Effect Of Temperature On Grain Refinement And Annealing Behavior Of The Multi-directionally Forged AZ21Magnesium Alloy

The low density and high specific properties of wrought magnesium make it an ideal choice for the light structure material. Their widespread application, however, is restricted because of their limited strength and poor ductility at ambient temperature. In this study multi-directional forging (MDF) at low temperature followed by annealing was imparted to Mg alloy, with the purposes of make an effective use of twinning and recrystallization to refine grain size and modify texture, strengthening and toughening of magnesium alloy.AZ21magnesium bulks were MDFed at room temperature,393K and448K followed by recrystallization annealing. The evolved microstructure, texture were studied by optical and SEM/EBSD metallographic observation and mechanical properties were evaluated by tensile tests at room temperature.{10-12} extension twinning is the dominant deformation mechanism at low cumulative strain during MDF at room temperature. With the increase of deformation pass, the high flow stresses that developed during compression enabled the activation of contraction twins even though the Schmid factor was quite low. Their intersections together with the gradual formation of fine grains along the contraction twins (strain-induced DRX) lead to the evolution of a high-density fine twins and ultrafine grained structure.It exhibited almost the same structure feature, microhardness and microtexture i.e. the strong basal texture parallel to the normal direction of sheet after4passes deformation at three temperatures, leading to an almost completely the same room temperature mechanical properties. As to the30passes samples, with the increase of deformation temperature, a deterioration of grain refinement, and ultimate tensile stress together with the ductility were detected. For the sample compressed at room temperature, it exhibited about130%and70%higher yield stress and ultimate tensile stress than that of the as-received one without destroying much ductility.The static recrystallization kinetics of the30-passes magnesium alloy MDFed at room temperature can be well described by JMAK model in the temperature range of473-548K, and the activation energy for recrystallization was estimated to be63.4KJ/mol. For the sample annealed at523K for150s, the average grain size is about3.75μm, increasing and reducing annealing temperature can both lead to grain coarsening. The SEM/EBSD results showed that the microtexture of this sample was severely randomized and weakened, result in a higher ductility of almost1.5times of the initial one and a superior stretch formability.Static recrystallization based on twinning was observed in the4passes MDFed samples, for the one compressed at a higher temperature, a poor ductility can be obtained. While the similar trend is not recognized for the30passes MDFed samples. For the one compressed at393K, it has the most excellent ductility (33.9%), the difference of recrystallization mechanism maybe the reason.To sum up, it is evident that multi-directional forging is an effective means to enhance not only the ambient strength but also ductility. The more deformation pass applied to a sample, the finer grain size together with better ductility can be obtained. While the tendency is not obvious if a high deformation temperature was applied to.