Aluminum is a lightweight metal, highly valued across many industries for its superior corrosion resistance and natural ductility. It is incredibly versatile, finding application in everything from beverage cans to high-performance aircraft structures. However, “hardness” does not refer to a single fixed value for aluminum, as its properties vary dramatically. The ultimate hardness of any aluminum product depends entirely on its precise chemical composition and the manufacturing processes it has undergone.
Understanding Material Hardness and Measurement Scales
Material hardness is resistance to localized plastic deformation, such as scratching, abrasion, or indentation. Since hardness is not a fundamental physical property but a composite one, standardized tests are essential for consistent measurement. These tests provide a reliable, quantifiable way to compare different materials.
The Brinell hardness test (HBW) is a primary method used to determine the hardness of metals, especially those with coarser grain structures like aluminum forgings and castings. This method involves pressing a hardened ball, typically tungsten carbide, into the material’s surface using a precisely controlled force. The resulting Brinell hardness number is calculated by measuring the diameter of the permanent indentation left behind.
Another common method, the Rockwell test (HR), measures hardness based on the depth of the indentation rather than its width. Both the Brinell and Rockwell scales rely on the basic principle of indentation testing, where a fixed shape is pressed into the material under a fixed load. The scale used is particularly important for aluminum, as its relatively soft nature requires lower test forces than those used for harder materials like steel.
Hardness Values of Pure Aluminum vs. Alloys
The Brinell hardness value of aluminum changes significantly depending on whether the metal is commercially pure or alloyed. Commercially pure aluminum (1000 series) is inherently soft and highly ductile. For instance, the Brinell hardness of pure aluminum in its fully annealed condition (1100-O) is very low, typically measuring around 23 HB.
This low hardness value reflects the material’s excellent formability, making it suitable for applications like foil or chemical equipment. Even when slightly strain-hardened, commercially pure aluminum remains relatively soft, with the 1100-H14 temper reaching only about 31 to 32 HB.
In contrast, structural aluminum alloys exhibit much higher Brinell hardness numbers, making them suitable for load-bearing applications. These alloys contain elements like magnesium, silicon, or zinc, making them significantly harder. For example, the popular general-purpose alloy 6061, when fully heat-treated to the T6 temper, reaches approximately 93 HB. The ultra-high-strength aerospace alloy 7075 (T6 temper) is much harder, typically registering around 150 HB. This maximum hardness makes 7075-T6 comparable to some mild steels, demonstrating the massive range of properties achievable in the aluminum family.
How Alloying and Treatment Change Aluminum’s Hardness
The wide range of aluminum hardness values results directly from controlled metallurgical processes, specifically alloying and heat treatment. Alloying involves adding specific elements like copper, zinc, or magnesium to the pure aluminum base. These foreign atoms integrate into the crystal structure, creating localized imperfections that disrupt the movement of dislocations within the metal.
These dislocations are microscopic defects whose movement allows the metal to deform; by obstructing this movement, the added elements strengthen the metal, a process known as solid solution strengthening. This initial increase in strength is then dramatically enhanced in specific alloy series through heat treatment, also called precipitation hardening.
This involves two main steps: solutionizing and artificial aging. During solutionizing, the alloy is heated to a high temperature to dissolve the alloying elements completely into the aluminum matrix. The material is then rapidly quenched, or cooled, to trap these elements in a supersaturated solution. Artificial aging involves reheating the material to a lower temperature, allowing the trapped atoms to precipitate out and form countless microscopic particles. These tiny precipitates act as effective roadblocks to dislocation movement, which is the primary mechanism for the dramatic increase in strength and hardness, resulting in tempers like 6061-T6 and 7075-T6.
Cold working, or strain hardening, is another method used to increase the hardness of both pure aluminum and non-heat-treatable alloys (“H” tempers). This mechanical process involves deforming the metal at room temperature through rolling or drawing. The deformation increases the density of dislocations within the material, causing them to tangle and increasing the overall hardness.