16 November 2016

An overview on aluminum and its alloys


Each to his own taste

natural aluminum occurrence

Aluminum is the third most common element in Earth’s crust (8.3%), after oxygen (45,5%) and silicon (25,7%). It’s found in many minerals, always combined with other elements.

Its main area of use is in the transport industry (28%): just think that a Boeing 747 is made using more than 66 tons of aluminum. Another 23% is used for the production of cans, tubes and various containers; about 14% is used in the building and construction industry (from windows to the construction of mobile homes); while the remaining 35% is used in a wide range of products, such as electrical cables, components, specialized equipment, etc.

 

English scientist Humphry Davy identified it for the first time in 1808 in alum. For decades it was considered more precious than gold: Napoleon III reserved a aluminum tableware for his most important guests. The other had to make do, so to speak, with gold tableware.

aluminum kitchenware

Aluminum is light yet durable, and above all self-passivating. Its silver-gray color is in fact due to the thin layer of oxide that forms when it is exposed to air, and that prevents further corrosion. For this reason it is used in moderately aggressive environments, or in kitchenware: tools, pans and coffee pots are made of aluminum because it leaves no aftertaste to foods.

Its specific weight, equal to approximately 2.70 kg/dm3 is equivalent to about a third of that of steel (7.9 kg/dm3) or copper (8,93 kg/dm3). It has a high malleability and thermal and electrical conductivity. It is a nontoxic element and has excellent weldability. Yield strength is almost always lower than that of steel. This disadvantage is, however, compensated for by its lower density, which therefore allows to increase the section of the piece until reaching the desired strength, while still maintaining a lower weight.

 

aluminum cylinder head
Alloy Specific weight  Yield strength Tensile strength Elastic modulus
2024-T3 2.780 kg/m3 345 MPa 483 MPa 73,1 GPa
7075-T6 2.810 kg/m3 503 MPa 572 MPa 71,7 GPa
7175-T66 2.800 kg/m3 520 MPa 590 MPa 72 GPa
2090-T3 2.590 kg/m3 210 MPa 320 MPa 76 GPa
6061-O 2.700 kg/m3 55,2 MPa 124 MPa 68,9 GPa

 

For comparison, these are the characteristic figures of some steel alloys:

Alloy Specific weight  Yield strength Tensile strength Elastic modulus
S275 mild steel 8.000 kg/m3   275 MPa 430 MPa 200 GPa
Low-carbon steel 7.850 kg/m3 400 MPa 700 MPa 200 GPa
High-carbon steel, tempered and annealed 7.800 kg/m3 1.500 MPa 1.700 MPa 205 GPa
316L stainless steel 7.900 kg/m3 > 200 MPa 500/700 MPa 200 GPa
aluminum sheet metal facade

When bending aluminum, it’s important to keep rolling direction in mind. Bending along the grain, the probability of cracks is much higher than with steel or other metals.

Another important characteristic of aluminum is its capacity to bind with other elements. Although pure aluminum is soft and ductile, its characteristics change dramatically when combined with other metals. Aluminum alloys are grouped into series, according to their composition.

1000 Series

Made of pure aluminum (99%). The key features of this series are: excellent corrosion resistance, high thermal and electrical conductivity, and good workability.

They are typically used for chemical plants, heat exchangers, electrical conductors, and architectural and decorative applications. A thin layer of pure aluminum is sometimes deposited over less resistant series to protect them from corrosion.

 

2000 Series

Also called Avional (or Duralumin). Since the ’30s, they are employed in the aviation industry for the construction of the structural elements of airplanes. Thanks to their lightness and hardness, they are also used for the construction of precision tools. Copper is the main alloying element; magnesium and manganese may be added, especially where lightness and good mechanical characteristics are needed. These alloys have poor resistance to corrosion, therefore they may be plated or painted. They are used for jobs that require high strength/weight ratios up to 150 °C.

 

3000 Series

Manganese is the main alloying element, which is responsible for the good mechanical resistance of this series, as well as its reduced ductility. This alloy is used for kitchenware, tanks and pressurized pipes.

 

4000 Series

The main alloying element of this series is silicon, that provides good resistance to wear. In commercial alloys, copper and magnesium are also added.

 

 

5000 Series

Also called Peraluman. The main alloying element is magnesium, shich provides good ductility. They are work hardening alloys, whose mechanical characteristics are increased by cold working. Because of their high resistance to corrosion and fusion, they are mostly used in marine environments and in the automotive industry for car body construction, where weldability is a key feature. Their overall characteristics are poorer than Duralumin, which precludes the usage in the aeronautical industry.

 

6000 Series

Also called Anticorodal; its main alloying elements are silicon and magnesium. Heat treatment provides intermediate mechanical characteristics, generally lower than those of 2000 series. They have good formability, workability, machinability and weldability. They possess good mechanical strength and good corrosion resistance. They are used for architectural applications, motorcycle and bicycle frames, and welded structures in general.

 

7000 Series

Commercially called Ergal. The main element of this series is zinc, with possible additions of magnesium, which increases the corrosion resistance. They have good machinability, but poor fusion weldability. This aluminum alloy is typically used when exceptional mechanical robustness and lightness are required. They are used in particular for aeronautical structures and for means of transport, especially in the motorcycle industry and in bicycle frames.


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