A modified acoustic-plastic constitutive model characterising both acoustic softening and residual hardening effects on aluminium alloys is developed and applied into a computational fluid dynamic model for the ultrasonic vibration enhanced friction stir welding (UVeFSW) process. The flow stress and material flow field in UVeFSW are quantitatively analysed. The effect of acoustic residual hardening on the streamlines density at rear advancing side is examined by comparing with that based on the model considering just acoustic softening. The numerical simulation results are experimentally validated, and it is found that the modified constitutive equation improves the prediction accuracy of the flow stress and material flow field in UVeFSW.
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