Hot Stamped Components Research Articles

Evaluation framework of digital characteristics (DC) enhanced lubricant: Consideration of essential geometric features for hot-stamped components

Innovative digitally enhanced technologies are revolutionising the manufacturing systems, including the metal forming sector, at an unprecedented speed and scale. By utilising voluminous metadata collected through sensing networks and experimentally verified simulations during daily production and academic activities, profound insights can be provided to facilitate the ongoing efforts on understanding metal forming processes from the perspective of data. Data science in metal forming has emerged as an interdisciplinary research challenge that requires the knowledge of both manufacturing and data science. To unveil the distinctive thermo-mechanical characteristics of metal forming processes, for example the hot stamping process, the Digital Characteristics (DC), defined as the metadata visualisation that contains essential information spanning over the design, manufacturing and application of the manufactured products, were developed for different geometric features based on a vast number of hot stamping metadata. The analysis on distinctive hot stamping DC led to the development of a digitally enhanced interactive friction model considering complex, transient contact conditions incorporating evolutionary interfacial thermo-mechanical characteristics, such as interfacial temperature, contact pressure and sliding speed. The lubricant limit diagram (LLD) was developed demonstrated by coefficient of friction (COF) and performance grade (PG) to enable the quantitative evaluation of different lubricants considering different geometric features of hot-stamped components. The results advanced the understanding of lubricant performance and further demonstrated a novel methodology that integrates the development of DC in the hot stamping process, data-guided LLD, and a quantitative evaluation of lubricant performance. With the advancements in DC and LLD, a new framework can be established to characterise alternative applications in other forming processes that demand rigorous lubricant evaluation.

Reinforcement of thin hot-stamped components by fiber reinforced plastic structures with optimized fatigue strength properties

Multi-material structures in the automotive industry hold great potential for lightweight design, body construction, and functionalization due to their favorable mechanical properties and reduced structural weight. The combination of metal and plastic, in particular, is commonly used to enhance the overall properties of the end product when compared to single-material structures.This paper describes a process development with a hot-stamping and an extrusion tool. By means of this tool, a thermally assisted extrusion process can be used to join GMT (Glass Mat reinforced Thermoplastics) and 22MnB5 steel in a single process step. Through adhesion, the GMT adheres to the rough surface of the AlSi(aluminum-silicon)-coated 22MnB5. Test components were manufactured and through static tests the influence of process parameters was evaluated. Assuming that the parameters determined for the reference component are already sufficient for a design in the vehicle, the reduction of the steel thickness of the structure from 1.5 mm to 1.2 mm can be recommended on the basis of the results obtained. This is accompanied by a reduction in the mass of the test structure used while maintaining or improving its dynamic and static properties. Further weight savings appear possible through further component and process optimization.

Examining the Impact of Intermediate Cooling on Mechanical Properties of 22MnB5 in a Tailored Tempering Process

Tailoring the properties of hot-stamped components offers the potential to enhance crash performance while simultaneously improving downstream joining processes. In recent years, an innovative technology suited for achieving tailored properties involving the utilization of a specialized furnace chamber, known as the TemperBox®, has been introduced. Within this chamber, a cooled aluminum mask shields specific areas of the blank from incoming heat radiation and concurrently absorbs the blank’s own radiation. The duration of the heat radiation exchange can influence the diffusion-dependent phase transformation and, consequently, the resulting mechanical properties. Hence, the intermediate cooling duration assumes a pivotal role as a parameter, as is investigated in this study. To examine the effects, specimens of the steel 22MnB5 AS150 are subjected to intermediate cooling of varying durations, followed by forming and partial quenching. The temperature profile of the blank during intermediate cooling prior to forming and quenching is analyzed. Subsequently, the tailored hot-stamped components are assessed for hardness, strength, ductility, and thickness strain. The study reveals that with increasing duration of partial intermediate cooling and targeted radiation exchange, a homogeneous ferritic–pearlitic structure is formed from an austenitic structure. This uniform structure of ferrite and pearlite is reflected in lower hardness and strength values, along with improved ductility. Additionally, this paper introduces a simulation methodology designed to calculate the dynamics of thermal radiation and the kinetics of phase transformation.

Partial resistance tempering of hot-stamped components made of 22MnB5 for subsequent bending

The manganese-boron alloy 22MnB5 is particularly used for structural and safety-relevant parts in the automotive industry. Parts made from this alloy are usually produced using the hot forming process. Here, the sheet is heated to over 950 °C using an industrial roller hearth furnace. The heated sheet is then simultaneously formed and quenched in a cooled tool with a temperature gradient of more than 27 K/s. This leads to the formation of a martensitic microstructure with a hardness value of over 450 HV10 and an elongation at break of less than 6%. The small strain potential of such components makes them difficult to form after hot-stamping. Due to the high temperature gradients of resistance heating, a sheet can be heat-treated locally without a large temperature transition zone. This can be used to locally soften already hot-stamped components for subsequent operations such as bending. Within the scope of this paper, resistance heating is used to soften a hot-stamped 22MnB5+AlSi sheet stripe of 3 mm width. The sheet could consequently be bent over an angle of 90° without cracking the substrate.

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.

Design and development of a micro-LDH apparatus to determine formability of sheet metal under hot stamping conditions using Gleeble

In the last few years, demand for hot stamped components has increased in the automotive industry. Determination of formability under hot stamping is challenging due to elevated temperature, fast cooling, and high punch velocity. Although there was various strive for formability determination, but there were limitations with experiments like non-uniform heating of specimen, non-uniform strain, and temperature distribution. Therefore, in this work, an experimental apparatus was developed to determine formability at room temperature, high temperature, hot stamping conditions, and any complex process cycle involving heating and cooling. New specimen was designed to produce different strain paths, uniform, and homogenous temperature distribution with the help of FEM software using thermomechanical and thermoelectrical simulation. A micro hemispherical dome-based experimental apparatus was designed using Solidworks. The designed apparatus was used in conjunction with the thermo-mechanical simulator (Gleeble-3800). Thermomechanical analysis was done in PAM STAMP software to optimize specimen size and shape to get uniform strain distribution and different strain paths. A thermoelectric FEM model was developed using Abaqus 6.14 to optimize the temperature distribution in the specimen. The developed model enables choosing the appropriate polarity of the electrical cable connection to achieve uniform temperature distribution in the specimen. Strain path and temperature profiles for experiment and simulation were compared. Further, a forming limit curve was developed using the designed apparatus to verify the feasibility of the apparatus. For feasibility test of apparatus, hot stamping process was selected. This new design apparatus can be used for a range of temperatures up to 1000 °C, hot stamping conditions, and for different materials (aluminum, magnesium alloys, different grade of steel, etc.). It concludes that the connection of different polarities of electrical cable was critical for homogenous and uniform temperature distribution in specimens.

A Tailored Preparation Method of Variable Strength for Ultra-High-Strength Steel Sheet and Mapping Mechanism between Process and Property.

The spatiotemporal phase transformation during hot stamping would considerably effect the microstructure and mechanical properties of steels. In order to manufacture hot-stamping components of ultra-high-strength steel with tailored mechanical properties, the effect of the quenching time and the temperature of the die on the phase-transformation characteristics and mechanical property of ultra-high-strength steel was deeply studied. A finite element (FE) model coupled with a thermomechanical phase was employed to perform a succession of simulations for hot stamping corresponding to different quenching times and the temperatures of die, and the corresponding hot stamping experiments were performed. The 3D mapping surfaces of the temperature; quenching time; and three microstructures, namely austenite, bainite, and martensite, were constructed, and the mapping relationships in such surfaces were further explained by microstructural observations. Subsequently, based on the test results of the mechanical properties, the relationship curves of hardness and tensile strength, hardness, and elongation at break were fitted respectively, and then the 3D mapping surfaces were constructed for hardness, tensile strength, and elongation at break, which varied with the temperature die and quenching time. Finally, the quenching parameters of the automobile B-pillar were designed according to the constructed mapping relationship, and the hot-stamping FE simulation of the automobile B-pillar was developed. The result shows that those constructed mapping surfaces are helpful for adjusting the local mechanical property of the steels by designing the parameters.

Effect of Combined Forming and Aging Processes on the Mechanical Properties of the Precipitation-Hardenable High-Strength Aluminum Alloys AA6082 and AA7075

The recently increasing demand for hot stamped aluminum components in the automotive and aerospace industries explains the necessity of designing efficient and resource-conserving thermo-mechanical processes. Within the thermo-mechanical process, the simultaneous effect of deformation and temperature accelerate the precipitation kinetics. Therefore, this study focuses on the combined effect of forming and aging processes on the mechanical properties of high-strength aluminum alloys AA6082 and AA7075. For this aim, two different thermo-mechanical aging process strategies after solution heat treatment and quenching in a water-dilutable polymer quenchant are proposed. The superpositioning of the forming step is either performed at the beginning or continuously during the aging treatment. The resulting mechanical properties are characterized using tensile tests. With increasing the plastic elongation, there is an increase in yield and tensile strength, which is accompanied by a significant decrease in strain after failure. Both thermo-mechanical aging strategies reveal mechanical properties similar to the conventional T6 peak aged condition with a significant reduction in process time from 24 h to 5 h.

Analysis, Prediction and Reduction of Emissions in an Industrial Hot Forming Process Chain for the Manufacture of Sheet Metal Components

Increasing demands for reducing greenhouse gases drive the metal processing industries to a CO2-neutral production. A thorough understanding of CO2 emission sources from the stage of material acquisition up to the final component is thus necessary to improve the CO2 footprint of sheet metal hot forming process chains. To emphasize on this, an exemplary hot forming process chain is assessed to identify the impact of each sub-process step on total CO2 emissions and the savings potential of individual measures is evaluated. Moreover, a mathematical model is proposed that enables for the prediction of the product specific CO2 emissions as early as in the product design stage. This model is tested to calculate the CO2 emissions resulted during the production of an exemplary hot stamped sheet component. The results point out that the heating stage is responsible for the second highest percentage of CO2 emissions in the process chain next to the material acquisition. Thus, as one of the most suitable measures, a concept to recover process heat from hot formed components to the cold initial blanks is proposed and evaluated analytically.

Upset bulging as a preforming operation for hot metal gas forming of 22MnB5 tubes

Hot Metal Gas Forming (HMGF) is a coupled process of gas forming and quenching of tubes. This process allows to obtain complex and accurate geometry due to the elimination of springback and high mechanical properties due to the formation of martensite, as a result, the weight of the parts can be reduced. HMGF is widely used in the aerospace and automotive industries to make critical parts from high hardenability steels such as 22MnB5. A typical hot stamped component has 1000 MPa yield stress and 1500 MPa ultimate tensile strength. The main challenge of HMGF process is a significant material thinning and cracking due to the biaxial tension stress state. The paper proposes a preforming method for increasing the forming limits of HMGF by the cold upset bulging of tubes by means of the additional volume of material in the deformation zone. This method allows to obtain one or several waves in the cross section of the tube, which helps to increase the minimum workpiece wall thickness after the forming process by more than 40% and to reduce a probability of the crack formation.

Investigations on the interfacial heat transfer coefficient during hot stamping of ultra-high strength steel with Al-Si coating

The ultimate mechanical properties and microstructures of hot-stamped components are highly related with the interfacial heat transfer coefficient (IHTC) between blank and tool. In this paper, an advanced temperature acquisition system was established to measure the temperature of boron steel Usibor 1500P with Al-Si coating during the hot stamping process, and an inverse algorithm was developed to calculate the IHTC. Moreover, the influences of contact pressure and tool roughness on IHTC were discussed. The results show that the IHTC only increases with pressure when it is less than 1MPa, while remains almost unchanged under higher pressure. Meanwhile, little effects on the IHTC are generated by tool surface roughness. Those phenomena are quite different from the commonly studied uncoated blanks. Further exploration demonstrates that the effect of high pressure and tool surface roughness on IHTC is not significant due to the oxidation resistance and the spalling of the Al-Si coating respectively.

Damage Evolution of Hot Stamped Boron Steels Subjected to Various Stress States: Macro/Micro-Scale Experiments and Simulations.

Hot stamping components with tailored mechanical properties have excellent safety-related performance in the field of lightweight manufacturing. In this paper, the constitutive relation and damage evolution of bainite, martensite, and mixed bainite/martensite (B/M) phase were studied. Two-dimensional representative volume element (RVE) models were constructed according to microstructure characteristics. The constitutive relations of individual phases were defined based on the dislocation strengthening theory. Results showed that the damage initiation and evolution of martensite and bainite phases can well described by the Lou-Huh damage criterion (DF2015) determined by the hybrid experimental–numerical method. The calibrated damage parameters of each phase were applied to the numerical simulation, followed by the 2D RVE simulations of B/M phase under different stress states. To study the influence of martensite volume fraction (Vm) and distribution of damage evolution, the void nucleation and growth were evaluated by RVEs and verified by scanning electron microscope (SEM). Three types of void nucleation modes under different stress states were experimentally and numerically studied. The results showed that with the increase of Vm and varying martensite distribution, the nucleation location of voids move from bainite to martensite.

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.

Increasing the energy absorption of monolithic manganese boron steels in oxygen-free environment

The heat-treatable steel 22MnB5 is used in hot stamping processes to produce high-strength body-in-white components. In this process, sheet blanks are conventionally heated in roller hearth furnaces and then hot-stamped, whereby strengths of 1,500 MPa can be achieved. Disadvantages of this process are the low plastic deformation of the material in hardened state and the poor energy efficiency of roller hearth furnaces. In a new approach, these disadvantages are eliminated by combining edge decarburisation with resistance heating. Due to a diffusion-controlled removal of the carbon in the edge layer of the blanks heated in an oxygen-free atmosphere, the energy absorption in bending tests was improved by 61 % compared to customary hot-stamped 22MnB5. Furthermore, with a subsequent resistance heating in an oxygen-free silane atmosphere, the sheet can be heated and coated. A hermetically sealed heating chamber was developed which allows to heat the blanks up to 950 °C without scale formation. The coating during heating further improves the corrosion properties of the component. With this approach, hot-stamped components with improved properties and coated in an energy-efficient resistance-heated process can be manufactured.

Constitutive modelling of Usibor® 1500 sheets after intercritical quenching

In this study, 0.9 and 1.8 mm thick Usibor® 1500 sheets were subjected to intercritical quenching by heating to 760-930°C and quenching at a controlled rate. The tensile behavior of as-quenched Usibor® 1500 was experimentally obtained using uniaxial tension tests at strain rates ranging from 0.001 to 0.25 s−1. The constants in the hardening models, including Johnson-Cook, were optimized using a Genetic algorithm and linear regression for each condition at each strain rate. Then, these models were numerically modified to account for heat treatment dependency. Uniaxial tensile tests were simulated using the fitted models and compared to experimental flow curves and strain distribution maps to determine the accuracy of the prediction of each model. Optical microscopy was used to determine the volume fraction of each phase using image processing tools and these characteristics were used to explain the behavior of Usibor® 1500 after intercritical quenching. It was found that intercritical quenching process parameters determine the distribution and morphology of each phase and consequently the range of mechanical properties. This model can be used to simulate the deformation of hot-stamped components with tailored properties produced under controlled austenitization.

Viscoplastic constitutive modeling of boron steel under large strain conditions and its application in hot semi-cutting

Abstract To overcome the long process time of laser cutting and the high costs for repairing cold blanking tools of ultra-high strength hot stamping components, the hot semi-cutting process was proposed. The reliable testing and parameter identification method of the constitutive behavior under large strain conditions are essential for an accurate simulation of hot stamping and hot semi-cutting process. In the present work, a new testing and parameter identification method for the viscoplastic constitutive behavior of boron steel is proposed based on the Gleeble system, which includes a new grip design, shear specimen selection, and its relevant high-temperature and high-speed digital image correlation (DIC) technology. To obtain the stress-strain curves under large deformation, the FEM-based optimization method is also proposed to identify the hardening parameters. To cover a greater strain range, a new viscoplastic hardening model is proposed based on the dislocation density evolution under elevated temperature deformation conditions. The reliability of the proposed testing method and hardening model is verified by the analysis of uniformity of temperature and strain rate, and the comparison of predicted stress-strain curves with parameters identified from shear and tensile testing. The thermomechanical tension of the center-hole and notched specimen, and the hot semi-cutting process are carried out to verify the proposed testing method and hardening model. The results show that the shear specimen has a more uniform temperature and strain rate distribution in the deformation zone than the tensile specimen. Shear specimens can be used to test the hardening behavior under various process conditions until failure at the large strain. The insensitivity to phase transformations of shear testing is another advantage for identifying the constitutive behavior of the product phase formed during continuous cooling. The constitutive behavior of the boron steel under a large strain range can be accurately identified by using the FEM-based optimization method proposed in this paper. Besides, the mechanical response of the center-hole specimen, notched specimen, and the hot semi-cutting deformation predicted by the proposed hardening model shows accurate results.

Steel sheets partnered with quenchable sheet in hot stamping of tailor-welded blanks and its application to separation prevention of fractured components

The phase transformation and mechanical properties of non-quenchable steels partnered with the quenchable boron steel in hot stamping of tailor-welded blanks were evaluated to produce tailored components with partially balanced strength and ductility. The effect of the forming start temperature after natural air cooling on the phase transformation and mechanical properties for 270 MPa mild steel, non-quenchable steel, 440 MPa high strength steel, and 22MnB5 steel sheets was examined, and the 270 MPa and non-quenchable sheets had enough ductility after hot stamping. Tailored components having a hardness of about 500 HV1 in the high strength zone and a total elongation of about 30% in the high ductility zone were hot-stamped from a tailor-welded blank composed of 22MnB5 and 270 MPa sheets. It was found that the 270 MPa mild steel sheet is sufficient as a partner sheet of tailor-welded blanks. In addition, the safety of hot-stamped components was heightened by welding a 22MnB5 main blank with a 270 MPa steel patch. Even if the main blank is fractured by a collision, the hot-stamped component is not separated by the 270 MPa patch having high ductility.

Numerical modeling of ductile fracture of hot stamped 22MnB5 boron steel parts in three-point bending

With the increasing demand in car safety and energy consumption reduction, the driving force in developing components made of UHSS steels has risen during the last decade, increasing the use of hot stamping process. While several research studies have focused on the modeling of the fracture occurrence during hot stamping, very few analyze the ductility and failure behavior of hot stamped components during static loading as well as crashworthiness events. In this framework, the paper intends to study the failure behavior of hot-stamped boron steel 22MnB5 under different loading conditions. Three-point bending tests are conducted on hot stamped U-shaped beam assemblies to evaluate the effect of complex loading on the damage evolution. Through SEM-based microscopic analyses, the material failure mechanisms, such as the mode of void coalescence, are explored, as well as the relation between the direction of the maximum shear stress and fracture surface. A FE-based model of the three-point bending tests incorporating a ductile fracture criterion is calibrated and implemented in ABAQUS/Explicit software using VUMAT subroutines. It is proved that the numerical model is capable to properly reproduce the deformation sequence and predict two symmetric macro-cracks under a circular punch. The load-stroke curve of FE simulation is finally compared with that of simulation to show its reliability.

Investigation of the influence of forming parameters on the springback of hot-stamped hat-shaped parts

The application of hot stamping parts has become a dominant approach to achieve lightweight design and increase crashworthiness performance in the automotive industry. Compared to the cold stamping parts, the main feature of the hot stamped component is its high strength and good size stability. However, recent studies have shown that the springback or size deviation of hot stamped parts caused some assembly problems. This paper aims to investigate the influence of forming parameters on the part’s springback via a hot stamping hat-shaped die. Three different forming parameters with the same blank heating temperature (930°C) were conducted in the study: (1) transfer time 8 sec/die quenching 15 sec (2) transfer time 8 sec/die quenching 8 sec and (3) transfer time 18 sec/die quenching 15 sec. The selected forming parameters are used to evaluate the effect of the before-forming blank temperature and die-opening blank temperature on the springback of the hot stamping parts. The results show that the case 3 with a prolonged transfer time causes a small portion of ferrite phase formation in the hot-stamped part, resulting in the decrease of the tensile strength and a more significant springback is observed.

Functional optimization of hot-stamped components by local carburization

One of the major challenges in modern car body construction is the reduction of component weight while still maintaining passenger safety. In terms of these requirements, the substitution of conventional steel grades by ultra-high strength materials is the most promising approach. Hot stamping of these steels has developed to a state of the art process for manufacturing safety-relevant car body parts such as B-pillars or front bumpers. By locally adjusting the mechanical properties, for example, increasing the ductility, passive safety can be further improved. For this purpose, the hot stamping process is adapted in different ways. Varying from these established methods, tailored carburization is an alternative approach for manufacturing functional optimized components by combining hot stamping with prior local carburization. Within this work, the potential of local carburization is analyzed by a fundamental investigation of the influence of the process parameters on the mechanical properties as well as the transition zones between carburized and non-carburized areas. Besides the mechanical properties, the heat transfer between carburized sheets and tools is studied. The process suitability is validated by a demonstrator.