Study Of Effects Of Novel Processing Methods On Microstructures And Mechanical Properties In Mg-Al-Zn Alloys

Abstract:Due to the hexagonal close packed structure, magnesium alloys have only two active slip systems at room temperature. As a result, the plasticity of Mg alloys is low. In order to improve the mechanical properties of Mg alloys at room temperature, this study presents two novel processing methods: compression-rolling-annealing and cross-wavy bending. The evolution of microstructures during deformation and annealing is observed by optical microscopy and the textural evolution is examined by electron backscatter diffraction analysis. The effects of these novel processing methods on the mechanical properties such as microhardness, tensile ductility and stretch formability at ambient temperature are analyzed and evaluated.Compression-rolling-annealing method is presented to solve the problem of deforming hard-to-form Mg alloys such as AZ91Mg alloy. The unidirectional compression was carried out at723K under an initial true strain rate of0.3s-1. Consequently, a large strain was achieved with no crack. Then, the compressed samples were rolled at623K under cold rollers. A large number of twins were introduced and they would accelerate the nucleation during static recrystallization by consuming stored energy. After annealing at different temperatures, the grains in the samples were obviously refined and the ductility and strength were enhanced simultaneously. Among different annealing conditions, the most prominent refinement was obtained via annealing at723K for2.5min, resulting in an average grain size about8.7μm. The optimized mechanical properties were18.2%in fracture elongation and314.2MPa in ultimate tensile strength through annealing at673K for5min.As a strong basal texture always exists in hot-rolled AZ31Mg alloys, the follow-up processing is used to be difficult. Thus, a novel method cross-wavy bending was come up to solve the poor plasticity induced by strong basal texture. After bending for4passes at673K, the tensile ductility, stretch formability and ultimate tensile strength were increased concurrently. And the average grain size was refined to about8μm. During the former3passes, dynamic recovery was the main work softening mechanism. However, at the4-pass bending, dynamic recrystallization became the dominant mechanism. Through the different directions and values of the introduced shear strain and work softening, the intensity of basal texture was weakened obviously and the distribution was diffused significantly. Specifically, during the former3passes, texture weakening was the main result, while the basal texture was randomized at the4-pass bending with the influence of dynamic recrystallization. Correspondingly, the tensile ductility and stretch formability were improved restrictedly during the former3passes but enhanced significantly at the4-pass bending. Furthermore, the ultimate tensile strength of the4-pass sample was almost equal to that of the hot-rolled sample. After bending for4passes, the fracture elongation and ultimate tensile strength were25.4%and232.2MPa, respectively. And the stretch formability reached2.6.The synchronous improvement of ductility and strength at ambient temperature was attained via novel processing methods in this study. Hence, the novel methods can contribute to the industrial production and application. Moreover, this study can enrich the fields of plastic deformation, recrystallization and texture in Mg alloys.