Study On Damage Model And Ductile Fracture Behavior For Tube Power Spinning

As a typical continuous partial plastic forming process,spinning technology has significant advantages for the forming of thin-walled rotating parts.However,since the metal needs to undergo a complex deformation process during the spinning forming process,ductile fracture may often occur in the process,which will severely restrict the application of spinning technology.For this reason,this thesis uses a combination of experimental research,theoretical analysis and finite element methods to conduct an in-depth study on the mechanism of damage evolution during spinning.Firstly,a modified GTN(Gurson-Tvergaad-Needleman)model was developed by re-defining the damage evolution law under the negative stress triaxiality.The inverse method based on the optimization technique was used to calibrate the modified GTN model parameters based on the different stress states mechanical experiments.Based on the modified GTN model in this paper,the tube spinnability test of 2024-T351aluminum alloy was simulated.The comparison of the maximum thinning rate between the simulation and the experiment shows that based on the modified GTN model,the error of the thinning rates between prediction and experiment was only 8.36%,which verifies the applicability of the modified GTN model in spinning.Based on the simulation result,it can be known that the location of maximum damage zone located in the outer layer when the thinning rate was small.With further increase of thinning rate,the maximum total damage to move from the outer to the inner surface,and eventually the shear damage will be too large to induce crack initiation.Through the multi-pass spinning experiment of 2024-T351 aluminum alloy,it is found that when the thinning rate of each pass is relatively low,the material fractured during the spinning process.However,when the thinning rate in each pass is appropriately increased,no cracks appear in the material during the spinning process process.If the thinning rate is further increased,the material will fracture during the spinning process.The experiment results show that the formability limit of 2024aluminium alloy in the multi-pass spinning process is closely related to the thinning rate in each pass.Based on the modified GTN model,the damage evolution in different spinning passes was simulated.The shear damage should be responsible for the final fracture during single-pass tube spinning process.During multi-pass tube spinning,the void damage should be responsible for the final fracture when the thinning rate was relative small.With further increase of thinning rate,the cracks would introduced by shear damage.Through analysis,it is found that the limit reduction rate achieved by multi-pass spinning is greater than that of single-pass spinning is that after the material is deformed in the first pass,the material is work hardened,while the relative reduction in the second pass is low,resulting in less deformation of the inner layer by the roller,so that its shear damage is not fully increased,and finally the limit thinning rate of multi-pass spinning is greater than that of single-pass spinning rate.To increase the spinnability in the multi-pass spin forming process,the thinning rate in each pass was recommended to be around 21%.Secondly,taking Mg-6Gd-5Y-0.3Zr alloy as the research object,the high-temperature flow stress-strain model of the alloy is constructed using the relationship between stress and dislocation and kinetic equation of dynamic recrystallization based on the hot compression experiment.But this kind of model can not be used to predict the material fracture during the deformation process.Through the analysis of the hot deformation of the material,it is known that the recrystallization will have a significant impact on the macro-mechanical properties of the material.In order to construct the damage evolution mechanism of the Mg-6Gd-5Y-0.3Zr alloy under high temperature deformation conditions,this paper uses the uncoupled model proposed by Lou as the basis,and for the first time defines the reference Zener-Hollomon value(Z₀)to construct the ductile fracture strain and Z The relationship between the parameters and the C value that affects the fracture cutoff surface in the model is defined as a function of temperature and strain rate,thereby constructing a high temperature fracture model coupled with recrystallization volume fraction.Then,based on the continuum damage mechanics framework,a high-temperature constitutive model coupled with recrystallization volume fraction and damage was constructed,and the VUMAT user material subroutine is developed.Based on the established thermal damage constitutive,a finite element model of Mg-6Gd-5Y-0.3Zr alloy hot spinning was constructed,and the reliability of the model under hot spinning conditions was verified by comparison with experiments.Finally,in order to have a deeper understanding of the influence of spinning deformation on the subsequent mechanical properties of the material,this thesis takes the hot spinned Ti-6Al-4V alloy as the research object,based on the finite element method of low-order strain gradient plasticity theory(CMSG),constructs a mesoscopic scale model.This model takes into account the strengthening effect brought by the interface.Compared with the classic plastic model,it can capture the influence caused by the size of the second phase,so it has higher accuracy.