The AddJoining process produces extremely strong joints without using adhesives or screws. Beech, oak, carbon fiber-reinforced polyamide and polyphenylene sulphide, stainless steel 316L and Ti-64 alloys were used as test materials.

“Our motivation is clearly environmental protection,” says Sergio Amancio, Ph.D., head of the Institute of Materials Science, Joining and Forming at TU Graz. “With new manufacturing processes, the renewable raw material wood could replace components made from energy-intensive or difficult-to-recycle materials.”

With the AddJoining technique, a component made of polymer composite is affixed to and printed directly onto a wood surface via a 3D printing process. The printed material penetrates into the wood pores, where a chemical reaction occurs, similar to the reaction of glue with wood. The resulting joints were highly successful in mechanical load tests.

“After the joint fractured, we were able to find polymer in the wood pores and broken wood fibers in the polymer, which suggests that the fracture occurred in the wood and polymer, but not at the joint,” explains Amancio. “These successful tests were carried out on the untreated wood surface. Even more durable joints could be achieved by introducing a micro- or nano-structure into the wood through laser texturing or etching, which increases the pores and enhances the bonding surfaces.

“But, we wanted to work with as few steps as possible and, above all, without chemicals,” notes Amancio. “We can use this technology particularly well with complicated 3D geometries, because the components are printed directly onto the surface in whatever geometry is required.”

Amancio and his colleagues also experimented with ultrasonic joining technology. They applied high-frequency vibration with low amplitude to wooden components using a sonotrode. In contact with the base component—polymer or a polymer composite material—the friction generates heat at the interface which melts the surface of the polymer part. Molten polymer infiltrates into the naturally porous surface of the wood.

“In this way, a very stable spot joint can be achieved, from a mixture of mechanical interlocking, because the melted plastic solidifies again in the wood and adhesion forces,” says Amancio. “This technique is particularly suitable for large components and 2D structures, since we achieve a precisely localized spot joint.”