Comparison of strength characteristics between aluminum alloy fasteners and metal fasteners

[China Aluminum] The weight of aluminum fasteners is 1/3 of the weight of its equivalent steel fasteners. This often used alloy has surprisingly good strength characteristics. In fact, in the strong-to-mass ratio, aluminum fasteners are higher than any other fasteners made from industrial and commercial materials. Aluminum is non-magnetizable. The thermoelectric conductivity of aluminum is very good, about 2/3 of that of copper in the same volume. Aluminum has good processing characteristics, easy cold forming and hot forging.

Comparison of strength characteristics of aluminum alloy fasteners and metal fasteners:

The strength characteristics of externally threaded fastener aluminum alloy materials 2024-T4, 6061-T6, and 7075-T73 are discussed in detail in ASTMF468 on page B-158; the strength of nut aluminum alloy materials 2024-T4, 6061-T6, and 6062-T9. The characteristics are discussed in detail in ASTM F467 on page B-184.

Here, it is necessary to explain the difference in mechanical properties between aluminum alloy threaded fasteners and fasteners made of other metal materials.

The front point is: When calculating the load capacity of a part, the area of the cross-section of the bottom of the tooth should be determined rather than the area of greater tensile stress. Only the tensile and yield strength values of mechanical test specimens given in Table 2 of ASTM F468 are the true strength values. When calculating the strength of a fastener of the entire size, appropriate adjustments can be made. In this way, when the stress value is multiplied by the area of the threaded force area to calculate the load capacity in pounds, the calculation result is approximately the product of the “true value” in the table and the area of the smaller tooth bottom area.

The second point is that the difference in hardness of aluminum alloys is small, and it does not make sense as the inspection criteria. As an alternative to the hardness test, shear strength tests are usually introduced.

The 2024-T4 aluminum alloy (containing 4.5% of copper, 1.6% of manganese, 1.5% of magnesium, and the balance of aluminum) is a heavy-duty alloy. It achieves a perfect balance of strength, corrosion resistance, manufacturability, and economy, and is widely used in the manufacture of threaded fasteners.

The bolts, screws, and studs made of 7075-T73 aluminum alloy (containing 1.6% copper, 2.5% manganese, 0.3% chromium, and the balance of aluminum) have made slight improvements in strength, and due to " T73" special heat treatment process, so that it can largely prevent the occurrence of stress corrosion. However, the expensive construction has limited its popularity.

6061-T6 aluminum alloy (containing 0.6% of silicon, 0.25% of copper, 1% of magnesium, 0.2% of chromium, and the balance of aluminum) can be used to design internally and externally threaded fasteners that are more resistant to corrosion .

Type 6062-T9 aluminum alloy (containing 0.6% of silicon, 0.25% of copper, 1% of magnesium, 0.09% of chromium, 0.5% of lead, and the balance of aluminum) is almost exclusively designed for nuts. This alloy is stronger than the 6061-T6 aluminum alloy and has relatively good corrosion resistance.

Full-thickness nuts made of 6062-T9 aluminum alloy have sufficient strength to fit bolts made of 2024-T4 or 7075-T73 aluminum alloys. Machine screws, nuts, and other 1/4-inch and smaller nuts are made of 2024-T4 aluminum alloy.

Aluminum Alloys for Advantages of Fastener Manufacturing The four types of aluminum alloys that have already been mentioned are widely used in the manufacture of threaded bearing fasteners, while other aluminum alloys are used in the manufacture of other types of fasteners. The small solid, half-pipe, and blind rivets were made of 1100-F, 5052-F, and 5056-F aluminum alloys, respectively. Heat treatable 2017-T4, 2117-T4, 2024-T4, 6061-T6 aluminum alloys and relatively newly developed 7075-T73 aluminum alloys have superior shear strength and do not require "pre-drive treatment" Can drive.

Plain washers are usually made of an aluminum-plated 2024-T4 alloy; spiral spring washers are usually made of 7075-T6 alloy; tapping screws are made of 7075-T6 alloy; tapping screws are obtained from the same material alloy by anodizing. The 2011-T3 aluminum alloy (containing 5.5% copper, 0.5% lead, 0.5% niobium, and the remainder aluminum) can be used to make parts for thread cutting machines.

Under normal circumstances, aluminum has sufficient corrosion resistance. And when the expected exposure environment is very harsh, its corrosion resistance can be greatly improved by anodizing. Anodic treatment is an electrical process that forms an oxide film on the metal surface. Anodic treatment not only enhances the ability to resist corrosion, but also enhances protection against wear and scratches. Anodic coatings are available in a variety of colors for decorative and identification purposes. In atmospheric corrosion, aluminum forms a light gray oxide film on the surface. These corrosion products do not contaminate the surface of the aluminum, or spread to adjacent surfaces. It is not the same as many other metals under the effect of corrosion.

The tensile strength of pure aluminum is about 13,000 psi. It is possible to increase the amount of alloying elements and greatly improve the strength. Aluminum alloys of 2XXX, 6XXX, and 7XXX are very effective for heat treatment. Therefore, virtually all threaded fasteners used for load transfer are made from these three major types of aluminum alloys. There are four kinds of aluminum alloys that are almost exclusively used.