Aluminum is a popular material choice for trailer and truck bodies. It’s light, strong and corrosion-resistant and can be given a range of finishes, or just left bare. These bodies though are typically constructed from both extruded and plate material, and that’s where complications arise.

Creating a durable weld between the two forms entails managing the metallurgy of the joint. That’s done by adding a filler material. In aluminum trailer and truck body manufacturing the default choice of is 5356 aluminum filler, but it’s not the only choice. Here’s a look at the advantages of 5356 aluminum filler and some observations on when to consider an alternative.

Drawn from different series

Extruded aluminum section provides rigidity, which makes it good for structural frames and trailer beds. Aluminum is most often extruded in the 6000 series. Aluminum plate, used to fill in between the framing elements, is usually 5000 series. Both are aluminum but their metallurgy and properties are different. When they’re welded the two metals melt and fuse together, and that has the potential to create weak points in the fabrication.

Challenges of welding aluminum

Aluminum is tricky to weld. First, while it has a relatively low melting point, aluminum oxide melts at a much higher temperature. So unless the oxide is thoroughly removed before welding it’s only too easy to burn right through.

Second, aluminum is an excellent thermal conductor. So while the welder is trying to put enough energy in to create a weld pool, that heat is quickly flowing away. That can cause distortion as the weld cools, but it also makes it hard to maintain the weld pool. And complicating matters further, 6000 series aluminum has significant higher conductivity than the 5000 series.

Third, the heat of welding creates a heat affected zone (HAZ.) This changes the temper of the aluminum, basically taking it back to its annealed state.

The role of filler

Welders almost always add filler metal to a weld. It takes up the gaps between the pieces being joined and forms a strengthening fillet. It also contributes material to the metallurgy of the joint. So for example, using a filler high in magnesium, like 5556, lets that element diffuse through the joint.

The filler material, just the like the plate and extruded material, must be matched to the application. Contact with corrosive materials, abrasive wear, high temperatures and the intended type of post-fabrication finishing must all be considered.

Strengths of 5356 Aluminum Filler

5356 is compatible with both 5000 and 6000 series aluminum. Used with 6000 series, 5356 has good strength and resistance to cracking. It has good levels of ductility and toughness and color-matches well after anodizing.

5356 Aluminum Filler Weaknesses

There are two situations where this filler may not be a good choice. The first is when it will contact corrosive substances. The second is when it will see elevated temperatures.


If you’re only welding 5000 series aluminum, as when joining plates together, 5556 filler creates a stronger joint thanks to it’s higher magnesium content. It is more expensive than 5356, although the potential to use less goes some way towards reducing the differential.

When corrosive substances are a factor – tanker trailers are a prime example – 5554 filler is a better choice. It also tolerates higher temperatures and will survive heat treatment after welding.

Consider the application

Welding different grades of aluminum, as are often used in trailer and truck body manufacturing, has some challenges. In particular, the choice of filler metal has a significant impact on the durability of the fabrication. 5356 is the default choice for many applications, but there are circumstances where a different filler might perform better.