Effect of selected rotary-swaging routes on the microstructure, residual stresses, and compressive behavior of boron-modified AZ91

Rotary swaging (R-SWG) was used to tailor the microstructure and residual-stress state of magnesium alloys through combined radial compression and shear deformation. This work compares five thermomechanical R-SWG routes applied to permanent mold-cast AZ91 containing 0.03 wt% B in order to establish route-dependent relationships between microstructure, texture, residual stresses and compressive response. The investigated routes combined different second-pass temperatures, feeding rates, prior T4 treatment and total reductions. The route-dependent differences in grain refinement and radial microstructure heterogeneity have been revealed using electron microscopy. All processed conditions developed pronounced textures with distinct central-peripheral variations. Neutron diffraction resolved the residual-stress distributions across the full specimen cross-sections and showed that the finest microstructure did not necessarily coincide with the largest residual-stress gradients. Compared with the initial cast state, rotary swaging increased the compressive yield strength to approximately 240–290 MPa and substantially enhanced the strain to fracture, while Young’s modulus remained within 42–47 GPa. Finite element modeling (FEM) provided a physically consistent interpretation of route-dependent material flow, effective-strain penetration and temperature evolution. The study demonstrates that different thermomechanical R-SWG routes generate distinct process-structure-stress-property relationships in boron-modified AZ91 and highlights the value of full-cross-sectional neutron-diffraction residual-stress mapping as a complement to local microstructure characterization.