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UNDERCUT. A groove melted into the base metal adjacent to the toe or root of a weld and left unfilled by weld metal.
ULTRASONIC WELDING. Ultrasonic welding is a solid-state process wherein coalescence is produced by localized application of very high frequency (10,000 to 200,000 cps) vibratory energy to the workpieces as they are held together under pressure. It uses an ultrasonic transducer which is coupled to a force-sensitive system that contains a welding tip on one end. The pieces to be welded are placed between this tip and a reflecting anvil, thereby concentrating the vibratory energy within the work. Stationary tips for spot welds or rotating disks for seam welds can be used.
UPSET. The localized increase in volume in the region of a weld, resulting from the application of pressure.
UPSET WELDING. A resistance-welding process wherein coalescence is produced, simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area of contact of those surfaces. Pressure is applied before heating is started and is maintained throughout the heating period.
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