Aluminum fasteners are significantly lighter than their steel counterparts, weighing about one-third as much. Despite this, they offer impressive strength characteristics. In terms of strength-to-weight ratio, aluminum fasteners outperform most other materials used in industrial and commercial fastener production. Additionally, aluminum is non-magnetic and has excellent thermal and electrical conductivity—about two-thirds that of copper for the same volume. Its good workability allows for easy cold forming and hot forging, making it a versatile material for manufacturing.
When comparing the mechanical properties of aluminum alloy fasteners to those made from other metals, there are key differences to consider. For example, when calculating the load capacity of a fastener, the cross-sectional area at the root of the thread should be used rather than the tensile stress area. The tensile and yield strengths listed in ASTM F468 Table 2 represent true values, and adjustments must be made when calculating the overall strength of a fastener. This means that multiplying the stress value by the threaded area gives an approximate result close to the product of the true value and the smaller root area.
Another important point is that the hardness of aluminum alloys varies only slightly, making it an unreliable indicator for quality control. Instead, shear strength tests are commonly used as a more accurate alternative.
Among the commonly used aluminum alloys, 2024-T4 (containing 4.5% copper, 1.6% manganese, 1.5% magnesium, and the rest aluminum) is known for its high strength and good corrosion resistance, making it ideal for threaded fasteners. 7075-T73, with 1.6% copper, 2.5% manganese, 0.3% chromium, and the rest aluminum, offers even higher strength due to a special heat treatment process that reduces stress corrosion risk. However, its higher cost limits its widespread use.
6061-T6 (with 0.6% silicon, 0.25% copper, 1% magnesium, 0.2% chromium, and the rest aluminum) is widely used for both internal and external threaded fasteners due to its superior corrosion resistance. 6062-T9, which contains similar elements plus 0.5% lead, is mainly used for nuts and offers better strength and corrosion resistance compared to 6061-T6. Full-thickness nuts made from 6062-T9 can accommodate bolts made from 2024-T4 or 7075-T73 alloys, while smaller 1/4-inch nuts are typically made from 2024-T4.
Other aluminum alloys are also used in various fastener types. For instance, 1100-F, 5052-F, and 5056-F are commonly used for solid, half-pipe, and blind rivets. Heat-treatable alloys like 2017-T4, 2117-T4, 2024-T4, 6061-T6, and 7075-T73 are preferred for their high shear strength, allowing them to be driven without pre-drilling.
Plain washers are often made from 2024-T4 aluminum, while spiral spring washers and tapping screws are usually made from 7075-T6. Tapping screws can also be produced using anodized 7075-T6. 2011-T3, containing 5.5% copper, 0.5% lead, 0.5% niobium, and the rest aluminum, is used for thread-cutting machine parts.
Aluminum generally has good corrosion resistance, and this can be further enhanced through anodizing. Anodizing creates a protective oxide layer on the surface, improving both corrosion and wear resistance. It also allows for a range of colors, useful for identification and aesthetics. Unlike many other metals, the oxide formed on aluminum does not flake off or spread to surrounding surfaces.
The tensile strength of pure aluminum is around 13,000 psi, but adding alloying elements can significantly increase this. Alloys such as 2XXX, 6XXX, and 7XXX are particularly effective for heat treatment, making them ideal for load-bearing fasteners. These four main types of aluminum alloys are almost exclusively used in the production of threaded fasteners.
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