Yes, you read that title correctly. Glue is not just for sticking pieces of paper together, or to fix toys. In this article we’re going to explain how adhesives can be used to bond metal (or nearly any material) parts together.
What You Should Know About Metal Gluing Systems
Adhesives are an entire science unto themselves, continuously growing and improving. We’d never be able to cover everything, but we can hit some important highlights. First, this article is about metal adhesives you can use for your DIY projects. There are plenty of adhesives for all sorts of materials, but here we’re focusing on metals. Different materials have different properties, so it’s important to use an adhesive that’s meant to work with materials you’re bonding together. The adhesive you choose will determine how well the joint performs, whether that’s strength, flexibility, temperature resistance (hot or cold) or any other criteria that may be important to you.
Adhesive bonds for metal generally work in two ways. There’s a chemical bond and a mechanical bond.
A chemical bond is the molecular bond between the contacting adhesive and substrates. This is commonly the strongest type of bond for an adhesive.
A mechanical bond relies on the geometry of the surface, creating an interlock between the adhesive and the substrate materials. The mechanical interlock can also help in providing more surface area for chemical bonding.
There are other types of adhesive bonds, but with metal substrates, these are the most prevalent.
The other aspect of adhesives worth discussing is two types of strengths when it comes to those bonds. There’s the adhesion strength and the cohesion strength.
Adhesion strength refers to the strength of the bond between the adhesive and the substrate. How well can the glue hold onto the metal?
Cohesion strength refers to the strength of the bond between the molecules of the adhesive itself. The glue may stick to the metal, but if the glue separates from itself the joint still fails.
Why Gluing Metal to Metal is Challenging
Metals have a relatively high surface energy. The physics of surface energy aren’t important for this article, just know that it’s an advantage for getting liquid adhesives to lay down onto the surface. It also unfortunately makes it easy for oils and dirt to get embedded into the metal surface. Metals also have a tendency to form oxides on their surfaces, which can have different mechanical and chemical properties than the base metal. This can make it a challenge to bond metals using adhesive.
The key to getting a good adhesive bond with metal is a well prepared surface. The exact details of what that means may vary depending on the adhesive you’re using, but in general it means a clean surface free of oils and grease. A slightly rough surface can also help (for that mechanical bond).
The prep is usually the hardest part. If properly preparing a surface for adhesive sounds like too much hassle, there are alternatives to join metals. Fasteners don’t require any surface preparation and hardware insertion and hole tapping are both available on parts directly from SendCutSend.
Metal Glue Bonding vs. Fasteners
A properly designed and executed glue joint can be extremely strong, but how does it compare to the more common bolted joint?
Pros for Adhesives vs Fasteners
- Size – Adhesives add minimal size and weight to an assembly compared to fasteners. Two thin plates can be joined together with a final thickness not much thicker than the two plates.
- Toolless – Adhesives can form a permanent bond without the need for tools. There are no proper torque specs when assembling with adhesives. It’s worth noting that not all adhesives are designed to be permanent.
- Fragile Parts – Adhesives can join thin or fragile pieces of metal together that would be difficult or even impossible for a fastener.
- Aesthetics – Adhesives can be virtually invisible, making them a great option for applications where aesthetics are important.
Cons for Adhesives vs Fasteners
- Size – Adhesives need surface area for adequate strength, which may require more space than an equivalent fastener. Depending on your application, size can be either a positive or a negative for adhesives vs fasteners. Yes, we listed size as both a pro and a con.
- Environment – Adhesives can be more affected by outdoor use than fasteners. Sunlight, moisture and temperature can all negatively impact some adhesives.
- Preparation – Adhesives require adequate surface preparation to be effective. Without it, the bond is more likely to fail.
Welding Metal vs. Gluing Metal Parts
Another popular form of joining metals is welding. Welding and adhesives have a lot of similarities. Let’s look at some pros and cons of adhesives compared to welding.
Pros for Adhesives vs Welding
- Cost – Adhesives don’t require expensive equipment like welding machines.
- Skill – Welding typically requires a higher level of skill to get quality results, where a quality bond with adhesive does not.
- Heat – Welding by definition puts a lot of heat into the material that can warp or damage parts and finishes. Adhesive can be applied after finishing with no damage. Your powder coated, anodized or plated parts from SendCutSend can be glued together without damage to the finish.
- Dissimilar materials – Some different materials can be welded together, like stainless steel and steel, but aluminum to steel is nearly impossible. With an adhesive it can be done with ease.
Cons for Adhesives vs Welding
- Strength – A good adhesive can be extremely strong, often as strong as the base metal. But in many applications, joining thin sections like the edges of panels, welding is the strongest option.
- Seamless – While almost any joint can be covered to appear seamless, a welded joint can be ground smooth to appear seamless without any covering. The bare metal itself appears to be one continuous part.
- Environment – A welded joint will typically behave the same way from environmental factors as the base material, which can’t always be said of adhesives.
Types of Glues for Metal Bonding
Adhesives come in nearly unlimited varieties. Some are specific to metals, some are for bonding metals and other materials, some are general purpose. For the best results, an adhesive designed specifically for your application is recommended. Let’s go over some of the more common types.
Epoxy adhesives are versatile. They can be designed for different applications. There are epoxies for high-strength, high temperatures, low temperatures, UV resistance, water resistance, chemical and solvent resistance and on and on. Epoxies usually come in two separate components (a resin and hardener) that get combined in the right ratios to start the curing reaction. This is referred to as a 2K system. Single component systems that cure in the presence of air or moisture are called 1K systems. Examples of epoxies are available from manufacturers like West Systems, 3M, Loctite, Gorilla Glue and J-B Weld.
When it comes to strength and durability of adhesives, epoxies are king. Modern vehicle manufacturers often use epoxies, such as panel bonding adhesives made by 3M, to bond structural panels of cars together. This can be done faster and for less cost than welding and provide a super-strong bond.
Sometimes called “CA glue” or better known by some of its brand names like Superglue and Krazy Glue, cyanoacrylate is what often comes to mind when people think of glue. Cyanoacrylate is a lighter duty adhesive, but can be used to bond metals together. The key to using lighter duty adhesives is sufficient surface area. A common use for CA glue on metal is for temporarily holding small parts during light machining. Parts too small for a chuck or vise can be glued to a larger part to make the cuts, then heat is applied to break the glue bond. Any residual glue can be cleaned up with solvent.
Acrylic adhesives are fairly common and can have comparable strength to epoxies. These adhesives are more flexible than most epoxies, which makes them widely used in industrial grade tapes like 3M VHB. They’re also a great option for dissimilar materials.
Urethanes and polyurethanes are known for their flexibility, which makes them good choices for bonding dissimilar materials and even rubbers. They aren’t as common for bonding metals, but can be used in the right applications. Many construction adhesives, like Liquid Nails Loctite Power Grab, that you’d find in retail stores are urethane based.
7 Steps on How to Glue Metal to Metal
You’ll want to review the instructions for your particular adhesive, but we’ve included some general steps here to help you as well.
1. Select an appropriate adhesive
Based on your application, decide on the best adhesive to bond your material. Higher strength isn’t always the top priority, sometimes strong enough is all you need and you can focus on other important factors.
2. Prepare an appropriate area to work
An appropriate area needs good ventilation if your adhesive contains anything you don’t want to breathe. It includes the appropriate method for holding parts, whether that’s a workbench and a clamp or a complex fixture to hold parts together while the adhesive cures. Finally, the part will need a safe place to be while the adhesive cures. Some adhesives are more sensitive to all these things than others. Some bond near instantly and can cure underwater, where some take hours or days to cure and must be held in position.
You may also want to do a dry fit of the parts to make sure you’re ready. Better to realize you need more clearance for a part before it’s covered in sticky goo.
3. Thoroughly clean both surfaces
Clean both parts well. Soap and water or alcohol may be sufficient, but sometimes degreaser or acetone are needed to get a part thoroughly clean. If your parts are steel, take care not to cause flash rusting on the surface.
4. Sand both surfaces
To get a mechanically strong bond, it can help to sand the surfaces with fine grit sandpaper. Each adhesive has different requirements, but most work better on a slightly rougher surface. Be sure to clean off any sanding grit or dust.
5. Apply the adhesive
Mix (if using a two-part) and apply the adhesive to one or both parts, depending on what you’re using. This step may also include a short waiting period. Some adhesives need to be joined immediately, some need a little time to get tacky before joining.
6. Join parts
Join your parts together and if the bond isn’t instant, use something to hold them that way until cured. In some cases the parts can bolt together or they may need to be clamped. There’s no rule saying you can’t have both fasteners and adhesives in an assembled joint.
7. Allow adhesive to cure
Depending on the cure time, your parts may need to sit without any stress for a period of time to reach full strength.
A Word from the Experts
Adhesives come with their own challenges, but can be an excellent method for bonding metal parts. Keep in mind that all assembly methods will come with their own pros and cons, adhesives are no exception. There are different types of adhesives, so be sure to find one that fits your application. Proper surface preparation is critical to getting a good bond. If this article has swayed you away from adhesives, alternatives for joining metal parts exist, such as welding and fasteners. In the case of fasteners, SendCutSend’s hardware installation service is available. For your next project, you’ve got plenty of options to join your metal parts.