Joining metal

A discussion on twitter started by Fastener News Desk encouraged me to put pen to paper and write a few notes on joining metal.

“Which method do you prefer – chemical #bonding or #fasteners?”

I have to confess that I don’t really have a preference. Given that I’ve a range of tools available, I normally choose based on the selected application. Obviously what skills/tools/materials are available could be a significant deciding factor. Other things to consider are:

  • Do you want to remove parts at a later date
  • Access when assembling
  • Environmental conditions
  • Dissimilar metals
  • Joint strength
  • Appearance

If you want to take your components apart at a later date then some form of fastener would be a likely first choice. However, some adhesives are soluble or can be removed by force or heat so can be considered.

If you can’t access the area to be assembled then that could be deciding factor. For example the trusty nut and bolt would not be appropriate if you could not hold the nut in place. That could be replaced with a screw and tapped hole or perhaps a pop rivet. If you are thinking of brazing or welding, would you be able to get a torch into the area to join?

Does your joint need to be water or air tight? If so then perhaps a welded or brazed joint will be appropriate? Adhesives can also have sealing properties. Will the joint be exposed to the sun or chemicals or sea water that could affect the joint? If your joint needs to have electrical properties then soldering is a good choice. As soldering works at a lower temperature it is good for delicate items such as electrical components but that can also work for joining thin metal sheets when done with a suitable flux.

Dissimilar Metals
It’s not always possible to weld dissimilar metals as they might have different melting points. This is where brazing and silver soldering can be used they a filler material at a lower temperature than both metals. Some metals like zinc or tin can’t be joined in this way as they oxidise at high temperatures.

Joint strength
It used to be the case that welding was significantly stronger than adhesives. However with modern “structural” adhesives these are now a good option for joining metals and can be as strong as the metal itself. As with welding, often the preparation is key to getting good strength with dirt and oxides needing to be cleaned off the metals first.

The visual end results, can also influence the choice of joint. If you want an invisible joint then welding followed by grinding or polishing could be appropriate. However a design such as a gate might use jointing techniques like riveting to match to a particular style.

More on structural adhesives and fasteners

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