Hot Stamped Parts Research Articles

Study on GISSMO fracture model of hot-stamped Tailored-Tempering-Properties parts

To accommodate the varying mechanical demands in different areas of vehicle components, the Tailored-Tempering-Properties technology of hot-stamped parts of high-strength steel came into being. This research crafted a segregated hot and cold partition die system to fabricate U-shaped parts from 22MnB5 with Tailored-Tempering-Properties. The sides and bottom surfaces of the hard and soft zones of the U-shaped parts were sampled, and uniaxial tensile experiments were carried out to obtain the stress-strain curve, tensile strength, and elongation. In addition, notch, shear, and tensile shear sampling were carried out in the soft area at the bottom flange of the U-shaped part, and the force-displacement curve and fracture strain under different stress conditions were determined by tensile test. The GISSMO fracture model was selected to simulate the failure fracture of the material, and the parameters were obtained by Hypermesh pretreatment, LS-DYNA solution calculation, and LS-OPT optimization. Finally, the GISSMO fracture model suit for the soft zone of 22MnB5 U-shaped parts was obtained to characterize and predict its fracture behavior, which is an essential basis for guiding the design and optimization of parts.

Estimation of Final Shape of Hot-Stamped Parts by Coupling CAE between Forming and Phase Transformation

Estimation of Final Shape of Hot-Stamped Parts by Coupling CAE between Forming and Phase Transformation

Analysis of process limits for partial hot stamping with controlled pre-cooling by radiation exchange

The increasing demands on part complexity and tailored properties in form of local strength and ductility cannot be achieved with conventionally manufactured hot stamped body components. To meet the requirements of safety-relevant body parts, partial hot stamping is increasingly used in the automotive industry. One approach to achieve tailored properties on hot stamped parts is based on controlled cooling of the blank prior to forming and quenching. The related furnace technology is based on a furnace chamber in which a cooled aluminum mask protects local areas of the blank from thermal radiation while simultaneously absorbing the blank’s own emitted radiation. The sheet thickness and utilized material significantly influence the controlled radiation exchange and present potential process limits. To analyse these lower process limits, blanks made of the materials 22MnB5 AS150 and 8MnB7 AS150 are partially pre-cooled and hot stamped with different sheet thicknesses. In addition, the process is simulated with AutoForm®R10 and validated by the obtained experimental results. The results show that the sheet thickness is a key influencing factor in the process and significantly affects the mechanical properties after partial hot stamping. Especially sheet thicknesses t0 of 2.25 mm can limit the process, as the effectiveness of the radiation exchange for controlled pre-cooling and phase transformation decreases.

Study of Microstructural Evolution of Press Hardening Steels using Dilatometer and In-situ Studies for a Simulated Hot Stamping Condition

Press hardening grades are widely used in automotive industries for safety-critical structural parts due to their unique combination of high strength, excellent formability, and crash performance. Considering various scenarios on thermo-mechanical profiles in different hot stamping lines, achieving the targeted strength and ductility / fracture strain in the hot-stamped parts is still challenging to some hot stampers for some grades. In this investigation, a dilatometer study for Usibor®1500 and two emerging grades Usibor®2000 and Ductibor®1000 under a given hot stamping condition was conducted with consideration of the entire hot-stamping processes (i.e., austenitization, blank transfer, forming and final quenching from 700°C) to understand the differences in critical cooling rates, and evolution of microstructures. Influence of large forming strain (15%) on final properties is also examined for Ductibor®1000 and Ductibor®500 by DIL 805 A/D dilatometer under tensile deformation mode. In-situ observation of microstructural evolution during hot stamping process for Usibor®1500 is explored using Confocal scanning laser microscope to uncover some physical phenomena for further refinement of hot stamping practices.

Carburization behavior at elevated temperatures and mechanical properties of a hot stamped complex phase steel

As a lightweight construction strategy, hot stamped parts of ultra-high-strength steels with tailored properties are increasingly used for crash relevant components in car bodies, e.g., as B-pillars. With the process of tailored carburization, parts can be reinforced locally without increasing the sheet metal thickness. Thus, this process has the potential to be applied to lightweight components for no deformation zones, e.g., in the battery housings. Standardly, carburization is performed at 950 °C, with the highest strengths achieved at long times of up to 6 h, which prolongs the total process time. By raising the temperature, the carbon diffusion increases, which enables a reduced heat treatment time and, consequently, a shorter process time. Therefore, the objective of this study is to investigate the influence of elevated carburization temperatures on the mechanical properties of a carburized and hardened complex phase steel, CP-W® 800. A single carburization step at the enhanced temperatures leads to an embrittlement of the samples. Hence, an additional diffusion-annealing step is implemented to homogenize the carbon content, without enlarging the total heat treatment time. Depending on the time allocation of carburization and diffusion, the application of the diffusion annealing step results in higher strengths and ductility compared to only carburized samples.

Development of steel for hot stamping of 22mnb5 steel by joule effect heating

With the increase in demand and development of the automobile sector, where it becomes important to improve the efficiency of energy consumption of vehicles, one can observe the need for materials that are both light and have high mechanical strengths. The Ultra High Strength Steel (UHSS), which include TWIP (Twinning-Induced-Plasticity) and Hot Stamping are characterized by unique microstructures and metallurgical properties that allow car manufacturers to meet the diverse functional requirements of vehicles. Boron steel is suitable for mechanical hot forming. In the last few years, hot stamped parts have occupied a prominent place in the bodies of bodies due to being in line with the demands mentioned above. There is a lot of research under development for this technology, be it in materials, means of production, coatings and applications. Currently, there is a patent that generates a technological restriction for the processing of hot stamping; in which it is to use boron steel with Al-Si metallic coating and heating the blank to be hot stamped through a radiation furnace. The objective of this work is to use a new processing alternative through another metallic diffusion coating of Fe-Zn and heating of the blank to be hot stamped by Joule effect; generating an industrial experiment and prototyping for characterization. The results showed the feasibility of using boron steel with Fe-Zn diffusion coating in hot forming applications by heating by Joule effect, with good surface quality, meets the mechanical properties and microstructure, without cracks in the steel, excellent adhesion of the paint layer and good resistance to corrosion.

Enhancement of shape accuracy and die quenchability of ultra-high strength steel hollow products in hot stamping of tubes using eco-friendly fiber-reinforced ice mandrel

A hot stamping process of quenchable steel tubes using a mandrel reinforced with eco-friendly fibers was developed to produce ultra-high strength steel hollow parts having enhanced lightweighting and crashworthiness. High internal pressure was generated to improve the die quenchability and shape accuracy of the formed parts by the fiber reinforcement. Wood sawdust, shredded copy paper, and plant fiber made of recycled toilet paper were chosen as the fibers, and not only the strength was evaluated from a uniaxial compression test but also the melting behavior of the mandrel was examined. The influence of the fiber reinforcement on the shape accuracy and die quenchability of hot-stamped parts was investigated. The generated internal pressure with the fiber-reinforced ice mandrel was higher than that with the pure ice mandrel without the reinforcement, and thus, the shape accuracy and die quenchability of hot-stamped parts were significantly improved even for a comparatively small change in internal volume of tubes.

Study on Hot Tensile Deformation Behavior and Hot Stamping Process of GH3625 Superalloy Sheet

Hot tensile tests of the GH3625 superalloy were carried out under the temperature range of 800-1050 °C and strain rates of 0.001, 0.01, 0.1, 1, and 10 s-1 on a Gleeble-3500 metallurgical processes simulator. The effect of temperature and holding time on grain growth was investigated to determine the proper heating schedule of the GH3625 sheet in hot stamping. The flow behavior of the GH3625 superalloy sheet was analyzed in detail. The work hardening model (WHM) and the modified Arrhenius model, considering the deviation degree R (R-MAM), were constructed to predict the stress of flow curves. The results showed that WHM and R-MAM have good prediction accuracy by evaluating the correlation coefficient (R) and the average absolute relative error (AARE). Additionally, the plasticity of the GH3625 sheet at elevated temperature drops with the increasing temperature and decreasing strain rate. The best deformation condition of the GH3625 sheet in the hot stamping is in the range of 800~850 °C and 0.1~10 s-1. Finally, a hot stamped part of the GH3625 superalloy was produced successfully, which had higher tensile strength and yield strength than the as-received sheet.

Investigation on the thermo-mechanical properties of hot stamped parts by using laser-implanted tool surfaces

In the automotive industry, hot stamping has been established as a key technology for manufacturing safety-related car body components with high strength-to-weight ratio. During the forming operation, however, hot stamping tools are highly stressed by cyclic thermo-mechanical loads, which encourage the formation of severe wear and high friction at the blank-die interface. Against this background, an innovative surface engineering technology named laser implantation has been investigated for improving the formability of the parts and the efficiency of the hot stamping process. The laser implantation process is based on the generation of highly wear resistant microfeatures on tool surfaces by embedding hard ceramic particles via pulsed laser radiation. As a consequence, the contact area of the tool and thus the tribological and thermal interactions at the blank-die interface are locally influenced. In previous studies, the improved tribological performance of the modified tool surfaces has already been proven. However, the thermal interactions between tool and workpiece have not been analyzed, which in turn have a significant impact on the resulting part properties. In this regard, quenching tests have been carried out under hot stamping conditions by using conventional as well as laser-implanted tooling systems. Based on these results, Vickers hardness test and optical measurements have been performed on the quenched blanks, to qualify the mechanical properties and clarify the cause-effect relations.

Multi-Objective Optimization of Process Parameters in 6016 Aluminum Alloy Hot Stamping Using Taguchi-Grey Relational Analysis

The hot stamping technology of aluminum alloy is of great significance for realizing the light weight of the automobile body, and the proper process parameters are important conditions to obtain excellent aluminum alloy parts. In this paper, the thermal deformation behavior of 6016 aluminum alloy at a high temperature is experimentally studied to provide a theoretical basis for a finite element model. With the help of blank stamping finite element software, a numerical model of a 6016 aluminum alloy automobile windshield beam during hot stamping was established. The finite element model was verified by a forming experiment. Then, the effect of the process parameters, including blank holder force, die gap, forming temperature, friction coefficient, and stamping speed on aluminum alloy formability were investigated using Taguchi design, grey relational analysis (GRA), and analysis of variance (ANOVA). Stamping tests were arranged at temperatures between 480 and 570 °C, blank holder force between 20 and 50 kN, stamping speed between 50 and 200 mm/s, die gap between 1.05 t and 1.20 t (t is the thickness of the sheet), and friction coefficient between 0.15 and 0.60. It was found that the significant factors affecting the forming quality of the hot-stamped parts were blank holder force and stamping speed, with influence significance of 28.64% and 34.09%, respectively. The optimal parameters for hot stamping of the automobile windshield beam by the above analysis are that the die gap is 1.05 t, the blank temperature is 540 °C, the coefficient of friction is 0.15, stamping speed is 200 mm/s, and blank holder force is 50 kN. The optimized maximum thickening rate is 4.87% and the maximum thinning rate is 9.00%. The optimization method used in this paper and the results of the process parameter optimization provide reference values for the optimization of hot stamping forming.

Comparative Analysis of Microstructure and Collision Performance of TWB Hot Stamped Parts with Various Strength Combinations

Hot stamping technology has been steadily developed because it provides both excellent formability and high strength. With the development of TWB hot stamping technology, it is now possible to freely apply the required strength and thickness in the right place. In this study, the microstructure and collision performance of TWB hot stamped parts were evaluated according to their combined strength. Through dilatometry analysis, the hot stamping heat treatment temperatures of 22MnB5 steel and 30MnB5 steel were set at 950 oC and 870 oC. The 5MnB8 steel was composed of Bainite + Martensite when heat-treated at 950 oC and provided a strength of 980 MPa grade. When heat-treated at 870 oC, it was composed of Ferrite + Bainite + Martensite and provided a strength of 780 MPa grade. Simulated rear impact testing showed the 30MnB5- 5MnB8 TWB combination had the best performance, because the 5MnB8 part of the 780 MPa grade absorbed enough energy and the 30MnB5 part of the 1.8 GPa grade fully served as an anti-intrusion.

Resistance Heating by Means of Direct Current for Resource-Saving CO<sub>2</sub>-Neutral Hot Stamping

Hot stamping is a well-established and frequently used manufacturing process in automotive body construction. The number of components manufactured in this way is continuously increasing. Hot stamping is used to produce components with a completely martensitic structure, resulting in high strength and hardness. These components are mainly used in safety-relevant areas of the passenger cell, such as the A-pillar, B-pillar, tunnel and sill. For hot-stamping processes, it is necessary to austenitize the blanks. Heating the sheet metal up to 930 °C in a furnace is very energy-intensive. In large-scale industrial applications, the sheets are generally heated in gas-fired roller hearth furnaces up to 60 m long. Apart from the poor energy balance and the high CO2 emissions of such furnaces, they are associated with high investment and maintenance costs, large space requirements and a long heating time. Rapid heating by means of the Joule effect and direct current instead of alternating current offer an energy-efficient and environmentally friendly alternative for sheet metal heating. Therefore, this technology can make a major contribution to environmental protection and resource saving. Within the scope of this work, parts were rapid-heated and subsequently hot-stamped by means of a novel heating system based on direct current with energy savings of up to 80 %. Using electricity guarantees a good CO2 balance. In addition, resistance heating with a new type of DC-heating system and an adapted process chain is compared with conventional furnace heating. In thermographic images and microstructural examinations of the hot-stamped parts, it can be demonstrated that this direct-current technique is well suited for achieving homogeneous hardness and strength in the whole sheet metal. Thus, this new heating system can enhance the efficiency of the hot-stamping technology.

Novel Approach toward the Forming Process of CFRP Reinforcement with a Hot Stamped Part by Prepreg Compression Molding.

The use of carbon fiber-reinforced plastics (CFRP) is markedly increasing, particularly for the manufacturing of automotive parts, to achieve better mechanical properties and a light weight. However, it is difficult to manufacture multi-material products because of the problems due to the adhesive between CFRP and steel. The prepreg compression molding (PCM) of laminated CFRP can reduce the production time and increase the flexibility of the manufacturing process. In this study, a new manufacturing process is proposed for CFRP reinforcement on a hot stamped B-pillar using PCM. A finite element (FE) simulation of the hot stamping process is conducted to predict the dimensions of the B-pillar. The feasibility of PCM manufacturing is explored by the simulation of the thermoforming of a CFRP set on a shaped B-pillar. The temperature conditions of the CFRP and B-pillar for the PCM are determined by considering the heat transfer between the CFRP and steel. Finally, the PCM of the B-pillar consisting of steel and CFRP was performed to compare with the analytical results for verification. The evaluation of the B-pillar was conducted by the observation of the cross-section for the B-pillar and interlayer by scanning electron microscopy (SEM). As a result, a steel/CFRP B-pillar assembly could be efficiently manufactured using the PCM process without an additional adhesive process.

Investigation of the impact of process parameters on the layer formation of AlSi coated boron-manganese steel

Hot stamping has established itself as a technology for manufacturing safety-relevant car body components. Its usage has found increased importance due to rising political and social pressure e.g. for improved fuel conservation. Due to reduced sheet thickness, hot stamping parts offer lightweight solutions while at the same time providing improved crash performance due to their high strength. Boron-manganese steel is commonly used in hot stamping processes. Due to hot scale formation during heat treatment and the subsequent forming process, additional coatings, typically AlSi-coatings, are applied. Since hot stamped parts are formed at temperatures of 600 °C to 800 °C no suitable lubricants have yet been found. This leads to increased friction and severe wear in the process, which negatively affect tool lifetime and overall part quality. Additionally, process temperatures influence the layer formation and surface roughness due to diffusion processes. A deeper process understanding is needed to acquire detailed knowledge of the resulting impact on tribological behavior. Within this study, the effect of coating thickness on the resulting layer is analyzed by comparing the resulting material composition and surface roughness. To this end, the layer formation and surface roughness are analyzed for two different layer thicknesses (AS150 and AS80) in relation to furnace temperature, holding time and heat up rate. Both are characterized by means of heating experiments conducted on an annealing furnace. The change in layer composition and surface roughness are determined via metallographic investigation and tactile measurement. The results of this study help to improve the process understanding of the tribological conditions within the hot stamping process and to develop future measures for reducing tool wear.

Deep learning based phase transformation model for the prediction of microstructure and mechanical properties of hot-stamped parts

Hot stamping of boron steel is an effective way for weight reduction of automobiles parts, but suffers from the loss of mechanical properties due to incomplete martensitic transformation. Different from general heat treatment, phase transformation in hot stamping relates to both thermal loading path and deformation history. Unfortunately, the existing models have difficulty in capturing the history-dependent phase transformation behavior of boron steel under hot stamping conditions. In the present work, the gated recurrent unit (GRU) are combined with fully connected neural network (FCNN) to capture the coupling effect of both nonlinear strain history and thermal loading path on phase transformation. A unified deep-learning based phase transformation (DLPT) model is developed to integrate both diffusive and diffusionless transformation to cover all feasible thermo-mechanical loading path in hot stamping process. A hybrid driven thermo-mechanical-metallurgical (TMM) framework is developed by integrating the DLPT model into general TMM framework, which providing a novel approach for two-scale simulation of hot stamping process. A comprehensive database of phase transformation is constructed for model training based on well-designed thermo-mechanical loading experiments to cover hot stamping conditions, and the optimal topology of neural network in DLPT model is obtained by training on this database. The accuracy and reliability of DLPT model is systematically demonstrated by dieless hot V-bending and hot stamping of T-shaped parts besides CCT and DCCT tests. The net effect of local plastic strain on subsequent phase transformation is clarified with accuracy by dieless V-bending with the removal of disturbing factors. Non-uniform distribution in hardness of the final hot stamped parts can be predicted with the DLPT model successfully, which is better in accuracy than in case of working with the conventional model.

Hot Stamping Parts Shear Mold Manufacturing via Metal Additive Manufacturing

Hot stamping uses boron (B) steel to simultaneously form parts at high temperatures while cooling the parts in a mold, which is advantageous because of the ability to freeze the forms. However, compared to conventional cold forming, this technique requires additional facilities that include heating devices and additional time for cooling after forming at high temperatures. Additionally, because of the high strengths of hot stamping parts, shear process operations after molding tend to be difficult to perform as a continuous operation via press processing; thus, most operations depend on separate laser processing, which results in lower productivity and increased manufacturing costs. This limitation continues to be the most significant problem with this technology, therefore, restricting its commercialization because of increased mold manufacturing costs and durability problems. This study investigated a low-cost, high-functionality shear mold manufacturing method for 1.5 GPa grade hot-stamped components using heterogeneous metal additive manufacturing. After the concentrated stress in steel during the shearing processes was analyzed using a multi-physical analysis, metal additive manufacturing was used to fabricate the shear mold. Its life was evaluated through trial molding and compared with that for conventional technology. Finally, the commercialization potential of the newly developed method was assessed.

成形-相変態連成CAEによるホットスタンプ部材の形状予測

The hot stamping method is established for manufacturing high-strength automotive parts with good shape fixability. Recently, the hot stamping method has been applied to higher performance parts with tailored properties. CAE is indispensable in designing such parts. For designing hot-stamped parts, the forming simulation must be coupled with thermal analysis considering the temperature effect on formability. The final shape need not necessarily be predicted because of its good fixability. However, in high-performance parts with tailored properties, it is important to predict how the final shape is affected by phase transformation. To predict the final shape of hot-stamped parts, a CAE method that takes into account phase transformation effects has been developed. The accuracy of the calculation of the final shape by the CAE method was investigated. Hot-stamping experiments were carried out to obtain parts of different shapes by cooling control, and the CAE analysis of the final shape for each experimental condition was conducted. It was clarified that the final shape was varied by changing the cooling process, and that the results of CAE including phase transformation were in good agreement with the tendency of shape change in cross section.

Design optimization and application of hot-stamped B pillar with local patchwork blanks

The demand for new structural concept design of automotive parts has grown with weight reduction and improvement of crash safety in automotive industry. In this study, the established local patchwork hot stamping technology was used in structural design of hot-stamped automotive parts and expected to obtain maximum lightweight efficiency while maintaining crash performance by using local patchwork blanks instead of conventional reinforcement. Firstly, the technical feasibility of this technology was verified in an experimental method, and an optimization method was proposed to determine the optimal position and shape of local patchwork blanks. Then it was used for the improvements of two parts as examples, a top-hat channel created based on a conventional B pillar cross section and a B pillar. Finite element (FE) analysis models of these two parts were established based on the deformation of B pillar during full vehicle side impact and validated through experiments. It was confirmed that both the top-hat channel and the B pillar optimized have the same crashworthiness but became lighter compared to the original parts with reinforcement. Furthermore, the effects of local patchwork blank thickness, material strength and outer thickness on the optimal position and shape of local patchwork blanks and lightweight efficiency were investigated. Finally, the effectiveness of a hot-stamped B pillar with local patchwork blanks was verified through full vehicle side impact simulations. It can be concluded that the established local patchwork hot stamping technology and the proposed optimization method can be used as a dependable tool to design and manufacture hot-stamped parts with local patchwork blanks.

Numerical approach for predicting hydrogen diffusion in dual-phase hot stamped boron steel with hydrogen embrittlement

The hot stamped boron steel parts are developed quickly due to the increasing demands of light-weighting for cars. However, hydrogen embrittlement always bothers the manufacture and service process of the hot stamped parts. Although several numerical models have been developed to predict the hydrogen diffusion which determines the susceptibility of hydrogen embrittlement, but they are hard to calculate the hydrogen distribution for the hot stamped boron steel because of the possible existence of the dual-phase structure. In this article the hydrogen transport in the dual-phase hot stamped boron steel is studied by a modified numerical method. The present model is developed by introducing the mean-field homogenization method into the classical equation for hydrogen transport. In order to verify the validation of the present approach, a numerical simulation of the hydrogen diffusion near the blunting crack tip is conducted. The overall and local hydrogen concentration are both predicted in trapping sites and lattice sites respectively. In addition, the inconstant occupancy of the local hydrogen concentration per phase near the crack tip is found in both the trapping sites and lattice sites, which is due to the variation of the local mechanical responding under the macro boundary condition. Furthermore, the coupling effects of the microstructure and the mesomechanical behavior on the local hydrogen concentration are analyzed. The present model realizes to multiscale describe the hydrogen diffusion in the dual-phase microstructure under the macro mechanical behavior, which is meaningful to the prediction of hydrogen induced crack in dual-phase microstructure under different boundary conditions.

Dual-frequency ultrasonic cleaning with diluted phosphoric acid solution for removing oxide scale of uncoated steel sheets in hot stamping

Dual-frequency ultrasonic cleaning with a diluted phosphoric acid solution was developed to remove oxide scales on surfaces of hot-stamped parts from uncoated steel sheets, and conventional shot blasting processes are omitted. The removal of the oxide scale by ultrasonic cleaning is accelerated by the phosphoric acid solution and the dual frequency. The removing time for the phosphoric acid solution was shorter than that for a hydrochloric acid solution, and rust appearing for leaving after cleaning was prevented by generating an iron phosphate layer. In dual-frequency ultrasonic cleaning with the diluted phosphoric acid solution, the oxide scale was dissolved, and then the oxide scales were exfoliated from the thin scale and high-pressure portions. The removing time decreased with decreasing pH and oxide scale thickness and with increasing solution temperature. The surface roughness and distortion of an ultrasonic-cleaned hot-stamped part were smaller than those for shot blasting, and the weldability and paintability were similar. The oxide scale of a hot-stamped part having a nonuniform distribution of oxide scale thickness was successfully removed by dual-frequency ultrasonic cleaning with the diluted phosphoric acid solution.