Analysis of the premature failure of some aluminum alloy sonotrodes for ultrasonic welding

A sonotrode is a manufacturing tool typically used for ultrasonic welding of light metallic alloys and plastics. A well designed sonotrode works at a frequency ranging from 20 to 40 kHz and for an average period of ten years, so performing a number of working and, consequently, stress cycles in the order of 1012. The present research is focused on the analysis and the understanding of the premature failures (happened within about 5x109 cycles) of some sonotrodes made of AA7075-T6 and used for the ultrasonic welding of thermoplastic materials. It is worth remarking that identical sonotrodes, produced at a different time, are still in service.