AbstractSeries of welds were made by friction stir welding (FSW) with various backplates made out of materials ranging from low diffusivity granite to high diffusivity copper in order to reveal the effect of backplate diffusivity on the joint microstructure and properties. The temperature, microstructure, microhardness and tensile properties of joints were compared and discussed. Results show that the backplate with high diffusivity effectively decreases the heat input to the workpiece during FSW. With decreasing the backplate diffusivity the sizes of equiaxed recrystallized grains in the nugget zone increase obviously, while the hardness of the nugget zone also increases a little. The interface between the thermo-mechanically affected zone and nugget zone at the retreating side disappears under the granite backplate. Moreover, the ductility of the joint is more excellent under the copper backplate, but under the granite backplate the failure has mixed fracture characteristics of quasi-cleavage and dimples.