Stability of nugget zone grain structures in high strength Al-alloy friction stir welds during solution treatment

Abstract

From studying the solution treatment behaviour of friction stir welds, in a typical high strength Al-alloy (7010), it has been established that the nugget zone grain structure is inherently unstable at high temperatures, despite the presence of Al 3Zr dispersoids that inhibit grain boundary mobility. Good agreement has been found between experimental observations and a unified theory of the stability of cellular microstructures, proposed by Humphreys, which has shown that the condition for instability is highly dependent on the welding parameters. Low heat inputs result 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, instability is more marginal, as the grain structure after welding is coarser relative to the dispersoid density. However, grains can still grow abnormally into the nugget zone with a planar front and this leads to very large, mm-scale, grains being formed.