Fasteners for aluminium
Unlock the secrets of mechanical fasteners for aluminium sheets and profiles. Discover the benefits and applications of screw connections, folded joints, and riveted joints.
Mechanical fasteners for aluminium
To connect pieces to each other is important and often a critical operation in the manufacture of articles and constructions made of aluminium sheets or profiles. Mechanical methods provide high bond strength, and it is convenient to manufacture and quality control. Moreover, mechanical methods of joining aluminium components do not require expensive equipment, easy to use and can be easily automated [1].
The main types of mechanical fasteners for aluminium parts are:
- screw connections;
- folded joints;
- riveted joints.
Aluminium bolts, nuts and washers
Screw connections
The types of screw joints
Screw fasteners are releasable connection. By their design, they can be (Figure 1):
- cross-cutting, protruding from both sides;
- through, protruding from one side;
- not end-to-end (blind), protruding from one side.
Figure 1 – Structural types of screw connections [1]
If appropriate measures are taken against corrosion, then screw connections are quite suitable for sheet aluminium components and aluminum profiles:
- Fasteners must be made of corrosion-resistant stainless steel (steel group A2 / A4).
- Since aluminum alloys have relatively low compressive strength, contact surface to be protected by applying a washer as a screw head, and a nut.
The types of screw connections for thin sheets
In addition to the methods type compound “screw-nut” thin sheets can be joined together using a variety of different threaded fasteners (Figure 2).
Figure 2 -Various types of screw connections for thin sheets [2]
With the type of fasteners “screw-nut” provides greater bonding force. On the other hand, special screws used for thin-walled parts, to eliminate the drilling operation for final assembly, as these screws themselves cut a hole for yourself.
The operating principle of self-tapping screws
The disadvantage of the majority of thin-walled compounds is very limited length of the screw, which carries the load. Can improve the formation of cylindrical collar around the opening, that occurs when installing tapping screw. Carbide tip tapping screw, has no thread, It acts as a conical piercer, which rotates at high speed and hit a metal through. As a result of plastic deformation of the sheet metal around the opening is formed massive collar. Then a self-tapping screw cuts a thread in this hole and makes a screw connection (figure 3).
Figure 3 – Self-tapping screw installation principle [1]
Screw connection for aluminum profiles
For connection parts of the aluminum alloys most commonly used screws of corrosion-resistant stainless steel with a head, which is adapted for fastening sheets. Aluminum profiles for connection to each other or to sheet materials, they can have longitudinal and transverse screw channels (or grooves) (Figure 4).
Figure 4 – Screw connections for aluminum profiles [1]
Folding
The principle of folding aluminum sheets shown in Figure 4.
Figure 5 – Stages of the rebate process
for sheets with straight edge [1]
Depending on the purpose can be performed a variety of forms of the compounds rebated. The width of the rebate is of particular importance for each specific case. Too narrow seam has a low strength and low leak, and too wide a fold leads to excessive consumption of sheet material (figure 6).
Figure 6 – Overlap width in folded joints:
wrong (top) and correct (bottom) [1]
Riveted joints
Indirect and direct riveting
In recent decades rivets was rediscovered as an effective technology, especially in aerospace engineering. For aluminum, they are used by indirect and direct riveting methods (Figure 7). In the process of indirect parts are joined by riveting attraction to each other via an additional coupling member – separate rivets. In direct riveting one of the parts is itself a rivet member and an additional separate rivet is required.
Figure 7 – Types of riveting: indirect and straight [1]
Types of aluminium rivets
Currently, four main types of blind rivets are used to make one-piece (permanent) riveted joints (Figure 8 and 9):
- solid rivets;
- blind rivets;
- screw (threaded) rivets;
- self-piercing rivets.
Figure 9 – Self-piercing rivets [2]
Application of different rivet heads
Rivets are distinguished by the type of head, which is formed when installing rivets. For sheet metal and light structures, which do not require thicker rivets, than 8 mm, commonly used fasteners such as the head and head on the original rivets. Aluminium rivet diameter up to 8 mm are easily amenable to cold plastic deformation (Figure 10).
Figure 10 – Various types of rivet heads and their applications [1]
The principle of operation of the blind rivet
Typically a blind rivet consists of a hollow shank and an extrusion core, which serves as a tool for forming the fastener head.
The rivet is set by pulling the mandrel with a special tool, “Riveters”. As a result, a fastening head is formed from the rivet shank (Figure 11). When the pulling force exceeds a certain level, Core is cut off at a predetermined location. the gap can place inside the sleeve or rivet head.
Figure 11 – Blind rivet installation principle [1]
Prevention of galvanic corrosion
Materials for mechanical fasteners for aluminium pieces must be compatible in terms of corrosion. It means, the details, that are in contact with each other must have close electrochemical potentials to prevent galvanic corrosion. for instance, fastening elements made of copper or brass is not suitable for joining aluminium parts.
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