How To Weld Aluminum

Aluminum is a sought-after metal globally, and its lightweight and corrosion-resistant qualities make it a perfect fit for many residential, commercial, and industrial applications. That said, component pieces of aluminum often need to be joined to create a larger and more complex part.

Can You Weld Aluminum?

To join aluminum parts, TIG, MIG and stick welding can all be used. Other complex processes like LASER welding can also weld aluminum; however, we will only discuss the standard procedures in this article. Many consider aluminum to be one of the more difficult metals to weld, especially in comparison to steel. Though there are a few extra sensitivities in working with aluminum, there are also many qualities that make it easier to weld. That said, whether the welding is done indoors or outdoors and the particular process used are the two most significant factors determining the difficulty of a given aluminum project.

how to weld aluminumStick Welding Aluminum

First off, welding aluminum outdoors should always be a last resort. Usually, the most appropriate process used for any welding outdoors is always stick. However, stick welding aluminum is known for being a very inconsistent process. During welding, the protective flux that shields the weld pool is nearly indistinguishable from the weld pool, making it very difficult to tell if both pieces of metal are properly joined. On top of that, stick welding aluminum is much faster than the pace for stick welding steel, making it all the easier to make mistakes. Another issue is that the flux shielding doesn’t seem to work as well for aluminum as it does on steel. An aluminum stick weld may look healthy on the outside, but it is not uncommon to find scattered porosity after grinding the surface down. Generally, most welders prefer to stay away from stick welding aluminum, though it will be sufficient for nonstructural applications outdoors in breezy conditions. The polarity and amperage you use will be dependent on the type of electrodes you buy, so be sure to read the package thoroughly. Usually, either DCEN or DCEP is used. Though either can be used, we recommend a pull inclination over push to allow for slightly better visibility.

TIG Welding Aluminum

TIG is often an excellent choice for welding thinner aluminum. It offers more control than the other processes and the best penetration, assuming your settings are correct. Also, there is no extra equipment required like for MIG. The only major downfall of this process is that it’s slow. Although you can use multiple passes for thicker metals, the extra time needed compared to MIG is normally prohibitive. AC is almost always used in TIG welding aluminum, and the purple lanthanated tungsten with a tapered end is your best electrode choice. The green pure tungsten electrodes used to be the go-to electrode. However, their main disadvantage is that they can quickly overheat to the point that the balled end will drop off into your weld. For shielding gas, use 100% pure argon. For higher production times, you can use an argon/helium mix to increase the welding heat. The only drawback to this gas type is the extra cost.

How To Weld Aluminum With a MIG Welder

A significant difference in the equipment required for MIG welding aluminum versus steel is the need for a spool gun. The aluminum wire is very soft and cannot be pushed 8 or more feet through a cable liner. Having a spool gun push the wire only a few inches prevents the wire from bird-nesting inside a liner. Another option is to use a push-pull machine setup. The advantage to this is that you will be able to use a much larger spool and avoid frequently changing the smaller spool from the spool gun. Also, purchasing one large spool is usually cheaper than multiple small spools. Machines with a push-pull setup are more expensive than the spool gun setup; however, they will provide cost savings in the long run in labour and filler metal costs. MIG welding aluminum uses DCEP (reverse polarity) and 100% argon or 50/50 argon/ helium. That latter aids in welding thicker metals. For thicker metals to be welded indoors, MIG is the process of choice. It can also be used for relatively thin metals, though there is a higher risk of burn-through for metals under 0.125 inches thick. That said, MIG is faster than the other processes and can provide consistently sound welds. However, a significant thing to note is that thicker aluminum must always be preheated before welding, especially when using MIG. MIG is susceptible to cold-starting as the wire melts before the workpiece can get hot enough. Making sure the metal is heated to roughly 450 C will remedy this. We’ll discuss more on preheating thicker metal later in this article.

Filler Metal Section

There is a lot to be said on aluminum filler metals. However, we will stick to general guidelines in this article. Firstly, the two most common filler alloys are 4043 and 5356. The former is softer with slightly less strength. For most applications that do not require any post-weld heat treatments, forming, or anodizing, either of the two is usually a safe bet. For special applications, 5356 is better for post-weld anodizing (provides better colour match) and forming.

At the same time, 4043 is better for workpieces exposed to elevated temperatures or for when cosmetic appearance is of higher importance due to improved weldability. Regardless of the grade of aluminum that you use, some are not weldable under most circumstances. The two primary examples are most grades in the 2XXX and 7XXX series. These are both uncommon, high-strength grades that invariably crack after being welded. If you are unsure of the grade of aluminum that you’re working with, going with 4043 will usually give you positive results. However, be sure to notify the client that there is always a small chance that the grade of aluminum to be welded is not weldable.

How Do You Weld Cast Aluminum?

It is very common to come across cast aluminum that requires modifications or repairs via welding. As you will rarely know which grade you’re working with, using filler metal grade 4043 and welding following the same steps you normally would. The only difference is that cast aluminum usually needs to be preheated before welding, as cast metals are usually thicker. Now, there is the chance that you unknowingly come across a casting grade 5XX, which is to be welded with 5356 filler metal. If you mistakingly use 4043 grade instead, you may produce reduced toughness in the weld metal. However, it won’t lead to significant failures in most applications.

Can You Weld Steel to Aluminum?

Another question we get asked often is whether aluminum will weld to steel. In a word, no. It is possible to join the two via silver soldering. However, soldering is categorized separately from welding as it does not involve melting to bind the metals and is, therefore, a much weaker joint.

How to Weld Thicker Material

how to weld aluminum

Aluminum has a high thermal conductivity, which means that the base metal quickly extracts the heat from the weld zone and spreads it through the workpiece. Its high thermal conductivity reduces penetration during welding, which requires much higher heat settings to offset this phenomenon. And with thicker pieces of aluminum, even appropriate heat settings will not be enough to get full penetration. For this reason, aluminum thicker than 0.25 inches should usually be preheated to roughly 450 C before welding. A simple way to do this without a temperature gun or crayons is to heat the metal with a pure acetylene flame from an oxy-fuel torch. Once the metal turns black, adjust the oxygen setting until the flame becomes neutral and heat the work until the black shade burns away. Once the metal turns white again, the aluminum will be roughly 450 C. Ensure to begin welding immediately, as aluminum doesn’t stay hot for long.

Difficulties in Welding Aluminum

As we mentioned at the beginning of this article, aluminum has a reputation for being difficult to use. Below are a few challenges of welding aluminum.

Techniques for Welding Aluminum

A significant change in welding techniques that steel welders need to adapt for aluminum is to learn to weld at higher temperatures and at a much faster pace. Too cold, and the penetration won’t be adequate. Too slow, and you’ll either blow a hole in the workpiece, or it will melt from an excessive buildup of heat. Once you get the hang of it, however, it becomes second nature. Depending on the thickness of aluminum you’re using, you will need a powerful enough machine to handle anywhere from 120 to 250 amps. Unfortunately, this eliminates most hobby welders. Another adaptation that can help improve welding aluminum with TIG is to feed filler metal while welding continuously. These adaptations are more necessary for aluminum than steel due to aluminum filler’s higher deposition rate. You’ll go through a whole piece of filler in twice the time, and learning to feed filler while welding will reduce the amount of time you spend stopping to adjust.

Oddities of Welding Aluminum

While steel is also susceptible to porosity from lack of shielding gas or contaminated metal, it is nowhere near as sensitive as aluminum is. Little pieces of dirt can halt an aluminum weld puddle in its tracks. The solution to this is to clean the areas to be welded to the best of your ability. Quickly running a grinder over the surface should do the trick. Another peculiarity of aluminum is that there is no colour change when it gets hot. You’ll want to pay close attention to whether it was just recently welded. It can be a painful mistake otherwise.

Cracking in Aluminum

The most common threat of welding aluminum, however, is hot cracking. Hot cracking is the development of cracks that form during the solidification of the weld metal. This is because specific quantities of magnesium, copper, silicon, and magnesium silicide in the weld metal can create an extra sensitivity to cracking. If the amount of any one or more of these alloying elements falls into a specific range, the probability of hot cracking will increase. To avoid this, you have to consider the grade of aluminum that you are welding on. You’ll want to use filler metal of similar composition for grades that naturally have low sensitivities to cracking. For these scenarios, the joint geometry is not as important. For welding on grades sensitive to cracking, you’ll need to use filler of a grade with a different mix of alloying elements to avoid pushing the quantity of a given element into dangerous territory.

Consulting a chart that sorts base metals with appropriate filler metals is an easy way to get it right. ESAB provides a great resource on how to select aluminum filler alloy.

Additionally, you’ll have to create a joint geometry that increases the amount of weld metal. Doing this will reduce the dilution factor and help ensure that the weld metal has a composition more similar to the filler metal than the sensitive base metal. An example of this would be creating a V groove rather than a square butt joint, or at least increasing the gap of the square butt joint.

Final Words on How to Weld Aluminum

Although some factors can make the learning curve for learning how to weld aluminum a little painful, aluminum can be a great metal to weld onto. It is much more malleable and thus easier to hammer and form than steel and welds much faster, and this can accelerate production times. If you keep an eye out for your heat settings, preheat, filler metal selection, metal cleanliness, and gas coverage, there’s no reason your aluminum welds shouldn’t be as pretty, if not prettier, as your steel welds.

To learn more about how to weld aluminum for professional welding services in the Greater Toronto Area, contact the Central Mobile Welding team today.

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