A welding apparatus has a guide rail arrangement with at least one guide rail attachable to a welding target. A carriage has a carriage housing and a rail follower assembly that is movably mountable to the guide rail arrangement for relative movement along the at least one guide rail. A feedstock source is disposed within the carriage housing and configured to deposit a feedstock material on a target surface of the welding target. An ultrasonic weld head is partially disposed within the carriage housing and has a sonotrode that extends toward the target surface so as to engage the deposited feedstock material and apply a normal welding force to the deposited feedstock material and the target surface. The sonotrode is operable to conduct ultrasonic vibrations into the deposited feedstock material and the target surface to weld the feedstock material to the target surface.