Aluminum | Definition, Manufacturing, Properties, Uses, and Alloys

Ahmed Samy

28 Jan, 2024

Aluminum and its alloys are lightweight metals that have become crucial in many industries due to their fantastic properties and wide range of applications. People discovered it in the 1800s, and since then, it has become one of the most common metals on Earth. It plays a key role in modern engineering and manufacturing processes.

What is Aluminum?

Aluminum is a chemical element with the symbol Al and Atomic weight 27 g/mol. It is a silvery-white, soft, non-magnetic, and ductile metal in A3 group. Aluminum is remarkable for its low density (2.7 g/cm3 as compared to 7.9 g/cm3 for steel), low melting temperature 660°C (1220°F), and ability to resist corrosion. These properties, along with its excellent conductivity and malleability, make it a highly versatile metal used in various applications.

What is the source of Aluminum?

Aluminum is electrochemically extracted through the electrolysis of bauxite ore (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆), with a small amount of fluorspar (CaF2) to reduce the melting point of the mixture from 2045°C to 950°C. Recently, cryolite has been replaced by a mixture of fluoride salts containing aluminum, sodium, and calcium. This mixture, when combined with bauxite, forms a molten substance characterized by a lower melting point and reduced density compared to the mixture with cryolite. The lower density of the molten substance facilitates the separation of aluminum from the molten residue, which settles at the bottom of the electrolytic cell.

The composition of the molten Aluminum usually contains from 99.5% to 99.9% Aluminum with iron and silicon being the major impurities.

Aluminum from the electrolytic cells is taken to large refractory-lined furnaces, where it is refined before casting. Alloying elements may also be melted and mixed in with the furnace charge. In the refining operation, the liquid metal is usually purged with chlorine gas to remove dissolved hydrogen gas, which is followed by a skimming of the liquid-metal surface to remove oxidized metal. After the metal has been degassed and skimmed, it is screened and cast into ingot shapes for re-melting or into primary ingot shapes such as sheet or extrusion ingots for further fabrication.

Aluminum electrolytic cell diagram

In this cell the cathode (the negative electrode) is the vessel body made of steel and lined with a layer of carbon (graphite), while the anode (the positive electrode) consists of cylinders made of carbon (graphite).

What are the properties of pure Aluminum?

Pure aluminum, in its elemental form, exhibits several properties that make it a versatile and widely used material. Here are the properties of pure aluminum:

Pure aluminum has a relatively low tensile strength compared to some other metals. Its typical tensile strength is around 70 MPa.

Aluminum is relatively soft on the Mohs scale, which measures the hardness of minerals, where its hardness number is 2.75 – 3.

Aluminum is highly malleable, meaning it can be easily shaped and formed without breaking. This property makes it widely used in cold working.

Aluminum is known for its high elongation, which means it can deform significantly before breaking. Pure aluminum can have an elongation of 30% to 40%.

Pure aluminum has good fatigue strength, allowing it to withstand repeated loading and unloading cycles without failure. However, this property can be affected by factors such as the presence of impurities or specific alloying elements.

The modulus of elasticity for aluminum is around 70 GPa. This property measures the material's stiffness, indicating how much it will deform under a given load.

Pure aluminum has relatively low creep resistance at elevated temperatures. Creep is the slow deformation of a material under a constant load over time.

Aluminum is generally considered non-toxic, and it does not release toxic fumes when exposed to normal temperatures.

Aluminum has a high reflectivity for both visible light and radiant heat, making it suitable for various applications, such as in reflective surfaces and coatings.

Aluminum forms a protective oxide layer on its surface, providing excellent corrosion resistance.

Aluminum is lightweight compared to many other metals like steels, making it a popular choice for applications where weight is a critical factor.

Note that:

The Young's Modulus of steel is approximately 3 times that of Aluminum, meaning Aluminum will elongate 3 times the elongation of steel (under same stress).

The mechanical strength of aluminum may be enhanced by cold work and by alloying; however, both processes tend to decrease resistance to corrosion.

Aluminum can be alloyed to a strength of about 690 MPa (100ksi).

Pure Aluminum at room temperature is in FCC crystal structure which contributes to its ductility.

What is Aluminum used for? (Aluminum uses)

Aluminum is a versatile metal with a wide range of uses across various industries and applications. Some common uses of aluminum include packaging, transportation, construction, electrical conductors, automotive, cookware, and aerospace.

Packaging:
Aluminum is widely used for packaging materials such as beverage cans, food containers, foil wraps, and aerosol cans.

Transportation:
Aluminum is used extensively in the transportation industry for vehicle bodies, aircraft structures, railcars, and marine vessels.

Construction:
Aluminum is used in construction for window frames, doors, roofing, siding, curtain walls, and structural components.

Electrical Conductors:
Aluminum is used as an electrical conductor in power transmission lines, overhead cables, wiring, and electrical components.

Manufacturing:
Aluminum is used in various manufacturing processes to produce components for appliances, machinery, equipment, and consumer goods.

Automotive:
Aluminum is increasingly used in the automotive industry for engine components, wheels, body panels, and structural parts to reduce weight and improve fuel efficiency.

Cookware and Utensils:
Aluminum is commonly used to manufacture cookware, including pots, pans, and utensils.

Aerospace:
Aluminum alloys are widely used in the aerospace industry for aircraft structures, fuselages, wings, and other components.

Note that:

Pure Aluminum can be used in various industrial applications, but its usage is somewhat limited compared to Aluminum alloys.

If aluminum cookware is scratched, it can expose the underlying aluminum metal to the food being cooked. Aluminum is a reactive metal, and when it comes into contact with acidic or alkaline foods, it can potentially leach small amounts of aluminum into the food. While small amounts of aluminum are generally considered safe, prolonged exposure to higher levels of aluminum may pose health risks.

Aluminum alloys

Generally, aluminum alloys are classified as either cast or wrought.

1. Wrought Aluminum Alloys

Wrought aluminum alloys are aluminum alloys that are worked or shaped through mechanical processes such as rolling, forging, extrusion, or drawing at room temperature. These alloys are typically used in applications where strength, ductility, and formability are important.

The manufacturing of Wrought Aluminum Alloys

The manufacturing process of wrought aluminum alloys involves several key steps, including alloy formulation, casting, hot working, cold working, and heat treatment. Here's an overview of each stage:

1. Alloy formulation

Where aluminum serves as the base metal, and various alloying elements are added to enhance specific properties such as strength, corrosion resistance, and formability. The composition of the alloy is meticulously designed based on the desired characteristics for the intended application.

2. Casting

Is carried out using the “semicontinuous” casting method. In this process, the alloy is melted in a furnace and poured into molds to form ingots of the desired shape. These ingots are then allowed to cool and solidify, forming raw material for subsequent processing.

3. Preheating

After casting, the ingots undergo preheating or homogenization at high temperatures for a specified duration to ensure uniform composition through atomic diffusion. This preheating is performed below the melting point of the constituent with the lowest melting temperature to avoid thermal degradation.

4. Hot Working

The preheated ingots are hot-rolled using a four-high reversing hot-rolling mill. Initially, the ingots are rolled to a thickness of about 3 inches, followed by reheating and further hot-rolling to achieve a thickness ranging from 3/4 to 1 inch using an intermediate hot-rolling mill. Further reduction in thickness is achieved through a series of tandem hot-rolling mills to produce metal sheets approximately 0.1 inch thick.

5. Cold Working

Cold working processes may be employed after hot rolling to further refine the microstructure and enhance mechanical properties. Cold rolling, drawing, or stretching techniques are utilized to fabricate the material into sheets, wires, or other forms with precise dimensions and improved strength.

6. Heat treatment

Is often employed to optimize the mechanical properties of wrought aluminum alloys. This process typically involves solution heat treatment, quenching, and aging to precipitate strengthening phases within the alloy, ensuring the desired mechanical properties are achieved.

7. Finishing operations

Such as machining, surface treatment, or coating may be performed to achieve the desired surface finish, dimensional accuracy, and corrosion resistance of the wrought aluminum alloy products. Through meticulous control and optimization of each manufacturing step, wrought aluminum alloys with tailored properties are produced to meet diverse industrial requirements and applications.

Classification of Wrought Aluminum Alloys with examples

Aluminum Alloys produced in the wrought form (i.e., sheet, plate, extrusions, rod, and wire) are classified according to the major alloying elements they contain. A four-digit numerical designation is used to identify aluminum wrought alloys. The first digit indicates the alloy group that contains specific alloying elements. The last two digits identify the aluminum alloy or indicate the aluminum purity. The second digit indicates modification of the original alloy or impurity limits.

Temper designations for wrought aluminum alloys follow the alloy designation and are separated by a hyphen (for example, 1100-0). Subdivisions of a basic temper are indicated by one or more digits and follow the letter of the basic designation (for example, 1100-H14).

Wrought aluminum alloys can conveniently be divided into two groups:

1. Non–Heat-Treatable Wrought Aluminum Alloys

Non–heat-treatable aluminum alloys cannot be precipitation-strengthened but can only be cold worked to increase their strength. The three main groups of non–heat treatable wrought aluminum alloys are the 1xxx, 3xxx, and 5xxx groups.

1xxx alloys: These alloys have a minimum of 99% aluminum, with iron and silicon being the major impurities (alloying elements). An addition of 0.12% copper is added for extra strength. The 1100 alloy has a tensile strength of about 13 ksi (90 MPa) in the annealed condition and is used mainly for sheet metal work applications.

3xxx alloys: Manganese is the principal alloying element of this group and strengthens aluminum mainly by solid-solution strengthening. The most important alloy of this group is 3003, which is essentially an 1100 alloy with the addition of about 1.25% manganese. The 3003 alloy has a tensile strength of about 16 ksi (110 MPa) in the annealed condition and is used as a general-purpose alloy where good workability is required.

5xxx alloys: Mn is the principal alloying element of this group and is added for solid-solution strengthening in amounts up to about 5%. One of the most industrially important alloys of this group is 5052, which contains about 2.5% magnesium (Mg) and 0.2% chromium (Cr). In the annealed condition, alloy 5052 has a tensile strength of about 28ksi (193 MPa). This alloy is also used for sheet metal work, particularly for bus, truck, and marine applications.

2. Heat-Treatable Wrought Aluminum Alloys

Some aluminum alloys can be precipitation- strengthened by heat treatment. Heat-treatable wrought aluminum alloys of the 2xxx, 6xxx, and 7xxx groups are all precipitation strengthened.

2xxx alloys: The principal alloying element of this group is copper, but magnesium is also added to most of these alloys. Small amounts of other elements are also added. One of the most important alloys of this group is 2024, which contains about 4.5% copper (Cu), 1.5% Mg, and 0.6% Mn. This alloy is strengthened mainly by solid-solution and precipitation strengthening. An intermetallic compound of the approximate composition of Al2CuMg is the main strengthening precipitate. Alloy 2024 in the T6 condition has a tensile strength of about 64 ksi (442 MPa) and is used, for example, for aircraft structural.

6xxx alloys: The principal alloying elements for the 6xxx group are magnesium and silicon, which combine to form an intermetallic compound (Mg2Si), which in precipitate form strengthens this group of alloys. Alloy 6061 is one of the most important alloys of this group and has an approximate composition of 1% Mg, 0.6% Si, 0.3% Cu, and 0.2% Cr. This alloy in the T6 heat-treated condition has a tensile strength of about 42 ksi (290 MPa) and is used for general-purpose structural.

7xxx alloys: The principal alloying elements for the 7xxx group of aluminum alloys are zinc, magnesium, and copper. Zinc and magnesium combine to form an intermetallic compound, MgZn2, which is the basic precipitate that strengthens these alloys when they are heat-treated. The relatively high solubility of zinc and magnesium in aluminum makes it possible to create a high density of precipitates and hence to produce very great increases in strength. Alloy 7075 is one of the most important alloys of this group; it has an approximate composition of 5.6% Zn, 2.5% Mg, 1.6% Cu, and 0.25% Cr. Alloy 7075, when heat-treated to the T6 temper, has a tensile strength of about 73 ksi (504 MPa) and is used mainly for aircraft structural.

2. Cast Aluminum Alloys

Cast aluminum alloys are aluminum alloys that are melted and poured into molds to achieve the desired shape. These alloys are commonly used in applications where complex shapes, high strength, and excellent castability are required.

The manufacturing of Cast Aluminum Alloys

Manufacturing cast aluminum alloys involves several steps to ensure the desired material properties and quality.

1. Alloy Design

Aluminum alloys are typically made by combining aluminum with other elements like silicon, copper, magnesium, zinc, and others to achieve specific mechanical, thermal, and chemical properties.

2. Melting

The raw materials, including aluminum scrap and alloying elements, are melted in a furnace. The temperature and composition of the melt are carefully controlled to achieve the desired alloy composition.

3. Degassing

Molten aluminum is susceptible to absorbing gases like hydrogen and nitrogen, which can weaken the final product. Degassing processes, such as vacuum degassing or fluxing, are employed to remove these gases.

4. Refining

To improve the homogeneity and cleanliness of the melt, refining processes like fluxing or filtration may be used. This helps remove impurities and solid particles from the molten metal.

5. Casting

The molten aluminum is poured into molds to form the desired shape. There are various casting methods, including sand casting, permanent mold casting, die casting, and investment casting, each with its advantages and suitable applications.

6. Solidification

As the molten metal cools inside the mold, it solidifies into the desired shape. Proper cooling rates and solidification practices are essential to ensure the desired microstructure and mechanical properties.

7. Heat Treatment

Many aluminum alloys undergo heat treatment processes to further enhance their mechanical properties. This may involve processes like solution heat treatment, quenching, and aging to achieve the desired strength, hardness, and other characteristics.

8. Finishing

After casting and heat treatment, the castings may undergo various finishing processes such as machining, grinding, polishing, and surface treatment (e.g., anodizing, painting) to meet the final product's specifications and aesthetic requirements.

Aluminum alloys are normally cast by one of three main processes: sand casting, permanent-mold, and die casting.

1. Sand Casting

Process: Sand casting is one of the oldest casting methods where a mold made of sand is used to create the desired shape. A pattern, typically made of wood, metal, or resin, is placed in the sand to form the mold cavity. Molten aluminum is then poured into the mold, where it solidifies to form the final casting.

Advantages:

Low tooling cost: Sand molds are relatively inexpensive compared to other casting methods.

Suitable for complex shapes and large parts.

Flexibility in design changes and modifications.

Disadvantages:

Lower dimensional accuracy and surface finish compared to other methods.

Longer lead times due to mold preparation and cooling.

Sand molds can only be used for a limited number of casts before they degrade.

2. Permanent Mold Casting

Permanent mold casting, also known as gravity die casting, involves using reusable metal molds to produce aluminum parts. The molds are typically made of steel or iron and have a cavity in the shape of the desired part. Molten aluminum is poured into the mold under gravity, where it solidifies to form the casting.

Advantages:

Higher dimensional accuracy and better surface finish compared to sand casting.

Faster production cycles compared to sand casting.

Suitable for medium to high-volume production runs.

Disadvantages:

Higher tooling costs compared to sand casting.

Limited to simpler part geometries compared to die casting.

Limited to smaller part sizes compared to sand casting.

3. Die Casting

Process: Die casting is a high-pressure casting method where molten aluminum is forced into a steel mold cavity under high pressure. The mold, called a die, is typically made in two halves and is precision-machined to produce complex parts with tight tolerances. After solidification, the mold opens, and the casting is ejected.

Advantages:

Excellent dimensional accuracy and surface finish.

High production rates and short cycle times.

Ability to produce complex shapes with thin walls.

Disadvantages:

High tooling costs due to the complexity and precision of the molds.

Limited to smaller to medium-sized parts.

Not suitable for materials with high melting points or high viscosity.

Classification of Cast Aluminum Alloys with examples

Aluminum casting alloys have been developed for casting qualities such as fluidity and feeding ability as well as for properties such as strength, ductility, and corrosion resistance. As a result, their chemical compositions differ greatly from those of the wrought aluminum alloys.

These alloys are classified in the United States according to the Aluminum Association system. In this system, aluminum casting alloys are grouped by the major alloying elements they contain by using a four-digit number with a period between the last two digits.

Silicon in the range of about 5% to 12% is the most important alloying element in aluminum casting alloys since it increases the fluidity of the molten metal and its feeding ability in the mold and strengthens the aluminum.

Magnesium in the range of about 0.3% to 1% is added to increase strength, mainly by precipitation strengthening through heat treatment.

Copper in the range of about 1% to 4% is also added to some aluminum casting alloys to increase strength, particularly at elevated temperatures. Other alloying elements such as zinc, tin, titanium, and chromium are also added to some aluminum casting alloys.

Note that:

In some cases, if the cooling rate of the solidified casting in the mold is sufficiently rapid, a heat-treatable alloy can be produced in the supersaturated solid condition. Thus, the solution heat-treatment and quenching steps can be neglected for precipitation strengthening the casting, and only subsequent aging of the casting after it has been removed from the mold is required. A good example of the application of this type of heat treatment is in the production of precipitation-strengthened automobile pistons.

A generation of new Aluminum–Lithium alloys has been developed recently for use by the aircraft and aerospace industries. These materials have relatively low densities (between about 2.5 and 2.6 g/cm3), high specific moduli (elastic modulus–specific gravity ratios), and excellent fatigue and low-temperature toughness properties. Furthermore, some of them may be precipitation hardened. However, these materials are more costly to manufacture than conventional aluminum alloys because special processing techniques are required as a result of lithium’s chemical reactivity.