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This study investigates the mechanical properties and microstructure of friction stir butt welded high strength/ductility multilayered steel consisting of 15 alternating layers of SUS 301 austenitic stainless steel (eight layers) and SUS 420J2 martensitic stainless steel (seven layers) with a total thickness of 1?2 mm. With optimised welding parameters, defect free welds with an ultimate tensile strength (UTS) of 1240 MPa and a fracture elongation of 13% were accomplished. This corresponds to a joint efficiency of 90%. In this case, fracture occurred in the heat affected zone as a result of a very pronounced hardness drop in the martensitic layers resulting from the formation of a large amount of grain boundary precipitates, which were formed at temperatures ,750uC slightly below A c1 . By applying post-weld heat treatment, the hardness drop in the martensitic layers was removed and the tensile properties were enhanced to UTS of 1310 MPa (95% joint efficiency) and a fracture elongation of 22%.

We investigate the influence of Al3(Sc,Zr) precipitates on the cold deformability and friction stir welding behavior of a novel AlMg4Sc0.4Zr0.12 alloy. To analyze the influence of the precipitate state on the flow stress, samples were pre-aged at various temperatures for up to 120 min prior to plane strain compression. It was found that the flow stress increased considerably with increasing pre-ageing temperature and duration. The formation of Al3(Sc,Zr) precipitates during friction stir welding was investigated on welds with high and low heat input by measuring the respective hardening response with and without post-weld heat treatment.

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