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A basic model was applied to pure Ni and then to a single-phase superalloy. The high temperature deformation of the superalloy, a solution treated IN718, was investigated by torsion testing in a high-temperature regime (1000-1100 °C) where no precipitation of secondary phases was expected. The material exhibited the classical behaviour of alloys which undergo dynamic recrystallization. The peak-flow stress dependence on strain-rate and temperature was described by a physically-based set of constitutive equations, which took into account both dislocation hardening and solid solution strengthening, and was previously successfully used for the description of creep deformation of Cu, Al alloys and austenitic steels. The model provided an excellent description of the experimental data and for this reason can be considered an excellent basis for further development, which should take into account the precipitation of secondary phases. [ABSTRACT FROM AUTHOR]

Friction stir welds are considered reliable joints for their lack of voids, cracks and distortions. When compared to the base material, friction stir welding (FSW) joints typically exhibit finer grain structured (especially at the nugget zone, NZ). Similarly, refined grain structure can also be obtained by severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP). In fact, the fine grain structures produced within the NZ of FSW or friction stir processed (FSP) materials are usually coarser than the ones achieved by ECAP. The former is characterized by lower dislocation density, higher high-angle boundary fraction and different mechanical strength, compared to what can be obtained by ECAP. In this study, a dedicated cold-rolling (CR) set-up, specifically designed to simulate an ECAP-equivalent shear deformation, was used to further refine the grain structure of FSW AA5754 sheets. The effect of ECAP-equivalent deformation induced by CR in a 2 mm-thick AA5754-H111 FSW joint was investigated. FSW was carried out at two different rotational ( ω ) and translational ( v ) welding speeds, 600 rpm, 200 mm/min and 1800 rpm, 75 mm/min, respectively. FSW sheets were then CR to obtain an equivalent shear strain of ε ~ 1.08, that is equivalent to 1-ECAP pass carried out with an internal die channels intersecting at an angle φ = 90° with a curvature extending over an angle Ψ = 20°. By CR, the sheet thickness reduced only by ~20%. The role of annealing on the FSW and CR plastically deformed AA5754 was also investigated. This was applied either prior or after FSW, and it resulted that whenever it follows the FSW, the mean volume fraction of dispersoids and Mg-rich particles is higher than the case of annealing preceding the FSW process. On the contrary, it was found that the annealing treatment had a minimal role on the dispersoids and particles mean size. The here reported post-FSW ECAP-simulated deformation, obtained by a customized CR process, showed sheet integrity and a significant concurrent grain size refinement.