Aluminium Fasteners

Aluminium has dominated the aerospace industry for over half a century. Now other industries are turning to the miracle metal.

Ford recently launched a revamp of its iconic F-150 pickup truck with an all aluminium body. In 2008, Apple launched an anodised aluminium casing for its new MacBook laptops, widely regarded as a design classic, while one year later, in the USA, the packaging industry began using more aluminium than the transport industry.

Why is aluminium the miracle metal?

David Harris from ALFED, the aluminium federation representing the industry in the UK, explains some of the metal’s benefits:

“Aluminium is light, strong and easily fabricated into extrusions and other casts. It has good corrosion resistance and can be joined by welding, adhesive bonding and mechanical methods. It can be anodised and painted to give a wide range of attractive finishes that improve appearance and further enhance corrosion resistance. Aluminium is a good conductor of electricity, better than copper by volume and a quarter of the price, and a good conductor of heat. Crucially, at the end of its very long life it is recyclable again and again with no loss of quality.”

Despite that long list of benefits, the money men of the automotive industry favour aluminium for one reason: it is lightweight. Considerable weight savings (Ford claims the new F-150 will be 700 lb, which is about 318 kilos, lighter) lead to improved energy efficiency and a more appealing product.

Pure aluminium is rarely used for industrial purposes. Instead it’s mixed with copper, magnesium, manganese, silicon and zinc to create aluminium alloys.

The extrusion process consists of pushing a pre-heated cylindrical aluminium ingot through a steel die, producing complex sections that slide together simply.

“Within the limit of the size of the extrusion press, the restriction in section design for extrusions is limited only by the imagination of the designer”, adds Harris.

As a base metal, pure aluminium has excellent corrosion resistance but when connected to one of the noble metals, such as gold, silver or copper, aluminium can corrode. Very acidic or very alkaline solutions (outside of the pH range 3.5 - 8.5) should be avoided or corrosion protection measures will be necessary.

“It is hard to give clear advice as it depends on the aluminium used”, suggests Frida Cullin, Project Manager at the Nord-Lock Group, with a background in material science, when asked what bolt grades are recommended for corrosive environments.

“Both steel bolts and stainless steel bolts can be used together with aluminium but we recommend that you check with your supplier what type of aluminium you are using and seek guidance regarding how to avoid galvanic corrosion,” she adds.

To join aluminium parts together there are various suitable solutions including welding, soldering, gluing and nailing, along with bolted connections.

During welding, all alloys suffer a loss of strength, particularly around the weld itself. Heat treatment can restore properties but this process risks causing internal element separation, which means the metal corrodes from the inside out. Alloy 5083 is especially suitable for welding since it offers the highest strength after welding of any other standard alloy.

For bolted connections, it is common to use standardised steel fasteners (for example, steel bolts class 8.8) since they have higher strength and are less expensive.

“Using aluminium bolts can be a good choice when the joint is exposed to temperature variations. Using bolts made of the same material as the clamped parts decreases the risk of having increased stresses in components and decreases the risk of dropped clamp load due to different thermal expansions”, says Cullin.

As aluminium deforms easily and does not support high pressures, consider reducing the load when clamping aluminium parts together with steel fasteners. Alternatively, placing a washer underneath the bolt head can help distribute the load over a larger area, thereby permitting a higher clamp load. Nord-Lock wedge-locking SP washers, with enlarged outer diameter, provide one possible solution.

“It is recommended to use the SP washers in conjunction with flanged fasteners, which are common in lower property classes”, adds Cullin.

If using a threaded hole in aluminium it is important to remember that the minimal threaded engagement length is longer for aluminium than for steel. A threaded insert might be required to increase the thread strength of the hole, particularly when the joint is re-tightened often. Self-pierce riveting is a method for joining two or more aluminium sheets together, or joining aluminium with other materials such as plastic, which can’t be done by welding. The potential for automation makes it an attractive option for industrial use. Advanced research into self-piercing rivets began at SIMLab, the Centre for Research-based Innovation at the Norwegian University of Science and Technology, back in 2000.


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