Hot Stamping Research Articles

Numerical and experimental research on localized induction heating process for hot stamping steel sheets

In this work, the localized induction heating for hot stamping steel sheets was introduced and studied through numerical and experimental methods. The electro-magnetic-temperature multi-field coupling simulation was performed with the ANSYS software. Simulation results showed that the higher temperature region and the transition region caused by the effect of the magnetizer piece should be minimized before industrial application. In order to solve this problem, two shape magnetizers (ring and U-shaped) were placed at different positions on the coils to minimize the effect. The simulation result was validated through the experiments. Experiment results showed that as-quenched part with tailored properties generated related to the different temperature regions of the steel sheet after the localized induction heating and water quenching. The simulation temperature distribution of the B-pillar shows that the localized induction heating can meet the requirements of the tailored temperature regions by using magnetizer rings. The localized induction heating promotes a new application of induction heating for hot stamping steel sheets.

Energy-Economizing Optimization of Magnesium Alloy Hot Stamping Process

Reducing the mass of vehicles is an effective way to improve energy efficiency and mileage. Therefore, hot stamping is developed to manufacture lightweight materials used for vehicle production, such as magnesium and aluminum alloys. However, in comparison with traditional cold stamping, hot stamping is a high-energy-consumption process, because it requires heating sheet materials to a certain temperature before forming. Moreover, the process parameters of hot stamping considerably influence the product forming quality and energy consumption. In this work, the energy-economizing indices of hot stamping are established with multiobjective consideration of energy consumption and product forming quality to find a pathway by which to obtain optimal hot stamping process parameters. An energy consumption index is quantified by the developed models, and forming quality indices are calculated using a finite element model. Response surface models between the process parameters and energy-economizing indices are established by combining the Latin hypercube design and response surface methodology. The multiobjective problem is solved using a multiobjective genetic algorithm (NSGA-II) to obtain the Pareto frontier. ZK60 magnesium alloy hot stamping is applied as a case study to obtain an optimal combination of parameters, and compromise solutions are compared through stamping trials and numerical simulations. The obtained results may be used for guiding process optimization regarding energy saving and the method of manufacturing parameters selection.

High temperature deformation behaviors of hot dip 55 wt% Al-Zn coated steel

The high temperature tensile test of hot dip 55 wt% Al-Zn coated steel was performed to study the fracture behaviors of hot dip 55 wt% Al-Zn coating as a protective coating for hot-stamping steel. During austenitization, the 55 wt% Al-Zn coating had transformed to an alloy layer composed an Fe2Al5Znx (with few FeAl and Zn) outer layer and an FeAl inner layer. When the austenitization heat treatment was fixed at 5 min, the sample heated at a higher heating rate (20 K/s) was subjected to a longer soaking time at 900 °C. This resulted in a thicker FeAl inner layer at the steel/Fe2Al5Znx (FeAl, Zn) interface. During the initial stages of deformation, cracks formed in Fe2Al5Znx (FeAl, Zn) layer. Subsequently, cracks developed in the FeAl inner layer. With continued deformation, cracks widened and debonding occurred at the FeAl/Fe2Al5Znx (FeAl, Zn) interface for the samples heated at lower heating rates (5 K/s or 10/s). Finally, crack widening accelerated the exposure and subsequent oxidation of the steel substrate. The absence of debonding can be beneficial for reducing the susceptibility to powdering during hot stamping.

Combined Effect of Pre-Strain at Elevated Temperature and Pre-Aging on Aging Strength and Microstructures of AA6082

With the increasingly serious energy and environmental issues, aluminum alloys are becoming increasingly desirable for transportation facilities due to their high specific strength. Their low formability results in the difficulty of forming complex-shaped panels and bodies. Hot stamping is a new technology developed to address this formability challenge by applying plastic deformation to the heated aluminum alloy sheet at the solution heat treatment temperature. The stamped panels gain high strength via the sequent aging treatment. Different panels, even different regions on the same panel, are subjected to different forming strains. The present paper is focused on the experimentally investigating the effects of the pre-strain at elevated temperature and pre-aging on the mechanical behavior and microstructures of AA6082 after the natural aging, paint-bake treatment, and artificial aging. Sample sheets of AA6082-T6 were pre-strained by uniaxial tension at different levels of strain, and then subjected to the pre-aging, natural aging, paint-bake treatment or artificial aging, respectively. The yield strength, tensile strength and elongation of post-aged samples were gained via the standard uniaxial tensile tests. The results show that both of the pre-strain at elevated temperature and pre-aging suppress the natural aging of AA6082. Pre-strain at elevated temperature induces weak bake softening, and has a detrimental effect on the artificial aging strengthening, while the sequent pre-aging could compensate the strength decrease.

Thermal Diffusivity Measurement of Laser-Deposited AISI H13 Tool Steel and Impact on Cooling Performance of Hot Stamping Tools

Additive manufacturing is a technology that enables the repair and coating of high-added-value parts. In applications such as hot stamping, the thermal behavior of the material is essential to ensure the proper operation of the manufactured part. Therefore, the effective thermal diffusivity of the material needs to be evaluated. In the present work, the thermal diffusivity of laser-deposited AISI H13 is measured experimentally using flash and lock-in thermography. Because of the fast cooling rate that characterizes the additive process and the associated grain refinement, the effective thermal diffusivity of the laser-deposited AISI H13 is approximately 15% lower than the reference value of the cast AISI H13. Despite the directional nature of the process, the laser-deposited material’s thermal diffusivity behavior is found to be isotropic. The paper also presents a case study that illustrates the impact of considering the effective thermal conductivity of the deposited material on the hot stamping process.

Evolution of Phase Transition and Mechanical Properties of Ultra-High Strength Hot-Stamped Steel During Quenching Process

Hot stamping process is widely used in the manufacture of the high strength automotive steel, mainly including the stamping and quenching process of the hot-formed steel. In the hot stamping process, the steel is heated above the critical austenitizing temperature, and then it is rapidly stamped in the mold and the quenching phase transition occurs at the same time. The quenching operation in the hot stamping process has a significant influence on the phase transition and mechanical properties of the hot-stamping steel. A proper quenching technique is quite important to control the microstructure and properties of an ultra-high strength hot-stamping steel. In this paper, considering the factors of the austenitizing temperature, the austenitizing time and the cooling rate, a coupled model on the thermal homogenization and phase transition from austenite to martensite in quenching process was established for production of ultra-high strength hot-stamping steel. The temperature variation, the austenite decomposition and martensite formation during quenching process was simulated. At the same time, the microstructure and the properties of the ultra-high strength hot-stamping steel after quenching at different austenitizing temperature were experimental studied. The results show that under the conditions of low cooling rate, the final quenching microstructure of the ultra-high strength hot-stamping steel includes martensite, residual austenite, bainite and ferrite. With the increase of the cooling rate, bainite and ferrite gradually disappear. While austenitizing at 930 °C, the tensile strength, yield strength, elongation and strength-ductility product of the hot-stamping steel are 1770.1 MPa, 1128.2 MPa, 6.72% and 11.9 GPa%, respectively.

Young Modulus at High Temperatures

The dependence of the young modulus of metals and alloys on the high temperature must be taken into account in metallurgy in the hot rolling of steel sheet on sheet-rolling mills, hot flattening of steel sheet in the sheet straightening machines, press forming forgings from hot steel sheet, hot volume stamping of metal products, and so on. For example, on the mill 5000 by SMS Siemag, the sheet-rolling of steel strips from the hot slabs at the stand Quarto (the maximum force of 120 MN) occurs at a temperature from 750 °С to 1050 °С, but the pre-flattening of hot steel strip on the five-roller sheet-straightening machine by SMS Siemag occurs usually in the temperature range from 600 °С to 760 °С. In this paper, based on the analysis of experimental data about the dependence of the young modulus for various steel alloys, the analytical dependence of the young modulus on temperature is obtained.

A comparison study on wear characteristics of Ni-based, Co-based and Fe-based alloys for heated hot stamping tools manufactured by surfacing technology

To meet the demanding requirement of ultra-high-strength steel parts with tailored properties, heated hot stamping tools are becoming widely used. However, due to severe working conditions, wear on tool surface is serious. Therefore, the service life of heated hot stamping tools is limited. In this paper, with the purpose of improving the service life of heated hot stamping tools, we aim to find a suitable alloy with better wear-resistant properties to manufacture heated hot stamping tools by surfacing technology. Thus, wear comparison tests between SDCM steel and specimens made of Ni-based, Co-based and Fe-based alloys were conducted under 500 °C and 180 N normal load to determine which kind of alloy has the best wear-resistant properties. Specifically, the mechanical properties, worn volume, worn morphologies and wear mechanisms of test materials were investigated. The results indicate that the Fe-based alloy has the highest compressive yield strength and hardness, but the Co-based alloy has the lowest worn volume, which is 0.49 mm3 at 20 min and 0.70 mm3 at 90 min. And there is no obvious positive relationship between the mechanical properties and the wear-resistant properties of the test materials under the test conditions. The common wear characteristic of the four test materials is that their main wear mechanisms are oxidation wear. The Co-based alloy has the moderate mechanical properties, and its wear mechanisms are oxidation wear and adhesive wear; thus, shear tearing, material transfer and severe abrasive wear are avoided, and the oxide layers adhere to its worn surface instead of spalling. Therefore, the Co-based alloy manifests the best wear-resistant properties and is chosen as the surfacing materials for heated hot stamping tools.

A Fast Approach for Optimization of Hot Stamping Based on Machine Learning of Phase Transformation Kinetics

Hot stamping is one of the key technologies to produce lightweight components for the automotive industry. Numerical simulations are an important tool to design hot stamping tools. However, the hot stamping simulations are highly complex due to the deformation as well as heat transfer and phase transformation mechanisms taking place simultaneously. An accurate prediction of these mechanisms leads to high computational cost. Conventionally, an elastoplastic material model with temperature and strain rate dependence is used to describe the forming behavior, while austenite decomposition models are used to predict the kinetics of the phase transformation. The time spent on calculating the phase transformations can reach up to 40% of the total calculation time. To overcome these computational time issues, the current study introduces an alternative fast approach, to reduce the total calculation time of the hot stamping simulation during the die design stage. The simulation is sped up by replacing austenite decomposition models with surrogate models. A machine-learning (ML) model is used as a candidate for calculating the final phase fraction of hot-stamped parts. To train the model, various simulations models with varying degrees of deformation and cooling rates are set-up in the commercial FEM program LS-DYNA®. The ML model learns from the deformation histories and the final phase fractions to achieve the desired accuracy. For validation, the proposed approach is tested and compared with fully integrated phase transformations models. The final phase fraction and the total amount of floating operations are used as indicators to evaluate the performance of the fast approach. The results indicate that the ML model performs well in predicting the final phase fraction after hot-stamping and a significant reduction of the calculation time evaluated from the counting of floating operation FLOPS is reduced from 5713 to 12.

Modelling of Friction in Hot Stamping

Hot stamping is used to produce structural automotive parts. In this process, high strength ( ̴ 1500 MPa) can be achieved with good formability and accurate geometrical tolerances due to the combination of a forming step at high temperature ( ̴ 700 °C) and the quenching step in the press. However, due to the forming at high temperatures the friction is high and the tool wear is severe. A huge number of measurements of the Coefficient of Friction (COF) have been published so far. However, contradictory findings regarding influence of different parameters such as temperature and pressure are observed and large deviations in the COF values are found. Furthermore, in finite element (FE) analyses of hot stamping processes, as yet the friction is modelled as one constant value, while it is known that the friction depends on temperature, pressure and strain. In this paper, the effect of different parameters on COF using strip-draw tests are investigated. Moreover, a multi-scale friction model for hot stamping is developed and calibrated relative to the experimental results. The friction model regards the surface characteristics of the sheet and tool, which are of major importance due to the build-up of galling. The friction model also takes temperature, pressure and strain effects into account. The friction model is finally coupled with the FE simulation of a hot stamping process.

Optimization Design of Insert Hot Stamping Die’s Cooling System and Research on the Microstructural Uniformity Control of Martensitic Phase Transitions in Synchronous Quenching Process

Ultrahigh‐strength steel BR1500HS was chosen as the research object. The parameters of insert hot stamping die’s cooling system in the synchronous quenching process were calculated, and the cooling system was designed based on the temperature distribution of formed part. Then, by combining FEM simulation and thermal‐mechanical coupling theory of synchronous quenching process of hot stamping, the cooling system was optimized. After that, the punch and die were optimally designed based on the simulation of temperature field of formed part as well as dies, and their temperature decreased by 15°C and 12°C, respectively. Next, the microstructural evolution in austenization and synchronous quenching processes was analyzed. By adopting a synchronous cooling system in hot stamping, by optimizing the parameters and control strategies, and by controlling the water velocity in various inserts, the homogeneity of martensitic phase transitions was improved. The results of the application test show that martensitic phase transitions of major parts are uniform under given working conditions, and hardness difference of each part was small.

Hot stamping of non-rectangular steel sheets using resistance heating by local preheating

Hot stamping of non-rectangular steel sheets using resistance heating by local preheating was developed. In resistance heating of a non-rectangular blank, the temperature distribution becomes non-uniform because of the difference in the current density. The zone having large cross-sectional area was preheated by contact heating before resistance heating, and then the preheated blank was resistance-heated and hot-stamped. The effect of preheating on the temperature distribution after resistance heating was estimated by FEM simulation, and a hot stamping experiment using resistance heating by local preheating was carried out based on the results of simulation. The temperature distribution in the blank became uniform by preheating, and the whole hot-stamped part was sufficiently hardened.

Cracking Behavior of Coating during Hot Tensile Tests of AlSi-Coated Press Hardening Steel

For industrial hot stamping applications, press hardening steel is usually coated with Al-10wt.%Si, in order to prevent substrate decarburization and oxidation at elevated temperatures. However, during hot stamping, the AlSi coating layer fractures, causing severe tool wear, substrate oxidation and increased friction coefficient between the tool and stamped part. The initiation of coating fracture can largely be attributed to the formation of several intermetallic compounds (i.e., FeAl & Fe2Al5) via Fe-diffusion, which also results in void formation throughout the coating layer. These intermetallics are formed mainly during the heating stage, with decreasing Fe-content from the coating-substrate interface. Due to distinctive thermo-mechanical properties of intermetallics compared to the steel substrate, the interaction between different intermetallics, including voids, causes high strain localization around the voids, leading to coating fracture. The goal of this study is to detect the initiation of cracks in a ~45 μm coating layer during uniaxial tensile deformation of a 1.5 mm AlSi-coated press hardening steel. For this purpose, isothermal tensile tests were performed at elevated temperatures. The coating cracks were detected by means of acoustic emission (AE) sensors during deformation. The distribution of coating cracks at hot stamping condition was examined via optical measurements. The tensile strain was measured from a strain grid on the sample gauge. The experiment involves heating the coated steel in a furnace to 920°C for 6 minutes, followed by uniaxial tensile deformation (at 600°C and 800°C), and finally quenching at ambient air. The first AE signal from the sample was observed during the tensile deformation at 600°C, indicating that tensile strain initiates fracture in the coating layer. At cooling stage, the temperature change with time triggered more AE signals, which may correspond to substrate phase transformation and additional fractures in the coated steel; the latter is owing to thermal expansion mismatch between the intermetallics in the coating layer, and steel substrate. Interestingly, no AE signals were observed during the heating stage; i.e., no coating cracks occur prior to deformation and quenching.

Технологічні особливості виготовлення дискових робочих органів ґрунтообробних та посівних машин

The purpose of this study is to improve the technology of manufacturing spherical disk working bodies of tillage machines to increase their wear resistance and strength. The world's leading manufacturers achieve high wear resistance and strength of discs made of boron-containing steels both in their chemical composition and the choice of rational heat treatment technology. Experimental studies of 30MnB5 steel parts revealed that heating for 15 minutes to a temperature of 850°C and quenching in water was sufficient to achieve maximum hardness values. To prevent hardness reduction and to ensure the necessary elasticity and viscosity of the discs, it is recommended to temper at a temperature of 180°C and combine with heat before painting with powder paint. The application of cutting blanks with cutting of depressions along the outer contour of the disk and internal holes on the laser machine, as well as the formation of the finished shape of parts in the process of hot stamping and subsequent heat treatment in quenching dies. 1. The market of disk working bodies of tillage and sowing machines is studied, technological processes of manufacturing of which it is expedient to improve. 2. Theoretical and experimental study of the structure, structure and material properties of samples of spherical disks of imported agricultural machinery. The choice of 30MnB5 pine steel for their production is substantiated. 3. The improved technological process of manufacturing spherical disks is developed. It is recommended to form them by hot stamping in cold dies and intensive hardening in a water bath. 4. The project development of the section of production of disk working bodies of tillage and sowing equipment is carried out.

Investigation of material properties of tailored press hardening parts using numerical and physical simulation

In order to assure good resistance performances, automobile manufacturers are looking for high strength steel, especially for structural parts of body in white, which are formed by hot stamping process. With the aim of guaranteeing specific crash behavior, various technologies have been developed to provide tailored properties.Typical tailored component is the B-pillar, which is studied in this work. In general, the pillar must be with greater resistance in some areas, while in others it must have greater toughness to absorb the energy of a possible impact. The tailored technology investigated in this work is the tailored tempering, in which different areas of the component experience different cooling histories leading to requested final mechanical properties.The methodology used to investigate custom properties involves a first designing phase implemented with Finite Element (FE) commercial programs; FE simulations were performed to investigate the Press Hardening process of a 22MnB5 boron steel blank and in particular the thermo-mechanical cycles related with both the high-resistance and high-toughness regions of the pillar.In the second phase of the proposed approach, the obtained thermo-mechanical cycles have been physically simulated, by using Gleeble 3185 system. The following consecutive steps was reproduced on home-designed specimens: (i) Blank heating for the complete austenitization (at a temperature of 930° C for 4 min). (ii) The heat loss due to the transport phase of the blank from the oven to the press. iii) The mechanical deformation of the blank due to the stamping phase. iv) The quenching phase of the part. (v) The cooling on air of the B-Pillar.Finally, micro-hardness tests have been performed on the specimens subjected to physical simulation. FE model predictions and micro-hardness tests are in good agreement, showing that tailored tempering effectively leads to differentiation of the mechanical properties.

Study on Preparation of Microlens Array Hydrophobic Flim by Ultraviolet Curing Micro-imprint

In this paper, the formation process of microstructure array hydrophobic film prepared by ultraviolet curing imprinting was studied. The UV-curing micro-imprinting method was used to replace the traditional hot stamping method to prepare the micro-structured hydrophobic film. The effects of illumination time and intensity on the surface hydrophobicity were investigated.Results shown that the micro-structure hydrophobic film prepared by UV-curing micro-imprint has high replication precision and good hydrophobic effect. With the increase of illumination time, the water contact angle of the product increased and reached to 125° at 30s illumination time. With the increase of the illumination intensity, the water contact angle of the micro-imprinted product was also gradually increased.

Development of Hot Stamping for Automotive Panel Parts with Step-shaped Wall

Development of Hot Stamping for Automotive Panel Parts with Step-shaped Wall

Effect of Al-Si Coating on Weldability of Press-Hardened Steels

Resistance spot welding (RSW) of Al-Si-coated PHS was undertaken, and the effect of Al-Si coating on nugget formation and mechanical properties was investigated. Press-hardened steel (PHS) has long been applied to automotive body structure construction to support mass and corresponding greenhouse gas emission reductions. PHS materials are often combined with an Al-Si coating applied as an oxidation barrier though unfortunately, the Al-Si coating poses a challenge to the resistance spot welding (RSW) of PHS containing stack-ups. As a newly developed coating for the hot stamping process, the property of Al-Si coating is different from base metal and traditional coatings, and the influence mechanism of Al-Si coating on the welding process is not clear. It would remain at nugget edge and might cause severe stress concentration. To investigate this problem, RSW of 1.5-mm Al-Si-coated PHS was undertaken, and the results indicate that a large portion of the Al-Si coating is extruded and forms a sharp notch close to the nugget edge during the welding process. During the post-weld cooling stage, a thin layer of residual coating is formed between the nugget and notch root. The mechanical performance of the welded joints is limited by the thin residual Al-Si layer which acts as a preexisting crack and supports the interfacial fracture. The presence of the Al-Si coating at the faying interface also significantly delays nugget formation, though it contributes to a larger nugget size by inhibiting expulsion events at the faying interface.

Localized dispersing of TiB2 and TiN particles via pulsed laser radiation for improving the tribological performance of hot stamping tools

The aim of this study is to increase the tribological performance of hot stamping tools by using a laser implantation process. This technique allows the fabrication of separated, elevated and dome-shaped microfeatures on the tool surface in consequence of a localized dispersing of ceramic particles via pulsed laser radiation. Hence, the topography and material properties of the tool are modified, which influences the tribological interactions at the blank-die interface. However, an appropriate selection of ceramic particles is an essential prerequisite, in order to obtain tailored and highly wear resistant surface features. In this regard, different titanium-based hard particles (TiB2 and TiN) were laser-implanted on hot working tool specimens and subsequently tested by means of a modified pin-on-disk test regarding to their wear and friction behavior.

Investigation on mechanical behavior and plastic damage of AA7075 aluminum alloy by thermal small punch test: Experimental trials, numerical analysis

The hot stamping process has demonstrated its high potential to fabricate complex-shaped automotive panel components with high strength AA7075-T6 (Al-5.37Zn-2.18Mg-1.25Cu) aluminum alloy, which conduces to product customization, cost reduction, and efficiency improvement. However, the damage-induced forming crack at high temperatures is more sensitive to the complicated process parameters. A self-developed thermal small punch test (T-SPT) was introduced to investigate the thermo-mechanical behavior and plastic damage of high-strength AA7075-T6 miniature specimens under elevated temperatures in this research. The mesoscopic damage Gurson-Tvergaard-Needleman (GTN) model was employed to predict the micro-area ductile fracture and to conduct formability prediction. An FEM inverse calibration approach coupled with the response surface method (RSM) and the genetic algorithm was developed to accurately identify the temperature-dependent GTN damage parameters based on a series of uniaxial hot tensile curves. Subsequently, numerical simulations of T-SPT regulated by GTN damage parameters were executed to compare with the experimental results. The effects on the mechanical behavior of critical process factors were further discussed, including the deformation temperature, the loading speed, and the surface friction coefficient. Finally, the GTN damage model was successfully applied to predict the formability and the damage distribution of actual AA7075-T6 B-pillar during the hot forming process. The results reveal that the numerical simulation based upon the GTN model owns high reliability and accuracy within the industrialized hot forming application.