Replacing mechanical fasteners and welds with metal adhesives
Where fasteners and welded bonds used to be of paramount importance, metal adhesives are now used more and more often. Glues and tapes bring along many benefits and cancel the disadvantages of mechanical bonds and seals. However, as there are different types of metals, also many bonding solutions are available including structural and machinery adhesives.
Bonding metal with glues and tapes is highly possible, even recommended, but the choice of the right metal adhesive may be a challenge due to the wide variety of different kinds of products which each have unique properties. Here you find the benefits of metal adhesives, the different types of glues and the suitable solutions for various types of metals.
Why metal glue
Metal adhesives create chemical bonds which, in many cases, are preferred to mechanical fasteners, soldering and welds. Not only are metal glues often the more cost efficient option, but they also reduce the steps in assembly lines and so speed up the process as a whole. The main reason for reducing the assembly steps is that adhesives not only bond but also seal. Thus the adhesive and sealant can be applied in one go. The bond (or seal) can also be flexible allowing for relative movement of the parts without affecting the bond. That is why metal adhesives are a go-to solution for products that are subject to vibration, temperature fluctuation and weather, such as trucks and other vehicles.
Metal adhesive vs. fasteners
Fasteners such as bolts and screws are a widely used method of bonding for temporary joints. Fasteners are also used for semi temporary joints such as those in automotive assemblies. Nevertheless, problems may occur as fasteners require carefully placed holes, are prone to corrosion and may loosen over time.
This is where metal adhesives come into picture: removable glues and tapes can be used for temporary bonding, and no trace of them remains after removal. Adhesives are also suitable for semi temporary joints as they seal from corrosion, are generally lighter and cheaper than mechanical fasteners, and distribute the stress throughout the bond making the assembly stronger. Metal glues can also be used for supporting fasteners: threadlocking and thread sealing are common applications where a glue makes sure the fastener does not loosen over time.
Gluing vs. welding
Welding is a common method of creating metal to metal bonds in assembly operations. In a way, welds resemble bonds created by adhesives: both bonds are uniform and the stress is distributed equally. They also do not require holes unlike fasteners. However, when welds are used the labor costs are likely to be higher as the application requires special training and certificates. The welds must also be inspected afterwards, bringing along more costs. With adhesives this is not the case as most applications require only basic training and inspection is not necessary.
In metal to metal bonding adhesives protect from corrosion whereas welds might even encourage it. Also the rapid heating a cooling required by welding is eliminated. The drastic temperature fluctuations can cause the metal to weaken or discolor.
Different types of metal adhesives
Generally, metal adhesives can be divided into two groups: machinery adhesives and structural adhesives. The latter refer to adhesives that are designed to bear structural loads. These include epoxies and acrylic metal adhesives. The other group is called machinery adhesives which include those used for threadlocking and thread sealing as well as some special application which are not required to carry structural loads.
The following 4 types of adhesives are the most common in bonding metal to metal and metal to other materials.
- Epoxy is one of the strongest adhesives for metal. It exists in different types which have unique properties such as chemical resistance and heat resistance. Epoxies come as one or two component systems.Single component epoxy for metal cures when heat is applied. A 2K system is a mixture of two parts with reach with each other starting the curing process.
- Acrylic metal glues exist as two types: surface activated and and bead on bead acrylics. The latter refers to applying a bead of adhesive on both substrates before connecting them. The bonding starts when the parts are pressed together. Surface activated acrylics, in turn, require a water thin initiator applied on one substrate and the resin to the other.
- Cyanoacrylate, also known as instant adhesive, is the super glue metal to metal bonding needs. Cyanoacrylate adhesives is suitable for most metals as long as they are reactive. Therefore it works better on brass and copper than on steel.
- Anaerobic metal adhesives are only used for metal to metal bonding as they require presence of metal and absence of oxygen to be able to cure. Anaerobic adhesives are ideal for securing fasteners in terms of threadlocking and thread sealing as well as gasketing and retaining.
Metal bonding can additionally, be realized by using MS (modified silane) polymers which are common when bonding metal sheets in vehicles like trains and buses. When invisible bonds are required, UV curable transparent systems should be considered. In case you are curious for more metal bonding solutions, contact us and let our experts guide you through the possibilities.
Bonding solutions for different types of metals
Not only are there many types of metal glues, but the range of different types of metals is also broad. Therefore, before choosing an adhesive system, you must know the metal that requires bonding: its properties have a great impact on the choice. Here you will find some common types of metal and suitable systems for different metal bonding applications.
When working with aluminum, it should be kept in mind that the metal is prone to corrosion and often has a thin oxide layer on it. The oxide layer forms a good surface for adhesives to adhere, however, the layer does not have a strong bond with the aluminium. Thus, to create strong bonds between aluminum and other materials, the adhesive should not be applied on the oxide layer, the layer should rather be removed. The best method to remove aluminum oxide is using solvent wipes, such as acetone or isopropanol, and abrading or grit blasting the surface.
Once the surface has been abraded or grit blasted, and cleaned appropriately, the adhesive can be applied. Let’s look at the types of adhesives:
ANAEROBIC ADHESIVES – Even though aluminum is a passive metal, some anaerobic systems can be used for close-fitting concentric parts of threaded components. For softer aluminum alloys, anaerobic glues can be used for gasketing and flange sealing.
CYANOACRYLATE GLUES – All grades of cyanoacrylate adhesive, instant glue, bond aluminum extremely well. Special formulations of super glue can be considered when bonding aluminum to dissimilar surfaces with different coefficients of thermal expansion.
EPOXY SYSTEMS – One component epoxies are ideal for aluminum bonding. They are easy to apply and cure fast when heated. Also 2K epoxy glues are suitable for aluminum. They generally form the strongest bonds.
ACRYLICS – Structural acrylic adhesives are very suitable for aluminum. Both types can be applied: surface activated and bead to bead acrylics.
As many types of adhesives meet the needs of aluminum, the choice should be based on the adhesives suitability to your process, desired curing method and the required resistance.
Which adhesives bond brass?
Brass is an alloy which consists of zinc and copper. Brass is non-magnetic and resists corrosion: therefore, it is commonly used in pipe joints, hydraulic fittings, heat exchangers, musical instruments and door locks and knobs.
For bonding brass, the options are open as the metal is rather easy to bond and many glues adhere well to it. The options include:
ANAEROBIC ADHESIVES – Brass is among the most reactive metals which causes anaerobic adhesives to cure much faster than when applied to, for instance, steel. On brass, anaerobic glues are used for threadlocking, thread sealing, form in place gaskets, retaining and weld sealing. These adhesives exist as permanent and removable systems depending on the application. The applications also include pipe connections for oxygen and potable water pipelines.
CYANOACRYLATE SYSTEMS – When bonding pads to musical instruments cyanoacrylate super glue is a preferred option. The strongest bonds are created with methyl cyanoacrylates. However, when the bond needs resistance to impact or polar solvents, structural adhesives should be considered.
STRUCTURAL ADHESIVES – Both epoxy and acrylic adhesives are highly suitable for bonding brass. Similarly to aluminum, all types can be used: heat curing 1K epoxy, reactive 2K epoxy, surface activated acrylic adhesive and bead to bead acrylic glue all do the job depending on the application.
Due to the wide variety of possible metal adhesives for brass, the choice should be based on the needs of the application, the adhesive´s compatibility with existing processes and the environment´s requirements.
Metal adhesives for copper
Copper and its alloys are readily bonded with a wide variety of different metal adhesives. For copper, adhesives are often the preferred option as they do not cause heat damage on the metal unlike soldering.
The most commonly used metal glues for copper include:
ANAEROBIC GLUES – As anaerobic adhesives require the presence of a metal and absence of oxygen to be able to cure fast, copper offers a perfect substrate due to the fact that it is one of the most reactive metals. That is why anaerobic systems cure much faster on copper than many other metals as steel. The uses include threadlocking, thread sealing, weld sealing, gasketing and munitions sealing.
CYANOACRYLATE ADHESIVES – Instant adhesive offers the fastest curing time and results in increased efficiency. It is also highly compatible with copper. Nonetheless, when polar solvent resistance and impact resistance are required, an epoxy or acrylic adhesive should be considered.
EPOXY METAL GLUES – Both one part, heat curing epoxies and 2K reactive epoxy glues can be used to bond copper. Epoxy glue for metal is ideal for forming strong metal to metal and metal to other material bonds.
ACRYLIC SOLUTIONS – When it comes to acrylics, copper can be bonded using either surface activated or bead to bead systems. Both result in strong structural bonds.
The strongest metal adhesives for mild steel
Mild steel, sometimes called carbon steel, is a common material used throughout industries such as automotive and machine building. The key to successful steel bonding lies in proper surface preparation. The surface should be appropriately cleaned und abraded where needed.
Note that mild steel is often powder coated or otherwise painted prior to adhesive application. Then the adhesive forms a bond with the coating layer, not the steel itself.
ANAEROBIC SYSTEMS – Threadlockers, thread sealants, gaskets and retaining compounds all work well on mild steel. These are realized using anaerobic metal adhesives.
CYANOACRYLATE GLUES – When strong bonds must be created fast cyanoacrylate systems should be considered. When polar solvent and impact resistance are required, structural adhesives may offer a more suitable solution.
STRUCTURAL SOLUTIONS – Structural adhesives such as epoxy and acrylics are known for their heat and chemical resistance. Generally, epoxy is the tougher solution, yet it always depends on the application and the environment.
Several adhesive systems are suitable for bonding mild steel: the right one is often recognized by the correct chemical, weather and temperature resistance.
Glues for stainless steel
Stainless steel is generally an easy metal to bond, however the mirror-finish stainless steel may be a tricky metal to apply adhesives to as it usually has a very low texture surface which prevents adhesion. Stainless steel without mirror-finish requires little to no surface preparation and can be bonded with the following systems:
ANAEROBIC SYSTEMS – threadlockers, thread sealants, gaskets and retaining compounds all work well on stainless steel. However, the curing time of anaerobic adhesives is rather long due to the fact that stainless steel is a passive metal.
CYANOACRYLATE ADHESIVES – All grades bond stainless steel rather well. The strongest bonds are made when using special products designed for metals like steel.
STRUCTURAL GLUES – Structural adhesives such as epoxy and acrylics provide heat and chemical resistance. Generally, epoxy is the tougher option, yet it always depends on the application and the environment.
If you are working with mirror-finish stainless steel, before applying any adhesives, you should create more texture on the surface. The surface must be roughened, but methods like sandpaper, steel wool or scotch brite are likely to fail due to the hard surface of the material. On the other hand, automated roughening methods may generate heat, which can alter the metal´s surface properties and prevent adhesion. The best way to abrade mirror-finish stainless steel is sandblasting: it is efficient enough and involves no heat.
Metal glues for magnets
Strong, even permanent magnets are used in many assemblies, the most important ones being electric motors and speakers. Magnet types differ from each other by strength, cost as well as temperature and corrosion resistance. Some types of magnets are neodymium, rare-earth, AlNiCo, samarium cobalt and ferrites. Usually, magnets can be bonded without surface preparation, however, to achieve the strongest bonds, isopropanol can be used for cleaning the surface.
There are many adhesives that bond magnets more than well. Here are some of the most common metal adhesives for magnets:
CYANOACRYLATE ADHESIVES – These instant adhesives create strong bonds at high speed as their curing time is reduced to minimum. When the bond must be resistant to polar solvents, high temperatures or impact, structural adhesives should be considered.
EPOXY SOLUTIONS – One and two component epoxies are both suitable for bonding most magnets. The compatibility of the adhesive and the specific magnet should always be confirmed by the adhesive manufacturer.
ACRYLIC GLUES – Due to the fact that structural surface activated acrylic adhesives have a fast set time, they are often a preferred solution for high speed motor assemblies. External 2K mix systems are the alternative.
Looking for the perfect metal glue for your application?
The range of metal adhesives and types of metals is broad making the choice of an adhesive extremely tricky. Our experts are always willing to share their expertise and guide you towards the ideal solution. Hit the button below and find the perfect metal glue for your application.