Research On Texture Evolution Mechanism Of Magnesium Alloy Cylinder Parts With Inner Ribs During Hot Power Spinning

With the continuous increase components requirements of lightweight,high precision and high performance in aerospace industry,the demand for thin-walled shell parts with remarkable structural benefits is rising sharply.Magnesium alloy cylindrical parts with inner ribs have the double lightweight advantages of both material and structure,and the development and improvement of its preparation technology and forming theory will greatly promote the core competitiveness of cutting-edge equipment.Hot power spinning is one of the most effective methods to fabricate thin-walled cylindrical parts of magnesium alloy and other difficult-to-deform materials.The easily formed deformation texture and the anisotropy of mechanical properties which is caused by deformation texture are the major factors restricting the performance improvement of magnesium alloy.Especially,the uniqueness of inner ribs in the cylindrical parts with inner ribs is not only bring the new challenges for the hot power spinning forming when contrast to the forming of cylindrical parts,but also lead to the different deformation condition which causes the difference of texture between cylindrical wall and inner rib.Therefore,the influence of hot power spinning process parameters on the texture evolution,the mechanism of texture evolution and the influence of texture on the mechanical behavior are the core scientific problems for the preparation of high performance magnesium alloy cylindrical parts with inner ribs.The extruded ZK61 magnesium alloy tubular was taken as the research object,and the construction of the texture evolution model for hot forming of magnesium alloy,the effect of hot power spinning process parameters on the texture evolution,the evolution mechanism of hot power spinning texture and the effect of spinning texture on the mechanical properties of spun workpiece were emphatically studied.According to the requirements of physical simulation test for the characteristics of hot power spinning deformation of cylindrical parts with inner ribs,uniaxial hot compression test for extruded ZK61 magnesium alloy was carried out,and the crystal plastic constitutive model of extruded ZK61 magnesium alloy under hot forming was constructed.The time integral algorithm and the subroutine of the constructed model were developed.The polycrystalline deformable geometrical model with the characteristics of microstructure and texture was reconstructed for the spinning blank with significant initial texture,and the construction method of crystal plastic finite element model(CPFEM)for magnesium alloy hot forming was obtained.In order to determine the parameters of the crystal plastic constitutive model and verify its reliability,the CPFEM was used to simulate the meso-deformation during uniaxial hot compression.Focus on the effect of hot power spinning process parameters on the texture evolution,a macro-meso coupled texture evolution model of hot power spinning process was established;and the deformation textures in the cylindrical wall and inner rib of the magnesium alloy cylindrical parts with inner ribs were predicted and analyzed under different spinning process parameters.The effects of spinning process parameters on the texture evolution and the mechanism of texture evolution were obtained.On the basis of the above theoretical research,the hot power spinning experiment was carried out to test the microstructure,texture and mechanical properties of spun workpiece,and to verify the predictive ability of macro-meso coupled model for texture evolution,and to analyze the influence of microstructure and texture changes on the mechanical properties during hot power spinning.Under the crystal plasticity framework,an improved Voce hardening model was proposed.By introducing the intermediate strain hardening rate factor to describe the significant softening phenomenon in the hot deformation process of magnesium alloy,the slip and the hardening evolution of the twinning deformation system and the grain orientation transformation were controlled,and the plastic constitutive model of magnesium alloy was constructed and solved.With the aid of ABAQUS,the crystal coordinate system was constructed to realize the identification of grain orientation in the macro coordinate system,and the bridge of data exchange between the main program and the subroutine was conducted by allocating the state variables,the subroutine of the constructed crystal plastic constitutive model was developed.The Voronoi theory based representative volume element modeling results show that the reconstructed model has the same texture type as the experimental results,and the relative errors of the maximum pole density in pole figure and inverse pole figure are 1.89%and 5.1%,respectively,which realizes the reconstruction of the meso-characteristics of spinning blank in the finite element simulation.By combining the reconstructed polycrystalline with the hot deformation crystal plastic constitutive,the CPFEM for the study of texture evolution of magnesium alloy was built,and the macroscopic stress-strain relationship and texture evolution during hot compression were predicted,and the model parameters were determined.The results show that the maximum average relative error of stress and strain prediction is only 3.36%in the temperature range of 250??350?and strain rate range of 0.01s^-1?1s^-1,and the texture prediction can also be realized under different temperature and deformation rate.The macro-meso coupled modeling for hot power spinning of magnesium alloy cylindrical parts with inner ribs was used to study the texture evolution in cylindrical wall and inner ribs of the spun workpiece under different thinning ratio of wall thickness and temperatures.The results show that under the same thinning ratio of wall thickness and temperature,the deflection angle of grain's c-axis at cylindrical wall is larger than that at the inner rib.The accumulative shear strain of the slip system increases linearly with the increasing of thinning ratio of wall,which will lead to the increasing of texture strength.The basal sheet texture characteristics of the blank are still retained at cylindrical wall and inner ribs under the lower spinning temperature.With the increase of spinning temperature,the grain's c-axis at cylindrical wall and inner ribs will deflect by a larger angle,and the texture strength will be strengthened.The sheet texture characteristics will be weakened.Compared with the cylindrical wall,the macroscopic deformation degree at inner rib fluctuates more severer with the temperature variation,which is the reason that the temperature has a greater effect on the texture at inner rib.The mechanism of texture evolution was revealed by tracing the grain orientation changes during hot power spinning.The results show that the grain's c-axis of blank gradually deflects from parallel to tangential direction to parallel to radial direction.Under the radial compressive stress,the texture at cylindrical wall is formed by the orientation deflection of the grains which have the large Schmid Factor(SF)of the base plane and located in the position with the maximum texture strength in blank,while the texture at inner rib is mainly caused by the grain orientation deflection with small(0.1?0.2)SF of the base plane.The results of the mechanical properties analysis of magnesium alloy cylindrical parts with inner ribs show that the elongation of the spun workpiece is significantly improved.The yield strength(YS)and ultimate tensile strength(UTS)are basically the same level as that of the blank,and the YS and UTS at cylindrical wall are slightly lower than that of the blank.In addition,the direction of the fracture surface of the tensile specimen taken from blank and spun workpiece are different.By the analyzing the reasons of the strength change and fracture direction before and after hot power spinning,the results show that the different direction of c-axis between the blank and the spun workpiece and the freedom of grains of spun workpiece around its c-axis were the reasons for the different direction of macro fracture surface.and the strength variety before and after hot power spinning is traceable in the competition of fine-grain strengthening and softening action of texture.