This research presents microscopic evidence of dislocation propagation and sub-grain
refinement in 3003-TO aluminium undergoing high frequency fully reversed loading
conditions during the Ultrasonic Consolidation process. Dual Beam Focused Ion
Beam etching techniques and Transmission Electron Microscopy were used to
characterize sub-grain size, morphology and dislocation structure in regions that were
subjected to high levels of multi-axial ultrasonic micro-strain and resultant plastic
deformation. This Deformation Affected Zone is characterized by regions of reduced
sub-grain sizes that form a gradual transition into larger equiaxed grains well below
While ultrasonic weldi~g has been explored for some time, there has been
little agreement on the specific softening mechanisms that allow an ultrasonic weld to
occur. Usually, the unexplained effect is vaguely referred to as "acoustic softening,"
or "ultrasonic softening", generic terms that do not identify a specific mechanism.
This uncertainty has led to many disparate theories as to the specific nature of
ultrasonic welding. The presented interface characterization discoveries challenge
many of these ideas and prescribes a fundamental operative "ultrasonic softening"
mechanism that is similar in character to the Bauschinger Effect.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University