From analysing the behaviour of friction stir welds, produced with low and high heat inputs, it has been found that the nugget zone grain structure is inherently unstable during solution treatment. A low line energy results in an exceptionally fine nugget grain structure, and abnormal grain growth occurs throughout the nugget zone, encouraged by the dissolution of soluble precipitates. When welds are produced with higher heat inputs, although the nugget grain structure is coarser relative to the dispersoid density, and is thus more stable during solution treatment, neighbouring recrystallised grains can still grow abnormally into the nugget zone with a planar front and this leads to very large, mm-scale, grains being formed. Good agreement was found with the model of Humphreys, and showed that the condition for instability is highly dependent on the welding parameters.