Basic Research On Hot Bulging Process Of Solid Granules Medium For Variable Diameter Tube Made Of AZ61 Magnesium Alloy

The SGMF(Solid Granules Medium Forming)process is a new method of tube bulging by using dispersed particles and free-flowing particles as the intermediate force medium to replace the role of rigid punch(or die)for plastic forming of tubes.This research mainly focuses on the solid granule medium hot bulging process of variable diameter tube made of AZ61 magnesium alloy,using the excellent plasticity of magnesium alloy at high temperature and the unique force transfer characteristics of solid particle media to realize the formation of the variable diameter tube.A new method of forming variable diameter tube to use external punch was presented.Through both of numerical sand experimental researches,the forming processes of two different bulging methods of external punch and internal punch were studied.The finite element simulation software Abaqus was used to simulate the solid granule medium bulging process of variable diameter tube made of the AZ61 magnesium alloy.The solid granule medium was regarded as a complex fluid,which were approximated by the Drucker-Prager expansion model.The magnesium alloy tube was divided into the deformable solid elements,the female die and punch are defined as analytical rigid bodies.The influences of different punch types,punch pressure,initial punch height and other process parameters on the bulging process were analyzed.The relationship between the special pressure transfer law of solid granules medium and the bulging process was discussed,and two different bulging styles were compared.The differences in force applying styles,stress and strain relationship,metal flow role,and wall thickness distribution have summarized the appropriate range of process parameters.Experiments and simulations have found that there is a maximum stress peak in the pressure transfer of solid granules medium,and the positional relationship between the maximum stress peak area and the maximum deformation zone in the process of punch downward movement largely affects the deformation results,and the external punch bulging method exists a clear passive axial feeding behavior.Therefore,controlling the deformation conditions is a key factor to avoid defects and uniform distribution of wall thickness.Based on the numerical simulation results,the experimental study on the hot bulging of the solid granules medium for variable diameter tube made of AZ61 magnesium alloy was carried out.The bulging mould corresponding to the two indenters were designed respectively,and small steel balls with a diameter of 1 mm were used as the solid particle media.The experimental results show that the suitable experimental temperature for thehot bulging of AZ61 magnesium alloy variable diameter tube is 370°C.For both of the external and internal punches,in order to study the metal flow role,the same initial punch height was used for the different amount of feed experiments to observe the changes in the profile of the tube during the entire bulging process.With 65 mm-125 mm at intervals of 10 mm the different initial punch heights of the hot bulging tests were used to study the effect of initial punch height on wall thickness distribution,forming pressure and internal surface roughness.The evolution of microstructure in the process of deformation was analyzed by using an optical microscope.The numerical results of thickness distribution are good agreement with the experimental results.When the external pressure head bulging initial pressure head height is 115 mm,the maximum difference between the experimental results and the numerical results is 13.27%,and the maximum difference between the built-in pressure heads is 12.03%,indicating that the simulation results can guide the experiment.