<p>This study systematically investigates the flow lines, microstructure, and mechanical properties of large AZ80 magnesium alloy support beams formed through isothermal die forging. A combination of finite element analysis and experimental methods was used to conduct this investigation. The results show that flow lines develop along the flow direction, with the most pronounced lines observed at the center. The forging develops a strong basal texture perpendicular to the principal stress direction, and changes in the principal stress direction weaken the texture intensity. Fine equiaxed recrystallized grains form in regions where the effective strain exceeds 0.8, providing additional nucleation sites for secondary phase precipitation. Dispersed short rod-shaped secondary phases offer better strengthening compared to lamellar secondary phases, though both types slightly reduce the plasticity of the forged piece. Additionally, while the bottom and corner regions display similar microstructures, when the flow lines are disrupted, both strength and plasticity decrease by more than 20%, leading to a sharp decline in mechanical properties. This decline is primarily attributed to stress concentration at the disrupted flow lines, which increases the susceptibility of the forgings to crack initiation and rapid propagation.</p> Graphical abstract <p></p>