Calculation Of Shot Peening Stress Field And Prediction Of Aluminum Alloy Wall Panel Forming Based On DEM-FEM Coupling Modeling

Shot peen forming is a forming method that uses a medium with kinetic energy to impact the surface of a workpiece at high velocity,promoting deformation of the workpiece in a new stress balance.The forming effect is determined by the physical properties,kinetic energy,and quantity of the medium.It has high forming flexibility,low manufacturing cost,and is suitable for producing large-sized,variable thickness,thin-walled and lightweight integral wall panel parts.The key to shot peen forming technology lies in the rational planning of shot peening parameters and peening paths.In the planning process,the difficulty lies in how to accurately predict the shot peening forming effect for a set of determined peening parameters and peening paths.The purpose of this article is to use analytical calculations to reveal the mechanism of high strain rate local deformation of aluminum alloy under high-velocity impact of shots;Propose a partition statistics and thickness classification method for calculating the average induced stress in a random shot impact model to obtain more accurate average induced stress,and explore the effects of peening intensity and coverage on the deformation of strip specimens during shot peening forming;A quantitative relationship between peening parameters and average induced stress is established based on the discrete element finite element coupling calculation method,which is applied to the path planning and parameter determination of shot peening forming of aluminum alloy integral wall panels.This provides a theoretical basis and implementation method for predicting shot peening forming of aluminum alloy integral wall panel parts.The main research content of this article is as follows:(1)Establish an analytical stress field for shot impact using peening velocity and coverage as variables,providing a theoretical basis for selecting reasonable peening velocity and coverage for forming sheet metal.Calculate the induced stress after a single shot impact in three stages: elastic,plastic,and unloading.Based on the spherical cavity expansion model,the influence of strain rate on deformation is used to characterize the mechanical response of materials during high-velocity impact of shots.Derive a functional expression for the relationship between shot velocity,induced stress,and load displacement.Taking into account the randomness of shot distribution and the complexity of multiple shot stress fields,it is proposed to approximate the average induced stress of multiple shot impacts as a function of the centerline stress a single shot and the coverage.(2)In order to obtain a more accurate average induced stress,a method for calculating the average stress considering boundary conditions is proposed.Firstly,divide the peening area into different regions,and then average the stresses at all integration points within the specified area according to the depth coordinates.By comparing the average stresses in different regions,the optimal region for calculating the average stress is obtained.Subsequently,the finite element model of random shot impact is used to discuss the effects of boundary conditions such as path number,specimen shape,impact sequence,and mesh size on the average stress,further improving this method.Finally,this method is used to investigate the effects of peening velocity and coverage on average stress.(3)Explore the impact induced grain refinement and propose a numerical simulation method for directly predicting peening stress based on grain size.By conducting electron backscatter diffraction(EBSD)experiments,the local surface morphology of the shot impact area is characterized,and the grain size and orientation are obtained.Based on the dislocation density evolution model,numerical simulation is used to calculate the grain size after peen forming with different parameters,and compared with experiments.Based on the results of tensile tests,the crystal plasticity finite element method is used to perform uniaxial tension on representative volume units to calibrate the crystal plasticity parameters of the material.The influence of grain size on the stress-strain relationship of aluminum alloy wall panels is considered using the Hall Petch formula.A stress update algorithm based on grain size correction plasticity increment theory is used to obtain the effect of grain refinement on induced stress,and further predict the induced stress under shot impact.(4)Propose a combined DEM-FEM shot peening forming simulation method to predict the forming effect of large-sized specimens during shot peening,and use it to determine the parameters for shot peening forming of aluminum alloy integral wall panels.The motion simulation of the nozzle in strip peening using particle generators for the first time can reproduce the process of continuous superposition and evolution of stress fields in real shot peening forming.Due to the large number of shots that can be generated in the DEM-FEM coupling method,it is necessary to consider the impact of collisions between shots on impact velocity and analyze the distribution of impact velocity of shots under different mass flow rates and feed rates.Subsequently,an estimation equation for induced stress is fitted based on the simulation results.Introduce induced stress as the initial stress into the peening area of the specimen to predict the forming effect in the new stress equilibrium.The surface morphology of the extracted specimen is compared with the experimental results,and the simulated curvature radius is in good agreement with the experimental results,verifying the effectiveness of this method.(5)For the first time,a multi-point prestressing loading method and fixture based on equal difference active loading is proposed.This method can gradually provide complex prestressing through multiple passes,thus forming aluminum alloy integral wall panels with complex structures and large deformation.The use of the DEM-FEM coupled model provides the possibility of directly calculating the stress field of multi-point prestressed shot peening forming using numerical simulation methods.Specific peening parameters are designed for simulation of prestressed shot peening forming,and the simulation results are used as sample vectors for orthogonal training.A neural network is used to predict the formation of prestressed shot peening forming.The above analysis methods have been applied to the shot peening forming experiment of a certain type of wing wall panel.Taking the high rib integral wall panel as an example,peening parameter optimization and peening path planning are completed,achieving accurate prediction of shot peening forming effect under different peening parameters and ensuring forming accuracy.