Friction welding is a fascinating process, able to join dissimilar metals and plastics together with bonds that are far stronger than their welded counterparts. As far as I was aware though it was limited to inorganic materials, mostly because other materials would simply catch fire and not fuse together. As it turns out that’s not the case and recent research has shown that it’s possible to friction weld pieces of wood together in the much the same way as you would metal.
What’s particularly interesting about the process is how similar it is to friction welding of metals or plastics. Essentially the rubbing of the two surfaces together causes the interfaces to form a viscous film that mixes together and, when the friction is stopped, fuse together. For the above video you can see some of the film produced escaping through the sides due to the large amount of pressure that’s applied to ensure the weld is secured. Like all other kinds of friction welding the strength of the joint is dependant on a number of factors such as pressure, period of the friction motion and duration of the weld. As it turns out friction welding of wood is actually an active area of research with lots of investigation into how to create the strongest joints.
Even cooler is the fact that some researchers have developed a technique that allows the welds to be done with no fibres being expelled out the sides. This means that there was no charring of the interface medium, enabling the resulting weld to be even stronger and much more resistant to intrusion by moisture. Whilst you’re not going to see a sub built of friction welded wood any time soon it does mean that, potentially, your house could be built without the use of fasteners or joiners and the first rain that came through wouldn’t make it all come unstuck.
Don’t think you can just go off and rub two pieces of wood together though, the frequency required to fuse the wood was on the order of 150Hz and a pressure of 1.9MPa, far beyond the capabilities of any human to produce. Still it’s not unthinkable that a power tool could be made to do it, although I lack the mechanical engineering experience to figure out how that would be done. I’m sure someone will figure that out though and I can’t wait to see what kind of structures can be made using friction welding techniques.
Kinematics was my least favourite part of physics, mostly because I always had a rough time wrapping my head around the various rules and principles that govern the way things move in our world. However one lesson always stuck with me in my head, the one relating to friction and it’s various forms. Whilst I’m sure the teacher delighted in tricking us all by asking us what kind of friction a rolling tire has (hint: it’s either static or kinetic and it’s not the one you’d first think it is) that example rooted the principle firmly in my head. Understanding that made further concepts a lot easier to grasp although I’d never really considered friction a powerful force until I saw this:
What you’re seeing happen here is a process called Friction Welding although in technical terms it’s actually not welding at all. Instead it’s actually a type of forging as in traditional welding two pieces of metal are joined via melting whereas in friction welding no such melt occurs. This process has a lot of advantages most notably allowing 2 dissimilar metals, say high grade aluminium and steel (a common pair in space fairing missions), to be joined together. Doing this process via other means is extremely difficult due to the different melting points of each material and would likely lead to a much weaker bond. Friction welding by comparison always creates a full strength bond without the additional weight introduced via other methods.
Interestingly enough this process can also be used with materials other than metals, specifically thermoplastics which are a type of plastic that becomes pliable under heat. Friction welding can then also be used to join said plastics onto metal surfaces, enabling cross material bonds that are far stronger than those that could be achieved via other methods.
Pretty fascinating, isn’t it?