Aluminum alloy 5056 rivets are the standard choice for riveting magnesium alloy structures. This specific aluminum alloy was selected because of its strong corrosion resistance when paired with magnesium, a metal notorious for corroding when it contacts dissimilar metals. In aerospace and defense manufacturing, 5056 rivets are essentially the default whenever magnesium sheet metal is involved.
Why 5056 Aluminum Works With Magnesium
Magnesium is extremely reactive. When it touches most other metals in the presence of moisture, a process called galvanic corrosion kicks in, gradually eating away at the magnesium. This happens because the two metals have different electrical potentials, and moisture acts as a bridge that lets ions flow between them.
Aluminum alloy 5056 sits close to magnesium on the galvanic scale, meaning the electrical difference between the two is small. That small gap dramatically slows corrosion compared to what you’d see with steel, copper, or even other aluminum alloys. For comparison, most aluminum alloy structures use 2017 aluminum rivets, but those would accelerate corrosion if used in magnesium. The composition of the material being joined dictates the composition of the rivet: 2017 for aluminum, 5056 for magnesium.
Common Rivet Head Styles
The 5056 alloy is available in the same head configurations used across aerospace riveting. The two most common are the universal (protruding) head and the 100-degree countersunk head. Which one you use depends on the application.
- Universal head (MS20470 style): Used where surface smoothness isn’t critical and clearance around the rivet isn’t an issue. The protruding dome provides a larger bearing surface, which can distribute load over a wider area.
- Countersunk head (100-degree): Selected when the surface needs to be aerodynamically smooth or when nearby components require clearance. The rivet sits flush with the skin of the structure.
Self-plugging and pull-through rivets also come in both head styles and are available with 5056 sleeves for blind installations where you can only access one side of the joint.
Installation Considerations
Magnesium alloys are more brittle than aluminum, which makes the riveting process less forgiving. Excessive force or poorly aligned holes can crack the surrounding material. Hole preparation matters: rivet holes in magnesium need to be cleanly drilled and properly sized to avoid stress concentrations that lead to cracking.
One important question is whether heat is needed during installation. Traditional solid (buck) rivets in 5056 are typically driven cold, meaning no pre-heating is required before forming the shop head. Research into friction-based riveting of magnesium alloys has confirmed that the heat and pressure generated through plastic deformation during the driving process is enough to form the rivet head without a separate heating step. This simplifies the process but still demands careful control of driving force to avoid damaging the magnesium workpiece.
Corrosion Protection During Assembly
Even with the galvanic compatibility of 5056 rivets, additional corrosion protection is standard practice for magnesium structures. The most common approach is “wet installation,” where a sealant or primer is applied to the rivet and hole before driving. This creates a moisture barrier at the metal-to-metal interface, blocking the pathway that galvanic corrosion needs to start.
Zinc chromate primer has historically been the go-to sealant for this purpose in aerospace applications. The primer coats the inside of the rivet hole and the rivet shank before assembly, sealing out moisture. Without it, even a 5056 rivet in magnesium will eventually corrode in humid or salt-spray environments. Testing has shown that “dry” installed fasteners (no sealant) represent a worst-case corrosion scenario, which is why wet installation is the accepted standard for any magnesium structure expected to see service in demanding environments.
Newer Joining Methods for Magnesium
Traditional solid rivets remain the workhorse for magnesium alloy sheet metal, but newer techniques are emerging for specialized applications, particularly when joining magnesium to dissimilar materials like carbon fiber composites.
One method called ultrasonic extruded weld-riveting (UEWR) skips the metal rivet entirely. Instead, through-holes are pre-drilled in the magnesium sheet, and a thermoplastic composite layer is melted using ultrasonic energy and squeezed into those holes. Once it cools and solidifies, the composite material itself forms the rivet structure, locking the two sheets together through mechanical interlocking. This avoids heating the magnesium directly, which prevents the high-temperature damage that magnesium alloys are susceptible to. Joints made this way have reached tensile shear strengths around 56.5 MPa.
These advanced methods are still largely in the research and specialty manufacturing stage. For conventional airframe and structural work involving magnesium alloy sheets, 5056 aluminum rivets with proper sealant remain the standard and most widely specified solution.