Application of Hot Forming High Strength Steel Parts on Car Body in Side Impact

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.

Influence of process parameters on properties of AA6082 in hot forming process

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.

Medium-Mn steels for hot forming application in the automotive industry

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.

Deformation Behavior and Microstructure Evolution of AA2024-H18 Aluminum Alloy by Hot Forming with Synchronous Cooling Operations

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.

SIMULATION OF HOT FORMING PROCESSION IN Mn18Cr18NSTEEL

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.

Study on Temperature Distribution of Specimens Tested on the Gleeble 3800 at Hot Forming Conditions

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.

CONTROLLED HOT FORMING OF RETAINING RING OF GENERATOR

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.

Discrete element and finite element coupling simulation and experiment of hot granule medium pressure forming

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.

Process parameters optimization of Ti-6Al-4V alloy sheet during hot stretch-creep forming

Hot stretch-creep forming （SCF） is a novel technique to produce hard-to-form thin-walled metal components. Comprehensively considering the analysis results of the springback angle, yield strength and microstructure, four hot SCF process parameters including temperature, stretch velocity, post stretch percentage and dwelling time of a Ti-6Al-4V alloy sheet were optimized using an orthogonal experiment. The results reveal that temperature is the most important factor on springback angle. The yield strength of the deformed material in 0° direction increases, while those in directions of 45° and 90° fluctuate around the original value. After hot SCF, the shape of some a phases changes from short thin grains to long slender ones, and the microhardness changes very little. The optimized parameters with temperature of 700 ℃, stretch velocity of 5 mm/min, post stretch percentage of 2% and dwelling time of 8 min are achieved finally.

Hot Granules Medium Pressure Forming Process of AA7075 Conical Parts

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.

Research on technological parameters of pressure forming with hot granule medium on AA7075 sheet

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.

Stress-strain analysis on AA7075 cylindrical parts during hot granule medium pressure forming

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.

PREDICTION OF MICROSTRUCTURE EVOLUTION DURING HOT EXPANDING-EXTRUSION FORMING OF RESTRAINING RINGS

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.

FEM analysis of vacuum hot bulge forming of Hastelloy C-276 thin-walled cylindrical workpiece

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.

Numerical study on forming complex fitting body on end of integrated stainless steel pipe without welds

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.

Effect of plastic strain and forming temperature on magnetic properties of low-carbon steel

Claw poles are a key component of automobile generators.The output power performance of the generator is very dependent on the magnetic properties of its claw poles.Plastic deformation is known to significantly change the magnetic behavior of ferromagnetic materials in claw poles.In this paper,changes in the magnetic properties of low-carbon steel,used for claw pole components due to their plastic deformation,were investigated for different strains and temperatures.Ring-shaped material samples were prepared by machining and their magnetic properties were measured.The surface roughness was first evaluated and a machining process with an arithmetic average of roughness Ra 1.6μm was selected as enabling the lowest measurement error.Hysteresis loops at different applied magnetic fields of the material were obtained for different plastic strains and forming temperatures.The magnetic parameters of magnetic flux density,coercivity,and remanence were obtained and compared with magnetic flux density as the primary focus.Results showed that machining,cold forming,and hot forming all led to lower magnetic flux density,larger coercivity,and smaller remanence.Magnetic flux density showed a sharp decrease at the start of plastic deformation,but as the strain increased,the decreasing trend gradually reached a constant value.The decrease was much larger for cold forming than for hot forming.For example,at 500 A/m,the degradation of magnetic flux density with a reduction percentage of 5%at room temperature was about 50%,while that of hot forming at 1200°C was about 10%.Results of this research may provide a reference for the future process design of hot-forged claw poles.

Fundamentals and Processes of Fluid Pressure Forming Technology for Complex Thin-Walled Components

A new generation of fluid pressure forming technology has been developed for the three typical structures of tubes,sheets,and shells,and hard-to-deform material components that are urgently needed for aerospace,aircraft,automobile,and high-speed train industries.in this paper,an over all review is introduced on the state of the art in fundamentals and processes for lower-pressure hydroforming of tubular components,double-sided pressure hydroforming of sheet components,die-less hydroforming of ellipsoidai shells,and dual hardening hot medium forming af hard-to-deform materiais Particular attention is paid to deformation behavior,stress state adjustment,defect prevention,and typical applications.In addition,future development directions of fluid pressure forming technology are discussed,including hyper lower-loading forming for ultra-large non-uniform components,precision for ming for intermetallic compound and high-entropy alloy components,intelligent process and equipment,and precise finite element simulation of inhomogeneous and strong anisotropic thin shells.

High-efficiency forming processes for complex thin-walled titanium alloys components: state-of-the-art and perspectives

Complex thin-walled titanium alloy components play a key role in the aircraft,aerospace and marine industries,offering the advantages of reduced weight and increased thermal resistance.The geometrical complexity,dimensional accuracy and in-service properties are essential to fulfill the high-performance standards required in new transportation systems,which brings new challenges to titanium alloy forming technologies.Traditional forming processes,such as superplastic forming or hot pressing,cannot meet all demands of modern applications due to their limited properties,low productivity and high cost.This has encouraged industry and research groups to develop novel high-efficiency forming processes.Hot gas pressure forming and hot stamping-quenching technologies have been developed for the manufacture of tubular and panel components,and are believed to be the cut-edge processes guaranteeing dimensional accuracy,microstructure and mechanical properties.This article intends to provide a critical review of high-efficiency titanium alloy forming processes,concentrating on latest investigations of controlling dimensional accuracy,microstructure and properties.The advantages and limitations of individual forming process are comprehensively analyzed,through which,future research trends of high-efficiency forming are identified including trends in process integration,processing window design,full cycle and multi-objective optimization.This review aims to provide a guide for researchers and process designers on the manufacture of thin-walled titanium alloy components whilst achieving high dimensional accuracy and satisfying performance properties with high efficiency and low cost.

Study of microstructure evolution of magnesium alloy cylindrical part with longitudinal inner ribs during hot flow forming by coupling ANN-modified CA and FEA

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.

Influence of Hot Press Forming Techniques on Properties of Vehicle High Strength Steels

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.