Joining Wood by Friction Welding - Fabrication of Multi- layered Components

Since decades traditional friction welding methods are applied industrially to metal and thermoplastic synthetics. Research showed that for different applications this method is promising for wood too. The process does not necessitate any additional material. One of the process’ advantages, compared to glued connections, is the short time, which is necessary to realise a connection. The consolidation or curing of the decomposed wood at the heat-affected zone, and thus the development of the interfacial shear strength, is of major importance for the efficiency of friction welding of wood laminates. Seven test series of ten samples each from Norway spruce (Picea abies) were welded and tested with regard to the evolution of the connection’s shear strength. Compression shear tests were carried out at different times shortly after the termination of the welding process. One minute after the termination of the frictional movement, the joints show average shear strength of about 2 MPa. During the following fifteen minutes, the shear strength increases to about 2.6 MPa. This demonstrates that the shear strength obtained immediately after the interruption of the welding process, reaches about 70% of the maximum value, reached after fifteen minutes with nearly ambient temperature at the heat-affected zone. According to measurement of the interfacial shear forces during welding, the maximum shear stress applied by the vibrational motion is approximately 0.5 MPa. Thus, the resistance of the connection, reached immediately after termination of the process, exceeds this value noticeably. As the initial shear resistance is higher than the shear stress applied by the vibration, continuous welding of wood laminates composed of various layers is possible. In spite of the rather low shear strength obtained for welded spruce wood samples, the method could potentially be applied to the lamination of wooden slabs by timber planks with low shear load at the connection, forming solid wall and ceiling elements. The experimentation carried out dealt with small surfaces of mainly 25 cm². However, the results gained, led to the development of a new friction welding machine, conceived to be able to weld significantly larger surface areas and to be more appropriate for the material than traditional machines.