apparatus for use with an ultrasonic plastic welder in which an ultrasonic horn on a movable welding head produces sonic vibrations along a first axis generally normal to the contacting surfaces of the workpieces to be welded together , the apparatus rendering the welder suitable for welding metals . the apparatus includes a coupling member removably connected to the distal end of the horn and extending transversely thereof and having coupled thereto a workpiece - engaging member capable of adjustment along a second axis parallel to the first axis . a force - applying member is connected to the welding head and extends therefrom toward the workpieces , with the workpiece - engaging member being adjusted to contact the lower end of the force - applying member at a fulcrum point so that axial vibration of the coupling means by the horn serves to pivot the workpiece - engaging member about the fulcrum point to effect a vibration transverse to the first axis at the contacted workpiece . static force is applied to the workpieces along the second axis by the movable head acting through the force - applying member and the workpiece - engaging member .