The question of whether an alloy is stronger than aluminum is based on a misunderstanding of materials science. An “alloy” is a category of materials, not a single substance; aluminum is the base element for many of these alloys. Pure aluminum (99% or greater) is soft, highly malleable, and lacks the structural integrity for demanding applications like aerospace or automotive components. Nearly all commercial and structural applications utilize aluminum alloys, which are engineered mixtures significantly stronger than the pure metal.
Understanding Pure Aluminum and Alloys
Pure aluminum is a soft metal valued for its low density, excellent conductivity, and natural corrosion resistance due to a self-forming oxide layer. Its mechanical properties are low; for example, the 1100 grade has a low yield strength and deforms easily under stress. This softness makes it suitable only for non-structural items like foil and electrical conductors.
An aluminum alloy is created by introducing small, controlled amounts of elements like copper, magnesium, silicon, manganese, and zinc into the pure aluminum melt. This transforms its properties, enhancing mechanical characteristics such as hardness, durability, and strength. Metallurgists produce materials with strength levels many times greater than the pure metal.
Wrought aluminum alloys use a four-digit naming system reflecting their composition. The first digit indicates the main alloying element (e.g., 2xxx for copper, 6xxx for magnesium and silicon, or 7xxx for zinc). This systematic approach allows engineers to select materials with specific, predictable properties.
The Mechanisms That Increase Strength
The increase in strength involves disrupting the orderly arrangement of aluminum atoms in the crystal structure. When stressed, metal deforms because internal defects, known as dislocations, move through the crystal lattice. Strengthening mechanisms create obstacles that impede or block this dislocation movement.
One primary method is solid solution strengthening, where alloying atoms are dissolved into the aluminum lattice. Atoms like magnesium or copper are either larger or smaller than aluminum atoms, causing local strain and distortion. These localized stress fields interact with moving dislocations, pinning them in place and requiring more force to continue deformation. This provides a moderate increase in strength for many alloys.
A more dramatic increase in strength is achieved through precipitation hardening (age hardening), which involves a precise, multi-step heat treatment. This process involves heating the alloy to dissolve the elements, followed by rapid cooling (quenching) to trap them in a supersaturated solid solution. The final step, aging, involves reheating the material, causing the trapped atoms to precipitate out and form millions of tiny particles within the aluminum matrix. These dispersed precipitates act as strong, physical barriers that dislocations cannot easily bypass, leading to a substantial increase in yield strength.
Comparing the Strength of Standard Aluminum Alloys
The strength differences between pure aluminum and its high-performance alloys are considerable. Commercially pure aluminum (e.g., 1100 series) has a yield strength of approximately 35 to 90 megapascals (MPa), depending on its temper. This low strength severely limits its use in load-bearing applications.
Common structural alloys like the 6000 series (magnesium and silicon) offer a significant jump in performance. The widely used 6061-T6 alloy, strengthened by precipitation hardening, achieves a yield strength of about 276 MPa. This strength makes it a staple for general engineering, structural components, and bicycle frames, providing a robust balance of strength and versatility.
For applications requiring the highest strength-to-weight ratio, the 7000 series alloys, primarily alloyed with zinc, are utilized. The aerospace-grade 7075-T6 alloy relies on precipitation hardening and is one of the strongest aluminum alloys commercially available. This material boasts a yield strength exceeding 500 MPa, making it comparable to some steels. Its strength is more than five times that of pure aluminum, justifying its use in highly stressed components like airframe structures and military equipment.
Other Critical Material Considerations
While strength is a primary factor, alloying creates trade-offs in other properties.
Corrosion Resistance
The elements that boost strength can negatively impact corrosion resistance. For instance, the high-strength 2xxx (copper) and 7xxx (zinc/copper) series alloys exhibit reduced resistance to environmental corrosion compared to pure aluminum or the 5xxx (magnesium) series.
Weldability
Weldability is affected by the alloying process and heat treatment. High-strength, heat-treatable alloys like 7075 have poor weldability using traditional fusion methods. Welding heat can dissolve or coarsen the strengthening precipitates, leading to a severe loss of strength. Conversely, non-heat-treatable alloys like 5052 and low-strength pure aluminum grades are much easier to weld without significant loss of mechanical performance.
Machinability and Cost
Machinability, the ease with which a material can be cut and shaped, varies with the alloy. Softer pure aluminum can be gummy and difficult to machine, while some high-strength alloys are more brittle and easier to chip away cleanly. The cost of an alloy increases with its complexity. Common, moderate-strength alloys like 6061 are inexpensive, while specialized, high-strength, aerospace-grade alloys like 7075 command a higher price due to their composition and necessary heat treatment.