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# 4.5 Adhesive joints – gluing

The essential feature of adhesive joining is that two parts are joined by placing a liquid between them, which then solidifies. If you think about it, that is exactly what happens during casting; the major difference between casting and gluing being that, in casting, it is important for the cast material to separate from the mould whereas in gluing the aim is the opposite. The strength of a glued joint depends not just on the strength of bond between each part and the adhesive layer, but also on the strength of the adhesive layer itself.

In Soldering , the layer is put in as a hot liquid that solidifies on cooling to room temperature as shown in Figure 65.

Figure 65 Soldering

## Soldering

Soldering is defined as the joining of metals using a separate filler metal that melts at temperatures below the melting temperature of the metals being joined. The bond strength is relatively low. The 'traditional' solder alloy was based on a tin/lead mixture, but all solders used commercially in developed countries are now free of lead, mostly being based on mixtures of tin and copper along with other metals, such as silver.

Typical features of the soldering process are:

• The solder alloy can be significantly different from the base material because the base material does not melt.
• The strength of the alloy is substantially lower than the base metal.
• Bonding requires capillary action, where the solder liquid is drawn into the joint.

And because of these differences, the soldering process has several distinct advantages over welding:

• Virtually all metals can be joined by some type of soldering metal.
• The process is ideally suited for dissimilar metals.
• The lower temperature than that needed for welding (welding is discussed shortly) means the process is quicker and more economical.
• The low working temperature reduces problems with distortion that can occur during welding, so thinner and more complex assemblies can be joined successfully.
• Soldering is highly adaptable to automation and performs well in mass production.

Although the principles of soldering are shared with all gluing processes, the word 'adhesive' is usually taken to mean a type of polymer glue. Adhesives now come in a vast array of different types; some stick in seconds (cyanoacrylate – Superglue TM ), some take a day or so to achieve their full strength (thermosetting epoxies), others stay permanently in a soft flexible state, like silicone adhesives. Thermosetting glues , like thermosetting plastics, are made by mixing together two ingredients, a 'resin' and a 'hardener', usually in liquid form, which react chemically to form a solid.

• Almost all materials or combinations of materials can be joined.
• For most adhesives the curing temperatures are low, seldom exceeding 180 °C.
• A substantial number cure at room temperature and provide adequate strength for many applications.
• Heat-sensitive materials can be joined without damage.
• No holes have to be made as with rivets or bolts.
• Large contact areas means high joint strength.
• The adhesive will fill surface imperfections.