A.F. Norman

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 3 Zr 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.

Effect of welding parameters on nugget zone microstructure and properties in high strength aluminium alloy friction stir welds

Abstract Theeffects ofthemainwelding variables,apart form tool design,on themicrostructureandproperties ofthenugget zone in friction stir welds have been investigated for a typical high strength Al alloy (AA7010). It has been found that there is an optimum rotational speed, for a given travel speed, that gives the highest strength and ductility for the nugget zone. As the travel speed is increased, it is necessary to increase the spindle speed to maintain this condition, although the optimum condition does notdirectlyrelatetoa constantratioof rotationalto travel speed. This condition coincides with a heat input, for a given travel speed, that gives the maximum level of solute in solution in the nugget and the lowest density of coarse second phase particles. For low travel speeds, this is limited at excessive heat inputs by reprecipitation occurringduringtheweldcoolingcycles,whereas for high travel speeds incipient melting within the nugget appears to be an important factor.