Simulation Of Clustering Balls Spinning Method Forming Magnesium Alloy Curved Surface By The GTN Damage Model

Magnesium alloys have a broad development potential due to their low density,high specific strength,and strong capacity of antivibration.However,the plastic deformation of the magnesium alloy plate is poor at room temperature,and the forming method is commonly hot forming.Not only the cost of hot forming is high,but also the magnesium alloy plate is vulnerable to the defects of oxidation and coarse grain size.A forming method at room temperature to form the magnesium alloy plate is highly desired.Clustering balls spinning(CBS)method is a novel flexible sheet forming technology that combines the advantages of multi-point forming,single-point incremental forming and spinning forming to meet the requirements of modern industry for low cost,small batches and personal curved parts.In this paper,complex curved surfaces of AZ31 magnesium alloy sheet will be formed at room temperature by the CBS forming process to solve the problem of poor formability and low forming accuracy of the AZ31 sheet.The main method of research in the paper is the finite element method(FEM)simulation that can simulate the whole forming process,and help to reveal the deformation mechanism of the plate during the forming process,and provide a guide for the experimental process.At the same time,a modified GTN model was combined with the FEM of CBS to investigate the forming limits,fracture mechanism,and optimize parameters of the process for magnesium alloys.The FEM was applied to the forming process to solve the problem of non-uniform stresses and local deviations in non-rotationally symmetric workpieces.In addition,the theoretical analysis provides the theoretical basis for the finite element simulation.The main work and research results are as follows:(1)Through tensile experiments,the constitutive parameters of the modified GTN damage model were determined.The modified GTN damage model was applied to simulate the shear-tension test of the AZ31 sheet,and the experimental results demonstrated the modified GTN damage model can effectively simulate the damage process of AZ31 sheet.(2)Comparing the simulation and the experiment results of the CBS spinning process with the no-spinning process,it is demonstrated that the spinning process can improve the forming limit of magnesium alloys.At the same time,the pre-deformation of the sheet during forming process is determined.Exploring the effect of rotational speed and friction coefficient on the forming limits of magnesium alloys in the CBS process.The results of simulation show that the speed of rotation of the steel ball decreases the forming limit of the material.The higher the coefficient of friction between the steel balls and the AZ31 sheet,the higher the forming limit of the material.The influence of the coefficient of friction on the forming limit is much greater than the influence of the rotation speed on the forming limit.The coefficient of friction changes the tangential stresses and the stress state in the sheet.Taking into account the surface quality and temperature,the friction coefficient in the CBS spinning process is selected as 0.15 and the rotation speed is set to 24 r/min.(3)In order to solve the problem of poor local forming accuracy for nonrotationally symmetrical workpieces in the CBS forming process,the asymmetrical forming method is proposed.The asymmetric forming method can locally change the local deviation from the target surface.When the bottom die moves 5 mm to the left,the local deviation from the target surface decreased from 0.57 mm to 0 mm.The stress components of non-rotationally symmetrical workpieces under asymmetric forming methods were analyzed and it was found that the asymmetric forming method can locally change the stress components of the sheet.By combining the stress components obtained from the simulations with the theoretical equations,it can be found that the stress components in the CBS process are related to the sheet thickness and the radius of rigid balls.(4)The microstructure of the magnesium alloy sheets in the experiment shows that the main deformation method of the sheets is{10(?)}tensile twinning.With the development of forming process,the sheet temperature was raised,and more slip systems were activated which improved the forming limit of the magnesium alloy.Meanwhile,it can be found that the c-axes in the grains had a small angle rotation which was caused by the shear deformation of rigid balls during the CBS process.