Best Adhesives for Bonding Aluminium: A Manufacturer’s Guide

A standout metal in modern engineering

Aluminium is one of the most popular materials in modern engineering, second only to steel. Its unique combination of strength, low weight and resistance to corrosion makes it ideal for use in a diverse array of applications across many industries. Aluminium is popular for several key reasons:

1. Aluminium has an excellent strength-to-weight ratio. Weighing in at around a third of the weight of steel, this lightweight metal is ideal for use in applications where strength is required without adding excessive mass. Despite its low weight, aluminium is very strong and able to withstand challenging environments.

2. Aluminium is naturally corrosion-resistant. Because it forms a protective oxide layer upon exposure to air (which can be a challenge for bonding – more on this shortly!), it directly resists corrosion and rusting. This makes it well suited to outdoor applications where the metal comes into contact with air, water or harsh weather conditions.

3. Aluminium has high electric and thermal conductivity. It conducts heat and electricity efficiently, making it ideally suited for use in heat exchangers, electronics and cooling systems, among others.

4. Aluminium can be endlessly recycled. With concerns growing around sustainability in engineering, the fact that this lightweight, strong metal can be recycled indefinitely without any loss of strength or quality is a key standout feature of this material.

Other benefits of aluminium include its easy machinability and its affordability when compared to other metals like stainless steel or titanium.

Grades of aluminium

Aluminium can be sourced pure, but is often used in the form of an alloy, i.e. mixed with another metal. The various grades (or series) of aluminium and their key benefits are summarized below.

1000 Series (99% pure)

This is aluminium in its purest form, unbound with any other metal. It offers all the benefits listed above and is particularly resistant to corrosion.

2000 Series (Copper)

Offers very high strength and machinability. These alloys can be heat treated to give them outstanding toughness and hardness. Often used for structural component bonding in the aerospace industry.

3000 Series (Manganese)

This series is not heat treatable and offers high flexibility and workability in exchange for moderate strength. Used in everyday aluminium applications such as cooking utensils.

4000 Series (Silicon)

The combination with silicon lowers the melting point of this alloy. Workable and with good electrical and thermal conductivity, this series is popular in the automotive industry.

5000 Series (Magnesium)

These alloys provide outstanding corrosion resistance, making them particularly suitable for marine applications as they withstand seawater exposure and do not rust.

6000 Series (Magnesium & Silicon)

These provide good corrosion resistance and high levels of strength, and are often used in the construction industry.

7000 Series (Zinc)

The strongest of all aluminium alloys, even stronger than many types of steel. Due to their high strength and excellent strength-to-weight ratio, they are often used in aerospace and high-performance vehicles.

Challenges when bonding aluminium

Bonding aluminium can be straightforward, but you have to know how to overcome its inherent challenges first! The main issue with this metal is its tendency to build up a layer of aluminium oxide on its surface upon exposure to the air around it. This layer is often weak and invisible, such that you may not even notice it when you come to bond your aluminium.

You may be wondering, “so what?” An oxide layer is harmless and doesn’t damage the metal in any way, so why can’t you just leave it there and bond as is? The problem when you bond aluminium with an oxide layer on it is that you’ll bond the oxide layer – not the aluminium surface! Since the oxide layer itself isn’t strongly attached to the aluminium, it can come off later, especially when exposed to bond stress – and it’ll take the adhesive along with it. Since you can’t see the oxide layer with the naked eye, it’ll simply look like the adhesive failed to bond to the surface of the aluminium.

Other issues faced when bonding aluminium mainly occur when joining it with other metals such as stainless steel. Factors to consider here include aluminium’s much higher rate of thermal expansion (which can cause bond stress and fracture) and the risk of galvanic corrosion under certain conditions. We’ll cover these issues in more detail later.

Surface preparation – the key to successful aluminium bonding

Proper surface preparation of aluminium is arguably the most important step in achieving a strong bond on this substrate. Surface preparation not only removes the aforementioned oxide layer, but also any oils and other contaminants that can hinder a strong adhesive bond. Follow the steps laid out below to prepare your aluminium for bonding:

1. 1. Remove contaminants.

    

   Use a solvent to clean the surface of the aluminium and remove grease, dust, release agents, fingerprints etc. Isopropanol, acetone or Permabond Cleaner A are all suitable for this purpose. Note: this step alone is NOT enough to remove the oxide layer.

1. 2. Remove the oxide layer.

    

   Now that your surface is clean, it’s time to remove the thin oxide layer on your aluminium. The best way to do this is via light abrasion or grit blasting. Abrasive paper or pads are suitable for this purpose. You don’t need to go in really hard on the aluminium – a light touch goes a long way here. Some people use angle grinders to rough up the surface of the aluminium, which we don’t recommend. While its scratches and grooves can create extra surface area for bonding, the heat generated by this process causes the aluminium to begin re-oxidizing straight away, and can cause particles and debris to embed themselves further into the surface.

    

   3. Use a surface primer (optional).

    

   After removing the oxide layer, you can now use a surface primer on your aluminium. Products such as Permabond 2K Primer, applied after abrasion, help to prevent re-oxidization on the aluminium surface. It also acts as an adhesion promotor and improves the long-term durability of the bond.

1. 4. Clean again.

    

   After abrading or grit blasting, clean the surface again with solvent to remove any debris.

1. 5. Bond.

    

   Your aluminium is now ready for bonding. Bear in mind that a new oxide layer will start to form on the surface of the aluminium after this process, so the sooner you can bond, the better your results will be.

The above process will give you the best results consistently when bonding aluminium. If you’re looking for the strongest, most durable bonds over time, this is what we recommend you do. However, if your application is less demanding or doesn’t need to meet ultimate strength requirements, it may be enough to simply clean the surface with a solvent before bonding. Or, as you’ll find out shortly, you can use a very special Permabond adhesive that is able to bond through oxide layers!

A note on acid etching

There’s another way that aluminium surfaces can be prepared for bonding that was very popular in the past, especially in the aerospace industry – acid etching. This process involves subjecting the surface of the aluminium to an acid (the ‘etchant’) to change the nature of the surface and render it suitable for bonding with adhesives.

This method removes the oxide layer and allows for a very strong bond with adhesives. However, chemical etching is far less popular today because of the nature of the acids used in this process – many have recently been found to be harmful to health and either banned or heavily regulated. Working with acids also means workers need to exercise extra care and wear protective clothing to prevent exposure. And with adhesive technology continuing to improve year on year, many manufacturers no longer see the need to take the potential health & safety or legislative risks involved in using acid etching to bond aluminium.

The best adhesives for bonding aluminium

Many adhesive chemistries are suitable for bonding aluminium, which we’ll discuss below. Ultimately, the best one to choose depends on the specifics of your application, such as the environment it’ll be in, whether you’re bonding aluminium to itself or to a different material, how quickly you need the adhesive to cure, etc.

Epoxies

Epoxy adhesives – both one-part and two-part – are an excellent choice for bonding this substrate. They offer the highest structural strength of all adhesives on aluminium, so if bond strength is your top priority, an epoxy is probably where you should look first. They also tend to offer good chemical resistance, and many are suitable for applications needing to withstand high temperatures.

For example, Permabond ET5422 and ET5424 are two-part epoxies that bond very well to aluminium. ET5422 offers one of the highest peel strengths on the market at 320 N/25mm (70 PIW), while ET5424 offers outstanding heat resistance. Both these adhesives are very well suited to aerospace applications, for example bonding aircraft honeycomb sandwich panels which make heavy use of aluminium.

Permabond ET5428 and ET5429 are two-part epoxies that offer high bond strength on aluminium, as well as excellent impact and vibration resistance. These products are also great for bonding dissimilar materials, for example aluminium to composites.

Structural acrylic adhesives 

Remember that special adhesive we mentioned that can bond through oxide layers on aluminium? It’s a structural acrylic adhesive, and one of the best adhesives in our range for bonding aluminium, as well as other materials.

Permabond TA4208 Black was developed for effective bonding of a variety of substrates and performs excellently on metals, plastics and composite materials. Its surface-activated technology gives it the ability to bond through oxide layers and also through oily surfaces. This means you could bond aluminium to another metal with little more than an acetone wipe beforehand, and still get optimum strength results.

The table below compares shear strength values on various types of aluminium using TA4208 Black:

As you can see, the strength values are still very high even on unprepared aluminium. During testing, we found that the addition of surface adhesion technology gives a 4-fold increase in lap shear strength on unprepared aluminium.

Permabond TA4246 is another structural acrylic that bonds well to aluminium. It’s a 2-part, no mix adhesive that cures at room temperature. While it doesn’t have the surface adhesion technology of TA4208, it offers very high peel strength and impact resistance and is ideal for replacing welding or fasteners.

Structural acrylics can not only offer the benefit of less surface prep, but also offer great impact and vibration resistance, a fast cure and are easy to optimize into a production process.

Cyanoacrylates

Cyanoacrylates, also known as superglue or instant adhesives, bond aluminium well. These adhesives are ideal if you need an instant cure – they reach fixture strength in a matter of seconds. They’re less ideal if you’re bonding large parts, since the adhesive may already start to cure before you can bring the parts together accurately. If you’re bonding small, close-fitting parts, such as in electronics, they can be an ideal choice.

If you’re interested in using a cyanoacrylate to bond aluminium, consider Permabond 910. This adhesive forms strong bonds with many metals, including aluminium and steel.

Other adhesive chemistries 

Anaerobic adhesives and sealants can be used to bond aluminium, but they may require the use of an activator first if a faster cure is required. Since aluminium is a passive metal, without an activator like Permabond A905, the curing process will take much longer compared to active metals like copper and brass. More reactive anaerobic adhesive formulations are available for use on aluminium.

UV adhesives also work on aluminium, as long as the UV light can reach the entire bond site. Since aluminium itself blocks UV light, the other substrate to be bonded would need to be clear, for example glass or a transparent plastic.

Bonding aluminium to other materials

Aluminium can be bonded to a range of other materials, from other metals, to plastics, glass and composite materials. However, there are two main issues to consider when doing this – thermal expansion and galvanic corrosion.

A material’s thermal expansion, signified by its coefficient of thermal expansion or CTE, tells us to what extent a material expands with heat and contracts with cold. A higher CTE means greater expansion and contraction, with a lower CTE signalling the opposite. This matters for adhesives because, should an application be subjected to high or low temperatures, or to thermal cycling, the bonded substrates will expand and contract. If the rate at which the substrates expand and contract is very different, this can cause stress at the bond site.

For example, aluminium has a CTE of 23, whereas mild or carbon steel’s CTE sits at around 11 – less than half that of aluminium. Therefore, in an aluminium-steel bond the aluminium would expand much more than the steel would if subjected to high heat, and subsequently contract much more when cooled down.

To counteract any negative effects this differential expansion and contraction may have on the bond area, it’s important to choose a toughened adhesive that can flex with the two substrates’ differing changes of state with temperature. Permabond ET5422, mentioned above, is ideal for this. Its extremely high peel strength and great temperature resistance give it the ability to hold its own under stress loads. Building in a slightly thicker glue line can also help absorb differences in expansion coefficients and reduce stress on components.

If you’re bonding dissimilar substrates, rigorous surface preparation is even more important to ensure the best, long-lasting bond.

Preventing galvanic corrosion

Galvanic corrosion is an electrochemical process where a metal begins to corrode in the presence of a dissimilar metal (and certain composites like carbon fiber). It happens as a result of small electric currents flowing between the two metals in the presence of an electrolyte such as saltwater or rainwater. If a less noble metal like aluminium comes into contact with a more noble one such as steel, it can begin to corrode in the presence of said electrolyte.

Note that galvanic corrosion can only happen in the presence of moisture. If your dissimilar metal bond area will be kept completely dry at all times, you don’t need to worry about this issue. However, if there’s even a slight risk of moisture ingress at the bond site, it’s worth taking steps to ensure that galvanic corrosion can’t take place. Simply choosing an adhesive to join your metals instead of mechanical fasteners is already a good choice!

This is because an adhesive, applied in the right area (i.e. where moisture would be more likely to ingress) will in itself provide a great deal of protection against galvanic corrosion. Make sure to apply it evenly over the entire bond area and ensure a good, tight seal. In terms of joint design, it’s also important to try and avoid creating an electrically conductive path between the two metals to be bonded.

Any exposed aluminium surfaces that could galvanically corrode can be protected through anodizing, paint baking or powder coating.

Frequently asked questions about bonding aluminium

• Is using adhesives better than welding for aluminium?

Both are possible, but adhesives present a lower-cost, lower-skill, more workplace friendly option that causes minimal disruption to the aluminium. Adhesives require no holes, drilling, bond dissimilar materials and distribute stress evenly across the joint. They also provide a cleaner, more aesthetically pleasing finish.

• What is the strongest adhesive for aluminium?

Epoxies tend to provide the strongest bonds on aluminium, especially structural or toughened epoxies like Permabond ET5428 and ET5422.

• Do you need to sand aluminium before bonding?

Ideally, yes. Sanding or abrading removes the thin oxide layer present on the surface of the aluminium, better priming it for bonding. It is recommended to use carborundum-based grit paper rather than sand paper.

• Should aluminium be primed before applying adhesive?

Priming is an extra step you can take to prepare aluminium for bonding after sanding/abrading. It’s not strictly necessary, but can be helpful in helping to prevent new oxidation occurring after you’ve abraded the metal. Permabond 2K Primer is ideal for this.

• Can cyanoacrylate bond aluminium?

Yes, cyanoacrylates bond aluminium very well. They’re ideal for bonding small, close-fitting parts that can be brought together instantly after applying the adhesive. Consider Permabond 910 for an aluminium bonding cyanoacrylate.

• What adhesive works best for aluminium outdoors?

Epoxy and structural acrylic adhesives work well for outdoor-based aluminium applications. Both of these offer good chemical and environmental resistance.

• Can aluminium be bonded to steel?

Aluminium absolutely can be bonded to steel. However, important to remember that aluminium’s rate of thermal expansion is twice that of steel. This isn’t a major issue if you’re only bonding small components, but on meter-long panels, for example, you would notice this. Therefore, it’s important to choose an adhesive that offers a degree of flexibility if your application will be exposed to high temperatures or thermal cycling. A toughened epoxy or structural acrylic will work well here.

If you have any questions about bonding aluminium or you’d like to discuss your project with us, please get in touch.