Friction Welding… Wood?

You’d think writing for Hackaday means we probably don’t get surprised very often by projects… but then we see something we never thought was possible — in this case, the linear friction welding of wood to join it.

Friction welding (also known as stir welding), is the process of taking two pieces of material (typically metal, or plastic), and vibrating one of them super fast while pushing against a stationary piece of the same material — the resulting friction causes a massive heat buildup that can then literally weld the two pieces of material together.

It’s an easy way to bond plastic parts together using a dremel and some 3D printing filament, and while doing it with metal is significantly harder, it is possible to do at home as well.

But according to the video after the break — it’s actually possible to do this with wood.

They’re using a linear friction welding machine which oscillates the sample at about 200hz. Their best guess is linen lignin cells within the wood are melting, interlocking and rejoining — resulting in a surprisingly strong bond.

They say further research is to be performed but the video is a few years old now and we still don’t see any updates on it — have you ever heard of using friction welding to bond wood?

[Thanks for the tip Karl Rosenqvist!]

62 thoughts on “Friction Welding… Wood?

  1. I’ve had limited exposure to friction welding plastics. In that scenario, we were forming a hollow assembly. I’ve also seen videos of metal wheel hubs welded together, which makes a lot of sense. The main application for wood friction welding that I foresee involves negating the use of adhesives. Perhaps assembling solid-wood door panels from many pieces without using adhesives? Applying expensive wood veneer onto cheap wood substrate? The ultimate dovetail joint? Assembling guitar bodies from many different pieces/types of wood without adhesives? If the machinery could be scaled to the size of a lathe or mill, this process could really take off.

  2. Friction welding is not stir welding. Stir welding, more correctly known as friction-stir welding, is a different but related process where a tool invades the pieces being joined and mechanically mixes them along with heating them up.

    In this case there is no such tool and no stirring. Come on, Hackaday editors, at least bother to open Wikipedia before jamming “post”

  3. sorry HaD but this sounds like utter BS. I’m a scientist and have been working with wood on a daily basis for the last 5 years. there is no such thing as a “linen cell” so either they misspelled something that does exist of the just made up something. also wood doesn’t melt, it burns…

      1. that’s heavily oversimplified but OK. but again lignin is an amorphous crosslinked polymer in its natural untreated state (as found in wood) it chars and decays under thermal load it does not melt.

        1. I think that if you see a burned down house or something you might see some wood stuck together too, but it’s just because the surfaces became rough and the scorched fibers are interlocking, but even a light shake will make it come apart because the surface is the brittle charcoal type.
          So I don’t take this serious at this point and just one of those youtube tomfooleries.

    1. Um…. TWI has a heck of a good rep. I doubt it is BS. There is (undoubtedly) more than you see in a short vid and HAD summary, but the claim “I’m a scientist” makes you sound like a 12 yr old in mom’s basement.

      I can see a number of mechanisms that could be at play here, having worked with wood for a good long time, among them active resins, chemical additives to convert the lignin to a thermoplastic form, or any of a number of other options.

      I need to track down the actual report (or reports) from the developers before I have an opinion, as, unlike some other cases on HAD, no natural laws seem to be being broken here. Given the history and reputation of TWI, I have faith that this is going to be interesting. Hint: TWI is one of the key places friction stir welding came from.

    2. maybe some pulp boiling althouht i might be mixing terms.
      linen cell as collagens and the term probaply used somewhere in the paper industry. there’s black stuff oozing out out so my guess would be that they are making adhesive with tar or whatnot is left as pine oil distillation procedure

    3. I would have expected a “scientist” so intimately familiar with wood to easily recognize that “linen” was simply a misspelling/misinterpretation of the word “lignin”.

    1. Agree, there are 0 details that show any actual usefulness of strength other than ‘they are stuck together see?’ doesn’t even attempt to touch the piece to show they are bonded well at all. A stab at funding by creating buzz?

  4. This certainly is interesting, but I doubt that the join would had much strength to it. From the postulated description, it is only the matrix material that is bonding together, not the fibers. With no fibers crossing the interface, the joint would be very weak.

    There are a lot of very interesting subtleties about fibrous composite materials. Leather is a good example: full thickness leather, unsplit, is far stronger than two thicknesses of the same leather that has been split. In the unsplit state, there is a large network of fibers throughout the material, but as soon as it is split, those fibers are cut, and the overall strength is reduced.

    I am kind of surprised that society hasn’t figured out synthetic wood yet. A bulk industrial process that could produce it, and still have all the advantages (nontoxic, easy to cut, lightweight, high stain to failure) could be a major boon in ecological preservation. Especially if we could grow the exotic hardwoods.
    Or, equivalently, if someone could figure out a way to rapidly accelerate the growth of trees, so whole lumber groves could be brought up in 1 year or less instead of 10 years.

    1. “Or, equivalently, if someone could figure out a way to rapidly accelerate the growth of trees, so whole lumber groves could be brought up in 1 year or less instead of 10 years.”

      Unfortunately there is an industry-wide shortage of handwavium at the moment.

      1. … are you really saying that because the technology doesn’t exist right now, it is an entirely absurd idea? Really? On Hackaday? Holy crap, ahahaha, *wow*. You are outright precluding the entirety of invention. On hackaday, nonetheless! “We don’t have it now, so therefore it will never exist” … bwahaha!

        I mean, a *cursory* Google search turns up plenty:

        I bet you’re one of those people who 5 year ago was saying, “No, solar power is a go-nowhere technology because it is just too expensive to ever be used. No, showing me that plot of solar prices falling continuously for the past 30 years doesn’t tell me anything.”

    2. we have, the lumberyards are full of plastic lumber. the problem is that it’s strength/weight ratio is worthless compared to wood so it’s only really used for decks and outdoor furniture and other items that have no load bearing.

      now using thin layers of wood with an epoxy makes a superior product. Glue-laminated beams are about 4-5 times stronger than the same beam made of 100% wood.

  5. Hmmm…taking wood fibers and bonding them together without adhesive to form a sheet of thin, relatively strong material. They should write a paper…wait…paper.

    For something to weld, it has to not be a thermoplastic – as cellulose is. You can dissolve it with chemicals (cf. cellophane), but that won’t help much. Heating wood with friction (or anything else) in the absence of air will make nice charcoal and a lot of coke gas by pyrolysis, but that’s not very sticky either. With air, you get a campfire.

    Seriously, the only adhesive affect here, if any, would be fibril entanglement (not strong). If this worked, everyone from Ikea to Weyerhaeuser would be all over it.

  6. That’s bizarre! But wood is full of all sorts of oils and proteins and stuff, maybe they’re providing a natural glue. Or maybe it’s a mechanical thing, grooves and the like are being formed, and squeezed into each other. Would be interesting to put under a microscope. Would also be interesting to test the strength of it, it would be amazing to think we’ve discovered a new way of sticking bits of wood together, be the first new one in centuries!

    1. Wood pellets are made by forcing ground up wood, coarser than sawdust, through a ring shaped die that has a lot of holes drilled radially through it. The die rotates around a pair of fixed rollers that are knurled. The knurling helps grab and force the wood through the die and it also forces the rollers to spin. If the rollers were smooth the wood could build up into a wedge that would just skid along the inside of the die and the roller wouldn’t turn.

      The rollers are mounted to a piece called a yoke. The yoke is attached to a large and very heavy shaft that under normal conditions does not spin. The shaft goes into a very heavy base and is supported in a pair of very expensive bronze bearings. If all goes right, those bearings never get any wear.

      At the end of the shaft opposite the die and rollers there’s a plate mounted to it. A shear pin keeps the shaft from spinning. If a chunk of metal gets into the die or something else packs it up, the shear pin is supposed to break, freeing the plate and allowing the shaft to spin. There’s a shutoff switch for the plate to trip, which either opens a lock-open relay or shuts down an electronic motor control.

      Some operators who try to run stuff through their mills that hasn’t been processed to the right size for the die they are running or has the wrong moisture content – but is ‘close enough’ that the mill can usually power through it – will replace the shear pin with a nice grade 8 bolt. Works great, saves down time from the shear pin breaking – until it eventually packs up bad enough and Bad Things happen like bent yokes, twisted shafts and broken cast iron.

      Then my friend Ned gets to make a ton of $$$$ rebuilding the mill, and trying once again to educate the person on the proper Care and Feeding of Pellet Mills. Nevermind his 60 or so years of experience with these things, he ends up spending a lot of time redoing mill setups after the people he built the plant for have a “better idea” about how they can improve things.

      Last year he got to completely rebuild some mills after a plant burned due to built up wood dust in the electric panels, after he repeatedly told the owners they needed to clean the place up before it caught fire and burned to the ground. After the building was rebuilt and the refurbished mills installed, the place was hit with a 70 MPH or so wind gust that literally blew the doors off the building. Considerably easier to repair than the fire damage.

  7. Woodturners (especially in Asia) have been using a similar process to permanently bond hardwood pieces together on a lathe for ages: you turn a socket and a conical pin and press the two together whilst one part is spinning. The result is a surprisingly strong bond … if you have a lot of practice. The secret is not to overdo it – you want the wood to “smear” but not to carbonize…

    From my experience (“wood technician in the prototype/model workshop of a furniture company), there appear to be indeed some thermoplastic properties to heated up hardwood when no oxygen is present. In the above video as in the “friction jointing” on a lathe, there is a good chance that a certain depletion process is taking place in which the “burning” of some of the wood is using up the oxygen in the air – thus creating a local environment which is mostly oxygen free. Also, it is interesting that we had a plasticky buildup on our tungsten drill bits that we used to drill dowel holes in beechwood (like, the bits got pretty hot and were in non-stop use). This material was not unlike the “arboform” resin which is a thermoplastic based on lignin…


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