TC4 ELI tapered stress joint by inertia friction welding used in offshore riser systems: Numerical simulation, microstructure, and mechanical properties

The TC4 ELI titanium alloy stress joints used in offshore riser system were welded by inertia friction welding with different rotational speeds (600-750 r/min), then the temperature field, stress field, axial shortening amount, microstructure and mechanical properties of joints were studied using a combination of experimental and simulation methods. Moreover, the simulation results indicated that the peak temperature, equivalent stress, and axial shortening amount all increased with increasing rotational speed, which is attributed to greater plastic deformation at the WZ induced by higher rotational speed. Furthermore, as the rotational speed increased from 600 to 750 r/min, the tensile strength of joints exhibited minimal variation, reaching 97.4%-97.9% (room temperature) and 101.2%-104.6% (350 °C) of the base metal, respectively. In contrast, the elongation first increased and then decreased. At 700 r/min, the tensile strengths reaches 99.5% (room temperature) and 104.6% (350 °C) of the base metal, respectively, which is attributed to the synergistic strengthening effect of brittle acicular α' martensite and dislocation pinning within the joint; and the elongation reaches 100.6% (room temperature) and 99.5% (350 °C) of the base metal, which is attributed to the lower geometrically necessary dislocations density, refined grains, and a high proportion of high-angle grain boundaries.