Abstract: Apparatus is described for welding of thermoplastic parts and the like where the heat to melt the surface or surfaces to be joined is produced by friction during high frequency rubbing. The parts are adapted to be placed on a platen which enables the mating surfaces of the parts to be brought into clamping engagement in a welding head. The welding head includes a seismic mass. Stiff flexural members support a plate, against which the parts are clamped with respect to the seismic mass, and which provides for relative vibratory motion of the mating surfaces to be joined. A hydroacoustic driver is coupled to the plate and causes the plate to execute linear vibratory motion at a high frequency, where the stiffness of the flexural members is resonant with the mass of the vibrated portions of the head. Due to the stiffness of the flexural member and the high frequency of vibration, the relative displacement of the mating surfaces to be joined during vibration can be extremely minute.