Hot Stamped Parts Research Articles

Process Monitoring And Real Time Algorithmic For Hot Stamping Lines

The Hot Stamping Process is a well-known manufacturing technology for safety and light weight parts in automotive applications. Several process routes and windows are defined in the past years including the virtual modeling. Nevertheless, deviations from ideal process conditions can be observed very often in serial production. This can be a result of an inconstant heating process, hydraulic defects in the cooling system or uneven tool wear, just to name a few possibilities. To improve the production safety this paper shows the development of a real time Process Monitoring system which is fully connected to the production system. The system is gathering data from the furnace, press, automation components, the thermographic cameras and other sensors to calculate thermal- and work balances during the production in real time. After a plausibility check of the data a feasible range of values can be defined. The preprocessed data is stored in a long term database for quality assurance and is a basis further analysis. The gained information and values can be used for a better definition of the boundary conditions and their deviations for virtual modeling as well as for predictive modeling to see upcoming malfunctions or failure in advance. With this valuable information countermeasures during production can be scheduled before sudden machine downtime. This paper also shows how this process information can be linked with hot stamped part identities to achieve tracking and tracing which will be a new standard for the hot forming technology.

Numerical modeling of the 22MnB5 formability at high temperature

Abstract It has been recognized in automotive sector that the use of hot stamped parts for the automobile body-in-white allows significant weight saving and reduction of CO2 emissions. The weight reduction is accompanied by high stiffness and increased crashworthiness, which improve the passengers’ safety and allow the development of more complex vehicle’s design. In direct hot stamping, the 22MnB5 sheets are heated above the austenitization temperature, and then, formed and quenched in cooled dies by using a minimum cooling rate of 27°C/s in order to obtain the direct martensitic transformation from the initial austenite. To evaluate part feasibility, minimize production cost and improve process robustness in hot stamping, numerical simulations are becoming increasingly important, where data regarding the formability of the material at high temperature plays a central role in the simulation. A survey of the literature shows that the complex aspects of the process have been extensively investigated from an experimental point of view, but limited studies concerning the numerical modeling of the process can be found. The aim of the present paper is to explore the critical aspects of the numerical modeling in terms of material formability in hot stamping. Numerical simulations of hot Nakajima tests at the critical temperature of 600°C were performed by using explicit numerical method, while experimental tests are carried out in the same condition. The comparison of the numerical results and the experimental data demonstrates the influence that the thermal cycle applied during the experimental tests may have, since it governs the phase transformations that affect the material formability.

Application of ultrasonic testing method for determining defects of hot-deformed powder materials

The article is devoted to the analysis of elastic and plastic characteristics of composite materials during hot stamping. The purpose of this work is to offer optimal conditions for hot plasticity of composite porous material with determination of temperature conditions of hot stamping excluding the appearance of defects in the structure. Production of details of the difficult form by method of hot stamping from preparations of the cylindrical form is followed by development of barrel on a peripheral surface. Sludge sintered porous blanks, and sediment compact material, accompanied by a nonuniform height lateral deformation. In connection with the action of friction forces on the contact surfaces, this leads to the formation of a “barrel”. The heterogeneity of the deformed state is associated with the appearance of tangential tensile stresses on the free surface of the workpiece. If they exceed some critical degree of transverse deformation, cracks appear on the side surface, which leads to gas saturation (oxidation) of the inner layers of the forging, to the ingress of grease into them and its pressing into the volume of the part during hot stamping. In the end, this significantly reduces the properties of hot-stamped parts. Conclusion: the methods of determining the elastic characteristics depending on the geometric parameters of the workpieces, the applied strain energy, body density and temperature dependence of the plasticity characteristics of the hot deformation of the powder material are сonsidered.

Hardness Prediction in Hot Stamping Process by Local Blank Heating Based on Quench Factor Analysis

Recently, the hot stamping process using local blank heating has been widely used to manufacture lightweight and crashworthy automotive parts. However, the hardness prediction of hot stamped parts produced using local blank heating is difficult because it involves many process variables, such as the heating temperature, heating time, and cooling rate. The purpose of this study was to predict the hardness of hot stamped parts fabricated using local blank heating based on quench factor analysis (QFA). The volume fraction of austenite was measured to consider the phase transformation in the heating stage, and it was expressed by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Additionally, a dilatometry test was performed to measure the hardness according to the cooling rates, which was used to determine the material constants for QFA. Finite-element simulation was performed to predict the temperature histories during the hot stamping process and the results were used to predict the hardness according to QFA with the JMAK equation. A hot stamping experiment with local blank heating equipment was performed, and the predicted and experimental results were compared for verification of the proposed hardness prediction method.

Two-scale simulation of the hot stamping process based on a Hashin–Shtrikman type mean field model

The hot stamping process of sheet metals into a w-shape profile is considered in experiments and numerical simulation using the FEM tool ABAQUS. The influence of the processing parameters of the hot stamping process on the final shape and the residual stress state are investigated. Tensile tests of thermomechanically treated specimens serve as a starting point for the modeling of the thermomechanical behavior of the steel grade 22MnB5. Based on the thermo-micromechanical (TMM) model for small deformations suggested by Neumann and Böhlke (2016), the updated Lagrangian formulation is used to render the finite deformation process.The model captures in a two-scale framework the temperature dependent elasto-plastic effect of the different phases, the phase transformation effect, and the phase transformation accompanying transformation strain occurring during the quenching of the austenitized sheet metal. Additionally, a model for the transformation induced plasticity (TRIP) effect in the context of the two-scale approach is suggested. The homogenization and localization of the thermomechanical properties is performed with a HashinöShtrikman (HS) type homogenization scheme. In this work, a reduced formulation for isotropic elastic behavior of the different phases is found reducing the amount of numerical costs. Furthermore, a self-consistent scheme based on the HS type method is introduced.Finally, the suggested TMM model is validated by considering the time-temperature-transformation- (TTT) and continuous-cooling-time-curves (CCT), dilatation tests under external load, and the hot stamping of w-shape profiles. The TMM model is compared to both experimental results and a phenomenological reference model usually used for the simulation of hot stamping processes.It is found that the suggested TMM model fits the final shape and the residual stress state of hot stamped parts better than the phenomenological reference model.

Enhancements on strength of body structure due to bake hardening effect on hot stamping steel

Body structure projects are designed to develop lightweight structures, which result in low fuel consumption and emissions for internal combustion engines and longer battery life in the case of electric vehicles. The structures need to be lightweight but also strong in order to provide maximum safety to the occupants. These premises have led to the development of special materials such as the hot stamping steels. The most used steel in this process, quenched 22MnB5, also exhibits the bake hardening effect: its yield stress increases after being exposed at temperatures close to 200 °C (typical heat treatment of paint lines in the automotive industry). The aim of this study was to verify the improvement in the mechanical strength of a body structure due to the bake hardening effect to which the hot-stamped steel components were submitted. Samples of annealed 22MnB5 steel were submitted to tensile and chemical analysis. A second group of the same steel was subjected to an additional heat treatment in a laboratory furnace simulating the heating that occurs in the body paint line. Subsequently, the same tests were performed with the samples from the first group. Tensile tests evidenced the increase of 6.5% in the yield stress of the samples that suffered the bake hardening effect. The stress-strain diagram curves were used as the input to the side crash simulation program. The simulations demonstrated that the body structure whose hot-stamped parts were subjected the bake hardening effect had a 2% reduction in side crash intrusion compared to body structure where the parts did not suffer this effect. In the case of the lateral protection bar test, the bake hardening effect provided a 5% increase in the maximum bending load.

Compensation for tool deformation and expansion in virtual try-outs of hot stamping tools

The mechanical properties of hot-stamped parts strongly depend on the tool’s cooling performance. The cooling rate hinges on sufficient temperature gradients and on an excellent contact conductance between tool and workpiece. A uniform distribution of the contact pressure is the key to an even cooling behaviour, and hence, a homogenous micro structure of the formed part. Elastic deformations of machine and tool components under load are a major influence on the pressure distribution between tool surface and hot-stamped part. This applies to hot and cold forming tools. An additional difficulty in hot stamping are superimposed thermal expansions and contractions of the tool, which also affect the part’s mechanical properties due to their influence on the normal contact pressure. Manual die spotting needs to compensate for all these undesired effects and makes tool try-out a large time and money-consuming factor in the development of hot forming tools. This paper presents methods to transform the spotting of hot forming tools into a virtual production reality in order to reduce manual labour and lower costs. It gives details on the numerical compensation of tool surfaces for elastic tool and machine deformations and for temperature-induced tool expansions and contractions. The authors critically analyse necessary and achievable accuracies of computed surfaces and point out required improvements for the future implementation of virtual try-outs into tool development and manufacturing processes.

Advanced induction heating system for hot stamping

The development of a rapid uniform heating process for blanks is needed to perform hot stamping processes. To solve the problem of inhomogeneous temperature distribution in conventional induction heating systems, a heating block has been added to an induction system and investigated to determine if it can achieve rapid and uniform blank heating. To characterize the effects of incorporating the heating block, the temperature distributions of various coil geometry have been experimentally analyzed. The results show that the proposed induction heating system makes it possible to achieve rapid and uniform blank heating, which is used to conduct stable hot stamping. The mechanical properties of a hot stamped part confirm that the proposed heating method is a feasible alternative method for hot stamping products, and that it yields excellent results. This system for the reduction of heating time requires less installation space compared to a conventional electric furnace and achieves uniform heating, which results in improved product quality and reduced manufacturing costs.

Combined hot stamping and Q&P processing with a hot air partitioning device

Combining the hot stamping process and a Quench and Partition (Q&P) heat treatment can improve the mechanical properties of hot stamped automotive parts. This paper investigates using a new heating device to heat or cool a part to a given temperature. Instead of performing the partitioning heat treatment in a furnace, this work proposes using a new hot air process that performs the partitioning treatment. The new device obtains a fast heating rate and a uniform temperature distribution, necessary for Q&P. The effect of partitioning time and temperature on the mechanical properties of parts were analyzed by the new processing method, which is suitable for use in the area of industrial hot stamping. The results show that the hot air partitioning device provides more control over the partitioning temperature because convection is more efficient than radiation. Control over the quench temperature and partition is critical for the Q&P process to enable the carbon diffusion to stabilize the austenite. Furthermore, the highest product of strength and elongation (1380 MPa × 13%) using the hot air process under the authors’ set-up condition is obtained when the partitioning time is 60 s and the temperature is 425 °C, thanks to the optimum ratio between martensite and austenite, which was observed through scanning electron microscopy.

Effect of the Martensitic Transformation on the Stamping Force and Cycle Time of Hot Stamping Parts

Stamping dies perform two functions in the hot stamping process of body-in-white components. Firstly, they form the steel sheet into the desired shape and, secondly, they quench the steel at a cooling rate that leads to hardening by means of the austenite-γ to martensite transformation. This microstructural change implies a volume expansion that should lead to a force peak in the press, which has yet to be detected in industrial practice. In this study, a set of hot stamping laboratory tests were performed on instrumented Al–Si-coated 22MnB5 steel flat formats to analyze the effect of the stamping pressure on the detection of the expected peak. Plotting the sheet temperature and pressure curves against time allowed us to identify and understand the conditions in which the force peak can be detected. These conditions occurred most favorably when the stamping pressure is below 5 MPa. It is thus possible to determine the exact moment at which the complete hardening transformation occurs by monitoring the local pressing force of the tool in areas where the pressure exerted on the metal format is below 5 MPa. This information can be applied to optimize the time needed to open the dies in terms of the complete martensitic transformation.

Tribological Behavior of Different Tool Steels and Surface Properties under Hot Stamping Conditions

Hot stamping is a well-established technology for producing safety relevant components. The use of hot stamped components in modern car bodies offers the possibility of improving the crash performance while reducing the fuel consumption by using thinner sheet thicknesses. Hot stamped components are mainly produced out of the boron-manganese-steel 22MnB5. To avoid oxide scale formation during the heat treatment and the subsequent forming process AlSi coatings are applied on the workpiece surface. Due to the high forming temperatures, the use of lubricants is not suitable for the hot stamping process. Consequently, high friction and severe wear occur during the forming process and affect the resulting quality of hot stamped parts as well as the tool wear. In order to improve the part quality and increase the efficiency of industrial hot stamping applications, measures for reducing the tribological load during the forming have to be found. Within this study, the tool-sided impact on the tribological conditions is analyzed. Three different hot working tool steels were characterized based on strip drawing experiments under hot stamping conditions. Based on these investigations the tool steel characteristics hardness, thermal conductivity as well as chemical composition have been identified as possible influencing factors on the tribological conditions. Furthermore, the influence of the surface finish on the tribological performance was investigated by analyzing tool surfaces with three different roughness values and two PVD coatings. The experiments indicate a significant reduction of friction and wear due to application of PVD coatings while the tool roughness did not affect the tribological behavior under hot stamping conditions.

Weight Reduction Study on BIW by Using Boron Alloyed Hot Stamping Solution of Front Frame Rail Instead Conventional HSS or AHSS Applications

With the increase of the performance requirements and homologative restrictions the correct way of choosing the right material on the related area is an inevitable manner for weight reduction of the vehicles. A hot stamped front frame rail which have roughly same performance with conventional cold stamped one was designed by using tailor welded and patch reinforcement. The part was optimized with full sized frontal crash (EUNCAP), full sized rear crash (Trias33) and fatigue simulations. Critic interferences-accelerations into the passenger cabin was considered for acceptance criteria of frontal crash, deformations on fuel system for acceptance criteria of rear crash and plastic deformations on the part, especially suspension fixing points, for acceptance criteria of fatigue test. At the end of all optimization the thicknesses of the system was designed as; first tailor welded part: 1.2mm, second tailor welded part:2.5mm and patch reinforcement: 1mm. Also some design critics (such as laser cut holes and trims, seal and barrier application for eliminating water infiltration into the bare surfaces of the hot stamped part) was clarified to guarantee the performance of the part during lifecycle of the vehicle. With final condition new hot stamped system was weighted 5 kg and gained a 2 kg weight reduction.

Size Effect Studies on Tensile Tests for Hot Stamping Steel

Tensile tests have been widely used to determine basic mechanical properties of materials. However, the properties measured may be related to geometrical factors of the tested samples especially for high-strength steels; this makes the properties’ definitions and comparisons difficult. In this study, a series of tensile tests of ultra-high-strength hot-stamped steel were performed; the geometric shapes and sizes as well as the cutting direction were modified. The results demonstrate that the hot-stamped parts were isotropic and the cutting direction had no effect; the measured strengths were practically unrelated to the specimen geometries, including both size and shape. The elongations were slightly related to sample sizes within the studied range but highly depended on the sample shape, represented by the coefficient K. Such phenomena were analyzed and discussed based on microstructural observations and fracture morphologies. Moreover, two widely used elongation conversion equations, the Oliver formula and Barba’s law, were introduced to verify their applicability, and a new interpolating function was developed and compared.

Constitutive and fracture models of hot stamped parts with multiphase using digital image correlate technology

The hot stamping technology is widely used because of the good formability, lower forming force, less spring back and high strength parts obtained. However, with the consistently increasing requirements for safety performance improvement, the fully martensitic hot stamped parts cannot meet the new requires, which consist high strength and energy absorbing. The compatibility of mechanical properties and safety performance of vehicle parts such as b-pillar has been proposed that different mechanical properties are needed in different regions of the same parts, namely tailored properties. Microstructure of hot stamped parts with tailored properties consist of multiphase, so in this paper, experiments and simulation have been performed to study the constitutive and fracture models of hot stamped parts with multiphase. The digital image correlation (DIC) technology was used to measure the strain over the whole specimen and the elastoplasticity constitutive models of martensite, bainite and ferrite/pearlite have been obtained. By combining the experiments and numerical simulation, the parameters of fracture model have been optimized, which are very hardly to be determined using only experiments. The constitutive and fracture models of multiphase has been obtained using the liner-mixture rule.

Investigation of diffusion behavior of carburized sheet metal in hot stamping

Today’s manufacturing of structural car body parts faces several challenges, like forming accuracy and passenger safety. Besides these two requirements, lightweight design plays an important role. One possibility to fulfill these partially rivalling demands is the application of hot stamped parts. The combination of hot forming and in die quenching reduces not only springback, but also results in tensile strengths of up to 1500 MPa. This makes a simultaneous reduction of sheet thickness and therefore weight reduction possible. Further development enabled a tailored adjustment of mechanical properties, for example by applying different cooling conditions along the parts. One of the biggest issues of these state of the art processes is the formation of a transition zone due to heat transfer. A promising approach to adjust the mechanical properties with a minimized transition zone is the carburization of sheet metal. Therefore, the parts are coated with graphite, heat treated and subsequently quenched. In this work, the time variant process of carbon diffusion is investigated. Sheets with two different thicknesses are carburized and quenched. The resulting mechanical properties are analyzed using uniaxial tensile tests and microhardness measurements. The results are correlated with the carbon content measured by EDX-analysis.

Investigation of the influence of process parameters on adhesive wear under hot stamping conditions

Current challenges like increasing safety standards and reducing fuel consumption motivate lightweight construction in modern car bodies. Besides using lightweight workpiece materials like aluminum, hot stamping has been established as a key technology for producing safety relevant components. Producing hot stamped parts out of ultra-high strength steels offers the possibility to improve the crash performance. At the same time the weight of car structure is reduced by using thinner sheet thicknesses. In order to avoid oxide scale formation and ensure corrosion protection, AlSi coatings are commonly deposited on the sheet surfaces used for direct hot stamping. This workpiece coating has a critical impact on the tribological conditions within the forming process and, as a consequence, influences the quality of hot stamped parts as well as tool wear. AlSi coatings have been identified as major reason for adhesive wear, which represents the main wear mechanism in hot stamping. Within this study, the influence of the process parameters on adhesive wear are investigated in dependency of workpiece and tool temperatures, drawing velocities and contact pressures. The tribological behavior is analyzed based on strip drawing experiments under direct hot stamping conditions. The experiments are performed with AlSi coated 22MnB5 in contact with the hot working tool steel 1.2367. For analyzing the amount of adhesion on the friction jaws, the surfaces are characterized by optical measurements. The experiments indicate that higher workpiece temperatures cause severe adhesive wear on the tool surface, while an increase of drawing velocity or contact pressure led to reduced adhesion. The measured friction coefficients decreased with rising amount of adhesion and remained at a constant level after a certain adhesive layer was built up on the tool surface.

Improving the Quality of Hot Stamping Parts with Innovative Press Technology and Inline Process Control

The increasing number of hot stamped parts in the automotive industry is challenging different process areas. This paper presents a method how to improve the production rates over the whole life cycle of a hot forming part. In the core element of a hot forming line, the hydraulic press, mainly two processing steps are performed. Forming and quenching of the sheet metal part. In addition to the forming operation, it is inevitable to optimize the quenching condition in the bottom dead centre in order to reach a fully martensitic structure and tight geometrical tolerances of the part. Deviations in the blank thickness, tool wear, polishing of classical tools impair the quenching condition and therefore the part quality over the time. A new press and tool design has been developed to counter this effect by providing homogenous contact pressure over the whole die. Especially with a multi cavity tool, the new method is advantageous. Test series have shown that the new tool and press concept can produce parts with a blank thickness of 1.0 mm within 8.0 s cycle time. The so called PCH flex principle makes it possible to produce such high output rates under reliable conditions.

Microstructure and performance evaluations on Q&P hot stamping parts of several UHSS sheet metals

Hot stamping (press hardening) is widely used to fabricate safety components such as door beams and B pillars with increased strength via quenching. However, parts that are hot-stamped from ultra-high-strength steel (UHSS) have very limited elongation, i.e., low ductility. In the present study, a novel variant of hot stamping technology called quenching-and-partitioning (Q&P) hot stamping was developed. This approach was tested on several UHSS sheet metals, and it was confirmed that this method can be used to overcome the drawbacks associated with conventional hot stamping. The applicability of Q&P hot stamping to each of these steels was also assessed. The part properties and performances of three widely used ultra-high-strength sheet metals, B1500HS, 27SiMn, and TRIP780, were evaluated through tensile testing and microstructural observations. The results demonstrated that the ductility of Q&P hot-stamped sheet metals was notably higher than that of the conventionally hot-stamped parts because Q&P hot stamping gives rise to a dual-phase structure of both martensite and austenite. Further, material tests demonstrated that the Q&P treatment had positive effects on all three selected materials, of which TRIP780 had the best ductility and the highest value of the product of strength and plasticity. Scanning electron microscopy images indicated that the silicon in the steels could limit the formation of cementite and would, therefore, improve the mechanical properties of Q&P hot-stamped products.

Artificial Neural Network (ANN) based microstructural prediction model for 22MnB5 boron steel during tailored hot stamping

Because of demand for lower emissions and better crashworthiness, the use of hot stamped 22MnB5 boron steel has greatly increased in manufacturing of automobile components. However, for many applications it is required that only certain regions in hot stamped parts are fully hardened whereas other regions need be more ductile. The innovative process of tailored hot stamping does this by controlling the localized microstructures through tailored cooling rates by dividing the tooling into heated and cooled zones. A barrier to optimal application of this technique is the lack of reliable phase distribution prediction model for the process.We present a novel Artificial Neural Network (ANN) based phase distribution prediction model for tailored hot stamping. The model was developed and validated using data generated from extensive thermo-mechanical physical simulation experiments and instrumented nanoindentation based phase quantification method. Advanced statistical techniques were used for preventing overfitting, for making the optimal use of available experimental data and for quantification of prediction uncertainty. The final predictions made by the ANN model during its independent validation have shown good agreement with the experimentally generated data and have a RMS prediction error of just 7.7%, which is a significant improvement over the existing models.

Supporting Lightweight Design: Virtual Modeling of Hot Stamping with Tailored Properties and Warm and Hot Formed Aluminium

Although in the past complicated and often considered ‘exotic’ process, hot stamping of steel is now widely used in the automotive industry and for some safety components it becomes obligatory. The hot stamped parts are used in regions where very high strength is required. But for some applications also ductility needs to be incorporated in parts to guide the absorption of energy under crash loading, for other the material thickness has to be reduced smoothly due to lightweight. This will be achieved by tailoring the material properties within the component. This new generation of hot formed parts is already standard in the market, but still difficult in the engineering. And the next one is already in start position – aluminum warm forming is planned as a substitution of the heavier steels for structural parts, but newly also for outer panels, where they have to fulfil all requirements on visible parts. Light, strong, accurate, high quality and warm formed to improve the formability of the material. This paper describes successful engineering methods in virtual hot stamping process with tailored properties and of warm and hot formed aluminum parts.