Deformation Prediction And Fixture Layout Optimization Of Thin Walled Workpiece

Thin-walled parts are widely used in aerospace industry,due to their higher structural efficiency and light weight characteristic.From the another point of view,these parts are complex in structure,weak stiffness,high precision demand and easy to distortion.During the machine process,their material properties and structural characteristics,the action of local elastic deformation,initial residual stress and residual stress redistribution in machining process,fixture clamping selection that will heavy affect the deformation of thin walled workpiece.This deformation problem has been plagued aerospace industry.In order to study the factors which cause deformation and then effectively reduce the distortion.In this dissertation,base on theoretic analysis,through applying of finite element simulation method and engineering experiments verification to predict the mechanism of thin walled wokpiece deformation,which make it possible to control the machining error.Based on the equivalent oblique cutting model,a finite element model of milling process in three-dimensional was established.Through this model,several key technologies of finite element simulation method was studied and analyzed,such as material constitutive equation,thermal and force coupled finite element equations,tool-chip separation criteria,tool-chip friction model and the energy of bulk consumption and the local heat transfer processes.Through the three-dimensional finite element simulation,the detailed process are studied and analyzed,which includes the chip's geometric evolution processes and its geometric shape under force action.Meanwhile,it reveals the dynamic performance of cutting phenomenon;focus on analysing the calculated results that including the distribution of stress,strain,equivalent strain rate and temperature.The data of cutting force and cutting temperature can be used for further study of residual stress and the mechanism of deformation that caused by fixture clamping,thus reducing the corresponding experiment.By analyzing the principle of residual stress and then compared the measurement methods,using crack compliance method reveals the pre-stretched aluminum alloy sheets initial residual stress distribution,the residual stress results showed that the exteriors are compressive residual stress and the interiors are tensile residual stress.Based on finite element method,an analysis model was obtained to study the cutting parameters and other factors will impact on the residual stress,it found that the proposed optimal cutting parameters can be affected on controlling residual stress.With the same experimental conditions,and using the X-ray diffraction to verify the cutting speed,feed rate and cutting depth factors on the distribution of residual stress.These works can be considered as a foundation for study processing routes and clamping scheme.Based on the location change model,the thin walled workpiece deformation control model was established.Considering the fixture contact with workpiece,the contact deformation error and location error was described with proposed qualitative criteria calculation Equation.Through the location change model to predict workpiece deformation,as the system stiffness changes,the contact force between location unit and workpiece was predicted;a rectangle unit finite element model of joint stiffness was established.We can find that select a large flat surface for location,increasing the effective contact area that will effect to control the workpiece deformation.Using topology optimization techniques,an assistant support fixture layout optimization method was established.To reach uniform deformation and reduce deformation as objective function,using the finite element method and topology optimization method,through introducing the pseudo-density method,the fixture layout optimization as pairs of approximate solution of continuous variable optimization process,and then the topology optimization model was established.Meanwhile,a method of topological fixture systhesis was introduced.This method based on the topology optimization,focusing on determining the position of locators and clamps to minimize workpiece deformation.Compared with traditional"3-2-1" locating principle,this method had more flexibility and non-localized.The topological fixture synthesis may result in innovative fixture configurations that substantially reduce workpiece deformation.