Influence of thermal history on microstructural evolution and semi-solid processability of AZ91D magnesium alloy

Semi-solid processing of AZ91D magnesium alloy depends on the thermal history preceding deformation, which influences the evolution of the solid–liquid microstructure and the subsequent processing response. This study examines the integrated thermal route comprising homogenization, partial remelting, induction reheating, and semi-solid extrusion. The as-cast alloy was homogenized at 455 °C for 12 h and subjected to partial-remelting treatments at 560 °C, 575 °C, and 585 °C for 10 and 20 min. The evolution of the primary α-Mg phase was characterized by optical microscopy and quantitative image analysis using equivalent diameter and shape factor. Additional microstructural changes were evaluated after induction reheating to 560 °C. Semi-solid extrusion was performed at billet temperatures of 500 °C–560 °C and ram speeds of 3.33–16.66 mm·s −1 , with maximum extrusion pressure and product integrity used as indicators of processability. Partial remelting modified the α-Mg morphology and promoted liquid-phase formation. At 560 °C and 575 °C, both equivalent diameter and shape factor increased with holding time, indicating simultaneous particle coarsening and changes in particle shape. Subsequent induction reheating produced further qualitative changes in the morphology and spatial arrangement of the primary α-Mg phase, showing that the microstructure formed during partial remelting was not retained unchanged before extrusion. The macroscopic extrusion response was governed predominantly by billet temperature and ram speed, whereas the preceding billet condition showed no measurable systematic effect on maximum extrusion pressure. Increasing ram speed reduced extrusion pressure but was accompanied by product failure at the highest speeds investigated. Sound products were obtained at ram speeds of 3.33–8.33 mm·s −1 , whereas product breakage occurred at higher speeds. A quadratic regression model fitted to the 27 experiments that produced sound products yielded R 2 = 0.867 , an adjusted R a d j 2 = 0.835 , an in-sample RMSE of 36 MPa, and an MAE of 23 MPa. Leave-one-out cross-validation of the fixed model form yielded Q L O O 2 = 0.821 and an RMSE of 42 MPa. Overall, the results establish an integrated framework for examining the relationships between thermal history, semi-solid microstructural evolution, and processability in AZ91D magnesium alloy. They also highlight the need to assess extrusion pressure and product integrity separately when defining the processing window.