Study On Residual Stress And Metal Flow Control During Hot Forming Of 2024 Aluminum Alloy Plates

Aluminum alloy 2024 is widely used in aerospace and automobile industries due to its excellent properties of high strength and good fatigue resistance.Since the ductility of 2024 aluminum alloys is not high enough to form parts with complex geometries at room temperature,hot forming processes must be adopted when dealing with 2024 aluminum alloy sheets.However,the forming of aluminum alloy sheets at elevated temperatures always induces inhomogeneous plastic deformation in the metal matrix.Hence,residual stresses remain in parts even after unloading.Residual stresses not only reduce the fatigue strength and dimensional stability of aluminum alloy sheet components,but also increase the risk of stress corrosion cracking,and result in unexpected distortion due to the release and redistribution of residual stresses in subsequent machining processes.Therefore,it is of great necessity to study the residual stress and its control and elimination technology during hot forming of high strength aluminum alloy sheets.In addition,aluminum alloy sheets are usually formed by tensile deformation during hot forming process,and the wall thicknesses around small corners of parts are seriously decreased,which cannot meet the requirement of high strength in actual application.Therefore,it is necessary to optimize the metal flow during hot forming of aluminum alloy sheets,so as to improve the metal filling ratio at small corners of parts.The main work done and the main resuts obtained are as follows.Firstly,the hot working parameters of 2024 aluminum alloy were optimized,and the hot forming limit diagram of the alloy was obtained.Base on the tensile and compressive experimental data of 2024 aluminum alloy at different temperature(300~450?)and strain rates(0.001~1s-1),the optimized hot working parameters of 2024 aluminum alloy were obtained by using hot processing map technology based on dynamic material mode theory,which could provide theoretical guidance for the development of hot working parameters of 2024 aluminu alloy.Through Erichsen cupping tests and finite element simulations,the hot forming limit diagram of 2024 aluminum alloy was obtained,which could provide the basis for the determination of fracture of 2024 aluminum alloy sheets during hot forming in finite element simulations.Secondly,the residual stresses during hot stamping of 2024 aluminum alloy plates were investigated,and the mechanism of residual stress generation during hot stamping of aluminum alloy plates was obtained.The effects of hot stamping parameters such as forming temperature,blank holder force,die entrance radius,etc.and blank local thickening process on the residual stresses of square cups were studied,by conducting both finite element simulations and experiment investigations of the hot stamping of 2024 aluminum alloy plates.The mechanism of residual stress generation during hot stamping of aluminum alloy plates was mastered,and the optimized hot stamping parameters with low residual stress were obtained.Comparing to flat plates,using local-thickened plates can significantly reduce the residual stresses in hot stamped square cups.The maximum residual stress value was reduced by 58.3%.A residual stress calculation modle based on a wide sheet stretch-bending theory was established.Residual stress measure results showed that the model is capable of describing the distribution features of residual stress in the bottom circular arc of hot stamped square cups.Again,the residual stresses during hot stamping-forging of 2024 aluminum alloy sheets were studied,the variation of residual stress field and the elimination mechanism of residual stress during stamping-forging processing were obtained.Firstly,the variation of residual stress field during stamping-forging processing was studied,by conducting finite element simulations of the bending-upsetting of V-shaped parts with 2024 aluminum alloy sheets.Then,the stamping-forging processing was employed in the forming of square cups with nonuniform thickness,which are widely used as support components in aerospace applications,so as to control and adjust the residual stress in the square cups.The research results showed that the stamping-forging processing could reduce residual stress by changing the plastic deformation mode of local fibers in the metal sheet.Compared to bended V-shaped part,the residual stress in stamping-forged V-shaped part was reduced significantly.The maximum tangential residual stress value was reduced by 52%.The residual stress values in square cups decreased gradually during stampingforging processing.The maximum residual stress value can be reduced from 190 MPa for a deep drawn square cup to around 60 MPa for a stamping-forged square cup.Residual stress in stamping-forged parts is closely related to stamping-forging paremeters.Larger punch radius and die corner radius,or smaller die entrance radius are conducive to obtaining lower residual stresses in the stamping-forged square cups.Then,the hot stamping-forging experiments of square cups with nonuniform thickness were carried out,and the square cups with good dimensional accuracy and lower residual stress were obtained.According to the forming characteristics of square cups,a set of die system compose of universal die sets,universal fixing components,special die cores and different height adjustment units were designed and manufactured.Then,hot stamping-forging experiments of square cups were carried out,and the square cups with good dimensional accuracy and lower residual stress were obtained.Experimental results showed that the side-wall thickness of stamping-forged square cups is about 3.5mm after ironing and upsetting,which achieves the dimensional accuracy requirement for actual parts.The residual stress value was reduced from 200 MPa in the machined square cup to around 70 MPa in the stamping-forged square cup,with a reduction magnitude of 65%.Thus,the dimensional stability and reliability of square cup can be improved significantly in actual application.Lastly,the metal flow during hot stamping-forging of square cups with localthickeded plates were investigated,and the metal flow was controlled and optimized.With the utilization of a commercial FEM program,ABAQUS,the metal flow during hot stamping-forging of square cups with local-thickened plates were investigated.Base on this,the metal flow was optimized by varying the shape of the local thickened section and adjusting the friction conditions at the interface between blank and tools,so as to improve the metal filling ratio and achieve a small radius at the bottom corner of square cups.Hot stamping-forging experiments confirmed that the filling ratio of the bottom corner was significantly improved,and the obtained square cup can meet the requirement of a small radius of 2mm at the bottom corner.