Effect of Tool Geometry and Heat Input on the Hardness, Grain Structure, and Crystallographic Texture of Thick-Section Friction Stir-Welded Aluminium

The effect of tool geometries on the microstructure and crystallographic texture of 32-mm-thick friction stir-welded AA6082 has been investigated. The use of a tapered probe tool results in a significant variation in the grain size from the top to the base of the nugget, whereas parallel probe tools produce a uniform grain size throughout the nugget. The grain size in the nugget reflects the amount of deformation experienced and the speed of deformation expressed in terms of strain rate. An approach is proposed to calculate the strain rate during FSW of aluminum for which values between 217 and 362 s⁻¹ were obtained. The strain rate can be either uniform or varied through the joint thickness based on the type of tool used. The tapered tool produces a variation of the strain rate, whilst the parallel tool has a uniform strain rate throughout, which can explain the obtained grain structure in each case. The tool geometries also influenced texture development with the tapered tool producing a tilt of the local shear reference frame by an angle to the normal direction equal to the taper angle.