Multiaxial cyclic behaviour of extruded and forged AZ80 Mg alloy

The quasi-static and cyclic response of as-extruded and closed-die extruded-forged AZ80 Mg alloy forged at 250 °C and 20 mm/sec was investigated. Servo hydraulic, fully reversed, strain controlled uniaxial “push-pull”, pure torsion, and biaxial fatigue testing were also conducted on the closed-die forged alloy. Two uniaxial loading paths (pure axial and pure shear) and three different biaxial loading paths were investigated (proportional loading, and non-proportional loading at phase angles of 45° and 90°). After forging of the as-extruded billet, the yield stress in the axial direction increased by 46%, the failure elongation by 35% and the ultimate strength by 13%. The spatial variation in both the local texture, microstructure and superficial hardness throughout the cross section of the forging was investigated and discussed. The web region in the center of the forged billet exhibited a virtually fully recrystallized and refined microstructure with intense basal texture, whereas the rib regions on each side exhibited a partially recrystallized and coarser microstructure with less intense texture. The fatigue life of the forged material in the axial direction showed dramatic improvement relative to the as-extruded material at strain amplitudes which were less than 0.35%. However, at higher strain amplitudes, there are significantly higher cyclic stresses (and associated fatigue damage) in the forged material (relative to the as-extruded), resulting in a decrease in life under fully reversed strain controlled fatigue loading. For the pure axial strain path, the total amount of strain energy density (SED) observed in the stabilized response was a function of number of cycles only, and was insensitive to material condition (as-extruded or forged). However, for the forged material, of all the strain paths investigated, only the pure shear path demonstrated a significantly different relationship of total SED vs. life. Furthermore, the amount of total SED for a given number of cycles decreases when biaxial loading is introduced, though following the same general relationship as the pure axial strain path. The effect of a decrease in total SED for a given number of cycles occurred in all biaxial strain paths, with proportional being the least prominent, then 45° and 90° out of phase being progressively more pronounced respectively. Both the Smith Watson Topper (SWT) and Jahed-Varvani (JV) models were utilized to predict the fatigue life of the uniaxial and biax