Microstructural refinement and mechanical property enhancement of AZ91 magnesium alloy via room-temperature multi-directional forging

This study investigates the microstructural transformation and mechanical behavior of an AZ91 magnesium alloy processed via multi-directional forging (MDF) at room temperature. The alloy was subjected to nine forging passes (three cycles) under low-stress conditions, with each pass inducing an 8% deformation. The microstructural transformations were characterized using optical microscopy (OM), scanning electron microscopy (SEM) energy-dispersive spectroscopy (EDS), and x-ray diffraction (XRD). Experimental results demonstrate that while the annealing increased the crystallite size from 44.2 nm to 59.4 nm due to coalescence, the subsequent MDF process effectively refined the crystallite size down to 34.8 nm. This refinement, coupled with increased dislocation density and the formation of new grain boundaries, led to a substantial enhancement in mechanical performance. Mechanical testing revealed a 22.48% increase in Vickers hardness (from 72.73 HV to 89.08 HV) and a 47.87% improvement in compressive strength (from 188.37 MPa to 279.18 MPa). The grain refinement and hardening behavior are shown to be consistent with the Hall-Petch relationship. Furthermore, EDS analysis confirmed a homogeneous elemental distribution and the presence of secondary phase precipitates (Al, Si, and Mn). These findings suggest that room-temperature MDF is a cost-effective and practical severe plastic deformation (SPD) route for producing high-performance AZ91 alloys with superior strength and toughness, suitable for aerospace, automotive, and defense applications.