Mechanically induced wood fusion welding, without any adhesive, is shown here to rapidly yield wood joints that satisfy the relevant requirements for structural application. The mechanism of mechanically induced vibrational wood fusion welding is shown to be due mostly to the melting and flowing of amorphous cells-interconnecting polymer material in the structure of wood, mainly lignin, but also some hemicelluloses. This causes the partial detachment ("ungluing") of long wood cells (wood fibers) and the formation of an entanglement network drowned in a matrix of melted material, which then solidifies. This creates a wood cells entanglement network composite with a molten lignin polymer matrix. During the welding period, some of the detached wood fibers that are not being held anymore by the interconnecting material are pushed out of the joint as excess fiber. Cross-linking chemical reactions of lignin and of carbohydrates-derived furfural also occur. Their presence has been identified by CP-MAS 13CNMR. These reactions, however, are relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, which explains why relatively longer holding times under pressure after the end of welding contribute strongly to obtaining a good bond.