Friction Welding of Wood. Improved Wet Welding Properties

Linear friction welding is a technique widely used to join metals and plastics in industry, and has recently been used to join wood without any adhesives.

By rubbing two pieces of wood together, lignin, a molecule that is a main component of wood, heats up and liquefies. Once cooled and solidified, it holds the two pieces together with the help of interwoven fibers, without the use of glue.

This technique requires short welding times, no special surface preparation and no use of adhesives. It appears to be a particularly cost-effective alternative to wood gluing, as it reduces production time and saves large quantities of glue. Wood welding has proved its worth for softwood, hardwood and bamboo.

There are two main types of assembly:

• planar joining, achieved by linear friction using a friction welding machine. This makes it possible to assemble sample parts from the same species or from several species, with different grain orientations. This makes it possible to produce standardized solid wood products on a laboratory scale;

• the assembly of wooden dowels in a solid support is achieved by circular friction using a conventional drill, making it possible to envisage applications in the furniture industry, as well as for the general public.

Studies to date have shown that the use of welded wood is limited to interior applications due to its low moisture resistance. Welded samples are suitable only for "interior" quality applications such as furniture, despite their good structural dry strength. The aim of this article is to present the improvements obtained in the moisture resistance of welded parts via different methods, namely physical-mechanical by modifying welding parameters and forming natural grooves during welding, and chemical by using natural products to increase the waterproofing of wood.

This welding technique has a number of attractive features:

• technical features: fast production times and precise part alignment;

• economical: no storage or use of adhesives;

• environmental: use only renewable materials such as wood and natural additives.

The resulting final product is entirely recyclable.