Lightweight structure is an important method to increase vehicle fuel efficiency. High strength steel is applied for replacing mild steel in automotive structures to decrease thickness of parts for lightweight. Howeve...Lightweight structure is an important method to increase vehicle fuel efficiency. High strength steel is applied for replacing mild steel in automotive structures to decrease thickness of parts for lightweight. However, the lightweight structures must show the improved capability for structural rigidity and crash energy absorption. Advanced high strength steels are attractive materials to achieve higher strength for energy absorption and reduce weight of vehicles. Currently, many research works focus on component level axial crash testing and simulation of high strength steels. However, the effects of high strength steel parts to the impact of auto body are not considered. The goal of this research is to study the application of hot forming high strength steel（HFHSS） in order to evaluate the potential using in vehicle design for lightweight and passive safety. The performance of HFHSS is investigated by using both experimental and analytical techniques. In particular, the focus is on HFHSS which may have potential to enhance the passive safety for lightweight auto body. Automotive components made of HFHSS and general high strength steel（GHSS） are considered in this study. The material characterization of HFHSS is carried out through material experiments. The finite element method, in conjunction with the validated model is used to simulate the side impact of a car with GHSS and HFHSS parts according to China New Car Assessment Programme（C-NCAP） crash test. The deformation and acceleration characteristics of car body are analyzed and the injuries of an occupant are calculated. The results from the simulation analyses of HFHSS are compared with those of GHSS. The comparison indicates that the HFHSS parts on car body enhance the passive safety for the lightweight car body in side impact. Parts of HFHSS reduce weight of vehicle through thinner thickness offering higher strength of parts. Passive safety of lightweight car body is improved through reduction of crash deformation on car body by the application of HFHSS parts. The experiments and simulation are conducted to the HFHSS parts on auto body. The results demonstrate the feasibility of the application of HFHSS materials on automotive components for improved capability of passive safety and lightweight.展开更多
Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles.Different from the popular cold stamping,hot forming of boron-alloyed manganese steels,such as 22MnB5,coul...Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles.Different from the popular cold stamping,hot forming of boron-alloyed manganese steels,such as 22MnB5,could produce ultra-high-strength steel parts without springback and with accurate control of dimensions.Moreover,hot-formed medium-Mn steels could have many advantages,including better mechanical properties and lower production cost,over hot-formed 22MnB5.This paper reviews the hot forming process in the automotive industry,hot-formed steel grades,and medium-Mn steel grades and their application in hot forming in depth.In particular,the adaptabilities of medium-Mn steels and the presently popular 22MnB5 into hot forming were compared thoroughly.Future research should focus on the technological issues encountered in hot forming of medium-Mn steels to promote their commercialization.展开更多
The influences of process parameters on mechanical properties of AA6082in the hot forming and cold-die quenching(HFQ)process were analysed experimentally.Transmission electron microscopy was used to observe the precip...The influences of process parameters on mechanical properties of AA6082in the hot forming and cold-die quenching(HFQ)process were analysed experimentally.Transmission electron microscopy was used to observe the precipitate distribution and to thus clarify strengthening mechanism.A new model was established to describe the strengthening of AA6082by HFQ process in this novel forming technique.The material constants in the model were determined using a genetic algorithm tool.This strengthening model for AA6082can precisely describe the relationship between the strengths of formed workpieces and process parameters.The predicted results agree well with the experimental ones.The Pearson correlation coefficient,average absolute relative error,and root-mean-square error between the calculated and experimental hardness values are0.99402,2.0054%,and2.045,respectively.The model is further developed into an FE code ABAQUS via VUMAT to predict the mechanical property variation of a hot-stamped cup in various ageing conditions.展开更多
Hot forming with synchronous cooling(HFSC)is a novel technique for heat-treatable,high-strength aluminum alloys,which allows the alloys to acquire good formability,negligible springback,rapid processing and better mec...Hot forming with synchronous cooling(HFSC)is a novel technique for heat-treatable,high-strength aluminum alloys,which allows the alloys to acquire good formability,negligible springback,rapid processing and better mechanical properties.However,the deformation behavior and microstructure evolution of the alloys during HFSC are complex and need to be studied due to the temperature and strain rate effects.Uniaxial tensile tests in a temperature range of 250—450℃and a strain rate range of 0.01—1 s-1 for AA2024-H18 aluminum alloy sheet are conducted with a Gleeble-3500 Thermal-Mechanical Simulation Tester.And based on metallography observation and analysis,AA2024-H18 aluminum alloy sheet in HSFC process exhibits hardening and dynamic recovery behaviors within the temperature range of 250—450 ℃.Strain rate shows different effects on ductility at different temperatures.Compared with traditional warm/hot forming methods,AA2024-H18 aluminum alloy achieves a better work-hardening result through HFSC operations,which promises an improved formability at elevated temperature and thus good mechanical properties of final part.After HSFC operations,the microstructure of the specimens is composed of elongated static recrystallization grain.展开更多
In the hot forming of Mn18Cr18N steel, such problems as easy cracking, difficult controlling of forming paramenters often occur. In this paper,the variation rule of the plasticity of the steel, the starting mechanis...In the hot forming of Mn18Cr18N steel, such problems as easy cracking, difficult controlling of forming paramenters often occur. In this paper,the variation rule of the plasticity of the steel, the starting mechanism of micro-crack and its generating characteristics were studied with the combination of thermodynamic simulation test, micro-simulation and FEM, the related data of microstructure change and hot forming parameters were produced. The hot forming process of 600MW generator retaining ring was analyzed as an example.展开更多
Taking Ti-6Al-4V specimens into consideration, the coupled thermal-electrical finite element model has been developed in Abaqus/Explicit to simulate the heating process in Gleeble 3800 and to study the temperature his...Taking Ti-6Al-4V specimens into consideration, the coupled thermal-electrical finite element model has been developed in Abaqus/Explicit to simulate the heating process in Gleeble 3800 and to study the temperature history and distribution in the specimen. In order to verify the finite element （FE） results, thermal tests are carried out on Gleeble 3800 for a Ti-6Al-4V specimen with a slot to in the centre of the specimen. The effects of the specimen size, heating rate, and air convection on the temperature distribution over the specimen have been investigated. The conclusions can be drawn as： the temperature gradient of the specimen decreases as the specimen size, heating rate, and vacuuming decrease.展开更多
The relevant results of thermodynamics simulation test, recrystallization study and FEM numercal simulation are described. A method is also introduced that the controlled hot forming is carried out by adopting the pro...The relevant results of thermodynamics simulation test, recrystallization study and FEM numercal simulation are described. A method is also introduced that the controlled hot forming is carried out by adopting the program of expansion-extrusion compound forming.展开更多
Hot flow forming(HFF)is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs(CPLIRs)made of magnesium(Mg)alloys,which has wide applications in the aerospace field.How...Hot flow forming(HFF)is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs(CPLIRs)made of magnesium(Mg)alloys,which has wide applications in the aerospace field.However,due to the thermo-mechanical coupling effect and the existence of stiffened structure,complex microstructure evolution and uneven microstructure occur easily at the cylindrical wall(CW)and inner rib(IR)of Mg alloy thin-walled CPLIRs during the HFF.In this paper,a modified cellular automaton(CA)model of Mg alloy considering the effects of deformation conditions on material parameters was developed using the artificial neural network(ANN)method.It is found that the ANN-modified CA model exhibits better predictability for the microstructure of hot deformation than the conventional CA model.Furthermore,the microstructure evolution of ZK61 alloy CPLIRs during the HFF was analyzed by coupling the modified CA model and finite element analysis(FEA).The results show that compared with the microstructure at the same layer of the IR,more refined grains and less sufficient DRX resulted from larger strain and strain rate occur at that of the CW;various differences of strain and strain rate in the wall-thickness exist between the CW and IR,which leads to the inhomogeneity of microstructure rising firstly and declining from the inside layer to outside layer;the obtained Hall-Petch relationship between the measured microhardness and predicted grain sizes at the CW and the IR indicates the reliability of the coupled FEA-CA simulation results.展开更多
The granule medium of discreteness is supposed to be continuous（Drucker-Prager model） in the existing finite element simulation analysis on the hot granule medium pressure forming（HGMF） process, so the granule med...The granule medium of discreteness is supposed to be continuous（Drucker-Prager model） in the existing finite element simulation analysis on the hot granule medium pressure forming（HGMF） process, so the granule medium may produce tensile stress in the process of pressure-transferring and flowing, which does not coincide with the reality. The analysis method, discrete element and finite element（DE-FE） coupling simulation, is proposed to solve the problem. The material parameters of simulation model are obtained by the pressure-transfer performance test of granule medium and the hot uniaxial tensile test of sheet metal. The DE-FE coupling simulation platform is established by adopting Visual Basic language. The features in the process that AA7075-T6 conical parts are formed by the HGMF process are analyzed and verified by the process test. The studies show that the results of DE-FE coupling simulation coincide well with the test results, which provides a new analysis method to solve the mechanics problem in the coupling of discrete and continuum.展开更多
A martensitic initial microstructure before hot forming was prepared by direct quenching after hot rolling of the hot formed steel and the effect of such initial microstructure on mechanical properties of steel was an...A martensitic initial microstructure before hot forming was prepared by direct quenching after hot rolling of the hot formed steel and the effect of such initial microstructure on mechanical properties of steel was analyzed. The process of direct quenching after hot rolling which replaced the steps of coiling and cold rolling was termed as compact process. As the temperature before direct quenching falls within the non-recrystallization range, the deformed austenite grains exhibit flattened morphology along the hot rolling direction, and the high-density dislocations and significant strain energy in deformed austenite are inherited by directly quenched martensite. Moreover, due to promotion of austenite nucleation and subsequent recrystallization during the reverse transformation process in hot forming, both reversed austenite grains and martensite laths are significantly refined. Compared to the conventional process with an initial microstructure consisting of fully recrystallized ferrite and cementite, the compact process reduces average prior austenite grain sizes from 12.5 to 5.5 μm and martensite lath widths from 202 to 123 nm. Additionally, the compact process results in a higher density of dislocations in test steel, leading to maximum yield strength (1294 MPa) and ultimate tensile strength (2266 MPa). Compared to conventional process, this compact process significantly improves the mechanical properties of the hot formed steels while simplifying the production.展开更多
High strength aluminum alloy plate has a low elongation at room temperature, which leads to the forming of its components need a high temperature. Liquid or gas is used as the pressure-transfer medium in the existing ...High strength aluminum alloy plate has a low elongation at room temperature, which leads to the forming of its components need a high temperature. Liquid or gas is used as the pressure-transfer medium in the existing flexible mould forming process, the heat resistance of the medium and pressurizing device makes the application of aluminum alloy plate thermoforming restricted. To solve this problem, the existing medium is replaced by the heat-resisting solid granules and the general pressure equipments are applied. Based on the pressure-transfer performance test of the solid granules medium, the feasibility that the assumption of the extended Drucker-Prager linear model can be used in the finite element analysis is proved. The constitutive equation, the yield function and the theoretical forming limit diagram（FLD） of AA7075 sheet are established. Through the finite element numerical simulation of hot granules medium pressure forming（HGMF） process, not only the influence laws of the process parameters, such as forming temperature, the blank-holder gap and the diameter of the slab, on sheet metal forming performance are discussed, but also the broken area of the forming process is analyzed and predicted, which are coincided with the technological test. The conical part whose half cone angle is 15° and relative height H/d0 is 0.57, is formed in one process at 250℃. The HGMF process solves the problems of loading and seal in the existing flexible mould forming process and provides a novel technology for thermoforming of light alloy plate, such as magnesium alloy, aluminium alloy and titanium alloy.展开更多
Based on the combination of materials science and mechanical engineering,hot press forming process of the vehicle high strength steels was analyzed. The hot forming process included：heating alloys rapidly to austenit...Based on the combination of materials science and mechanical engineering,hot press forming process of the vehicle high strength steels was analyzed. The hot forming process included：heating alloys rapidly to austenite microstructures,stamping and cooling timely,maintaining pressure and quenching. The results showed that most of austenite microstructure was changed into uniform martensite by the hot press forming while the samples were heated at 900 ℃ and quenched. The optimal tensile strength and yield strength were up to 1530 MPa and 1000 MPa,respectively,and the shape deformation reached about 23%. And springback defect did not happen in the samples.展开更多
Hot granule medium pressure forming （HGMF） process is a new process in which granule medium replaces the medium in existing flexible-die hot forming process, such as liquids, gases or viscous medium. Hot forming of ...Hot granule medium pressure forming （HGMF） process is a new process in which granule medium replaces the medium in existing flexible-die hot forming process, such as liquids, gases or viscous medium. Hot forming of light alloy sheet parts can be realized based on the properties of granule medium, such as withstanding high temperature and pressure, filling well, sealing and loading easily. In this work, the forming of AA7075 cylindrical parts by HGMF process is taken as an example to establish the constitutive relation and forming limit diagram （FLD） of AA7075 sheet which is related to temperature by hot uniaxial tensile test of sheet metal. Based on the assumption that granule medium is applied to extended Drucker-Prager linear material model, the finite element model of HGMF process is established and the effect of technological parameters, such as forming temperature, blank-holder gap and drawing ratio, on the sheet metal formability, is studied. The limit drawing ratio curve of AA7075 cylindrical parts at forming temperature of 175-300 ℃ is obtained by HGMF process test, and the limit drawing ratio reaches the maximum value of 1.71 at 250 ℃. The results of numerical simulation are consistent with the results of process test, and the forming force, distribution of wall thichness and form of instability are predicted correctly, which provides reference for the application of HGMF process.展开更多
Hot granule medium pressure forming(HGMF) is a technology in which heat-resistant granules are used to replace liquids or gases in existing flexible-die forming technology as pressure-transfer medium. Considering the ...Hot granule medium pressure forming(HGMF) is a technology in which heat-resistant granules are used to replace liquids or gases in existing flexible-die forming technology as pressure-transfer medium. Considering the characteristic of granule medium that seals and loads easily, the technology provides a new method to realize the hot deep-drawing forming on high strength aluminum alloy sheet. Based on the pressure-transfer performance test of granule medium and the material performance test of AA7075-T6 sheet, plastic mechanics analysis is conducted for the areas, such as the flange area, force-transfer area and free deforming area, of cylindrical parts deep-drawn by HGMF technology, and the function relation of forming pressure is obtained under the condition of nonuniform distribution of internal pressure. The comparison between theoretical result and experimental data shows that larger deviation occurs in the middle and later period of forming process, and the maximum theoretical forming force is less than the experimental value by 24.6%. The variation tendency of the theoretical thickness curve is close to the practical situation, and the theoretical value basically agrees well with experimental value in the flange area and the top area of spherical cap which is in the free deforming area.展开更多
An analysis of the hot flow forming of Mg-3.0Al-l.0Zn-0.3Mn （AZ31B） alloy was conducted by experiments and numerical simulations. The effects of different thickness reductions on the microstructure and mechanical pr...An analysis of the hot flow forming of Mg-3.0Al-l.0Zn-0.3Mn （AZ31B） alloy was conducted by experiments and numerical simulations. The effects of different thickness reductions on the microstructure and mechanical properties were investigated at a temperature of 693 K, a spindle speed of 800 rev/min and a feed ratio of 0.1 mm/rev. Thickness reductions have great influence on the uniformity of microstructure along the radial direction （RD） and the grain sizes become refined and uniform when the thickness reduction reaches 45%. The c-axes of most grains are approximately parallel to the RD, with a slight inclination towards the axial direction （AD）. The best mechanical properties with UTS of 280 MPa and YS of 175 MPa near the outer surface while 266 MPa and 153 MPa near the inner surface have been achieved due to grain refinement and texture. Moreover, the material flow behavior and stress/strain distributions for singlepass reductions were studied using the ABAQUS/Explicit software. The calculated results indicate that the materials mainly suffer from triaxial compressive stresses and undergo compressive plastic strain in RD and tensile strains in other directions. The higher stress and strain rate near the outer surface lead to more refined grains than that of other regions along the RD, whereas the orientation of the maximum principal compressive stress leads to a discrepancy of the grain orientations in RD.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.展开更多
Anisotropy of mechanical property is an important feature influencing the service performance of titanium(Ti)alloy tube component.In this work,it is found that the hot flow formed Ti alloy tube exhibits higher yield s...Anisotropy of mechanical property is an important feature influencing the service performance of titanium(Ti)alloy tube component.In this work,it is found that the hot flow formed Ti alloy tube exhibits higher yield strength along circumferential direction(CD),and larger elongation along rolling direction(RD),presenting significant anisotropy.Subsequently,the quantitative characteristics and underlying mechanism of the property anisotropy were revealed by analyzing the slip,damage and fracture behavior under the combined effects of the spun{0002}basal texture and fibrous microstructure for different loading directions.The results showed that the prismatic slip in primaryαgrain is the dominant deformation mechanism for both loading directions at the yielding stage.The prismatic slip is harder under CD loading,which makes CD loading present higher yield strength than RD loading.Additionally,the yield anisotropy can be quantified through the inverse ratio of the averaged Schmid Factor of the activated prismatic slip under different loading directions.As for the plasticity anisotropy,the harder and slower slip development under CD loading causes that the CD loading presents larger external force and normal stress on slip plane,thus leading to more significant cleavage fracture than RD loading.Moreover,the micro-crack path under RD loading is more tortuous than CD loading because the fibrous microstructure is elongated along RD,which may suppress the macro fracture under RD loading.These results suggest that weakening the texture and fibrous morphology of microstructure is critical to reduce the differences in slip,damage and fracture behavior along different directions,alleviate the property anisotropy and optimize the service performance of Ti alloy tube formed by hot flow forming.展开更多
A mathematic model for dynamic and static recrystallization of Mn18Cr18N steel is presented.Hot expanding extrusion forming and air cooling process of 600 MW retaining ring has been simulated by the combination of th...A mathematic model for dynamic and static recrystallization of Mn18Cr18N steel is presented.Hot expanding extrusion forming and air cooling process of 600 MW retaining ring has been simulated by the combination of thermos coupled rigid viscoplasitic FEM with this model,and grain size distribution on cross section of ring is shown.The essential results can be provided for use of the new technique.展开更多
A stress relaxation test has been carried out for Hastelloy C-276 at temperature of 800 ~C and initial stress level of 250 MPa. Based on the experimental stress relaxation curve, the relationship between creep strain ...A stress relaxation test has been carried out for Hastelloy C-276 at temperature of 800 ~C and initial stress level of 250 MPa. Based on the experimental stress relaxation curve, the relationship between creep strain rate and stress has been derived. Then, a set of creep constitutive equations has been built and the values of constants arising in the constitutive equations have been determined by fitting the creep strain rate-stress curve. Close agreement between computed results and experimental ones is obtained for stress relaxation data. The creep constitutive equation set has been integrated with the commercial FE （finite element） solver MSC.Marc via the user defined subroutine, CRPLAW, for the vacuum hot bulge forming process modelling of Hastelloy C-276 thin-walled cylindrical workpiece. The temperature field, the radius-direction displacement field and the stress-strain field are calculated and analyzed. Furthermore, the bulging dimension and the final internal diameter of workpiece are predicted and the test results verify the reliability of the finite element method.展开更多
It is necessary to use the integrated stainless steel pipe having two fitting bodies without welds while train travelling at high speed. In order to form this type of integrated stainless steel pipe, the method of pre...It is necessary to use the integrated stainless steel pipe having two fitting bodies without welds while train travelling at high speed. In order to form this type of integrated stainless steel pipe, the method of preforming combined finish forming process is developed. The preforming process is characterized by flaring combined upsetting for left fitting body which is like a flange, and is characterized by tube axial compressive process under die constraint for right fitting body which is like a double-wall pipe. The finite element simulations of the processes are carried out by software package DEFORM, and the results indicate that: 1) left or right fitting body can be formed by a two-step forming process without folding and under-filling defects; 2) by using two-step forming, strain and stress in left fitting body are larger than those in right fitting body, and deformation in right fitting body is more homogenous than the deformation in left fitting body; 3) two or more preforming steps may be needed for left fitting body considering the distributions of strain and stress.展开更多
The hot stamping processing parameters are of critical importance in transforming ultra-high-strength steel （UHSS） into high-quality parts, which were studied by mechanical properties tests, metallographic observati...The hot stamping processing parameters are of critical importance in transforming ultra-high-strength steel （UHSS） into high-quality parts, which were studied by mechanical properties tests, metallographic observations and calculation analysis method based on hot stamping experiments and numerical simulation technology, the mechanical properties, thickness, dimensional accuracy, and microstructure of the hot formed parts are analyzed to determine the influence of different processing parameters for UHSS parts formed from BR1500HS. The results indicate that the quenching time had the most significant impact on the mechanical properties of the parts, and longer quenching time resulted in better mechanical properties. In addition, the pressing speed had a significant influence on the thick ness of the formed parts, and the part-opening temperature had the most significant effect on the dimensional accura cy of the parts. And to get hot stamped parts with excellent quality, the optimum process conditions should be set as heating temperature of 930 ℃, soaking time of 4 min, stamping force of 7 MPa, pressing speed of 75 mm/s, quench ing time of 15 s, and water-flow rate of 1.1 m/s.展开更多
基金supported by National Natural Science Foundation of China(Grant No.19832020)National Science Fund of Outstanding Youths of China (Grant No.10125208)+1 种基金Chongqing Municipal Programs for Science and Technology Development of China(Grant No.CSTC, 2007AA4008)National Key Technology R&D Program of China(Grant No.2006BA104B04-2)
文摘Lightweight structure is an important method to increase vehicle fuel efficiency. High strength steel is applied for replacing mild steel in automotive structures to decrease thickness of parts for lightweight. However, the lightweight structures must show the improved capability for structural rigidity and crash energy absorption. Advanced high strength steels are attractive materials to achieve higher strength for energy absorption and reduce weight of vehicles. Currently, many research works focus on component level axial crash testing and simulation of high strength steels. However, the effects of high strength steel parts to the impact of auto body are not considered. The goal of this research is to study the application of hot forming high strength steel（HFHSS） in order to evaluate the potential using in vehicle design for lightweight and passive safety. The performance of HFHSS is investigated by using both experimental and analytical techniques. In particular, the focus is on HFHSS which may have potential to enhance the passive safety for lightweight auto body. Automotive components made of HFHSS and general high strength steel（GHSS） are considered in this study. The material characterization of HFHSS is carried out through material experiments. The finite element method, in conjunction with the validated model is used to simulate the side impact of a car with GHSS and HFHSS parts according to China New Car Assessment Programme（C-NCAP） crash test. The deformation and acceleration characteristics of car body are analyzed and the injuries of an occupant are calculated. The results from the simulation analyses of HFHSS are compared with those of GHSS. The comparison indicates that the HFHSS parts on car body enhance the passive safety for the lightweight car body in side impact. Parts of HFHSS reduce weight of vehicle through thinner thickness offering higher strength of parts. Passive safety of lightweight car body is improved through reduction of crash deformation on car body by the application of HFHSS parts. The experiments and simulation are conducted to the HFHSS parts on auto body. The results demonstrate the feasibility of the application of HFHSS materials on automotive components for improved capability of passive safety and lightweight.
基金The authors acknowledge the financial supports from the National Natural Science Foundation of China(Nos.51861135302 and 51831002)Fundamental Research Funds for the Central Universities,China(No.FRF-TP-18-002C2).
文摘Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles.Different from the popular cold stamping,hot forming of boron-alloyed manganese steels,such as 22MnB5,could produce ultra-high-strength steel parts without springback and with accurate control of dimensions.Moreover,hot-formed medium-Mn steels could have many advantages,including better mechanical properties and lower production cost,over hot-formed 22MnB5.This paper reviews the hot forming process in the automotive industry,hot-formed steel grades,and medium-Mn steel grades and their application in hot forming in depth.In particular,the adaptabilities of medium-Mn steels and the presently popular 22MnB5 into hot forming were compared thoroughly.Future research should focus on the technological issues encountered in hot forming of medium-Mn steels to promote their commercialization.
基金Project(P2014-15)supported by the State Key Laboratory of Materials Processing and Die and Mould Technology,Huazhong University of Science and Technology,ChinaProject(20120006110017)supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China+1 种基金Project(2015M580977)supported by China Postdoctoral Science FoundationProject supported by Beijing Laboratory of Metallic Materials and Processing for Modern Transportation,China
文摘The influences of process parameters on mechanical properties of AA6082in the hot forming and cold-die quenching(HFQ)process were analysed experimentally.Transmission electron microscopy was used to observe the precipitate distribution and to thus clarify strengthening mechanism.A new model was established to describe the strengthening of AA6082by HFQ process in this novel forming technique.The material constants in the model were determined using a genetic algorithm tool.This strengthening model for AA6082can precisely describe the relationship between the strengths of formed workpieces and process parameters.The predicted results agree well with the experimental ones.The Pearson correlation coefficient,average absolute relative error,and root-mean-square error between the calculated and experimental hardness values are0.99402,2.0054%,and2.045,respectively.The model is further developed into an FE code ABAQUS via VUMAT to predict the mechanical property variation of a hot-stamped cup in various ageing conditions.
基金supported by the National Natural Science Foundation of China(No.51175252)
文摘Hot forming with synchronous cooling(HFSC)is a novel technique for heat-treatable,high-strength aluminum alloys,which allows the alloys to acquire good formability,negligible springback,rapid processing and better mechanical properties.However,the deformation behavior and microstructure evolution of the alloys during HFSC are complex and need to be studied due to the temperature and strain rate effects.Uniaxial tensile tests in a temperature range of 250—450℃and a strain rate range of 0.01—1 s-1 for AA2024-H18 aluminum alloy sheet are conducted with a Gleeble-3500 Thermal-Mechanical Simulation Tester.And based on metallography observation and analysis,AA2024-H18 aluminum alloy sheet in HSFC process exhibits hardening and dynamic recovery behaviors within the temperature range of 250—450 ℃.Strain rate shows different effects on ductility at different temperatures.Compared with traditional warm/hot forming methods,AA2024-H18 aluminum alloy achieves a better work-hardening result through HFSC operations,which promises an improved formability at elevated temperature and thus good mechanical properties of final part.After HSFC operations,the microstructure of the specimens is composed of elongated static recrystallization grain.
文摘In the hot forming of Mn18Cr18N steel, such problems as easy cracking, difficult controlling of forming paramenters often occur. In this paper,the variation rule of the plasticity of the steel, the starting mechanism of micro-crack and its generating characteristics were studied with the combination of thermodynamic simulation test, micro-simulation and FEM, the related data of microstructure change and hot forming parameters were produced. The hot forming process of 600MW generator retaining ring was analyzed as an example.
基金supported by the Fundamental Research Funds for the Central Universities of China under Grant No.A03007023801073
文摘Taking Ti-6Al-4V specimens into consideration, the coupled thermal-electrical finite element model has been developed in Abaqus/Explicit to simulate the heating process in Gleeble 3800 and to study the temperature history and distribution in the specimen. In order to verify the finite element （FE） results, thermal tests are carried out on Gleeble 3800 for a Ti-6Al-4V specimen with a slot to in the centre of the specimen. The effects of the specimen size, heating rate, and air convection on the temperature distribution over the specimen have been investigated. The conclusions can be drawn as： the temperature gradient of the specimen decreases as the specimen size, heating rate, and vacuuming decrease.
文摘The relevant results of thermodynamics simulation test, recrystallization study and FEM numercal simulation are described. A method is also introduced that the controlled hot forming is carried out by adopting the program of expansion-extrusion compound forming.
基金supported by the National Nat-ural Science Foundation of China(Grant Nos.51775194 and 52090043).
文摘Hot flow forming(HFF)is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs(CPLIRs)made of magnesium(Mg)alloys,which has wide applications in the aerospace field.However,due to the thermo-mechanical coupling effect and the existence of stiffened structure,complex microstructure evolution and uneven microstructure occur easily at the cylindrical wall(CW)and inner rib(IR)of Mg alloy thin-walled CPLIRs during the HFF.In this paper,a modified cellular automaton(CA)model of Mg alloy considering the effects of deformation conditions on material parameters was developed using the artificial neural network(ANN)method.It is found that the ANN-modified CA model exhibits better predictability for the microstructure of hot deformation than the conventional CA model.Furthermore,the microstructure evolution of ZK61 alloy CPLIRs during the HFF was analyzed by coupling the modified CA model and finite element analysis(FEA).The results show that compared with the microstructure at the same layer of the IR,more refined grains and less sufficient DRX resulted from larger strain and strain rate occur at that of the CW;various differences of strain and strain rate in the wall-thickness exist between the CW and IR,which leads to the inhomogeneity of microstructure rising firstly and declining from the inside layer to outside layer;the obtained Hall-Petch relationship between the measured microhardness and predicted grain sizes at the CW and the IR indicates the reliability of the coupled FEA-CA simulation results.
基金Projects(5130538651305385)supported by the National Natural Science Foundation of China+1 种基金Project(E2013203093)supported by the Natural Science Foundation of Hebei ProvinceChina
文摘The granule medium of discreteness is supposed to be continuous（Drucker-Prager model） in the existing finite element simulation analysis on the hot granule medium pressure forming（HGMF） process, so the granule medium may produce tensile stress in the process of pressure-transferring and flowing, which does not coincide with the reality. The analysis method, discrete element and finite element（DE-FE） coupling simulation, is proposed to solve the problem. The material parameters of simulation model are obtained by the pressure-transfer performance test of granule medium and the hot uniaxial tensile test of sheet metal. The DE-FE coupling simulation platform is established by adopting Visual Basic language. The features in the process that AA7075-T6 conical parts are formed by the HGMF process are analyzed and verified by the process test. The studies show that the results of DE-FE coupling simulation coincide well with the test results, which provides a new analysis method to solve the mechanics problem in the coupling of discrete and continuum.
基金financial support from the National Natural Science Foundation of China(No.52274372)the National Key Research and Development Program of China(No.2021YFB3702404).
文摘A martensitic initial microstructure before hot forming was prepared by direct quenching after hot rolling of the hot formed steel and the effect of such initial microstructure on mechanical properties of steel was analyzed. The process of direct quenching after hot rolling which replaced the steps of coiling and cold rolling was termed as compact process. As the temperature before direct quenching falls within the non-recrystallization range, the deformed austenite grains exhibit flattened morphology along the hot rolling direction, and the high-density dislocations and significant strain energy in deformed austenite are inherited by directly quenched martensite. Moreover, due to promotion of austenite nucleation and subsequent recrystallization during the reverse transformation process in hot forming, both reversed austenite grains and martensite laths are significantly refined. Compared to the conventional process with an initial microstructure consisting of fully recrystallized ferrite and cementite, the compact process reduces average prior austenite grain sizes from 12.5 to 5.5 μm and martensite lath widths from 202 to 123 nm. Additionally, the compact process results in a higher density of dislocations in test steel, leading to maximum yield strength (1294 MPa) and ultimate tensile strength (2266 MPa). Compared to conventional process, this compact process significantly improves the mechanical properties of the hot formed steels while simplifying the production.
基金Supported by National Natural Science Foundation of China(Grant Nos.51305386,51305385)Hebei Provincial Natural Science Foundation of China(Grant No.E2013203093)
文摘High strength aluminum alloy plate has a low elongation at room temperature, which leads to the forming of its components need a high temperature. Liquid or gas is used as the pressure-transfer medium in the existing flexible mould forming process, the heat resistance of the medium and pressurizing device makes the application of aluminum alloy plate thermoforming restricted. To solve this problem, the existing medium is replaced by the heat-resisting solid granules and the general pressure equipments are applied. Based on the pressure-transfer performance test of the solid granules medium, the feasibility that the assumption of the extended Drucker-Prager linear model can be used in the finite element analysis is proved. The constitutive equation, the yield function and the theoretical forming limit diagram（FLD） of AA7075 sheet are established. Through the finite element numerical simulation of hot granules medium pressure forming（HGMF） process, not only the influence laws of the process parameters, such as forming temperature, the blank-holder gap and the diameter of the slab, on sheet metal forming performance are discussed, but also the broken area of the forming process is analyzed and predicted, which are coincided with the technological test. The conical part whose half cone angle is 15° and relative height H/d0 is 0.57, is formed in one process at 250℃. The HGMF process solves the problems of loading and seal in the existing flexible mould forming process and provides a novel technology for thermoforming of light alloy plate, such as magnesium alloy, aluminium alloy and titanium alloy.
基金Item Sponsored by National Natural Science Foundation of China (50901011)National Science Fund for Distinguished Young Scholars of China (10125208)Liaoning Province Doctor Startup Fund of China (20071090)
文摘Based on the combination of materials science and mechanical engineering,hot press forming process of the vehicle high strength steels was analyzed. The hot forming process included：heating alloys rapidly to austenite microstructures,stamping and cooling timely,maintaining pressure and quenching. The results showed that most of austenite microstructure was changed into uniform martensite by the hot press forming while the samples were heated at 900 ℃ and quenched. The optimal tensile strength and yield strength were up to 1530 MPa and 1000 MPa,respectively,and the shape deformation reached about 23%. And springback defect did not happen in the samples.
基金Projects(5130538651305385)supported by the National Natural Science Foundation of ChinaProject(E2013203093)supported by the Natural Science Foundation of Hebei Province,China
文摘Hot granule medium pressure forming （HGMF） process is a new process in which granule medium replaces the medium in existing flexible-die hot forming process, such as liquids, gases or viscous medium. Hot forming of light alloy sheet parts can be realized based on the properties of granule medium, such as withstanding high temperature and pressure, filling well, sealing and loading easily. In this work, the forming of AA7075 cylindrical parts by HGMF process is taken as an example to establish the constitutive relation and forming limit diagram （FLD） of AA7075 sheet which is related to temperature by hot uniaxial tensile test of sheet metal. Based on the assumption that granule medium is applied to extended Drucker-Prager linear material model, the finite element model of HGMF process is established and the effect of technological parameters, such as forming temperature, blank-holder gap and drawing ratio, on the sheet metal formability, is studied. The limit drawing ratio curve of AA7075 cylindrical parts at forming temperature of 175-300 ℃ is obtained by HGMF process test, and the limit drawing ratio reaches the maximum value of 1.71 at 250 ℃. The results of numerical simulation are consistent with the results of process test, and the forming force, distribution of wall thichness and form of instability are predicted correctly, which provides reference for the application of HGMF process.
基金Projects(51305386,51305385)supported by the National Natural Science Foundation of ChinaProject(E2013203093)supported by the Natural Science Foundation of Hebei Province,China
文摘Hot granule medium pressure forming(HGMF) is a technology in which heat-resistant granules are used to replace liquids or gases in existing flexible-die forming technology as pressure-transfer medium. Considering the characteristic of granule medium that seals and loads easily, the technology provides a new method to realize the hot deep-drawing forming on high strength aluminum alloy sheet. Based on the pressure-transfer performance test of granule medium and the material performance test of AA7075-T6 sheet, plastic mechanics analysis is conducted for the areas, such as the flange area, force-transfer area and free deforming area, of cylindrical parts deep-drawn by HGMF technology, and the function relation of forming pressure is obtained under the condition of nonuniform distribution of internal pressure. The comparison between theoretical result and experimental data shows that larger deviation occurs in the middle and later period of forming process, and the maximum theoretical forming force is less than the experimental value by 24.6%. The variation tendency of the theoretical thickness curve is close to the practical situation, and the theoretical value basically agrees well with experimental value in the flange area and the top area of spherical cap which is in the free deforming area.
基金finanically supported by the National Key Research and Development Program of China (Nos. 2016YFB0101604 and 2016YFB0301103)National Natural Science Foundation of China (No. 51601112)Shanghai Rising-Star Program (No. 17QB1403000)
文摘An analysis of the hot flow forming of Mg-3.0Al-l.0Zn-0.3Mn （AZ31B） alloy was conducted by experiments and numerical simulations. The effects of different thickness reductions on the microstructure and mechanical properties were investigated at a temperature of 693 K, a spindle speed of 800 rev/min and a feed ratio of 0.1 mm/rev. Thickness reductions have great influence on the uniformity of microstructure along the radial direction （RD） and the grain sizes become refined and uniform when the thickness reduction reaches 45%. The c-axes of most grains are approximately parallel to the RD, with a slight inclination towards the axial direction （AD）. The best mechanical properties with UTS of 280 MPa and YS of 175 MPa near the outer surface while 266 MPa and 153 MPa near the inner surface have been achieved due to grain refinement and texture. Moreover, the material flow behavior and stress/strain distributions for singlepass reductions were studied using the ABAQUS/Explicit software. The calculated results indicate that the materials mainly suffer from triaxial compressive stresses and undergo compressive plastic strain in RD and tensile strains in other directions. The higher stress and strain rate near the outer surface lead to more refined grains than that of other regions along the RD, whereas the orientation of the maximum principal compressive stress leads to a discrepancy of the grain orientations in RD.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.
基金financially supported by the National Natural Science Foundation of China(No.51875467,52005313)the National Science Fund for Distinguished Young Scholars of China(No.51625505)+1 种基金the Young Elite Scientists Sponsorship Program by CAST(No.2018QNRC001)the Research Fund of the State Key Laboratory of Solidification Processing(NPU)of China(No.2019TS-10)。
文摘Anisotropy of mechanical property is an important feature influencing the service performance of titanium(Ti)alloy tube component.In this work,it is found that the hot flow formed Ti alloy tube exhibits higher yield strength along circumferential direction(CD),and larger elongation along rolling direction(RD),presenting significant anisotropy.Subsequently,the quantitative characteristics and underlying mechanism of the property anisotropy were revealed by analyzing the slip,damage and fracture behavior under the combined effects of the spun{0002}basal texture and fibrous microstructure for different loading directions.The results showed that the prismatic slip in primaryαgrain is the dominant deformation mechanism for both loading directions at the yielding stage.The prismatic slip is harder under CD loading,which makes CD loading present higher yield strength than RD loading.Additionally,the yield anisotropy can be quantified through the inverse ratio of the averaged Schmid Factor of the activated prismatic slip under different loading directions.As for the plasticity anisotropy,the harder and slower slip development under CD loading causes that the CD loading presents larger external force and normal stress on slip plane,thus leading to more significant cleavage fracture than RD loading.Moreover,the micro-crack path under RD loading is more tortuous than CD loading because the fibrous microstructure is elongated along RD,which may suppress the macro fracture under RD loading.These results suggest that weakening the texture and fibrous morphology of microstructure is critical to reduce the differences in slip,damage and fracture behavior along different directions,alleviate the property anisotropy and optimize the service performance of Ti alloy tube formed by hot flow forming.
文摘A mathematic model for dynamic and static recrystallization of Mn18Cr18N steel is presented.Hot expanding extrusion forming and air cooling process of 600 MW retaining ring has been simulated by the combination of thermos coupled rigid viscoplasitic FEM with this model,and grain size distribution on cross section of ring is shown.The essential results can be provided for use of the new technique.
基金Project(2009CB724307)supported by the Major State Basic Research Development Program(973 Program)of China
文摘A stress relaxation test has been carried out for Hastelloy C-276 at temperature of 800 ~C and initial stress level of 250 MPa. Based on the experimental stress relaxation curve, the relationship between creep strain rate and stress has been derived. Then, a set of creep constitutive equations has been built and the values of constants arising in the constitutive equations have been determined by fitting the creep strain rate-stress curve. Close agreement between computed results and experimental ones is obtained for stress relaxation data. The creep constitutive equation set has been integrated with the commercial FE （finite element） solver MSC.Marc via the user defined subroutine, CRPLAW, for the vacuum hot bulge forming process modelling of Hastelloy C-276 thin-walled cylindrical workpiece. The temperature field, the radius-direction displacement field and the stress-strain field are calculated and analyzed. Furthermore, the bulging dimension and the final internal diameter of workpiece are predicted and the test results verify the reliability of the finite element method.
基金Project(51305334)supported by the National Natural Science Foundation of ChinaProject(51335009)supported by the National Natural Science Foundation of China for Key Program+1 种基金Project(CXY1442(4))supported by the Science and Technology Planning Project of Xi’an,ChinaProject supported by Shaanxi Province Postdoctoral Science Research Program of China
文摘It is necessary to use the integrated stainless steel pipe having two fitting bodies without welds while train travelling at high speed. In order to form this type of integrated stainless steel pipe, the method of preforming combined finish forming process is developed. The preforming process is characterized by flaring combined upsetting for left fitting body which is like a flange, and is characterized by tube axial compressive process under die constraint for right fitting body which is like a double-wall pipe. The finite element simulations of the processes are carried out by software package DEFORM, and the results indicate that: 1) left or right fitting body can be formed by a two-step forming process without folding and under-filling defects; 2) by using two-step forming, strain and stress in left fitting body are larger than those in right fitting body, and deformation in right fitting body is more homogenous than the deformation in left fitting body; 3) two or more preforming steps may be needed for left fitting body considering the distributions of strain and stress.
基金Sponsored by National Key Basic Research Program of China(2012CB724301)Program of International S and T Cooperation of China(2011DFA50810)
文摘The hot stamping processing parameters are of critical importance in transforming ultra-high-strength steel （UHSS） into high-quality parts, which were studied by mechanical properties tests, metallographic observations and calculation analysis method based on hot stamping experiments and numerical simulation technology, the mechanical properties, thickness, dimensional accuracy, and microstructure of the hot formed parts are analyzed to determine the influence of different processing parameters for UHSS parts formed from BR1500HS. The results indicate that the quenching time had the most significant impact on the mechanical properties of the parts, and longer quenching time resulted in better mechanical properties. In addition, the pressing speed had a significant influence on the thick ness of the formed parts, and the part-opening temperature had the most significant effect on the dimensional accura cy of the parts. And to get hot stamped parts with excellent quality, the optimum process conditions should be set as heating temperature of 930 ℃, soaking time of 4 min, stamping force of 7 MPa, pressing speed of 75 mm/s, quench ing time of 15 s, and water-flow rate of 1.1 m/s.
