The described embodiments relate generally to ultrasonic welding and more particularly to performing an ultrasonic welding operation while rotating one part relative to a mating part. A non-uniform energy director can be disposed along a mating surface of a first part. The energy director can maintain a constant cross-sectional area while having a smaller height near a pivot point for the rotation and a larger height away from the pivot point. The varying height of the energy director can allow the tip of the energy director to come in contact with a second part rotating relative to the first part at approximately the same time during the ultrasonic welding process.