Adhesive bonding versus welding: an age-old choice

A common question that encountered in the adhesives industry is "why should I use an adhesive instead of welding?" While both methods are age-old behemoths in the world of joining - welding has existed in some form or other as a binding method since around 3000BC, while adhesives are even older - welding is sometimes more trusted by engineers and manufacturers in industry. As humans, we tend to stick with the tried and true = what we know and trust and welding remains a very reliable method of joining today.

Early Egyptians and Sumerians started joining metals together using heat and pressure, a process known as forge welding. This eventually became widespread and was the only welding method used until the end of the 19th century. Most welding techniques today, however, are based on arc welding - the use of electricity to heat and melt metals before joining and cooling them.

There are several different kinds of welding that are used in modern industry, but let's look at the four mostly commonly used in industry today.

MIG Welding (Metal Inert Gas/GMAW) - A continuously fed wire electrode melts to form the weld, while an inert or semi-inert shielding gas protects the molten metal. It offers fast welding speeds, is easy to learn and highly automatable and is excellent for production environments. Common applications include automotive manufacturing, general fabrication, structural steel and sheet metal.

TIG Welding (Tungsten Inert Gas/GTAW) - This uses a non-consumable tungsten electrode to create the weld arc. A filler metal is added separately if required. It is extremely clean and precise, offers excellent end appearance, and is easier to control. Ideal for thin materials, common applications include aerospace, medical equipment, stainless steel fabrication, aluminium components and food processing equipment

Laser Welding - Here, a focused laser beam melts the material to produce a narrow, precise weld. It offers very high precision, minimal distortion and high speed. Easily automated, it is characterised by a small heat-affected zone. Common applications include electronics, medical devices, automotive manufacturing, aerospace and battery production.

Resistance Spot Welding - Electrical current passes through overlapping sheets while electrodes apply pressure, creating heat that forms a weld nugget. It is extremely fast, highly repeatable and easily automated, with no filler material. Common applications include automotive body assembly, appliances, metal cabinets and white goods. This is the process structural adhesives most commonly complement or replace in vehicle body construction.

Benefits of welding

Welding offers many advantages that make it still highly popular in industry today. Its standout features include:

Long-established, proven method of joining - Welding has stood the test of time. It has been used for thousands of years to join metals together.

High trust and confidence - Engineers and the wider public trust welding as a reliable way of joining parts together. Welded joints are seen as strong and permanent.

Works on butt and other simple joint geometries - Welding is ideal for bonding metals that are in a butt joint (L-shape). Adhesives, on the other hand, don't tolerate this type of joint well.

Creates a continuous structure - Welding effectively makes two metals become one, with nothing in between them. This is ideal for maintaining electrical and/or thermal conductivity between parts.

Provides optimised temperature resistance - Especially if the weld has joined identical substrates, (like steel to steel) the joint will be well equipped to handle high or low temperature extremes.

Drawbacks of welding

Requires training and certification - Welding is a skill that requires proper training and certification in order to do professionally. This means that if you need welding done, especially on a regular basis, you'll need to find (and pay) someone qualified to do it.

Requires equipment - Likewise, welding also requires a welding machine, safety gear, a workstation, gas etc. These all cost money, and heavy duty industrial welding gear can be expensive.

Presents hazards - Welding carries inherent risks. Since it involves sparks, immense heat and often gas, welding is a fire and explosion risk. This means it can't be carried out in ATEX areas (European legislation) or anywhere where there is a risk of fire or explosion.

Requires space. Welders need space to work. Unlike with an adhesive, for example, you couldn't weld plumbing pipes together very easily under a sink.

Can cause tarnishing or distortion - Some metals, particularly stainless steel, can be affected visually by welding. Exposure to the very high heat from welding can cause the metal's surface to oxidize and discolour, leaving an unsightly finish to your product.

Limited substrate compatibility - Welding is generally only suitable for joining metals to metals, or plastics to plastics. You can't weld metals to plastics or composites. Even welding dissimilar metals together (like aluminium to steel) can be a challenge, because individual metals have different melting points. In the case of direct welding aluminium to steel, the aluminium would melt away before the steel even began to get hot.

Concentrates stress at the weld area - Welding focuses physical loads, forces, and vibrations onto tiny, localized rigid points, such as the edges of a bead or small weld nuggets. This creates high stress concentration peaks and fatigue risks.

Hard to form a complete seal - Pinprick holes in welded joints make it difficult to create a 100% seal against liquids and gases. Further processing may be required.

Adhesives - even older than welding

Welding has a very long history, but adhesives go back way further - Neanderthals were using birch bark tar as an adhesive for tools and hunting equipment 200,000 years ago! The first commercial glue manufacturing plant was founded in the Netherlands in the late 1600s, with modern adhesives gaining advent in the second half of the 19th century and continuing into the 20th century.

As a manufacturer of industrial adhesives for over 60 years, Permabond produces a wide variety of glues that vary in their chemistry, technology and substrate compatibility. From epoxy adhesives to structural acrylics, cyanoacrylates (also known as superglue) and adhesives that cure using UV light, adhesives cover a broad spectrum depending on the nature of the application and parts to be bonded.

Adhesives can bond a wide variety of materials including metals, plastics, composites, rubbers, ceramics and many more. They're also well suited to bonding dissimilar materials together, for example metals to plastics.

Benefits of adhesives versus welding

No extensive training needed - Unlike welding, bonding with adhesives requires minimum training and expertise. A certification or professional qualification is not required to use adhesives in industry.

Much safer to use - While basic PPE is highly recommended when using adhesives, most do not pose a fire or explosion risk. As a result, they can safely be used in ATEX areas. Good ventilation is still important (as with welding) to vent out any fumes.

Distribute stress evenly - Adhesives distribute stress over the entire bond area. This leads to better overall structural integrity, as well as impact and vibration resistance.

Can bond dissimilar substrates - Adhesives can bond metals to plastics, plastics to composites, etc. They also excel at bonding different alike materials together - for example aluminium to steel. However, some adhesives are better than others at doing this. Differences in thermal expansion and contraction between substrates need to be considered, with the right adhesive selected that can handle any stress at the bond site as a result of this.

Can be done ad-hoc and in small spaces - Adhesive bonding doesn't always require workstations or extensive space, particularly for small jobs.

Often provide a cleaner, nicer-looking finish - Adhesive bonding, when done well, often leaves no visible trace. Unlike welding, it doesn't tarnish the substrate surface.

Form a complete seal against liquids and gases. Many adhesives form a complete seal against liquids and gases.

Drawbacks of adhesives versus welding

May require joint redesign - Modern structural adhesives are capable of achieving exceptionally high bond strengths. However, replacing a welded joint with an adhesive is not always a simple like-for-like substitution. If the joint has been designed specifically for welding, it may not be suitable for adhesive bonding without modification. By considering adhesive bonding during the design stage and optimising the joint geometry, comparable - or in many cases even greater - joint strength can often be achieved.

Adhesives need time to fully cure - Welding provides more or less immediate strength development. Once metals are cooled, a welded item can be handled and used for its final purpose. Many adhesives, on the other hand, require several hours to reach full strength. Fixtures or clamps are also often required to achieve this. Cyanoacrylates and UV-curing adhesives, however, cure and reach full strength rapidly.

Temperature limitations - Most adhesives based on organic chemistry (such as Permabond's and those of many other manufacturers) cannot withstand very high or low temperatures. A welded steel joint, on the other hand, has no such issue.

Surface preparation is often needed - To achieve a strong, durable bond, the surfaces to be bonded often need to be cleaned, abraded and primed before bonding. Substrates such as steel, copper and aluminium as well as many plastics and composites fall into this category.

Joint design is more important - Adhesives work best when kept in shear or compression, and don't do well when faced with peel or cleavage forces (see image below.) With welding, joint design is much less of an issue.

So, with all of that said, which is better? As you can see, when comparing adhesives versus welding neither option is inherently 'better' than the other. Both methods have their pros and cons, and which one you use depends heavily on your application and design possibilities.

While adhesive bonding offers significant advantages in joining dissimilar substrates, distributing stress, and reducing workplace safety risks, it also introduces considerations such as cure time, surface preparation, and the right joint design. For this reason, the choice you make ultimately comes down to what best meets the mechanical, manufacturing, and environmental requirements of your application.