Processing Of Superplastic Forming By Direct Resistance Heating For AZ31 Magnesium Alloy

Magnesium alloys are the lightest structural metal in practice. They have high specific strength ,high elastic modulus, excellent electro-magnetic shielding effectiveness, damping and heat-dissipating properties. They have a wide application prospect in the fields of national defense and civilian areas. But Magnesium alloys have poor ductility because of their hexagonal close-packed(HCP) structure. And it is difficult to make productions by stamping, which limits their application in structural field greatly. SPF is different from traditional plastic forming processing. Magnesium alloys'the ability of plastic forming increase a lot in the situation of SPF. We can use Magnesium alloys to make parts by SPF methods.The purpose of this paper is to research the microstructure evolution, fracture behavior, cavitation by free bulging in diffident conditions, and determine the optimum process conditions for the best deformation properties of this alloy, thus supplying a reliable foundation for the development and application of the SPF technology of Mg alloy sheets. For this reason ,the following have been done:Hot rolling was used to refine the as-extruded AZ31 Mg alloy sheets and AZ31 Mg sheets with a average grain size of 4.5μm and a thickness of 0.8mm were obtained. ANSYS was used to simulate the temperature distribution in the sample in c different current density and time conditions. The temperature in the middle of sample is higher than the two ends. With the temperature increasing ,the area of Uniform temperature of the middle became bigger. At first ,the temperature increased fast . With increasing temperature, due to increased heat loss caused by radiation, heating rate decreased. Finally ,the temperature became trend to the line which parallel to the x axis. With the current density of 25A/mm2, the magnesium alloy samples would be heated to 400℃in about 40 seconds. At the temperature range of 350℃and 450℃and different strain rate ,SPF free bulging was done . the microstructure evolution and cavitation during SPF were observed by optical microscopy and scanning electron microscopy.at 400℃,the limiting dome height reached the maximum value of 0.52. the grain at the top of the dome is fine, about 4-6μm.The deformation degree of this area is bigger, so and the driving force of recrystallization is lager , and the force induced dynamic recrystallization in the release processing. Original twin organizations in the raw material disappear in the process because of dynamic recrystallization. Central grain grew up at a certain degree, and it is about 20μm. Grain at the bottom of were greater than the middle and top. There exist many O shaped cavities and filaments at the boundaries near fracture surface. The fracture of filaments results in intergranular cavity and the model for the formation of intergranular cavities is proposed. The growth of cavities are plasticity-controlled and the serrated boundaries of intergranular cavities agree with the results of surface relieves.