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The aim of this investigation was to study the influence of tool geometry on material flow during heterogeneous friction stir welding in 1 mm thick plates of AA 5182-H111 and AA 6016-T4 aluminium alloys. Two types of tool shoulders were used: a shoulder with a conical cavity and a scrolled shoulder. Pin-driven flow was predominant in welds produced with the conical cavity shoulder, which are characterized by an onion ring structure. The interaction between pin-driven and shoulder-driven flow is restricted to the crown of the weld, at the trailing side of the tool, and extends throughout the weld thickness, at the leading side. Although no onion ring structure was formed in welds done with the scrolled shoulder, extensive mixing of the base materials occurred in a plasticized layer flowing through the thickness around the rotating pin. Shoulder-driven flow is intense and continuous around the tool.

Morphological, metallographic and structural analyses of aluminium-copper dissimilar welds produced under different friction stir welding conditions were conducted in order to analyse the mechanisms of intermetallic phases formation, its relation with welding conditions and its consequences in the weld structure and morphology. Under lower heat input conditions, only a thin intermetallic layer distributed along the aluminium/copper interface was depicted inside the nugget. Increasing the heat input promoted material mixing and formation of increasing amounts of intermetallic rich structures. The intermetallic phase content and the homogeneity of the mixed area increased with increasing heat input, evolving from structures containing Al, Cu, CuAl 2 and Cu 9 Al 4 to structures predominantly composed of Cu 9 Al 4 and Cu(Al). In order to explain these results, the mechanisms of intermetallic phases formation are discussed, taking into account the process parameters and material flow mechanisms in friction stir welding. Important relations between intermetallic formation and weld surface morphology were also found.

a b s t r a c tThe tensile behaviour of similar and dissimilar friction stir welds in 1 mm thick sheets of two aluminium alloys (AA5182-H111 and AA6016-T4) is analysed in this paper. The heterogeneity in properties across the welds was studied by performing microhardness tests and microstructural analysis. The tensile tests were performed in samples extracted longitudinal and transverse to the weld direction. It was found that the tensile behaviour of the welds depends mainly on the grain size in the TMAZ, for the AA5182-H111 alloy, and on precipitate distribution, for the AA6016-T4 alloy. In all types of welds, the HAZ preserves the same properties of the base materials. The global mechanical behaviour of the AA5182-H111 similar welds is very similar to that of the base material. However, for the AA6016-T4 similar welds and for the AA6016-T4-AA5182-H111 dissimilar welds a 10-20% strength reduction relative to the base materials and important losses in ductility were reported.

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