Characterization of complex, solid-state flow and mixing in the friction-stir welding (FSW) of aluminum alloy 6061-T6 to magnesium alloy AZ91D using color metallography

Semi-solid-cast magnesium (Mg) alloy AZ91D (3% solid fraction) was friction-stir welded (FSW) to aluminum (Al) alloy 6061-T6 in continuous non-porous welds. The resulting welds were analyzed in order to understand the solid-state mixing mechanisms that create the chaotic intercalated weld zone microstructure characterized by swirls and vortexes of dynamically recrystallized material. Severe plastic deformation in the solid-state is accommodated by the dynamic recrystallization of the mechanically mixed materials which generates a fine-grained weld zone microstructure, along with dislocation substructures and precipitates. Mg–Al weld systems are also characterized by eutectic/intermetallic regimes that are thrice as strong as the base materials, and nearly six times stronger than some of the softer recrystallized regimes abutting them. Color metallography was used to gain additional information about the varying regimes within the intercalated microstructure. The variation in coloration indicated energy differences between the various dynamically recrystallized microstructural regimes, in turn providing correlations between the general coloration of these regimes, and their nature and strength. The weld zone is strengthened by the coexistence of the hard eutectic/intermetallic weld regimes with the abutting softer dynamically recrystallized regimes across a continuous interface.