An example electrode cap includes a bored portion of an electrode cap. The bored portion establishes a bore that extends longitudinally from one end of the electrode cap and terminates at a surface having a radius relative to the longitudinal axis. A tip portion of the electrode cap extends from the surface toward another end of the electrode cap. An example method of maintaining a focal point of an ultrasonic wave includes propagating an ultrasonic wave from a transducer through a bore and a tip portion of an electrode cap and receiving a reflection of the ultrasonic wave. The method further includes determining information about a welded area using the reflection and adjusting a radius of a surface of the electrode cap to position a focal point of the ultrasonic wave within the tip portion.