Study On Prediction And Mechanism Of Welding Distortion For Large Scale Thin-walled Structure Of Propellant Tank

Propellant tank is an important component of launch vehicles.The parts of the tank are mainly joined by the welding process.The geometrical accuracy of the tank after the welding process directly influences the manufacturing quality and the performance of launch vehicles.The structural stiffness decreases significantly due to the increase of the dimension of the new generation propellant tank.Thus,the obvious welding distortion easily appears when the clamps of welded structures are released,which seriously influences the manufacturing precision.The decrease of the structural stiffness enhances the effect of geometric nonlinearity on the out-of-plane distortion.Meanwhile,the mechanical parameters of the material change non-uniformly induced by the thermo-mechanical coupling effect.These two factors are main obstacles in the prediction of the welding distortion for large scale thin-walled structure.The welding process involves the material,the process and the microstructural parameters.The evolution process of temperature and stress could not be described accurately.Consequently,the welding distortion control is restricted for large scale thin-walled structures.Thus,it is of great significance to predict the welding distortion accurately and study on the mechanism and the control method of the welding distortion for the large scale thin-walled structure.A prediction method of welding distortion is proposed for large scale thin-walled structures to solve the problems of geometrically nonlinear and non-uniform change of material parameters in the welding process.The prediction accuracy of the welding distortion is improved compared with the inherent strain method.The welding stress model is developed for the friction stir welding process.The evolution of the stress gradient is studied,which could help to reveal the mechanism of welding distortions with various process parameters.A critical stress model of buckling analysis is established for large scale thin-walled structures by using the non-uniform stress distribution.The influence of geometrical parameters on the critical stress is studied.In addition,the welding stresses induced by various process parameters are compared with the critical stress,which is beneficial to the process parameter optimization.The proposed method is applied to predict the welding distortion of the propellant tank.The geometric quality of the tank is improved with the optimized process parameters.The main contents and conclusions of this paper are listed as follow.(1)Accurate prediction method for nonlinear welding distortion of large scale thin-walled structureAccording to the structural stiffness,the weld is separated into several parts to take the edge effect into consideration.In addition,the constraint effect of the elastic region is introduced in the numerical simulation.The finite deformation theory is applied in the simulation of the welding process in a local model.Considering the feature of the welding stress distribution,the stress tensor is mapped on the welded region of the full-size model to predict the welding distortion.In addition,the change of the elastic modulus is introduced into the full-size model.Thus,a three-dimensional stress mapping method is proposed to predict the welding distortion for the large scale thin-walled structure.The characteristics of welding distortion are investigated for the friction stir welding process.The proposed method is verifyed by the welding experiment.Comparing with the inherent strain method,the prediction accuracy of the welding distortion is improved with the three-dimensional stress mapping method.(2)Welding distortion mechanism of thin walled structure induced by friction stir weldingThe temperature distribution in the welding process is obtained for the friction stir welding(FSW)process.The welding stress in the process is solved based on the equilibrium equation and the yield criterion.The shrinkage effect induced by the cooling process is also taken into consideration to calculate the welding stress before releasing clamps.The analytical result can present the feature of the M-type distribution of the welding stress in friction stir welded joints successfully The influences of the process parameters on the evolution of the welding stress are studied.Then,the stress gradients along the thickness direction are investigated with various process parameters.The correlationship between the stress gradient and the welding distortion induced by FSW is established.The mechanism of the welding distortion on the thin-walled structure is revealed by using the gradient of welding stress.(3)Theoretical model of critical stress of buckling distortion under non-uniform welding stressThe non-uniform distribution of welding stress is studied quantitatively based on the model of welding stress before releasing clamps.Considering the distribution characteristics of welding stress in different directions,the theoretical model of critical stress of buckling distortion is built with the energy method.The deflection function of structural instability under different boundary conditions is described by Chebyshev polynomials.The buckling critical conditions for the straight and circumferential welded joints are calculated.The variation of the critical conditions of the buckling distortion with different geometrical parameters and process parameters is studied.This investigation is beneficial to the rational selection of welding process parameters to control the buckling distortion.(4)Prediction and analysis of welding distortion for propellant tankThree types of the weld is classified according to the structure characteristics of the propellant tank.The three-dimensional stress mapping method is applied to predict the welding distortion of the whole tank.The numerical results are compared with the experimental data to verify the effectiveness of the welding distortion prediction method.The welding distortion of the tank is predicted with the optimized welding parameters.The shape accuracy,location accuracy and dimensional accuracy are analyzed for the propellant tank.Results show that the welding distortion of the structure is obviously reduced,and the geometric dimensions meet the requirement of the manufacturing precision.