Failure analysis of two aluminium alloy sonotrodes for ultrasonic plastic welding

•The premature failure of two AA7075-T6 sonotrodes was due to the initiation of VHCF cracks.•Cracks showed competing initiations (ODAs and subsurface defects) caused by a high segregation at grain boundaries.•ODAs originated from the oxygen trapped inside the segregation.•Careful dynamical FEM analyses allowed the derivation of the service stresses at initiation regions.•The defect-dependent fatigue endurance strength and the service stresses allowed for the understanding of the premature failures.

A sonotrode is a part of the manufacturing system used for the ultrasonic welding of light alloys and polymeric materials. A well designed sonotrode vibrates at a frequency ranging from 15 to 70 kHz (typical values being 20 or 35 kHz), for an average period of ten years, and performs an expected number of working (stress) cycles equal to about 1012 at 20 kHz.The following research is focused on analysing the premature failure (which occurred within about 5 × 109 cycles at 20 kHz) of two sonotrodes made of AA7075-T6 and used for the ultrasonic welding of thermoplastic materials. It is worth mentioning that geometrically identical sonotrodes, produced from different batches of the same material and used for the same welding procedure, did not suffer any failure for a much higher number of service cycles.