Aluminum fasteners: bolts, nuts and washers

Aluminum as a material for fasteners

The use of aluminum alloys for the manufacture of fasteners - bolts, nuts and washers - provided by the special properties of aluminum compared to other construction materials:

  • Resistance to atmospheric corrosion; does not form rusty stains. In contact with atmospheric oxygen, aluminum instantly forms a protective oxide film on any fresh surface.. Since aluminum oxides are colorless, the, unlike carbon steel, aluminum oxidation products do not form rust spots and do not spoil the appearance of building structures.
  • Weight / strength. Aluminum alloy fasteners have the highest strength to weight ratio compared to any other metals, which are used for the manufacture of fasteners. The strength of aluminum fasteners is comparable to the strength of fasteners made of low carbon steels, and weigh aluminum bolts, nuts and washers are three times lighter than carbon or stainless steel counterparts.
  • electrical conductivity. The high electrical conductivity of aluminum makes it suitable for electrical connections.. Depending on the aluminum alloy used, aluminum fasteners have an electrical conductivity in the range from 38 to 43 % from the "copper standard" – annealed copper.
  • Non-magnetic. All fasteners made of aluminum alloys are non-magnetic and do not form sparks..
  • Compatibility / Contact (galvanic) corrosion. The use of aluminum fasteners in aluminum joints virtually eliminates the risk of galvanic corrosion.
  • Color Identification / Corrosion Resistance. The natural resistance of aluminum to corrosion can be increased by anodizing fasteners for those cases, when they are used in particularly corrosive conditions. During anodizing, aluminum bolts, nuts and washers can be painted in various colors. This can be convenient for quick identification of fasteners during assembly of structures., as well as serve decorative purposes.

Selection of aluminum alloys for fasteners

The choice of aluminum alloys for fasteners with external and internal threads - bolts and nuts - is quite limited. If an improper aluminum alloy is used for the manufacture of fasteners or subjected to improper heat treatment, this can result in serious problems with corrosion resistance and fastener strength.

In the United States, bolts are most often made from aluminum alloy. 2024 T4 condition. Such bolts have a level of strength., viscosity and corrosion resistance, which are suitable in most applications in structures and products. 2024-T4 alloy bolts are often anodized, to increase their natural resistance to atmospheric corrosion [1].

If the structural joint requires a slightly higher strength, then bolts from alloys 7075-T73 can be used.

If possible, reduce bolt strength, to obtain higher resistance to general corrosion, then apply bolts from 6061-T6 alloy.

To use the full strength of aluminum bolts, and also prevent stress corrosion cracking, aluminum nuts must be from aluminum alloys 6061-T6 or 6062-T9. Nuts can be manufactured by cold forging or by turning from a cold drawn rod.

For the manufacture of flat aluminum washers, an aluminum-clad sheet of alloy 2024-T4 is most often used [1].

Aluminum fasteners in aluminum structures

SP 128.13330.2012 (SNiP 2.03.06-85)

JV Building Code of Practice 128.13330.2012 (updated SNiP 2.03.06-85) "Aluminum structures" provides for the use of carbon steel bolts for various categories of bolted connections, stainless steels and aluminum alloys.

This document prescribes the use of aluminum alloy bolts (in the appropriate states):

  • AMg5p and
  • ABT1.

It is allowed to use the following aluminum alloys for bolts (in appropriate conditions):

  • AD33T1,
  • D18T,
  • V65T i
  • B94T1.

As for aluminum alloys for aluminum nuts and washers, there are no indications.

Eurocodes 9

The European standard EN 1999-1-1, which is included in Eurocode 9 "Aluminum structures", provides for the use of bolts from the following aluminum alloys in bolted joints:

  • 5754,
  • 5019 and
  • 6082

Aluminum bolts only apply to category A bolts. Category A includes connections without monitoring the level of bolt tightening.. High-strength carbon steel bolts are used for connections with bolt tightening control.

Aluminum alloys for bolts, nuts and washers to EN 28839

The European standard EN 28839 includes six aluminum alloys, which are used for the manufacture of bolts, screws, studs and nuts:

  • AlMg3
  • AlMg5
  • AlSi1MgMn
  • AlCu4MgSi
  • AlZnMgCu0,5
  • AlZn5,5MgCu

US Aluminum Bolts and Nuts

ASTM F standard 468

Technical requirements for bolts from aluminum alloys from M6 to M36 are set out in the American standard ASTM F 468. The standard provides for the manufacture of bolts from the following aluminum alloys, delivered in the following conditions:

  • 2024-T4
  • 6061-T6
  • 7075-T73.

ASTM F standard 467

Technical requirements for nuts from aluminum alloys from M6 to M36 are set out in the American standard ASTM F 467. The standard provides for the manufacture of bolts from the following aluminum alloys, delivered in the following conditions:

  • 2024-T4
  • 6061-T6
  • 6262-T9.

Specially noted, that aluminum nuts must have a yield strength equal to or greater, than the minimum tensile strength of the bolt (or stud), on which they are installed.

Aluminium Construction Manual

The American Aluminum Construction Manual clarifies the use of aluminum alloys for the manufacture of washers:

  • small nuts - diameter 6 mm and less - from aluminum alloy 2024 in the T4 state (hardening and natural aging);
  • larger nuts – from alloys 6061-T6 and 6262-T9;
  • for the manufacture of flat washers, sheets of 2024-T4 aluminum alloy with technical aluminum cladding are used;
  • spring washers are made of aluminum alloy 7075-T6.

All aluminum fasteners - bolts, nuts and washers - to increase corrosion resistance it can have an anode coating up to a thickness of 5 micrometers.

Sources:

  1. Handbook of bolts and bolted joints / J. H. Bickford, 1998.
  2. materials firm BÖLLHOFF.
  3. Aluminium Construction Manual, AAA, 1967