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.
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