Adhesive bonding is widely used for joining aluminium and is capable of providing excellent performance for joints in service. There are a variety of adhesives available to complete the joint, which are selected for use based on cost, strength and ductility requirements. In all cases, they need to be used correctly, which will typically involve some surface preparation before applying the adhesive, especially for structural joints.
There are a number of options for aluminium surface preparation, but they will typically include cleaning and degreasing, etching or roughening the surface, and – in the case of aluminium alloys - anodising.
Surface preparation will usually begin with surface cleaning to remove any impurities, although passivation can also be used. Passivation is when a material is coated to make it ‘passive.’ This can, for example, involve using rubbing alcohol or ultrasonic cleaning of the materials. Once cleaned, the surface of the parts to be joined can be roughened with an abrasive, such as sandpaper, to create asperities and increase the surface area for adhesive bonding.
Another way to prepare metallic surfaces is through the electron beam technique, Surfi SculptTM, which can create high aspect ratio protrusions in a matter of seconds that can be used to improve surface bonding.
A chemical treatment may also be required to remove the oxide layer and increase the adherent’s surface energy. A weakly-bonded aluminium oxide coating over the underlying aluminium will dramatically weaken the adhesive joint, so needs to be treated before the adhesive is applied. The oxide layer can be strengthened by anodising the material, which creates a strong hexagonal oxide layer with additional surface areas to improve adhesive joining.
It is advisable to conduct a dry fit before bonding to ensure the components fit together correctly. During bonding, a combination or heat and/or pressure may be required during curing.
Mechanical fastening provides a range of simple and inexpensive options for joining aluminium. Widely used across a range of industries and applications, examples of mechanical fastening techniques include:
1. Bolts, Nuts and Rivets:
This technique usually involves a hole being drilled through the base material before a bolt is inserted and a fastening nut applied, although the bolt can also screw directly into a pre-made fastening thread in the part. Alternatively, a rivet can be inserted and secured. This type of join usually requires the materials to overlap, which can bring some challenges in certain applications. The bolts, nuts and rivets can also be aluminium, but higher stress applications require higher strength materials like steel to be used. Using different materials with different electrochemical potentials for the fasteners can lead to galvanic corrosion, which will weaken the assembly and could lead to joint failure. Thermal fatigue is another potential problem when using different materials to fasten aluminium as heated stresses can enlarge the mounting hole over time.
2. Self-Pierce Riveting and Press Joining:
Both of these mechanical fastening techniques can be automated, which makes them of interest to industries such as the automotive sector. Self-piercing rivets work by being driven into aluminium sheets by a punch and die. The rivet expands in the lower sheet, usually without piercing it, to create a metal interlock. Press joining, also known as clinching, uses a punch to deform the sheets into a die that is designed to interlock the sheets by forming a button on one side. Again, the sheets are not typically pierced in the press joining process. Both of these techniques can be used to join aluminium to other materials, such as steel and some plastics, neither of which can be resistance spot welded to aluminium.
Mechanical fastening processes can be combined with adhesives to overcome potential drawbacks of both techniques, such as problems related to alignment while curing or for sealing joints. The combined use of mechanical fastening and adhesives is currently being used by different industries, including for the manufacture of aluminium alloy-based vehicles.
Brazing and soldering provides a flexible option for aluminium joining, allowing it to be joined to a wide range of materials, including ceramics. While the processes can be manual or automated, manual aluminium brazing can be difficult as there is no observable change in colour before melting. As with other processes, the presence of aluminium oxide can prevent proper bonding, and therefore a flux or vacuum-based process is required. Brazing alloys must melt below 660°C, which is the melting temperature for pure aluminium, although aluminium alloys have a lower melting point, therefore a suitable braze filler metal must be selected. Magnesium, which is present in many aluminium alloys, can ‘poison’ some fluxes, which leads to the formation of high melting points compounds, and can lead to poor joint quality. It is also possible to clad aluminium parts with a braze filler metal for brazing and soldering processes. Brazed joints commonly require overlapping parts, with the amount of overlap often affecting the final joint strength.
Aluminium alloys can be joined through welding, although some aircraft-grade aluminium and alloys cannot be welded using conventional methods. The welding process typically creates a softening in the weld metal and heat affected zone. Post-weld heat treatments are sometimes required to obtain acceptable material properties for specific applications. Common types of welding used for aluminium include arc welding, laser welding, resistance spot welding, and friction stir welding.
1. Arc Welding:
Aluminium can be welded using metal inert gas (MIG) / gas metal arc welding (GMAW) and tungsten inert gas (TIG) / gas tungsten arc welding (GTAW). It is important to use a positive polarity to break up the aluminium oxide layer and ensure a proper weld. Alternating current (AC) settings are used to provide penetration and a containment-free weld.
2. Laser Welding and Laser/Arc Hybrid Welding:
High power density welding processes such as laser welding can produce deeply penetrating welds with a low overall heat input. However, using laser technologies has proven challenging for welding aluminium alloys. This is due to the high reflectivity (low absorption) of aluminium when compared with other metals, like steel. In addition, the relatively high thermal conductivity of aluminium has also caused problems for the initiation and stability of laser welds for aluminium. The introduction of higher powered Neodymium:Yttrium Aluminium Garnet (Nd:YAG) and Ytterbium fibre lasers means that laser welding of aluminium is now not only possible but is a viable option for many applications. Laser welding can be combined with arc processes, offering the benefit of being able to add filler metal during welding and thereby control the weld properties while also increasing welding speeds.
3. Resistance Spot Welding:
This process is used to join thin sheet aluminium by clamping two or more overlapping sheets together between electrodes before passing a high current between the electrodes. Heating at the faying surfaces in the aluminium sheets causes a small nugget of molten material to be formed. Appropriate clamping forces and good electrical contact is required to deliver high quality, repeatable joints. Electrodes typically need frequent replacement as a result of tool wear and metal pick-up.
4. Friction Stir Welding:
Invented at TWI in 1991, friction stir welding (FSW) is a significant process for welding aluminium alloys. It is a solid-state process that uses a non-consumable tool that generates frictional heat as it rotates against the workpiece material. This frictional heat softens the material without melting it. Using mechanical pressure to join the softened materials, rather than melting them as with other welding techniques, FSW avoids the degradation of properties such as fatigue strength, tensile strength and toughness, providing very high weld strengths. Variants, such as friction stir spot welding (FSSW) can also be used with thinner aluminium sheets.
Material properties including corrosion resistance, high electrical and thermal conductivity, and an excellent strength-to-weight ratio mean that aluminium offers a wealth of benefits to different industries.
However, the effective use of aluminium is often dependent on joining processes, whether joining aluminium to other metals or to dissimilar materials like composites. Each of the available techniques, whether adhesive bonding, brazing, mechanical fastening, soldering, or welding has its own advantages and drawbacks.
Choosing the best aluminium joining process for a given application depends upon the required strength, whether the joints need to be permanent or temporary, and the cost of the process.
While each of these processes can be used on their own, combinations of different techniques, such as adhesives with mechanical fastening, or arc welding alongside laser welding can help overcome challenges and increase the benefits of a given aluminium joining method.