, we will discuss in more detail exactly which machines and methods are well-suited for welding aluminum.

What makes Aluminum so difficult to work with?

Oxidation

Aluminum has natural non-corrosive properties that can make the welding process a bit trickier. On top of the aluminum sits a layer about 4nm thick, created in reaction to exposure to water or air. This is aluminum oxide, a very stable material that prevents further corrosion or oxidation of the metal. Remember how aluminum’s melting point is a nice, low 1,221 F (660.3 C)? Well aluminum oxide melts at a significantly higher temperature—3,762 F (2,072 C) to be exact.

The aluminum oxide layer serves as an insulator for the aluminum inside, making the direct application of heat tricky. Melting through the oxidized layer requires high heat, but that heat can just as quickly burn holes in the aluminum underneath. This is the reason for the extremely small window of workability highlighted earlier.

The extreme case of this protective layer is called anodized aluminum. Anodization is a highly controlled, manufactured oxidation process applied to aluminum. The thicker, more porous layer provides even more corrosion protection and creates an ideal surface for paint and sealant. Baseball bats are a great example of anodized aluminum. While there are many benefits to using anodized aluminum for certain projects, it is extremely difficult to weld. In fact, it’s highly recommended that you don’t.

Impurities

Aluminum is a very sensitive metal. It is far more susceptible to impurities than many other materials, making it very easily compromised. There are a number of ways aluminum can become tainted and create problems during welding. To avoid this, you must clean aluminum thoroughly to remove impurities as well as its oxidized layer prior to welding. If the clean metal isn’t used right away, it must be stored correctly to prevent air, dirt, and water from contaminating it again. Keeping the aluminum at room temperature helps prevent any oxidation that may occur in reaction to condensation. Aluminum that has been correctly cleaned and stored will keep for a few days at the very most, although it is always a good idea to clean the material right before using it to ensure best quality.

You run the risk of weakening the structural stability of your weld if the aluminum hasn’t been properly cleaned and/or stored. This is true for the base material as well as filler metals.

Porousness & Hot Cracking

Molten aluminum absorbs hydrogen at increasing speeds as it heats up—the hotter the liquid aluminum, the more hydrogen it will take in. The absorbed hydrogen begins to separate back out as the metal solidifies, forming bubbles inside the material. These air bubbles cause the solidified metal to be porous and weak. Without the use of a proper shielding gas, pinholes formed by the impurities will sabotage the joint strength.

While the hydrogen won’t cause cracking in aluminum as it does in steel, aluminum does present the risk of hot cracking. The correct choice of filler metal and joint design can minimize this risk. Considerations for choosing the best filler metal for your project should include desired finish, strength, and non-corrosive properties among others. Consult an aluminum filler metal chart to ensure the best option for a given project and desired result is selected.

Visualization & Thickness

As with any project, the thickness of the material affects the way you approach the welds. Aluminum can be particularly tricky in this department. Due to its low melting temperature and lack of clear visual cues, working with different material thicknesses can tend to be a frustrating experience at first. Because so much heat is needed to break down the aluminum oxide and create fusion, thinner material can go from ready to ruined in a matter of seconds.

On the other side of the spectrum, thicker metal can give off the appearance of a good weld even when the metal wasn’t penetrated enough to properly fuse. The result is a weak weld that is likely to break. Experience is the biggest asset in navigating working with different thicknesses of aluminum. The better your understanding of how the metal behaves, the better chance you’ll have at getting the weld just right for whatever thickness the material is.

Best Practices for Welding Aluminum

Cleaning

The first step toward successfully welding aluminum is to clean the material thoroughly. Because aluminum is so susceptible to impurities, prepping the metal correctly can make a huge difference in weld quality. Start by degreasing the metal before using an oxide-remover or stainless-steel brush to strip the outer layer of aluminum oxide. If using a stainless-steel brush, make sure it has been/will be used only for aluminum to prevent contamination. Remember to store prepped aluminum correctly otherwise you will have to start the process over again. It is also important to ensure the aluminum is at room temperature prior to welding. This prevents condensation from forming on the metal, causing the aluminum to reform an oxidized layer.

Using the Right Welding Process

Most welders are made for steel, so it is important that you make sure you have the right machine for welding aluminum—TIG and MIG welders are the best options for the job.

  • TIG: AC TIG is a great tool to use for welding aluminum because the alternating current essentially “cleans” the oxidized layer off the aluminum as it welds. High frequency AC TIG ensures proper heat penetration resulting in a much more even weld.
  • MIG: Wire feed (MIG) is another tool well-suited for working with aluminum. However, you will need to find separate tools with special capabilities, such as a spool gun, to effectively weld the metal. It is also important to choose a shielding gas that will combat aluminum’s risk of becoming porous. Helium and argon are two options that will minimize this risk. When using helium, a higher voltage must be applied due to the gas’ higher ionization potential. This applies to TIG as well as MIG

Filler Metals

Selecting the correct filler metal to match your base alloy and give you the right properties for your project is essential to avoid cracking. There are 4 aluminum alloy families from which to choose your filler metal.

  • 1XXX – 99% or more aluminum
  • 2XXX – copper alloys
  • 4XXX – silicon alloys
  • 5XXX – magnesium alloys

Consulting a chart with details regarding the properties of each alloy family will help you best select your filler metal.

Grinding

Choosing the right grinding disc, specifically one made for softer metals like aluminum, brass, and copper, will also help ensure your welds are finished properly. Because aluminum is such a soft metal, it will melt if you don’t use the correct disk. This can not only ruin your weld, it can also gum up the disk and ruin your tool. The same grinding disks used for steel projects cannot be used for aluminum. Beyond the risk of melting the aluminum and ruining the tool, discs already used on steel will contaminate the aluminum. Never share grinding discs between projects using aluminum and other metals.

In the same way that aluminum presents less obvious visual cues during welding, it also looks different during grinding. Aluminum doesn’t throw off sparks the same way steel does. This can make it difficult to tell how much you are grinding down. Being aware of this and adjusting accordingly is important to ensure correctly finished welds.

Techniques

  • Stringer Bead: Using a stringer bead technique allows for a greater buildup of heat. This results in a much better weld than you would achieve using a weaving technique on aluminum. As mentioned before, aluminum’s excellent thermal conductor properties make it necessary to hit the metal with very high heat. This techniques will ensure heat penetration and proper fusion.
  • Push, Don’t Pull: It is also important to avoid using a drag angle or pull technique when MIG welding on aluminum. The insufficient gas coverage these techniques create poses significant problems for aluminum as it is so susceptible to impurities. The resulting welds will be porous and visually unappealing. While the sooty exterior of the weld can be made more aesthetically pleasing, the pinholes created by trapped impurities weaken the structure of the weld irreversibly.

A push technique solves these issues by ensuring the shielding gas fully protects the molten aluminum from impurities.

Accessories

Because aluminum’s visual cues are so subtle, having a good helmet with a clear, quality lens can make a huge difference. It’s already hard enough to read the material’s behavior, a blurry lens only makes the process more difficult to gauge.

It is also important to note that aluminum is not magnetic. If you are relying on any of your tools—such as weld clamp magnets—to stick to the material, they won’t.

Why use Aluminum?

You may be asking, “If aluminum is so tricky to work with, is it really worth investing the time and resources into learning how to weld?” The answer is a resounding “yes.”

Difficult as it may be to work with at first, there are many incentives to learning to weld aluminum. The same unique properties that make it challenging to weld are also significant benefits to choosing the metal for certain projects.

  • Non-corrosive properties: The same natural oxidation that can create headaches during the welding process is actually a significant benefit to using aluminum. This is especially true when fabricating pieces that will be exposed to corrosive elements.
  • Lightweight Material: Aluminum has a better strength to weight ratio (SWR) than steel. While steel is stronger and less prone to warping under extreme duress, aluminum’s SWR makes it the perfect choice for many projects requiring a good balance of strength and weight.
  • Conductor: Although aluminum isn’t as good a thermal conductor as copper, its low price and light weight make it a very popular material for fabricating heatsinks.
  • Cosmetics: Aluminum is often chosen for its cosmetic qualities, including its ability to effectively absorb paint and sealant once anodized.
  • Recyclable: Aluminum is both easy and cheap to recycle, making it a preferred alternative to more expensive and difficult materials like steel.
  • Temperature: While aluminum has a significantly lower melting point than steel, it becomes stronger and more stable as surface temperature decreases. Steel, on the other hand, becomes more brittle as temperatures drop. This is less of a consideration considering the extreme conditions required to highlight the different temperature qualities of aluminum and steel. However, it is still important to note regarding marine and aviation applications.

Go Forth and Weld Aluminum.

Though it can be challenging to learn how to weld aluminum, the metal has a lot of beneficial properties that make it well worth the effort. Understanding exactly what it is about the metal that makes it challenging to work with is essential in determining how you approach a project. With the right tools and techniques, as well as a decent amount of practice, welding aluminum will become far less daunting of a challenge. Also, as with any skill, guided practice can be a huge help in navigating and implementing the information above. If possible, consider taking classes or working with someone who has experience welding aluminum. Knowledge is important, but nothing beats experience.

Comments

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