Directly welding aluminum to steel is not possible due to metallurgical incompatibility. Instead, use a bimetallic transition insert, which is a piece of material combining aluminum and steel. It can be joined to each base metal using compatible welding processes, such as MIG or TIG, and filler materials.
Aluminum and steel are very different from each other when it comes to physical properties. This is the main reason why direct welding is difficult.
In this article, I’ll explain those different physical properties and why they hinder welding different metals together.
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Why Is It Difficult to Join Aluminum to Steel?
Here’s why traditional welding between aluminum and steel together is still out of reach:
This is often the most significant obstacle when trying to weld dissimilar materials.
The melting point is the temperature required to change the state of a material from solid to liquid. The closer the melting points of the two materials you’re joining, the easier the process will be.
Aluminum’s melting point is around 1220 °F. This might initially seem high, but it’s pretty humble compared to steel’s 2500 °F. The lower melting point of aluminum will make it melt long before the steel reaches its much higher melting point.
Aluminum has an approximate density of 2.70 g/cm3. Compared to the remarkably higher 8.05 g/cm3 of steel, aluminum’s density is around 3 times lower than steel.
Aluminum, being far less dense, is a lot more likely to bend and deform before it can be welded to steel.
If you can get past the density and melting point challenges, you’ll face a new problem; solubility.
Unfortunately, materials generally don’t always dissolve in each other. Aluminum and steel fall into such a category. So, even in their molten state, they are insoluble in each other.
Because of that, they form what’s known as intermetallic compounds. Those are usually brittle and affect the overall integrity of the final product.
Why Should You Join Aluminum to Steel?
Now that you have an idea of how difficult it is to join aluminum to steel, you might wonder: why is welding highly desirable? Why not just settle for brazing since it already works?
Joining aluminum to steel is often done to use them for structural applications that require high tensile strength. That tensile strength will be lowered if you use brazing instead of welding.
That’s because brazing doesn’t melt the base metals themselves; it melts a filler material between them, creating a joint. This is why it’s fairly faster and easier but also weaker and less reliable.
Welding, on the other hand, relies on the extremely high temperature to melt and join both metals together, effectively creating a much stronger bond between them.
How to Join Aluminum to Steel
Now that you understand why welding is difficult, it’s time to understand how to join aluminum to steel using the next best thing; brazing. There are six types of brazing techniques you can use, they are:
- Torch brazing (the most common)
- Furnace brazing
- Induction brazing
- Resistance brazing
- Dip Brazing
- Laser brazing
The method I’ll discuss is torch brazing since it’s the most popular and cost-effective. However, before I tell you the steps, here’s the equipment you’ll need:
- A brazing torch to act as your primary heat source
- A propane cylinder to supply your brazing torch
- Brazing filler rods that will melt and join your metals together
- A deburring tool to smoothen or bevel the metal surfaces before joining
- Abrasive cloth to clean the metal surfaces
- A cylinder trolly (optional but helpful to avoid accidents)
- Welding gloves
- Welding boots
- Safety glasses or face shields
- A fire-resistant coat
Joining Aluminum and Steel via Brazing
After you put on your safety equipment, follow these steps:
Bevel the Metals
Since you’re about to incorporate a third type of metal between two others, you must polish and bevel these two surfaces before starting your work.
This will allow the filler material to reach further into the base metals, eventually providing you with a stronger bond.
Use the deburring tool sharply and aggressively over the surface-to-be-joined. This should scrape off any protrusions that may reduce the quality of your joint.
Clean the Metals
This is a straightforward step of using abrasive cloth or sandpaper to smoothen out and clean the metals’ surface from debris. This will furtherly enhance the bond later on.
Fix the Metals in Place
You don’t want your metals to move anytime during the brazing process. Use whatever method you prefer to fix the metals in place.
Begin the Joining Process
Begin your brazing process by starting up your torch. Before holding your filler rods, heat the surfaces you intend to join.
This will raise their temperature enough to quickly melt your filler rods as soon as they get in touch.
Then, hold your filler rods and braze the two ends of the to-be-joined surface. This will keep both surfaces of metal together and prevent any movement caused by metal heat expansion.
Keep melting and soldering using the filler rods until you combine both metals.
Wait for the Joint to Cool Down
Did you notice that I didn’t say rinse with water or use a damp cloth? While it may be time-consuming, waiting for your joint surface to cool down is actually better for the final result.
Using water or any method to cool down your freshly brazed surfaces quickly can cause microcracks in the joint, reducing the overall resistance to tensile strength.
That’s a Wrap
As wonderful as it could have been, welding aluminum to steel is virtually impossible without adding filler material.
The melting temperature of steel is just too high for the aluminum to remain solid until they both join. Additionally, the different densities and insolubility make it even more challenging to achieve successful welding.
Luckily, brazing can get the job done without compromising too much in the tensile resistance. Just clean the metal surfaces, fix them, heat them, solder them, and then leave them to cool down.