Friction stirring results in very fine grain microstructure in the stirred region (often referred to as ‘nugget’). The grain refinement results from intense plastic deformation associated with the movement of material from the front to the back of the rotating pin. The fact that friction stirring of material leads to very fine grain size can be used to obtain very fine grained microstructure in desired regions to develop new concepts of superplasticity, such as selective superplastic forming and thick plate superplastic forming. In addition, the grain boundary misorientation distribution of friction stirred zone shows predominantly high angle grain boundaries in a FSP 7075-Al alloy. The combination of very fine grain size and high angle grain boundaries is ideal for superplasticity. Table 3 summarizes some examples of high strain rate superplasticity in FSP aluminum alloys. It shows the possibility of using a simple FSP to produce a microstructure conducive for high strain rate superplasticity in commercial aluminum alloys. Based on these results, a three-step manufacturing of components can be envisaged: Cast + Friction Stir Process + Superplastic forge or form.

Table 3. Superplastic ductility at high strain rates in FSP aluminum alloys.