Strain behaviour of a friction stir processed superplastic aluminium alloy sheet during free inflation tests

In this work, the strain behaviour of friction stir processed aluminium superplastic sheet was investigated. The aim was to study the feasibility of the combination of the friction stir processing technique with the superplastic forming process. Main friction stir process parameters (tool rotational speed and transverse speed) were varied in order to study the effect of these changes on the strain behaviour and on the formability of the processed sheet. For this purpose, free inflation tests were carried out on the processed sheet at a temperature at which the base material exhibits an optimal superplastic behaviour. A strong influence of the stirring parameters on the strain behaviour of the processed sheet was found denoting only a specified set of friction stir parameters made the processed sheet behave like the base material. The microstructure evolution of the stirred zone played an important role in this direction. Higher tool rotational speeds led to abnormal grain growth and, consequently, to a slower deformation in the processed material. Thickness measurements made after free inflation tests also confirmed this behaviour: the material processed with a higher tool rotational speed had a thickness that is twice the unprocessed one.