Welding Wood Is a Wonderful Idea

The University of Cambridge and TWI Ltd figure out how to rub two sticks together

welded wood

University of Cambridge

One of the questions often asked about cross-laminated timber (CLT) is: "What about the glue?" The boards in CLT are held together by a layer of polyurethane glue. It doesn't off-gas but it would be nice if it wasn't there and we had solid wood, with fewer worries about health hazards, flammability, or end of life issues.

We have shown Holz Thoma, held together by dowels, but Craig Rawlings of The Forest Business Network points us to something completely different:

It's welded wood, where researchers from TMI Ltd and the University of Cambridge's Construction Innovation Lab developed "a sustainable process to rapidly join timber elements using linear friction welding."

"In this energy-efficient process, joints are produced by pressing and rubbing two timber surfaces together at high frequency (50-150 Hz). The resulting friction and heat softens and re-sets lignin, the natural ‘glue’ in plant materials, as well as mechanically interlocks the cellular material, causing the ‘welding’. In just two to three seconds, the fused timber joint is stronger than conventional adhesives and even stronger than the native wood."

TMI is an association of welding industries and has been using friction welding in other materials, and started applying it to wood in 2019. They claim that "the process has many potential benefits, including environmental factors as the joining doesn’t require the addition of any other materials to the wood."

This little video shows two little pieces of wood being rubbed together and in seconds, there is a puff of smoke and a solid piece of wood. The University of Cambridge press release notes: "This technique has the potential to be applied not only to planed/sawn wood but also to CLT (Cross Laminated Timber)."

Dr. Darshil Shaw of the University of Cambridge compares it to rubbing your hands together, according to the release:

"How do you produce more heat?’ asks Dr Shah. ‘Rub your palms faster (frequency), push your palms against each other with more force (pressure), rub your palms for longer (time) and move your palms over a longer distance (amplitude). Similarly, in wood welding, to generate more friction and heat, these are the 4 principal manufacturing parameters we can control."

There are things that are perhaps harder to control; it would seem that as the pieces get larger, it takes a lot more energy to vibrate and press. In CLT the boards are laid up in layers at 90 degrees to each other; this is critical to making it stable. After posing the questions to Dr. Shaw, he told Treehugger:

"You are right that the power requirements scale in magnitude with the welded area, and therefore certain geometries may not be feasible. It is both the pressure and the fact that the samples have to be gripped and moved relative to each other at 75Hz that is the challenging bit!"

Dr. Shaw also informed Treehugger that the process does work at 90 degrees orientation, but that "bond performance is relatively weaker than in the parallel directions." There are also challenges in getting machine time; TWI's welder is used almost exclusively in the high-performance aerospace sector. "So we are breaking new ground here with the technique and its use in a totally different sector… which presents its challenges!

So it may be a while before we are building with welded CLT. In the meantime, here is a much longer webinar on the process: