Thermoplastic Deformation Of Az80Magnesium Alloy And Research On Microstructure And Properties After Forging

Due to the strong demand for weight-reduction in modern industry, it is meaningful to research and develop the forging technology of high strength magnesium, improve the comprehensive performance of magnesium alloy products, broaden the process method of high strength magnesium, promote the industrialized production and widen the application rage of magnesium in the aerospace, rail transportation, automotive and energy industry.Firstly, author observed the elevated-temperature applications of extrusion and cast AZ80magnesium alloy respectively, with the tensile and compressive behavior at573-673K and strain rate ranges of10-4～10-2s-1, though analyzing the microstructure evolution and calculating the constitutive equation to deduce the deformation mechanism at elevated-temperature. Then, according to the features of AZ80magnesium alloy at elevated temperatures, designed a scheme to forge the vane and through finite element numerical simulation, combined with the existing experimental conditions to forge the vane. Finally, by using optical microscope (OM), scanning electron microscope (SEM), energy spectrum analysis (EDS), micro-hardness and universal testing machine, analyzed relationship between microstructure and mechanical properties of the forged hub.The flow behavior of AZ80alloy was investigated at strain rate ranges of10-4～10-2s-1, and the result showed:1) The constitution equation for elevated-temperature deformation of the AZ80magnesium alloy could be described by ε=A[sinh(αб)]"exp(-Q/RT). For tensile deformation of extrusion alloy, the stress exponent n=3.13and the activation energy Q=147kJ/mol, while for compressive deformation of extrusion alloy, the stress exponent n=3.14and the activation energy Q=110kJ/mol. For tensile deformation of as-cast alloy, the stress exponent n=5.83and the activation energy Q=128kJ/mol, while for compressive deformation of as-cast alloy, the stress exponent n-5.39and the activation energy Q=147kJ/mol, the deviation of the peak stress predicted by the above equation from the experimental ones was under5%.2) The values of n and Q indicated that the self diffusion controlled dislocation climbing was the rate-controlling mechanism, supplemented by dynamic recrystallization deformation mechanism to coordinate the deformation.3) The dynamic recrystallization fine grain priority to nucleate at the triangle grain boundary and the interface between two phases.4) At elevated-temperatures, the dominant fracture mode was intergranular cracking which tended to nucleate at the triangle grain boundary and the interface between the a-Mg matrix, the cracking developed with further deformation.According to the above characteristics of AZ80magnesium alloy at elevated-temperatures, choose the appropriate deformation temperature, deformation rate and deflection to forge the extrusion alloy. Serious cracking happened in the forge that influent the finally forming of blade. This paper analyzed the reasons of failure and put forward the following suggestions:1) modify the die which is designed for Al alloy;2) decrease the velocity under1mm/s in the pre-forging process;3) optimize the extrusion technology, make the origin grain size is less than10μm.The result shows that, as the sequence of rim, outer rim, spokes and core, the ultimate strength decrease and the elongation almost unchanged, the ultimate strength and yield strength of rim was best as the twinning; a large number of lamella structure which is similar to pearlite in the forged hub, the lamella structure of two phase is formed with discontinuous precipitation which has obvious orientation relationship, and the precipitation grow toward the transgranular following some orientation, there is obvious interface between precipitine region and unprecipitine region; the hardness of lamella structure is in proportion of the mass fraction of second phase, with the mass fraction increase of β-Mg17Al12, the hardness of lamella structure arise; the forged AZ80product has uniform structure and excellent mechanical performance, obvious forging streamline can be found, the mechanical properties of parts parallel to the forged direction (rim) will be better than the parts vertical the forged direction (spoke). Magnesium alloy is poor to resistant corrosion so it must be protected carefully during the forging, control the temperature strictly to avoid grain coarsening that can improve the structure and properties further.