Comparison of strength characteristics between aluminum alloy fasteners and other metal fasteners
Aluminum fasteners weigh 1/3 of the weight of their equivalent steel fasteners. The strength characteristics of such frequently used alloys are surprisingly good. In fact, in terms of the strength-to-mass ratio, aluminum fasteners are higher than fasteners made from any other trade-trade material. Aluminum is not magnetizable. The thermal conductivity of aluminum is very good, about 2/3 of the copper conduction performance of the same volume. Aluminum has good processing characteristics and is easy to cold form and hot forge.
Comparison of strength characteristics between aluminum alloy fasteners and metal fasteners:
External thread fasteners The strength characteristics of the aluminum alloy materials 2024-T4, 6061-T6 and 7075-T73 are discussed in detail in ASTM F468 on page B-158; the strength of the nut aluminum alloy materials 2024-T4, 6061-T6 and 6062-T9 The characteristics are discussed in detail in ASTMF 467 on page B-184.
It is necessary to explain the difference in mechanical properties between fasteners made of aluminum alloy and fasteners made of other metal materials.
The first point is: When calculating the load capacity of a part, it is necessary to determine the area of the cross-section of the bottom of the tooth rather than the area of the tensile stress. Only the tensile and yield strength values of the mechanical test specimens given in Table 2 of ASTMF468 are true strength values. Appropriate adjustments can be made when calculating the strength of the entire size of the fastener. Thus, when the stress value is multiplied by the area of the threaded force area to calculate the load capacity in pounds, the calculated result is approximately the product of the true value in the table and the area of the smaller root area.
The second point is that the hardness of the aluminum alloy is very small and does not make much sense like the inspection guidelines. As an alternative to the hardness test, a shear strength test is usually introduced.
The 2024-T4 aluminum alloy (containing 4.5% copper, 1.6% manganese, 1.5% magnesium, and the balance 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.
Bolts, screws and studs made of 7075-T73 aluminum alloy (1.6% copper, 2.5% manganese, 0.3% chromium, the balance of aluminum) have made a slight improvement in strength, and due to T73 The special heat treatment process makes it possible to prevent stress corrosion to a large extent. But the high cost makes its popularity limited.
6061T6 aluminum alloy (containing 0.6% silicon, 0.25% copper, 1% magnesium, 0.2% chromium, and the rest aluminum) can be used to design internal and external threaded fasteners with higher corrosion resistance.
The 6062T9 aluminum alloy (containing 0.6% silicon, 0.25% copper, 1% magnesium, 0.09% chromium, 0.5% lead, and the balance aluminum) is almost exclusively designed for the nut. This alloy is stronger than the 6061-T6 aluminum alloy and has relatively good corrosion resistance.
The full thickness nut made of 6062T9 aluminum alloy has sufficient strength to match the bolts made of 2024-T4 or 7075-T73 aluminum alloy. Machine screws, nuts and other 1/4" and smaller nuts are made of 2024-T4 aluminum alloy.
Advantages of aluminum alloys for fastener manufacturing
The four aluminum alloys already mentioned are the most widely used in the manufacture of thread-loaded fasteners, while other aluminum alloys are used in the manufacture of other types of fasteners. Small solid, half-tube and blind rivets are made of 1100-F, 5052-F, 5056-F aluminum alloy, respectively. The heat treatable 2017-T4, 2117-T4, 2024-T4, 6061T651 aluminum alloy and the relatively newly developed 7075-T73 aluminum alloy have superior shear strength and can be driven without pre-drive processing.
Flat washers are usually made of aluminized 2024-T4 alloy; coil spring washers are usually made of 7075-T6 alloy; tapping screws are made of 7075-T6 alloy; and self-tapping screws are obtained by anodizing the same material alloy. The 2011-T3 aluminum alloy (containing 5.5% copper, 0.5% lead, 0.5% niobium, and the balance aluminum) can be used to make parts for thread cutting machines.
Aluminum has sufficient corrosion resistance under normal conditions. Moreover, when the expected exposure environment is very harsh, its corrosion resistance can be greatly improved by anodizing. Anode treatment is an electromachining process that forms an oxide film on a metal surface. Anode treatment not only enhances corrosion resistance, but also enhances protection against wear and scratches. Anode plating has a wide 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, and it does not differ from many other metals in corrosion.
The tensile strength of pure aluminum is about 13,000 psi, and it is possible to increase the strength by a small amount of alloying elements. The aluminum alloys of 2XXX, 6XXX, and 7XXX have a good effect on heat treatment. Therefore, virtually all threaded fasteners used for load transfer are made of these three types of aluminum alloys. There are four aluminum alloys that are almost exclusively used.
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