Researchers at TU Graz in Austria have made significant strides in developing an adhesive-free process for joining wood to metal and polymer composites. This advancement utilises 3D printing and ultra-sonic joining techniques, resulting in exceptionally strong bonds that could transform manufacturing in industries such as aerospace, automotive and furniture.

The research team from the Institute of Materials Science, Joining and Forming has tested two innovative methods: ‘AddJoining’ and ‘Ultrasonic Joining’. Both approaches create durable joints while emphasising environmental sustainability.

“Our motivation is environmental protection,” explains Sergio Amancio, the research leader. He highlights the potential for using renewable materials like wood to replace components traditionally made from energy-intensive or hard-to-recycle materials.

The AddJoining technique involves directly 3D printing a polymer composite onto a wooden surface. This process allows the printed material to penetrate the wood’s pores, where a chemical reaction occurs akin to that of traditional adhesives. The results from mechanical load tests indicate these connections are remarkably robust.

Durable connections

Gean Marcatto, a post-doctoral researcher, notes that after joint failure, they found polymer embedded in the wood and broken wood fibres in the polymer, suggesting the fracture occurred within the materials rather than at the joint interface.

Although the initial tests utilised untreated wood surfaces, the potential for even stronger joints exists. Introducing micro- or nano-structures into the wood through laser texturing or etching could enhance bonding surfaces and improve joint durability.

However, Amancio emphasizes the team’s commitment to simplicity: “We aimed to work with as few steps as possible and, above all, without chemicals.”

This focus on minimal processing is particularly advantageous for complex 3D geometries, allowing components to be printed in any desired shape directly onto the wood.

Vibration effect

The Ultrasonic Joining method employs high-frequency vibrations applied to the wooden component via a sonotrode. This vibration generates friction at the interface with the polymer or polymer composite, creating heat that melts the surface of the polymer.

The molten material infiltrates the wood’s natural pores, forming a stable joint through mechanical interlocking and adhesive forces. Awais Awan, who dedicated his doctoral research to this technique, emphasizes its suitability for large components and 2D structures, enabling precise localised spot joints.

Both joining methods have undergone rigorous mechanical testing, showing impressive results. As with AddJoining, the strength of ultra-sonic joints can also be enhanced through surface pre-treatment, such as laser texturing.

The research from TU Graz not only demonstrates a promising alternative to traditional joining methods but also aligns with a broader movement towards sustainable manufacturing practices.

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