Aluminum is highly valued in modern engineering, but the term often refers to an alloy—a blend of aluminum and other metallic elements. Pure aluminum is soft and pliable, but combining it with elements like copper, zinc, magnesium, and silicon transforms it into a material with a wide range of mechanical properties. The strength of an aluminum alloy is not a fixed value; it varies dramatically based on its specific chemical composition and manufacturing processes. This variation allows engineers to select an alloy tailored to the demands of a particular application, from beverage cans to high-performance aircraft.
Quantifying Strength in Aluminum Alloys
The strength of any metal is defined by a set of distinct mechanical properties that describe its resistance to different types of force. Engineers primarily rely on two metrics: yield strength and tensile strength. These values are typically expressed in units of stress, such as megapascals (MPa) or pounds per square inch (psi).
Yield strength is the maximum stress an alloy can withstand before it begins to deform permanently. Once a load exceeds the yield point, the component will not return to its original shape, which often constitutes structural failure. High-strength aluminum alloys, such as those used in aerospace, can have yield strengths exceeding 500 MPa.
Tensile strength, also known as ultimate tensile strength, represents the absolute limit of the material’s integrity. This is the maximum stress the alloy can endure before it fractures and breaks apart. Tensile strength is always higher than yield strength. A third property, hardness, describes the alloy’s resistance to localized plastic deformation, such as scratching or indentation.
How Alloying Elements and Temper Affect Strength
The two primary methods used to maximize the strength of aluminum alloys are the careful selection of alloying elements and the application of specific thermal or mechanical treatments, known as temper. The addition of other metals creates distinct alloy series, each possessing a different strength profile.
The highest strength aluminum alloys are found in the 7xxx series, where zinc is the primary alloying element, often combined with magnesium and copper. These zinc-rich alloys are valued in applications demanding maximum strength, achieving yield strengths greater than 500 MPa. The 2xxx series, which uses copper, also yields very high-strength alloys suitable for aircraft structures. In contrast, the 6xxx series, alloyed with magnesium and silicon, offers moderate strength but is favored for its excellent weldability and corrosion resistance.
Once the chemical composition is set, the temper designation dictates the final mechanical properties through processing. Alloys are categorized as either heat-treatable or non-heat-treatable, determining the strengthening mechanism used.
Non-Heat-Treatable Alloys
Non-heat-treatable alloys, designated with an “H” temper, are strengthened primarily through strain hardening. This involves cold working processes like rolling or drawing. This mechanical deformation introduces defects into the metal’s crystalline structure, making it harder and stronger.
Heat-Treatable Alloys
Heat-treatable alloys, marked with a “T” temper, achieve high strength through solution heat treatment and artificial aging. This involves heating the alloy to dissolve the alloying elements, rapidly quenching it, and then baking it at a lower temperature for a specific period. This aging causes the alloying elements to precipitate out as microscopic particles within the aluminum matrix, which impedes the movement of dislocations and significantly increases the metal’s strength. The resulting T-temper alloys, such as 7075-T6, are the strongest available.
Relative Strength Compared to Other Materials
Aluminum alloys are often compared to steel, the most widely used structural material. In terms of absolute strength, steel generally possesses higher yield and tensile strength numbers than aluminum. A steel part of the same size is typically stronger; for example, common carbon steel has a tensile strength around 500 MPa, compared to approximately 310 MPa for high-strength aluminum alloy 6061-T6.
The comparison shifts dramatically when considering the strength-to-weight ratio. Aluminum is approximately one-third the density of steel. This means an aluminum alloy can provide superior strength for a given unit of weight. This metric is why aluminum dominates industries where weight reduction is paramount, allowing for lighter structures that still withstand necessary forces.
Aluminum also offers natural corrosion resistance. When exposed to air, aluminum forms a thin, tough layer of aluminum oxide. This passive protection blocks further oxidation, contributing to the material’s long-term durability and structural integrity in marine or outdoor applications.
Real-World Applications Leveraging Aluminum Strength
The superior strength-to-weight ratio of certain aluminum alloys makes them indispensable in the aerospace industry, where every kilogram saved translates directly into fuel efficiency and performance. High-strength 7xxx and 2xxx series alloys are widely used for critical components like airframe structures, wings, and fuselage sections. These heat-treated alloys provide the necessary high yield strength to resist stress without permanent deformation during flight.
In the automotive sector, aluminum alloys are increasingly used to reduce vehicle mass for better fuel economy and reduced emissions. Moderate-strength alloys from the 5xxx and 6xxx series are frequently chosen for body panels, engine blocks, and chassis components. Their combination of adequate strength, formability, and good corrosion resistance makes them ideal for mass-produced vehicles.
The corrosion resistance of aluminum also makes it a preferred material for marine and maritime applications. Alloys from the 5xxx series, which contain magnesium, exhibit excellent resistance to saltwater environments and are used in shipbuilding and for constructing docks or offshore structures.