Microstructural and Residual Stresses Development in Ni-Based Superalloys Linear Friction Welds

Linear Friction Welding (LFW) is a novel solid-state joining process that has been successfully used in joining aerospace components through the relative motion of two components under compressive force. Recent work has focused on the microstructural, mechanical properties, and residual stress development in titanium and ferrous alloys. Nonetheless, limited work is available on the weldability of Ni-based superalloys, especially for dissimilar alloy/processing route combinations (e.g. cast to forged). In this study, the LFW weldability of a cast Ni-Al-Mo alloy to a forged Ni-Cr-Fe-Nb alloy is assessed using electron microscopy and neutron diffraction for residual stress characterisation. Stereological studies were performed to quantify the grain size, γ' and γ'' volume fraction and size variations across the weld zones in the as-welded (AW) and the post-weld heat treated (PWHT) and to link it to the microhardness distribution across the weld. Generally, the weld was divided into a number of regions, displaying variations in the γ' and γ'' precipitates distributions, as well as the grain size. The high strain rate thermomechanical deformation close to the weld interface led to the creation of a narrow dynamically-recrystallised (DRX) region in both alloys. Still, no chemical mixing was observed across the weld line. Beyond the DRX region, a thermomechanically-affected zone (TMAZ) was observed, where the thermomechanical deformation was sufficient to deform the parent structure, but not to recrystallise the parent structure. Moreover, the localised thermal field created during the process led to the development of strong tensile residual stresses towards the centre of the weld.