How to Weld Cast Aluminum: Recommendations

Yes, you can weld cast aluminum, but it requires careful preparation and technique. Clean the surface, preheat the material to 350-650°F (177-343°C), and use AC TIG welding with a pure or zirconiated tungsten electrode. Select a compatible filler metal, like 4043 or 5356 aluminum alloy, for optimal results.

TIG welding works just fine for cast aluminum with a few technical and safety tips to keep in mind.

When Can You Weld Cast Aluminum?

You can determine if an aluminum casting is weldable according to two key aspects. The first is the composition, and the second is the casting method. 

Alloying Elements

Aluminum castings alloys can be one of eight types:

• Pure aluminum 1XX alloys
• 2XX alloys (aluminum and copper)
• 3XX alloys (silicon, copper, magnesium, and aluminum)
• 4XX alloys (silicon and aluminum)
• 5XX alloys (aluminum and magnesium)
• 7XX alloys (zinc and aluminum)
• 8XX alloys (aluminum and tin)

Each alloy type has a different weldability level. You can easily weld the 1XX alloys with 1100 fillers. However, the 7XX and 2XX alloys have poor welding characteristics.

You can also weld 3XX with 4043 fillers. You can use the 5356 fillers for the 5XX alloys.

Casting Method

Another factor that determines the weldability of cast aluminum is how it’s made. There are four common methods for making cast aluminum: die casting, investment casting, green sand casting, and permanent mold casting.

Die aluminum castings are pretty difficult to weld. This is because when you die-cast aluminum, you pressure-inject it into a steel mold that cools with water. This makes the casting cool in an instant.

The quick cooling of the cast prevents gasses from escaping. These pockets of air can be difficult to deal with during welding.

You can readily weld the remaining three casting types—permanent mold casting, investment casting, and sand casting.

When you make aluminum castings with these methods, they cool slowly. That’s because these castings mold metal into a low thermal conductor. This allows the gasses to escape freely from the casting.

Why Weld Cast Aluminum?

Cast aluminum has versatile applications, from small appliances to car parts.

Knowing how to weld cast aluminum can come in handy for repair jobs that include jet pumps, bolt holes, boat motor parts, and more.

How to Weld Cast Aluminum

The prep phase for cast aluminum is just as important as the welding.

You’ll need to remove the grease and dirt with a solvent like acetone. Use a stainless steel wire brush to tackle the oxide layer. You can then rinse and dry before welding.

TIG and MIG welding are the most common ways to weld cast aluminum.

Let’s check their techniques and benefits below.

TIG Welding Cast Aluminum

TIG weld is great for aluminum castings.

It can automatically remove the oxide layer. You don’t need mechanical wire feeding. TIG can also protect cast aluminum from contamination.

Here are some tips to consider:

• Prepare a 3/32 2% lanthanated tungsten electrode.
• Set the AC frequency to 50hz.
• Set the AC balance to 65% electrode negative to get more penetration.
• Turn on the shielding gas (pure argon works best for cast aluminum).
• Do another cleaning pass and preheat 200°F if the cast aluminum is still contaminated. 
• Weld with a forward and backward movement (I recommend starting from the outer edges to build up more heat).
• Add the filler of your choice, depending on the alloy composition.

MIG Welding Cast Aluminum

Welding with MIG is another option for cast aluminum, but it can be complicated for beginners.

MIG isn’t versatile enough for cast aluminum. The welder could act up if your wire-feeding speed is a bit low or high.

MIG welds still have an advantage since it works faster for thicker cast aluminum.

Here’s how to do it:

• Get your choice of wire (ER5356 can be hard enough to facilitate wire feeding). 
• Start the wire-feeding process.
• Set the MIG machine to DCEP.
• Set the voltage to 21-24V, too. 
• Use a 100% argon shielding gas.
• Use a contact tip a size larger than the thickness of the wire.
• Weld with a swift, forehand movement and don’t move in circles. The key here is maintaining a steady speed.

Equipment

Here’s what you’ll need depending on your welding method.

I have written a guide to my favorite aluminum welding machines.

TIG Equipment

You need to use a pure argon shielding gas for cast aluminum. You can also mix the argon with up to 25% helium for thick metal pieces.

Keep the gas flow at around 17 cfh for every 1/2 inch of cast aluminum. You can turn up the flow to 20 cfh if the arc is unstable from the ambient airflow.

Get 4043 fillers since you’ll most likely use 3XX cast aluminum alloys. The filler’s thickness should be around 1/16 to 3/16 inches in diameter.

An AC TIG welder with a cleaning action should do the job.

MIG Equipment

You’ll also use pure argon shielding gas for the MIG welder, just like with TIG welding. 

Get a welding wire like 4043 or 5356 that works for most cast aluminum alloys. I recommend a 5356 aluminum MIG wire instead of a 4043 one to avoid possible wire-feeding problems.

You’ll need an oversized tip for cast aluminum. This is because the aluminum wire expands with heat. A normal tip may get stuck when the wire is hot.

Safety Precautions

Safety precautions are crucial when welding cast aluminum.

Here’s why:

Cast Aluminum Doesn’t Change Color When It’s Hot

It’s hard to tell the difference between hot and cold cast aluminum. So, always wear your PPE.

Cast Aluminum Fumes Are Hazardous

The fumes that come from welding cast aluminum can harm your lungs in the long term. Neglecting these fumes can eventually lead to aluminosis, an irreversible lung disease.

I recommend getting a fume extractor to protect your health.

Cast Aluminum Is Very Reflective

Cast aluminum has high reflective properties. This puts you at risk of UV-related injuries such as welder’s eye.

Wearing goggles can help reduce the risks.

Conclusion

The alloy elements and the casting method determine the weldability of cast aluminum.

You can weld 3XX and 1XX alloys. You can also weld cast aluminum that is made by sand casting, investment casting, or permanent mold casting. Die castings won’t weld easily.

I recommend TIG welding cast aluminum simply for its convenience.