No matter what you design, from computers to cars, assembly is always a challenge. What is the lowest cost, highest performance technique for joining parts and subassemblies? When it comes to metal-to-metal bonding, options include thermal joining techniques such as welding, brazing or soldering; mechanical fastening with hardware; and bonding using a liquid adhesive or tape.
Each method comes with its own advantages and disadvantages, though they share many of the same applications. For example, when mounting metal panels to the metal frame of a machine housing, solutions include metal-bonding adhesive tapes, such as 3M VHB Tapes; welding; or mechanical fasteners, such as rivets.
So, how do you decide which method to use for metal assemblies?
Adhering Metal to Metal
Bonding is fast.
Tapes designed for metal-to-metal bonding can be applied as quickly and precisely as a screw or rivet, and do not require a pilot or tapped hole. Structural adhesives can also be applied quickly and easily, such as with a dispensing gun, though they require curing time before the bond reaches full strength. Using faster, simpler assembly techniques can reduce labor and training costs, consequently reducing physical demands on human workers.
Another advantage of using adhesives or tapes in metal-to-metal applications is that they seal, potentially eliminating unnecessary steps in the assembly process. This seal can even be flexible, allowing for relative movement of parts without weakening the bonds. This makes adhesives suitable for applications involving temperature changes, weather and vibration, such as with many automotive and consumer goods.
The number of instances where adhesively bonding metal to metal is the most cost-effective solution may surprise you.
Metal-Bonding Adhesives vs Welding
Welding is a very common technique for metal-to-metal assembly operations, and in some ways using adhesive for metal is very similar. For example, the lack of protruding fastener heads gives both welded parts and glued metal parts a more streamlined appearance.
However, welding also has a number of disadvantages that would not apply to a metal-to-metal adhesive bonds. These include higher labor costs due to necessary training and certifications, inspection cost, damage and potentially flawed joints that are difficult to check by quality assurance (QA) personnel.
The rapid heating and cooling in welding can also cause damage such as warping or discoloration of parts and assemblies.
In contrast, most adhesive products can be correctly applied with minimal training. This translates to more consistent quality and lower overall production cost.
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Metal-Bonding Adhesives vs Fasteners
Fasteners such as machine screws or bolts are widely used for temporary joints. When a strong but removable metal-to-metal bond is needed, some metal-bonding adhesives, like Scotch ATG Adhesive Transfer Tapes, can serve equally as well as traditional fasteners. Keep in mind, though, that removed fasteners leave an empty hole at their former position, while tape does not.
The disadvantages of fasteners mirror their strengths. Since nuts, bolts and screws are often designed to be removable, they can be prone to loosen over time. For long-term or high-stress applications, this may require thread locking products or additional hardware, such as lock nuts.
With adhesives for metal assemblies, all the temperature performance and various chemical resistance data are often published by the adhesive maker, so it’s easier to identify the best adhesive for bonding metal in your application and predict subsequent maintenance costs (if any).
Another problem with fasteners is that they require precisely positioned holes, which can compromise the strength and rigidity of a design. Fasteners also distribute force differently than adhesives or welding, in that they concentrate force at discrete points, rather than disperse force continuously along the entire bond. That stress concentration can weaken the overall strength of the assembly, and if stressed, it can cause the metal to buckle or tear, pulling away from the joint in open areas but held in place at the fasteners.
If aesthetics or aerodynamics of the finished product are important, you’ll also need to consider how those exposed fastener heads will fit into your design. If your design uses fasteners, click here to receive more information about other adhesive options.
Surface Preparation for Metal-to-Metal Adhesive Bonding
Selecting the best adhesive for your metal-to-metal application can be difficult. Different metals subjected to different environmental conditions will consequently behave very differently. As a result, there is no such thing as the strongest adhesive for bonding metal. That’s why it’s important to choose the right adhesive based on a number of design considerations.
Most metals have relatively high surface energy, allowing adhesives to wet the surface and thereby improving bond strength. However, painted and powder-coated metals present a different surface.
On a painted surface, the adhesive is not bonding to the metal but instead to the paint coating. Nevertheless, it’s possible to achieve a successful bond to the metal or its surface coating, depending on the bond strength you need. One option is to mask the mating surfaces before coating, preserving the metal surface. Another is to choose an adhesive solution appropriate for bonding directly to the painted metal.
A “powder coat” is an electrostatically applied resin to a metal surface, heat-cured to form a solid layer. Powder coats are significantly more difficult to bond than paint because they often present a lower-surface-energy substrate. This will affect the adhesive you choose. Adhesive manufacturers such as 3M provide guides for choosing the best adhesive based on your substrate.
Adhesives form the best bond to clean, dry, oil-free surfaces, including oils from workers’ hands touching the surface.
It’s also important to consider the surface condition of your substrates. Rough surfaces can make for stronger mechanical bonds by increasing the surface area of the joint or removing residual contaminants much better than cleaning alone.
However, this benefit is only realized if the adhesive can penetrate the small cracks and fissures and wet the entire surface. A bonding tape, for example, is not able to wet deep inside cracks or around raised scratches of aggressively abraded surfaces. Therefore, a liquid adhesive is best for highly abraded surfaces. Tape is a great solution for lightly abraded surfaces.
Metal Bonding - Which Process?
When selecting a method for metal-to-metal bonding, the most important thing to keep in mind is how your chosen method will fit into your assembly process.
Are you looking for a solution that will fit into your current workflow, or can the workflow change to accommodate the new solution?
For example, welding requires skilled, certified welders, as well as using NDT or destructive weld inspections to your quality process. In some cases, welding can be automated though this may be a costly solution.
Mechanical fasteners call for less training, but these solutions require inventory management as well as tools and equipment, such as screwdrivers, torque wrenches or robotic end effectors. Where torque matters, QA procedures must be implemented and torque sensing tools calibrated.
Using an adhesive for metal-to-metal assemblies can reduce or eliminate many of these troublesome issues, although bonding metal to metal does require surface preparation, which could potentially add a step to your manufacturing process. Still, it’s hard to beat the speed of applying a strip of tape to a part.
In addition to assembly considerations, you should also think about how the final product will be used.
In automotive applications, for example, joints may need to withstand exposure to high and low temperature cycles, extreme weather, ultraviolet (UV) radiation or solvents. Protruding screw heads may present a hazard or require costly trim to hide their presence. Welds can be brittle or undergo changes in local metallurgical composition. This can make them a poor choice for joints that will flex and undergo stress during the lifecycle of the product.
All of these considerations—the assembly process, end use and overall cost—need to be taken into account when selecting an adhesive for metal bonding.
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While the term “glue” may be colloquially used to refer to adhesive products of any type, the adhesive bonding solutions mentioned in this article are created with synthetic materials and are therefore not technically considered “glue.” Glue is made with animal or plant by-products as the base.
3M has sponsored this article. All opinions are mine. –Isaac Maw