The Effect of the Welding Conditions on the Nugget Zone in Friction Stir Welds in an AA7010 Alloy

摘要

The effect of the welding conditions on the structure, properties, and stability during solution treatment, of the nugget zone in AA7010 friction stir welds (FSW) has been systematically investigated. As the spindle speed is increased, relative to the traverse speed, the line energy increases and the temperature gradient from top to bottom of the nugget zone is reduced, causing the microstructure to become more homogeneous and the grain size to increase. The density of second phase particles also reduces as the line energy rises leading, to improved tensile properties. However, at high heat inputs the properties deteriorate, due to re-precipitation occurring during cooling, giving in an optimum condition for the best nugget zone properties. The stability of the nugget zone grain structure during solution treatment is also strongly affected by the processing conditions. The exceptionally fine grain structure readily leads to abnormal grain growth occurring. With higher heat inputs, although the grain structure is coarser relative to the dispersoid density, and is thus more stable, neighbouring recrystallised grains can still grow abnormally into the nugget zone and this leads to very large, mm-scale grains, being formed.