Study On Annealing Process Of AZ61Magnesium Alloy Sheet And Its Microstructure And Properties

Magnesium alloys are the lightest metals that have been used in practice. Due to their excellent properties such as high specific strength and stiffness, good damping, shock resistance, electromagnetic shielding, convenient reclaiming, magnesium alloys are deemed as one of the most potential materials, and have received more and more attention. Magnesium alloys are widely applied to defence, aerospace,3C and automobile and so on. Developing manufacture process of wrought magnesium alloy sheets is a challenging technology, which is important for applications in the future.In this thesis, AZ61magnesium alloy roll-casting sheet was chosen to investigate the effect of hot-rolling, warm-rolling and annealing on its annealing process. Moreover, the microstructure and properties of AZ61magnesium alloy sheets were determined using optical microscopes, scanning electron microscopy (SEM) and mechanical properties measurement method. Major conclusions were summarized as follows.(1) The microstructure of AZ61magnesium alloy was composed of refine dynamic recrystallized grains, coarse grains and deformed twins after it was warm-rolled.(2) On annealing at225-325℃, along with the extending of holding time, the recrystallized grains grew up slowly. Especially, on annealing at325℃×3h, the microstructure of AZ61magnesium alloy sheets were mainly composed by equiaxed grains, and the grains were uniform. When it was annealed at350℃×6h, many super grains appeared in the microstructure. Therefore, annealing temperature should not exceed350℃.(3) Annealing at325℃×3h was the best among the annealing process of this paper. The horizontal tensile strength reached255MPa, elongation reached19.78%, the vertical tensile strength reached275MPa, and elongation reached20.88%.(4) As the annealing temperature and holding time increasing, horizontal and vertical mechanical properties of the sheets tended identical. On annealing at225℃×16h, horizontal tensile strength and elongation was significantly lower than the vertical; on annealing at325℃×3h, horizontal mechanical properties of the sheets were similar as the vertical. (5) Scanned images of tensile specimen fracture showed obvious fracture cleavage plane accompanied by a large number of dimples in the fracture plane when annealed at325℃×3h, and it is concluded that the tensile fracture of AZ61magnesium alloy was transitional fracture in which both brittle and ductile fracture occurred.(6) Under the same annealed process, more excellent mechanical properties could be obtained by cross-rolling than by the one-way-rolling.