Thixoextrusion of AZ91D magnesium alloy: effect of processing parameters on flow behavior and product properties

Abstract The magnesium alloy AZ91D was investigated to evaluate its microstructural evolution, flow behavior, and mechanical response during semi-solid extrusion (thixoextrusion) in a pilot-scale plant. Prior to extrusion, globular microstructures suitable for semi-solid processing were obtained through partial remelting heat treatment performed at 560 and 575 °C for 10 min. The influence of billet condition, extrusion temperature, and piston speed (3.33–16.66 mm·s –1 ) on extrusion behavior, product integrity, microstructure, and hardness was systematically analysed. The results showed that piston speed is the most influential processing parameter governing material flow and defect formation during thixoextrusion. Sound products were obtained at piston speeds up to 8.33 mm·s –1 , whereas speeds of 10.00 mm·s –1 and above promoted liquid-phase segregation, cracking, and porosity formation. Increasing extrusion temperature reduced the maximum extrusion pressure by increasing the liquid fraction available during processing, while billet conditioning improved semi-solid flowability and microstructural homogeneity. Furthermore, a predictive second-order regression model was developed to estimate the maximum extrusion pressure (Pmax) as a function of the processing parameters, providing a practical tool for process optimization. Microstructural and hardness analyses revealed a clear relationship between local deformation conditions, microstructural heterogeneity, and hardness distribution across the extruded specimens. The results contribute to a better understanding of AZ91D thixoextrusion and provide useful guidelines for the industrial implementation and optimization of semi-solid magnesium alloy processing.