How to Bond Metals?

Posted by Pedram Mojarrad on

Chemical adhesives in some cases, offer several advantages that make them an attractive alternative to mechanical fastening or thermal joining, such as welding, soldering, and brazing. They can be effective on dissimilar metal substrates with dissimilar melting points; they do not cause distortion, discoloration, or weld worms; holes do not need to be drilled into the material in preparation for fasteners; and they distribute stress load evenly over a broad area.


Epoxy adhesives used in bonding metal substrates and components fall into one of two categories: structural adhesives; and machinery adhesives.


Structural Adhesives

An adhesive's heat resistance and chemical resistance are important considerations for metal bonding. Metal substrates and adhesives have widely different coefficients of thermal expansion (CTE), which, during repeated heating and cooling, can cause the bonded joint to weaken. Some adhesives are sufficiently resilient to accommodate these stresses. Also, for metals such as aluminum and steel that can form a loose oxide layer in moist environments, some adhesives prevent moisture from reaching the metal surface in the bond joint.
The four adhesive technologies most commonly used for metal bonding—two-part no-mix acrylics; two-part mix acrylics; two-part epoxies; and polyurethanes—deliver high bond strength and can withstand harsh environments.

Two-part no-mix acrylics are durable, tough, and can develop bond strength very rapidly, but offer limited depth of cure.
Two-part mix acrylics offer improved depth of cure and are durable and tough, but tend to cure slowly.
Two-part epoxies offer high strength, durability, excellent depth of cure, and temperature resistance, but are slower to cure than either of the acrylic families.
Polyurethanes deliver superior flexibility and toughness, but cure more slowly than the acrylics and can withstand temperatures only up to 300 degrees F.
A structural adhesive is applied by hand to a metal assembly. Because adhesives can be dispensed relatively easily and cure rapidly, they are suitable for high-speed manufacturing processes.

The best structural adhesive for a specific application should be determined by considering critical factors such as substrates to be bonded, end-use operating environments, and the manufacturing process.

Machinery Adhesives

The term machinery adhesives refers to the family of anaerobic adhesives that are typically used in applications such as threadlocking; retaining rigid, cylindrical assemblies; and sealing between flanges. 
When anaerobic products are used to lock threaded assemblies together, they prevent loosening and corrosion of the assembly, maintain proper clamping force, and offer controlled torque for fastener removal. As the liquid threadlocker cures, it forms polymer chains that find their way into every tiny imperfection in the threads. The adhesive completely fills the microscopic gaps between interfacing threads to positively lock and seal threaded assemblies, preventing lateral movement and protecting the joint from corrosion that can result from moisture, gases, and fluids.
In manufacturing operations in which application of a liquid threadlocker is not desirable, preapplied dry threadlockers are available that cure when the threaded unit is assembled. Thicker paste versions of these anaerobics find wide applicability in sealing threaded pipe sections together. In addition to eliminating loosening of the assembly, these materials prevent galling, offer improved lubricity, and seal the joint to prevent leakage.
For rigid, cylindrical assemblies, such as joining a bushing to a shaft, anaerobic adhesives, or retaining compounds, enable you to bond assemblies that once could be assembled using only press fits or shrink fits. Press fits and shrink fits often require tight manufacturing tolerances and costly surface finish requirements; involve cumbersome heating and cooling of components before assembly; and can result in distorted assemblies. Retaining compounds allow you to open your tolerances, ease surface finish requirements, and produce high-strength assemblies.
Anaerobic adhesives also are widely used as formed-in-place gaskets that provide a seal between flange faces. Because they allow you to create joints with a line-to-line fit between metal flanges, they eliminate the need for tightly controlled flange surface finishes and the problems associated with compression gaskets, such as compression set, bolt relaxation, and fastener loosening. Contact us for more information about these adhesives (

Surface Preparation Is Critical

No matter which type of adhesive is used, surface preparation is critical in ensuring a lasting and stable adhesive bond, because bond strength is determined to a great extent by the degree of adhesion between the substrate and the adhesive.
Surface oxidation or rust impedes adhesion and must be removed to ensure an optimal bond. Some metals, such as carbon steel, are treated with oils or other rust preventives that also can adversely affect the bonding process. While certain adhesives can bond through surface contaminants, others may require a preliminary cleaning process before bonding.

GESS PioneerPro supplies a wide range of adhesives for bonding metals to metals, as well as metals to other substrates. Surface preparation plays a key role in ensuring that good bond strength is achieved. The following techniques are typically used as practices for proper surface preparation. 


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