外文翻译原稿.pdf

Multiple iteration springback compensation of tailor welded blanks during stamping forming process H Wang J Zhou T S Zhao L Z Liu Q Liang College of Material Science and Engineering Chongqing University Chongqing 400030 China a b s t r a c ta r t i c l ei n f o Article history Received 24 November 2015 Received in revised form 8 April 2016 Accepted 10 April 2016 Available online 19 April 2016 The forming and springback of tailor welded blanks TWBs are more complicated than those of conventional sheet metals because of the infl uence of weld seam The current study investigated the microstructure and frac tureofTWBsthrough metallographic microscopy and scanning electronmicroscopy Tensile tests show that thin and thick materials possess similar mechanical properties The maximum elongation and yield strength of the TWBs decrease whereas the tensile strength of the TWBs is larger than that of the base materials We proposed the use of multiple iteration springback compensation to improve the accuracy of springback compensation for the TWBs The simulation of the entire forming processes which includes drawing trimming and piercing as wellasspringbackanalysis wasusedtoexplorethestampingformingoftheTWBsbyautomotivefl oorreinforce ment Forming parameters were optimised in drawing process In addition springback calculation was per formed to predict the springback of the formed part Furthermore multiple iteration springback compensation was applied to drawing tools and verifi ed by experiments The experimental results agree with the simulation results 2016 Elsevier Ltd All rights reserved Keywords Tailor welded blanks Microstructure Fracture Springback compensation Finite element method 1 Introduction Tailor welded blanks TWBs are composedof two or more pieces of welded sheets which can be formed together in a single forming operation the welded sheets may have different thicknesses mechanical properties or surface coatings 1 3 With the trend of lightweight automotive TWBs forming has become an effective method to achieve lightweight automobiles The technique can enhance theintegralautomotivebodystiffnessandsecuritywhilesavingmaterials and relieving vehicle weight 4 Therefore the application of TWBs in automobile has been increasing annually Numerous materials including low carbon steels and high strength steels HSS and aluminum alloys can beused as TWBs 5 6 HSS TWBs are rapidly developed in the automobile industry because of their high yield strength and good welding performance TWBs forming has become a hot issue in recent years Abbasi et al 7 analysed the wrinkling behavior of TWBs in deep drawing Nguyen et al 8 improved the drawability of high strength differential TWBs by using the fi nite element method FEM Moreover Bandyopadhyay et al 9 proposed that the limiting drawing ratio and deep drawing behavior of dual phase steel TWBs could be improved with restricted weld movement by controlling the initial weld line position Mamusi et al 10 introduced a novel approach to acquire the forming limit diagram FLD for TWBs Song 11 and Safdarian 12 investigated the effect of thickness ratio on the formability of TWBs The researchers emphasised that the formability and the level of FLD decrease as the difference in the thickness ratio of TWBs increases Previous studies on TWBs mainly focused on the microstructure mechanical properties wrinkling behavior FLD drawing ratio and formability of these materials Few studies reported on the springback compensation of TWBs As is well known springback is a common and ubiquitous problem in the sheet metal forming 13 Springback appears when tools are removed from the formed part 14 An exces sivelylargespringbackcancauseconsiderableshapedeviationbetween the fi nal and designed parts and may lead to certain diffi culties in the subsequent forming operation or assembly Although the FEM has been successfully used to simulate the stamping forming of complex parts the approach still does not offer a satisfactory solution regarding springback 15 To date previous studies on springback mainly focused on simple geometries such as cylindrical tooling L bending U bending and V bending 16 However studies on springback compensation for complex parts are limited Furthermore numerous tool manufacturers deal with theproblem of springbackbasedonexperience of designers Numerous small enterprises neglect springback at the beginning of die design The initial tryout parts are inspected by a special fi xture the die surface is modifi ed on the basis of inspection results Complex parts usually require three or more modifi cations of tool shape and die tryout before the formed part meets the allowable tolerance requirement 17 Evidently the traditional approaches may result in uncertainty in predicting costs and lead times Materials and Design 102 2016 247 254 Corresponding author E mail address jz2012 J Zhou http dx doi org 10 1016 j matdes 2016 04 032 0264 1275 2016 Elsevier Ltd All rights reserved Contents lists available at ScienceDirect Materials and Design journal homepage Therefore thepresentstudyaimstoinvestigatethesimulationofthe entire forming processes and springback compensation for TWBs The FEM was used to simulate the forming process and predict springback and multiple iteration springback compensation was introduced to improve the accuracy of springback compensation Finally the proposed method was verifi ed by experiments 2 Materials 2 1 Microstructure The TWBs used in the present study were made by laser welding of two steel sheets having same chemical compositions and different thicknesses 1 2 and 1 6 mm The chemical composition of the base material HC340 590DPD Z used in TWBs is presented in Table 1 The base material belongs to dual phase HSS Given their good perfor mance thesetypes of TWBsare widelyused as structuraland reinforce ment components in the automobile industry The base materials and the welded material signifi cantly differ in microstructure and mechanical properties 18 The microstructure of the base materials and the weld seam zone was determined via metal lographic microscopy 19 Microstructures of the welded joint the base material the heat affected zones HAZ and the weld seam of the TWBs are presented in Fig 1 The weld joints possessed a narrow weld seam and a small HAZ and a clear boundary exists between the basematerials and theweld seam The microstructure differedindiffer entregions becausethebasematerialswereintherollingstate theHAZ was a gradient structure region caused by the welding thermal cycle and the weld seam was in the casting state The microstructure of the base materials were composed of ferrite and martensite and the martensite structure was distributed in the ferrite matrix with island dispersion Fig 1b The microstructure of the HAZ was mainly com posed of ferrite pearlite and a small amount of martensite Fig 1c The microstructure of the weld seam was composed of bainite and martensite Fig 1d The mechanical properties of the TWBs were determined by the microstructure of the base materials and the weld seam 2 2 Tensile tests Tensile tests according to ASTM E8 standard test 20 were used to obtain the mechanical properties of the base materials while samples of welded material based on Ref 19 21 were used to get the mechanical properties of the TWBs material Tensile tests were conducted at room temperature with a controlled displacement rate of 5 mm min The true stress strain curves of the base blanks and the TWBs are presented in Fig 2 The result shows that the true stress strain curves of the thin and thick sheets are similar The yield strength of the base materials thin and thick materials are 442 09 and 432 47 MPa respectively whereas that of the TWBs is 373 15 MPa The elongation of the base materials thin and thick materials are 26 17 and 26 46 respectively whereas thatoftheTWBsis18 48 FromFig 2 wecanconcludethatthemechan ical properties of the thin and thick materials are similar Moreover the infl uence of the weld seam can be clearly observed The welded blank decreases the maximum elongation of the tested material In addition the strength of the TWBs is larger than that of the base materials From these properties we can conclude that high strength and low elongation reduce TWB formability increase the risk of wrinkling cracking and springback and increase the need for forming forces 22 The failure conditions of the material should be identifi ed to minimise the faults during production 23 The fracture surfaces Table 1 Chemical composition of the base material used in TWBs wt CSiMnPSAlFe b0 15b0 6b2 5b0 04b0 015N0 01Balance Fig 1 Microstructures of a cross section of the welded joint b base material c HAZ and d weld seam Fig 2 True stress strain curves of the base materials and the TWBs 248H Wang et al Materials and Design 102 2016 247 254 were characterised by TESCAN VEGA3 LMH scanning electron micros copy SEM equipped with an energy dispersive X ray spectrometer and magnifi cation of up to 1 000 000 SEM micrographs of the fracture surfaces after the tensile test are presented in Fig 3 The images show numerous isometric dimples on the fracture surface of the base material This result indicates that the fracture type of the base materials is a typical ductile fracture Fig 3a and b As illustrated in Fig 3c the fracture surfaces of the weld seam of TWBs show the river pattern and numerous dimples surrounding the cleavage facets evidently indicating the presence of ductile and cleavage fractures 3 Methods The forming process simulation and multiple iteration springback compensation are illustrated as a fl owchart in Fig 4 Firstly analysis and designoftheprocesswerecarried out on thebasisof the character istics of the part In the fi rst stage the forming processes of the part should be planned Once the process design is fi nalised the addendum and binder designs which are called die surface design can be carried out The simulation of the entire forming processes which includes drawing trimming piercing and fl anging is performed to verify the formability of the designed processes and determine whether or not the designed surface can be formed without any defects Tooling geom etry and process parameters may need to be modifi ed or redesigned through several iterations based on feedback from the simulation Simu lation results are fed back to designers and the designs of the process anddiesurfaceareadjustedandmodifi ed inaccordancewiththesimula tionfeedback Meanwhile processparameters suchasblankholderforce anddrawingvelocity areoptimisedinthefi nite element FE simulation Springbackbecomesamajorproblemwhenformingdefects suchas wrinkles and cracks were solved Springback calculation was carried out to predict the springback after the tools are removed from the formed part The deviation distance between the springback part and the design model can be obtained and used to estimate whether or not the springback part can meet the allowable tolerance If the tolerance requirement cannot be satisfi ed the die surface should be compensated Fig 5 shows the schematic of the displacement adjustment method in multiple iteration springback compensation Displacement adjustment method is an effective approach toward springback compensation 24 The principle of this method includes calculating the distance between the springback and reference shapes The compensation shape is obtained with the same distance in the opposite direction of the springback As shown in schematic of the displacement adjustment method the geometry of the part is defi ned as a FE mesh with n nodes in 3 the coeffi cient i is a node in Fig 3 SEM micrographs of the fracture surfaces after tensile test a thin material b thick material and c weld seam of TWBs Fig 4 Flowchart of forming process simulation and multiple iteration springback compensation 249H Wang et al Materials and Design 102 2016 247 254 theFEmesh R isthereferenceshape ordesignedshape ofthepart S isthespringbackshape and C isthecompensationshape Thereference and springback shapes can be defi ned as follows R ri r 3 1 i n no 1 S si s 3 1 i n no 2 Theshapedeviation oftheformedpartcanbecalculatedasfollows S R 3 Ifthemaximumshapedeviation exceedsthedesignrequirements of the component tolerance the tools should be compensated The compensated shape C is expressed as C R aS R 4 ci ri a s i r i 1 i n 5 The factor a is called the compensation factor which is generally negative and varies between 2 5 and 1 The fi rst compensated shape C is defi ned as C 1 With this compensated geometry a new forming process simulation is carried out Ct is the compensation shape in the t th times S t is the springback shape in t th times of springback analysis in the FE simulation C 1 R aS 1 R 6 C 2 C 1 aS 2 R 7 C t C t 1 aS t R 1bt btmax 8 As shown in Fig 4 the process needs to be repeated for two or more times to meet the design requirements of component tolerance Multiple iteration springback compensation is completed when the maximum shape deviation meets the design requirements of the component tolerance The discriminant function can be expressed as follows S t R max b 9 4 Simulation 4 1 Finite element model AsshowninFig 6a theautomotivefl oorreinforcementcomposedof HSSTWBswasstudiedintheexperiment Theinitialblankoutlineofthe part can be obtained by unfolding the part adding a certain trimming allowanceand rounding the outline Fig 6b The leftsideisa rectangu lar blank with a thickness of 1 6 mm and the right side is an isosceles trapezoidal blank with a thickness of 1 2 mm On the basis of the structural characteristics and forming process analysis of the automotive fl oor reinforcement the forming process of this part includes drawing trimming and piercing The addendum and binder surfaces of the part were designed using a 3D software To analyze and optimize the forming process the FE model of the entire forming processes were built in Fig 7 using the FE software Autoform The initial blank was set as a dual phase HSS HC340 590DPD Z withthicknessesof1 2and1 6mm TheMaterialpropertiesandprocess parameters used in the FE simulation are shown in Table 2 25 The fl ow curve which describes the hardening behavior of the material was obtained in the tensile tests Triangular element was used in the FE simulation and the nodes and elements number of the initial blank were 285 and 487 respectively To achieve the accuracy and save Fig 5 Schematic of the displacement adjustment method a springback compensation in the fi rst time b springback compensation in the t th time Fig 6 a Automotive fl oor reinforcement and b blank outline 250H Wang et al Materials and Design 102 2016 247 254 calculating time the adaptive mesh was carried out to refi ne the elements of part during the forming process The FE model of the TWBs is more complicated than the conventional sheet metal forming model because the mechanical properties of the TWBs differ from those of the base materials 26 The weld seam was considered as rigidlinks spotwelds intheFEmodelbecausethismodelisparticularly effi cient for dealing with laser welded blanks especially the high strength of the weld metal in HSS TWBs 27 The simulation of the entire forming processes which includes drawing trimming and piercing as well as springback analysis was used to explore stamping forming The drawing process was divided into three stages gravity loading stage blank holder closing stage and drawing stage The gravity loading stage was adopted to elastic shell elements In addition the elastic plastic shell elements were applied to the remaining two stages The die punch and binder were set as rigid bodies in the drawing process A constant coeffi cient of friction 0 125 was used for the contact interface between blank and rigid stamping tools moreover the velocities of closing and drawing were both 1 mm s during the simulation Fig 7b demonstrated the trim and piercing lines in the trimming and piercing processes As shown in Fig 7c three points F1 F2and F3 were used to constrain the formed part by compulsory restraint condition during springback analysis 4 2 Multiple iteration springback compensation Springback is a common problem in the stamping forming process and TWBs forming is no exception To solve this problem two kinds of methods are developed in practice The fi rst method is controlling and optimizing the forming parameter i e forming temperature blank holder force contact friction tool radius 28 The second method is called springback compensation which is conducted by modifying the shape of stamping tools 29 Springback compensation is commonly used to develop these tools The springback may remain after forming but the fi nal shape of the part after springback would closely conform to the designed shape Springback compensation can be usually divided into three types namely compensating all stamping tools compensating a portion of stamping tools or only compensating drawing tools In general the strategy of compensating drawing tool is more economical than the other strategies Hence we select this approach in this study As shown in Fig 8 the drawing tool surface is defi ned by three regions the direct compensation region the fi xed compensation region and the transition region The fi nal shape of the part is defi ned as the direct compensation region and this region is compensated with the inverse vector of the springback simulation result The binder surface is defi ned asthefi xedcompensationregion whichisstationaryandunchangeable inspringbackcompensation Theareabetweenthedirectcompensation region and the fi xed compensation region is defi ned as the transition region which is used to guarantee the continuity of the compensation surface without a negative draft angle Fig 7 FE model of the entire forming processes a drawing b trimming and piercing and c springback analysis Table 2 Material properties and process parameters used in the simulations Material properties Yielding stress MPa 432 Work