The weight of aluminium fasteners is one third of that of their steel counterparts. The strength characteristics of this frequently used alloy are surprisingly good. In fact, in terms of strength-to-weight ratio, aluminium fasteners are higher than those made of any other industrial and trade materials.
Aluminum has always been synonymous with lightness. As engineering increasingly emphasizes maintaining energy by reducing weight, the use of aluminum is bound to increase.
Aluminum is non-magnetic. It has good thermoelectric conductivity, approximately two-thirds that of copper in the same volume. Aluminum also has excellent processing characteristics, being easy to cold form and hot forge.
Under normal conditions, aluminium has sufficient corrosion resistance. Moreover, when the expected exposure environment is very harsh, its corrosion resistance can be greatly improved through anodizing. Anodizing is an electrochemical process that forms an oxide film on the metal surface. Anodizing not only enhances corrosion resistance but also provides better protection against wear and scratches.
Anodized coatings come in many colors for decorative and identification purposes. In atmospheric corrosion, aluminium forms a light gray oxide film on its surface. These corrosion products do not contaminate the aluminium surface or spread to adjacent surfaces, which is different from the behavior of many other metals under corrosion.
Aluminum fasteners
The tensile strength of pure aluminium is approximately 13,000 psi. It is possible to significantly enhance the strength by adding a small amount of alloying elements. Alloys of the 2XXX, 6XXX, and 7XXX series respond well to heat treatment. Therefore, practically all threaded fasteners used for load transmission are made from these three major categories of aluminium alloys. There are four aluminium alloys that are almost exclusively used.
The 2024-T4 aluminum alloy (containing 4.5% copper, 1.6% manganese, 1.5% magnesium, and the rest aluminum) is a high-load alloy. It achieves a balance in the combination of strength, corrosion resistance, manufacturability, and economy, and is widely used in the manufacturing of threaded fasteners.
Bolts, screws and studs made of 7075-T73 aluminum alloy (containing 1.6% copper, 2.5% manganese, 0.3% chromium, and the rest aluminum) have made a slight improvement in strength. Moreover, due to the special heat treatment process of “T73”, it can largely prevent stress corrosion. However, its high cost limits its popularity.
Aluminum alloy 6061-T6 (containing 0.6% silicon, 0.25% copper, 1% magnesium, 0.2% chromium, and the rest aluminum) can be used to design internal and external thread fasteners with higher requirements for corrosion resistance.
6062-T9 aluminum alloy (containing 0.6% silicon, 0.25% copper, 1% magnesium, 0.09% chromium, 0.5% lead, and the rest aluminum) is almost exclusively used for designing nuts. This alloy is stronger than 6061-T6 aluminum alloy and has relatively better corrosion resistance.
The full-thickness nuts made of 6062-T9 aluminum alloy have sufficient strength to be used with bolts made of 2024-T4 or 7075-T73 aluminum alloy. Machine screws, nuts and other nuts of 1/4 inch and smaller sizes are made of 2024-T4 aluminum alloy.
The four types of aluminium alloys mentioned above are most widely used in the manufacturing of threaded load-bearing fasteners, while other aluminium alloys are used in the manufacturing of other types of fasteners.
Small solids, half-tubes and blind rivets are respectively made of 1100-F, 5052-F and 5056-F type aluminium alloys. Heat-treatable 2017-T4, 2117-T4, 2024-T4, 6061-T6 type aluminium alloys and the relatively newly developed 7075-T73 type aluminium alloy have superior shear strength and can be driven without “pre-driving treatment”.
Flat washers are usually made of 2024-T4 alloy with an aluminum coating; spring washers are typically made of 7075-T6 alloy; self-tapping screws can be made of 7075-T6 alloy; self-tapping screws are obtained by anodizing the same material alloy. 2011-T3 aluminum alloy (containing 5.5% copper, 0.5% lead, 0.5% bismuth, and the rest aluminum) can be used to manufacture parts of thread cutting machines.
Marking of aluminium fasteners
There are approximately 100 grades of aluminium alloys, each of which is identified by a four-digit code. The first digit of the four-digit code indicates the basic alloy type; the last two digits identify different alloys within the same type; and the second digit indicates the method of alloy improvement.
For example, 1XXX alloys are mainly pure aluminium; 2XXX alloys use copper as their main alloying element; 5XXX alloys contain magnesium; 6XXX alloys contain magnesium and silicon; and 7XXX alloys are aluminium-zinc alloys.
The subscript of an alloy grade is a letter or a number, used to identify tempering and metallurgical treatment methods. Letters represent basic metallurgical treatments, while numbers indicate post-treatment processing procedures.
For instance, “O” stands for full annealing; “F” for pre-fabrication; “T” for heat treatment; in heat treatment tempering, “T3” indicates cooling after hot melting; “T4” represents natural cooling to a stable state after hot melting; “T9” means cooling by artificial means after hot melting followed by cold working. “T73” is a special heat treatment method developed by the Aluminum Company of America (ALCOA).
Strength Characteristics of Aluminum Fasteners
The strength characteristics of the aluminum alloy materials 2024-T4, 6061-T6 and 7075-T73 for external thread fasteners are specifically discussed on page B-158 of ASTM F468; the strength characteristics of the aluminum alloy materials 2024-T4, 6061-T6 and 6062-T9 for nuts are specifically discussed on page B-184 of ASTM F467.
Here it is necessary to explain two differences in mechanical properties between aluminum alloy threaded fasteners and fasteners made of other metal materials.
The first point is: when calculating the load capacity of parts, the area of the tooth bottom part of the cross-section should be measured instead of the larger tensile stress area. Only the tensile and yield strength values given in Table 2 of ASTM F468 for mechanical test samples are the true strength values.
When making strength calculations for fasteners of the entire size, appropriate adjustments can be made. Thus, when multiplying the stress value by the area of the thread force-bearing region to calculate the load capacity in pounds, the calculation result is approximately the product of the “true value” in the table and the smaller area of the tooth bottom.
The second point is that the hardness difference of aluminium alloy fasteners is very small and, like the inspection criteria, is of little significance. As a substitute for hardness testing, shear strength testing is usually introduced.
If you are looking for quality fasteners, please visit us here https://hktl-fastener.com/.
