Aluminum, the silvery-white metal, is one of the most abundant elements found in the Earth’s crust. When encountered in its pure, elemental state, the answer to whether it is a soft metal is definitively yes. However, the aluminum used in common products, from beverage cans to aircraft frames, is almost always a different material. This material is intentionally hardened to withstand the stresses of everyday use, giving the metal a dual nature: naturally soft but engineered for strength.
How Scientists Define Metal Hardness
Hardness in metals is defined by a material’s resistance to localized permanent deformation, which scientists call plastic deformation. This resistance is measured using various standardized tests that quantify the material’s ability to resist being scratched or indented. One well-known method is the Mohs scale, which measures scratch resistance and assigns pure aluminum a low value of approximately 2.75. For industrial applications, engineers rely on indentation tests such as the Brinell or Rockwell methods. These tests press a hardened sphere or cone into the material surface under a specific load and then measure the size or depth of the resulting permanent mark. A soft metal is one that shows a large, deep indentation under a relatively light load, indicating a low resistance to having its shape permanently changed.
The Characteristics of Pure Aluminum
The inherent softness of pure aluminum stems from its crystalline structure and weak internal atomic bonds. This metal has a very low density of 2.7 grams per cubic centimeter, which is roughly one-third the density of steel. This low mass contributes to its characteristic lightness, a property highly valued in aerospace and transportation. The weak bonds allow the atoms to slide past one another with relative ease when a force is applied. This atomic behavior results in high malleability, meaning the metal can be rolled or hammered into extremely thin sheets, such as household aluminum foil. Similarly, pure aluminum exhibits excellent ductility, allowing it to be stretched into fine wires without fracturing. Pure aluminum also possesses a relatively low melting point of approximately 1,220°F (660°C). These combined physical properties result in a low tensile strength, with pure aluminum only able to withstand a pulling force of about 90 megapascals (MPa) before permanently deforming. This low strength is why it is unsuitable for most structural applications in its unrefined state.
Why Aluminum Alloys are Stronger
The aluminum used in most manufactured goods is not pure but is instead an alloy, a mixture of aluminum and small amounts of other elements. This process of alloying dramatically transforms the metal’s mechanical properties by disrupting the pure metal’s orderly atomic structure. Elements like copper, magnesium, silicon, and zinc are dissolved into the aluminum to create this new, much harder material. These added atoms introduce imperfections into the crystal lattice, which act as obstacles that prevent the aluminum atoms from easily sliding past one another. By restricting this movement of atoms, the alloy gains a significantly higher resistance to plastic deformation. For example, a common structural alloy like 7075 can achieve a tensile strength that is over six times greater than that of pure aluminum. Further strength enhancements are achieved through heat treatment, known as precipitation hardening. This technique involves controlled heating and cooling cycles that cause the alloying elements to form tiny, hard particles within the aluminum structure. These microscopic particles lock the material’s internal crystalline structure into place, giving the finished alloy the necessary strength for demanding applications like modern aircraft components.