Development of friction welding of wood : physical, mechanical and chemical studies

For several decades friction welding technology has found broad application in many fields of the metal and plastic connection industry. The fabricated joints are laminar or punctiform depending upon the shape of the parts to be welded. The work presented here deals with the application of this technology to wood connection. Its application to the material wood is still very young and the process is to a large extent unexplored. The type of connections investigated in this study is of laminar shape. The wood in the contact zone is heated by means of frictional energy. This causes a thermal alteration of the wood cell structure, which leads to the formation of a viscous layer. After cooling, the material in this layer forms the connection. The welding machine used for these investigations applies a circular movement to the parts to be welded. Within the scope of the work presented here the behaviour of the material during the welding process is examined. Measurement of the frictional force and interfacial temperature during the welding process shows that this process bears distinct resemblance to friction welding of metals and thermoplastic synthetics. As with metals and plastics, this process can be divided into different phases by means of measuring the frictional force. These phases reflect the particular states of the interface (dry friction, transition phase, viscous state). The influences of the machine settings, welding frequency and welding pressure, on the process cycle are examined by means of the friction force and the interfacial temperature as a function of welding time and welding displacement. To investigate the influence of the annual rings and their orientation these investigations are carried out for samples with different cut direction. Welding frequency and welding pressure affect the welding duration as well as the friction force in a distinct manner. The results show that circular friction welding is influenced by the orientation of annual rings in a significant way. The moisture content of the material is part of the investigations as well. Its influence on the behaviour of the material and the resistance of the connection is of substantial importance. This result arises from investigations on test pieces welded with different moisture contents. In accordance with the experiments carried out, the atmosphere of the welding environment (ambient air, argon atmosphere) seems to be not very important with regard to the process flow. Investigations concerning the evolution of the interfacial resistance showed that the welded connections do not meet the resistance achievable by conventional glues. However, the relatively high initial resistance permits a continuous welding of multilayered laminates. The initial strength of the newly formed connection clearly exceeds the load applied by the newly introduced vibration. Therefore the existing joint is not damaged by welding of an additional layer. Microscopic examinations reveal that the cell structure at the contact layer is completely destroyed. It becomes apparent that the decomposed wood forms a viscous layer during the welding process. This layer encloses the adjacent cell structure, embeds the cell walls, and contributes in this manner to the adhesion. Thermal degradation of the adjacent cell structure occurs only within a thin layer close to the contact zone. This is due to the good insulation properties of wood. The thermal influence becomes evident by a visible dark discoloration of the cell structure. The joint consists of a consolidated mass of thermally-altered wood decomposition products. This is also reflected in the results of chemical analyses carried out within the scope of this research. This study shows that the main components of wood (cellulose, polyoses and lignin) experience a thermal degradation. Within the material, which forms the interfacial layer the cellulose was detected to remain in a relatively high proportion. Due to its predominantly crystalline structure the cellulose behaves in a thermally relatively stable manner. Polyoses are thermally significantly less stable. Compared with thermally unchanged spruce wood, polyoses remain only in a small percentage in the joint substance. The molecular structure of lignin, the third chemical compound, experiences distinct changes. However, its total mass stays relatively stable. The investigations indicate reactions between the decomposition products of polyoses and cellulose and the thermally altered units of lignin, which could lead to a cross-linking of the joint material and thus contribute to cohesion. Advantages of this technology are seen particularly in the rapidity of the joint formation. In addition, wooden compounds, which are fabricated by this method consist of natural wood and thermally altered wood only. Compared to glued compounds where the adhesives usually contain solvents, this leads to advantages with regard to environmental compatibility. With respect to machining, welded connections offer advantages when compared to traditional methods used for wood connection. Disadvantages result from the resistance of the joint, which is significantly less stable than the resistance of most conventional glues. One application of this new method is seen in the fabrication of solid wood elements with rather small stress loads at the interface.