Aluminum Aircraft Parts: Clad Aluminum Vs Bare AluminumMatt Kerster
Anyone working with aircraft or aircraft parts is likely quite familiar with aluminum. For the most part, all aircraft are composed of significant quantities of aluminum parts. While composite materials are becoming more and more common, aluminum is still the mainstay of aircraft material. Especially in the older planes.
Take the Boeing 747-400, for example. According to a post in the Science-Based Life blog, the 747-400 consists of 147,000 pounds (66,150 kg) of high-strength aluminum. That’s almost 75 tons of aluminum (along with 171 miles (274 km) of wiring and 5 miles (8 km) of tubing!)
According to some sources it’s estimated that 75 to 80 percent of the materials used in modern-day aircraft is aluminum. And, according to Aluminum Leader, 27 percent of all aluminum is used in the transportation industry.
The history of aluminum for flying craft goes back to Count Ferdinand Zeppelin who used aluminum to make the frames of his famous dirigibles. In 1903, the Wright brothers Flyer-1 made its historic heavier-than-air controlled flight powered with a custom-made engine that had a cast aluminum cylinder block.
The challenge today is working effectively with the multitude of alloy options, forming options, and finishes.
A common attribution to certain types of aluminum parts is “clad” aluminum. While many people routinely make use of clad aluminum, not all of them are familiar with the purpose behind cladding, as it’s called.
According to a text entitled Aviation Maintenance Ratings:
“Pure aluminum has considerable corrosion resistance compared to aluminum alloys. but it has little or no structural strength. An extremely thin sheet of pure aluminum laminated onto each side of an aluminum alloy sheet improves the corrosion resistance with little impairment of strength. The trade name of this aluminum laminate, as originated by the Aluminum Company of America, is Alcad. From this trade name the adjective clad and the verb cladding have been derived.
An example of clad aluminum is the surface of unpainted aircraft. Not all aircraft sheet aluminum is clad, especially those alloy sheets from which small brackets, gussets, and fittings are made. The pure aluminum is very soft, and fabrication processes would severely damage or destroy the clad surfaces.”
The previous excerpt mentions the use of aluminum alloys. The most common alloying approaches are aluminum alloyed with copper to increase strength, magnesium for both strength and corrosion resistance, and manganese to increase strength and ductility.
However, aluminum-copper alloys make up the largest types of aluminum alloys used in aircraft construction.
An aluminum-copper allow displays significantly greater strength and it can be heat treated. In addition, even with the copper alloy, it is still very workable and quite corrosion resistant. However, if additional corrosion resistance is needed, the aluminum-copper alloy can usually be coated, or “clad”, with a layer of commercially pure aluminum.
The thin layer of 99 percent pure aluminum is virtually impervious to corrosion from all likely elements. The downside is that scratches or other damage to this aluminum coating can lead to corrosion that can begin to spread underneath the protective layer.
Along with the possibility of surface damage leading to corrosion, clad aluminum has proven increased resistance to corrosion, but at the expense of increased weight when compared to bare sheet aluminum.
A paper published at Free Patents Online, points out that:
“The fuselages of civil aircraft are, for the most part, made from 2024 alloy sheet, clad on either surface with a low composition aluminum alloy, a 1050 or 1070 alloy for example. The thickness of the cladding on each surface may, according. to the thickness of the core sheet, typically represent between 1% and 15% of the total thickness. The purpose of the cladding alloy is firstly to provide sufficient corrosion resistance. Slightly generalized or pitting corrosion is tolerable, but it must not be penetrating so as not to attack the core alloy.”
Baring it With Pure Aluminum
Pure bare aluminum is available, but not as common as aluminum alloys, for aircraft parts. The commercial grade of pure aluminum is known as 1100. According to Aircraft Spruce:
“It is soft and ductile and has excellent workability. It is ideal for applications involving intricate forming because it work hardens more slowly than other alloys. It is the most weldable of aluminum alloys, by any method. It is non heat-treatable. It has excellent resistance to corrosion and is widely used in the chemical and food processing industries.”
Another version, if you will, of pure aluminum is an alloy known as Alloy 3003. It’s probably the most widely used of all aluminum alloys and consists of commercially pure aluminum with manganese added to increase its strength. In fact, it is 20 percent stronger than the 1100 grade, it also has high corrosion resistance and workability like 1100.
Heat-treated aluminum, on the other hand, is alloyed with copper, magnesium, zinc or silicon. These materials can be heat-treated, or solution heat-treated. As noted already, the addition of these alloying elements provides greater strength, but it also reduces corrosion resistance in the materials.
Ultimately, the use of clad or heat-treated aluminum alloys is best determined by the parts being made. With eight aluminum grades to work with, the choice often comes down to the various qualities of each grade.
These include formability (or workability), weldability, machining, corrosion resistance, heat treating, and the need for strength. Of course, with all these factors there is always the end use applications to be considered.
2024, for example, is one of the best known of the high strength aluminum alloys. Because of its high strength and significant fatigue resistance, it is best for structures and parts where good strength-to weight ratio is desired.
Because the corrosion resistance of 2024 is relatively low, it is commonly used in an anodized finish or clad form. Common applications for 2024 include aircraft structural components and aircraft fittings.
Other common aviation spec aluminum grades include Aluminum Alloy 6061, 5052, 7075 and 3003. These alloys were specifically engineered for use in the aerospace industry and were designed to meet the requirements of aircraft specifications.
Making a Choice
No matter whether you need 100 feet of an aluminum alloy product, or just a few sheets of clad aluminum, AAA Air Support promises to always ship out your part orders quickly and efficiently. Our goal always is to provide your company with the means to complete that project or get that airplane off the ground and back in the air.