Experimental research and application of cold cutting to hot stamping parts based on multiphase microstructure

Among the bottlenecks that hinder the improvement of the production efficiency of hot stamping are high strength and difficulty in edge cutting and hole punching.Starting from the preparation of hot stamping multiphase microstructure materials,this paper developed a plate quenching die system with controllable surface temperature and prepared four types of hot stamping plates with different martensite volume fractions.Then,straight edge cold cutting experiments were performed to study the influence of cutting clearance and cutting force on fracture quality.The results show that the bright zone is the largest when the cutting clearance is 0.14 mm,and the cutting experience coefficient of the hot stamping sheet with each martensite volume fraction is obtained when the cutting clearance is 0.14 mm.The research results of this paper were applied to the production of hot stamping parts.

Application of new generation low-strength hot stamping steel to improve the crash performance of BIW

With the improvement of safety performance,car parts have different requirements for material strength and energy absorption performance.The conventional 1500-MPa hot stamping steel cannot well meet the requirements.Considering the new generation 600-MPa hot stamping steel,this study investigates the applicable car parts and hot stamping process,then designs a new body-in-white(BIW)crash test for obtaining the crash performance of the new material.Through the actual part development and crash test,it is verified that the application of the new generation hot stamping steel can improve the crash performance of BIW.

Forming defects in aluminum alloy hot stamping of side-door impact beam

The forming defects, including thinning, rupture, wrinkling and springback, usually arising in producing a side-door impact beam, were investigated by trial and numerical simulation. A temperature-related constitutive model specific to the temperature range from 350 °C to 500 °C was established and used for the numerical simulation. The trial and numerical simulation were conducted to clarify the quantitative characteristics of forming defects and to analyze the effects of process parameters on the forming defects. Results show that the rupture situation is ameliorated and the springback is eliminated in the aluminum alloy hot stamping. The wrinkling severity decreases with increasing blank holder force (BHF), but the BHF greater than 15 kN causes the rupture at the deepest drawing position of workpiece. The forming defects are avoided with lubricant in the feasible ranges of process parameters: the BHF of 3 to 5 kN and the stamping speed of 50 to 200 mm/s.

Formability of aluminum-silicon coated boron steel in hot stamping process

Flow behavior of the Al-Si coated boron steel was investigated with Gleeble-3500,in comparison with the uncoated one.Effect of deformation conditions on the coating integrity was characterized by optical microscopy.Facture surfaces of the coated steels were inspected under SEM.Experimental results indicate that the ultimate tensile strength and ductility of the Al-Si coated boron steel are lower than those of the uncoated steel under test conditions.Extensive cracks occur in the coating after tensile tests;the width and density of cracks are sensitive to the deformation temperatures and strain rates.The bare substrate exposed between the separate coating segments is oxidized.Appearance of the oxide degrades the Al-Si coating adhesion.Remarkable difference between formability of the coating layer and the substrate is confirmed.The formability of the Al-Si coating could be optimized by controlling the phase transformation of the ductile Fe-rich intermetallic compounds within it during the austenization.

Springback analysis of 6016 aluminum alloy sheet in hot V-shape stamping

Springback behavior of 6016 aluminum alloy in hot stamping was investigated by a series of experiments under different conditions using V-shape dies and a finite element model which was validated reliable was used to further elucidate the springback mechanism. The effects of initial blank temperature, blank-holding force, die closing pressure and die corner radius were studied. It is found that springback decreases remarkably as the initial blank temperature rises up to 500 °C. The springback also reduces with the increase of die holding pressure and the decrease of die corner radius. Under different initial temperatures, the influence of blank-holding force is distinct. In addition, the bending and straightening of the side wall during the stamping process is found to interpret the negative springback phenomenon.

Influences of hot stamping parameters on mechanical properties and microstructure of 30MnB5 and 22MnB5 quenched in flat die

The influences of hot stamping parameters such as heating temperature,soaking time,deformation temperature and cooling medium on the phase transformation,microstructure and mechanical properties of 30MnB5 and 22MnB5 are investigated and analyzed in this work.The quenching experiment,tensile testing,hardness measurement and microstructure observation were conducted to obtain the mechanical and microstructural data.The results indicate that 30MnB5 possesses a higher tensile strength but a lower elongation than 22MnB5,if hot stamped at the same process parameter.The tensile strength and hardness of the hot stamped specimens decrease under inappropriate heating conditions for two reasons,insufficient austenitization or coarse austenite grains.The austenitic forming rate of 30MnB5 is higher than that of 22MnB5,because more cementite leads to higher nucleation rate and diffusion coefficient of carbon atom.More amount of fine martensite forms under the higher deformation temperature or the quicker cooling rate.

Effect of solution treatment time on plasticity and ductile fracture of 7075 aluminum alloy sheet in hot stamping process

The effect of solution treatment time on the post-formed plasticity and ductile fracture of 7075 aluminum alloy in the hot stamping process was studied.Tensile tests were conducted on the specimens subjected to the hot stamping process with different solution treatment time.The digital image correlation(DIC)analysis was used to obtain the strain of the specimen.Based on the experiments and modeling,the Yld2000-3d yield criterion and the DF2014 ductile fracture criterion were calibrated and used to characterize the anisotropy and fracture behavior of the metal,respectively.Furthermore,the microstructure of specimens was studied.The experimental and simulation results indicate that the 7075 aluminum alloy retains distinct anisotropy after the hot stamping process,and there is no obvious effect of extending the solution treatment time on the material anisotropy.However,it is found that a longer solution treatment time can increase the fracture strain of the aluminum alloy during the hot stamping process,which may be related to the decrease of the second-phase particles size.

Springback prediction of TC4 titanium alloy V-bending under hot stamping condition

In this paper,the springback of TC4 titanium alloy under hot stamping condition was studied by means of experiment and numerical analysis.Firstly,an analytical model was established to predict the V-shaped springback angleΔαunder the stretch-bending conditions.The model took into account of blank holder force,friction,property of the material,thickness of the sheet and the neutral layer shift.Then,the influence of several process parameters on springback was studied by experiment and finite element simulation using a V-shaped stamping tool.In the hot stamping tests,the titanium alloy sheet fractured seriously at room temperature.The titanium alloy has good formability when the initial temperature of the sheet is 750–900°C.However,the springback angle of formed parts is large and decreases with increasing temperature.The springback angleΔαdecreased by 50%from 0.5°to 0.25°,and the angleΔβdecreased by 46.7%from 1.5°to 0.8°when the initial temperature of sheet increased from 750°C to 900°C.The springback angle of titanium alloy sheet increases gradually with the increase of the punch radius,because of the increase of elastic recovery,the complex distribution of stress,the length of forming region and the decreasing degree of stress.Compared with the simulation results,the analytical model can better predict the springback angleΔα.

Integrated Modelling of Microstructure Evolution and Mechanical Properties Prediction for Q&P Hot Stamping Process of Ultra‑High Strength Steel

High strength steel products with good ductility can be produced via Q&P hot stamping process,while the phase transformation of the process is more complicated than common hot stamping since two-step quenching and one-step carbon partitioning processes are involved.In this study,an integrated model of microstructure evolution relating to Q&P hot stamping was presented with a persuasively predicted results of mechanical properties.The transformation of diffusional phase and non-diffusional phase,including original austenite grain size individually,were considered,as well as the carbon partitioning process which affects the secondary martensite transformation temperature and the subsequent phase transformations.Afterwards,the mechanical properties including hardness,strength,and elongation were calculated through a series of theoretical and empirical models in accordance with phase contents.Especially,a modified elongation prediction model was generated ultimately with higher accuracy than the existed Mileiko’s model.In the end,the unified model was applied to simulate the Q&P hot stamping process of a U-cup part based on the finite element software LS-DYNA,where the calculated outputs were coincident with the measured consequences.

Research status and progress of hot stamping

In this article,current application,materials,key equipments,finite element(FE) simulation and parts properties of hot stamping are introduced.The investigations of all processes and further excellent processes are described.The survey of existing works,especially key equipments has revealed several gaps.Some new ideas and programs are proposed on the basis of traditional process.This article aims at providing an insight into a whole process backgrounds and pointing out the great potential for further investigations and innovations of hot stamping.

Numerical simulation of hot stamping of side impact beam

Ls-DYNA software is adopted to conduct research of numerical simulation on hot stamping of side impact beam to calculate the temperature field distribution, stress field distribution, forming limit diagram （FLD） figure, etc. in the course of hot stamping so as to predict and analyze the formability of parts. ProCAST software is employed to conduct research of numerical simulation on solid quenching course concerning hot stamping to calculate temperature field distri- bution of tools and component of muhiple stamping cycles. The results obtained from numerical simulation can provide significant reference value to hot stamping part design, formability predication and tools cooling system design.

Optimization of Process Parameters for Cracking Prevention of UHSS in Hot Stamping Based on Hammersley Sequence Sampling and Back Propagation Neural Network-Genetic Algorithm Mixed Methods

In order to prevent cracking appeared in the work-piece during the hot stamping operation,this paper proposes a hybrid optimization method based on Hammersley sequence sampling( HSS),finite analysis,backpropagation( BP) neural network and genetic algorithm( GA). The mechanical properties of high strength boron steel are characterized on the basis of uniaxial tensile test at elevated temperatures. The samples of process parameters are chosen via the HSS that encourages the exploration throughout the design space and hence achieves better discovery of possible global optimum in the solution space. Meanwhile, numerical simulation is carried out to predict the forming quality for the optimized design. A BP neural network model is developed to obtain the mathematical relationship between optimization goal and design variables,and genetic algorithm is used to optimize the process parameters. Finally,the results of numerical simulation are compared with those of production experiment to demonstrate that the optimization strategy proposed in the paper is feasible.

Error Analysis of FLC Experimental Data at Warm/Hot Stamping Conditions

Forming limit curves（FLCs） are commonly used for evaluating the formability of sheet metals. However, it is difficult to obtain the FLCs with desirable accuracy by experiments due to that the friction effects are non-negligible under warm/hot stamping conditions. To investigate the experimental errors, experiments for obtaining the FLCs of the AA5754 are conducted at 250℃. Then, FE models are created and validated on the basis of experimental results. A number of FE simulations are carried out for FLC test-pieces and punches with different geometry configurations and varying friction coefficients between the test-piece and the punch. The errors for all the test conditions are predicted and analyzed. Particular attention of error analysis is paid to two special cases, namely, the biaxial FLC test and the uniaxial FLC test. The failure location and the variation of the error with respect to the friction coefficient are studied as well. The results obtained from the FLC tests and the above analyses show that, for the biaxial tension state, the friction coefficient should be controlled within 0.15 to avoid significant shifting of the necking location away from the center of the punch; for the uniaxial tension state, the friction coefficient should be controlled within 0.1 to guarantee the validity of the data collected from FLC tests. The conclusions summarized are beneficial for obtaining accurate FLCs under warm/hot stamping conditions.

Study on the resistance spot welding technology of 22MnMoB hot stamping quenched steel

In this paper, the spot welding technology of a new kind of 22MnMoB hot stamping quenched steel sheet was systematically studied by power frequency spot welder. Through a series of technology and test experiments, we have obtained the optimal spot welding technological parameter condition. According to the results, the relations among spot welding technological parameter, welding nugget, mechanical property and fracture mode were discussed. The effects of all the welding parameters such as welding current, welding time and electrode force on the quality of joint can be boiled down to one thing--the diameter of welding nugget. The experimental results showed that welding nugget diameter determines the mechanical property of spot welding joint and the relation between welding nugget diameter and the mechanical property of joint presents a kind of linear mathematic representation. There are two typical fracture models of 22MnMoB hot stamping quenched steel sheet, i.e., interracial fracture and nugget pullout. Other than mild steel or normal high strength steel, in the shearing tensile test, hot stamping quenched steel has a great tendency to fail in interfacial mode due to the effects of high strength matrix structure, welding soft zone and the porosity level of fusion zone.

Comparative analysis by simulating and testing pole impact for hot stamping front crossbeam of automobile

Hot stamping 22MnB5 steel plate with ultra-high strength has been widely used for body structural members in consideration of automobile safety and lightweight.This paper presents a verification program of simulating and testing pole impact in order to verify if the front crossbeam reinforcement assembly can meet the design requirements,reduce the overall vehicle simulation and test cost and shorten the operation period.In the same condition,the simulation proved conforming to the design requirements;however,the bumper cracked at the impacting point in the course of pole impact test.The analysis of the crack by optical microscope,Vickers and scanning electron microscope indicates that mixture of ferrite,bainite and martensite was produced in the weld heat-affected zone of the 340/590DP tow hook holder and the 22MnB5 front crossbeam;therefore,their hardness and mechanical property were reduced obviously,so that they fractured when impacted.No welding process with continuous weld but spot welding or other bonding method may be employed for the reason that the capacity of the 22MnB5 steel plate in the weld heat-affected zone was reduced obviously.

Microstructure design and feasibility of hot stamping for new generation high strength multi-phase steels

Optimization of microstructure for new generation advanced high strength steels(AHSS ) for automobiles was briefly reviewed.Two different heat treatments(quenching partitioning austempering/QPA and quenching partitioning tempering/QPT) have been investigated to obtain optimal microstructures,which are made up of martensite(hard phase),retained austenite(soft phase),and carbide or nano-bainite.Combination of hot stamping and newly developed heat treatments is discussed.

Microstructure Evolution and Simulation in 22MnB5 Steel during Hot Stamping

Hot stamping components with 1500 MPa ultra-high strength are obtained by press hardening during hot stamping, and the properties depend on the microstructures. It is very important that the microstructure evolution rule is found out during hot stamping process. To characterize the microstructure evolution during hot stamping, a method combining finite element and experiment is carried out. Samples were heated to 950°C and held for 300 second at a induction heating furnace, then taken out from the furnace and stayed in the air at different time (7 s, 11 s, 13 s, 22 s), respectively, finally the specimens were formed and quenched at a die. Microstructural observation as well as surface hardness profiling of formed specimens was performed. And the numerical simulation to predict the austenite transformation into ferrite, pearlite, bainite, and martensite and the volume fraction of each phase during the hot stamping process was made with ABAQUS software. The results show that the ferrite is observed when the specimen stays in the air for 22 s, and the temperature drops to 325°C when the dwell time increases from 7 s to 22 s. The results of numerical simulation and experimental results are in good agreement. So the method finite element can be used to guide the optimization of hot stamping process parameters.

Temperature distribution of boron-manganese sheet metal blank by induction heating in application for hot stamping

In order to speed up the production and save more energy in hot stamping process,the induction heating technology as a new effective heating method is considerable.Finite element(FE)-simulation and a series of experiments are carried out to research the temperature homogenization of induction heating with the face inductor.It is found the edge effect has a notable influence on the temperature distribution.Results concerning the mechanical properties of the stamped part as well as surface characteristics will be presented and discussed.

Simulation about hot stamping of ultra-high strength steel on the basis of lightweight technology

With the development of automobile lightweight,it is very necessary to apply the ultra-high strength steel parts manufactured by hot stamping,which offers the possibility to reduce the weight of automobiles and maintain the safety requirement.In order to complete hot stamping,it is important to design the structure of parts reasonably,which is related with reasonable matching of strength.The objective of this paper is to guide the design of parts manufactured by hot stamping and find the forming technical requirements of vehicle performance.Through experiments,the paper obtains the stress and strain curves at different deformation temperatures and strain rates.Based on experimental data, the constitutive relationship model is established which can reflect the deformation capacity of ultra-high strength steel during the process of hot stamping.Combined with finite element simulation results of hot stamping by commercial software AUTOFORM,transfer path of load and matching law of strength,the paper determines the design criteria and forming technical requirements of parts manufactured by hot stamping.At the same time,the impact performance of front cross member internal plate is taken into consideration.

Study of front bumper process on the basis of hot stamping

The fitting method is used for obtaining the constitutive relation of ultra-high strength steel in this paper. At the same time, the forming limit of material is researched under different de/brmation conditions. Through experiments, the paper obtains stress-strain curve under different deformation temperatures and strain rates. On the basis of experimental data, the constitutive relation model is established which can reflect the de- fomlation capacity of ultra-high strength steel during process of hot stamping. Combining with finite element simulation results of hot stamping by the commercial software AUTOFORM, transfer path of load and matching law of strength, the paper determines the design criteria and forming process requirements of parts manufactured by hot stamping. Through setting different process parameters, such as punch velocity and friction coefficient, and blank shape, the paper finds that process parameters and blank shape have an essential effect on the quality of parts.