Effect of annealing treatment on the homogeneity of microstructure and mechanical properties in Mg-Gd-Y-Zn-Zr rotary backward extruded tubular

The tubular is an essential part of equipment and civilian products, and it is essential to develop an appropriate production process. The new severe plastic deformation (SPD) process of rotary backward extrusion (RBE) increases strain value during deformation, but there is still a gradient microstructure. In the present work, we investigated the effect pattern of different annealing parameters to get a homogenous microstructure and rotary backward extruded tubular properties. The results showed that the best annealing parameter was 460 °C × 5 h, with a recrystallized (RXed) percentage of 73.6%. The maximum tensile strength difference (ΔUTS) at different locations along the wall thickness is reduced from the initial 76.93 MPa to 6.31 MPa, and the Coefficient of Variation (C.V) value decreased from 9.58 to 1.18. Static recrystallization (SRX) is associated with dislocation accumulation and release. Based on the results of this experiment it is predicted that the grain boundaries of neighboring grains with a misorientation angle close to 30° are preferentially migrated and the RXed grains are gradually transformed from equiaxed to elliptical. The initial annealing stage produces abnormal texture (<0001 >//Extrusion Direction), mainly associated with activating non-basal slip systems. As annealing proceeds, the abnormal texture disappears, and basal slip dominates, forming basal texture and rare earth (RE) texture. The decrease in strength after annealing is mainly due to grain coarsening and the release of dislocations within the grains.