Researchers have developed a sustainable process to rapidly join timber elements using linear friction welding, in a proof-of-concept collaboration between TWI Ltd and the University of Cambridge, which was enabled by the Construction Innovation Hub.
The team has joined pieces of wood in two to three seconds by rubbing them together at high speeds, producing a welded joint as strong as the timber itself. The technique could be scaled up and applied in building construction and high-volume manufacturing of products such as furniture and packaging.
Timber construction is very sustainable, as trees capture CO₂ from the atmosphere and store it as carbon, and when the wood is harvested and used in buildings the carbon is stored for its entire lifecycle.
As well as its environmental credentials, advantages of mass timber construction using engineered timbers include speed of construction through off-site prefabrication and on-site rapid assembly. Both these benefits of sustainability and speed can be further enhanced through the development of rapid timber joining techniques.
In contrast with traditional methods such as adhesives, linear friction welding of wood can reliably produce high quality joints with high tolerances and does not require any filler or adhesive material.
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 inter-locking 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.
“Imagine you’re out walking on a cold day and want to warm up your hands; you’re likely to rub your hands together to generate heat,” explains Dr Shah. How do you produce more heat? 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 four principal manufacturing parameters we can control.”
Linear friction welding of Cross Laminated Timber
This technique has the potential to be applied not only to planed/sawn wood, but also to Cross Laminated Timber (CLT) – a wood panel product made from gluing together layers of solid-sawn lumber, which has been used in many high-profile tall buildings.
The team which comprises industry partners TWI Ltd, Dr Darshil Shah and Dr Michael Ramage from the Department of Architecture and the team behind the HappyShield open-source PPE design, continues to explore how these parameters change the process and why it seems to work differently with different woods. This will help them find the right materials and manufacturing parameters to optimise the process and scale it up for different purposes, such as construction and manufacturing.
In this initial proof-of-concept study, the team demonstrated the suitability of linear friction welding on a variety of hardwoods, softwoods and engineered woods (e.g. MDF), using various grain orientations. More recently, they have focused on beech, a European hardwood that is widely-used in construction and furniture-making. While this process has been tested at smaller scales, the team is hopeful that it is suitable for joining much larger pieces of wood and will therefore be well-suited to off-site timber construction for buildings.
Since October 2020, the team has continued to work together through studentships to advance the science and technology of wood welding. Led by Dr Darshil Shah (CNMI), Dr Eleni Toumpanaki (now at Bristol University) and Dr Kate Franklin (TWI), two final-year undergraduate project students at Cambridge, Mads Studholme and Jason Chang, are investigating joining bamboos, softwood Pines and even welding dissimilar woods, as well studying their resistance to moisture and their fire performance.
This research forms part of the Centre for Digital Built Britain’s (CDBB) work at the University of Cambridge. It was enabled by the Construction Innovation Hub, of which CDBB is a core partner, and funded by UK Research and Innovation (UKRI) through the Industrial Strategy Challenge Fund (ISCF).