Friction tapered plug welding is a solid-state welding process in which a bulled-shaped plug is rotated and forced axially into a prepared hole in a workpiece. The resulting friction and plastic work deforms the plug and the workpiece producing a high quality spot weld. An extension of this technique, called friction stitch welding, employs a series of overlapping plug welds to repair linear defects, such as cracks in structural components. A test program was undertaken to develop the stitch welding technique for repair of cracks in titanium structure for aircraft applications and to characterize the properties of the repairs produced. Welds were produced in 0.25-inch and 0.1-inch thick Ti 6Al-4V plate using plugs of the same material. The effect of process parameters and plug design on the required axial force and torque was determined experimentally. The welds were macroscopically examined to determine weld quality and structure, and the weld metallurgy was studied to characterize the microstructure and oxygen content. Mechanical testing consisted of static and fatigue testing, and characterization of the residual stress condition in the repaired region.