Hot Stamping Tools Research Articles

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.

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.

Performance of die cooling system design in hot stamping process

ABSTRACTThis study examined the cooling channel design of hot stamping tools to provide an effective method for the cooling system design. A flat plate tool model was investigated to determine the effects of cooling channel and manufacturing parameters on the cooling rate and cooling uniformity of the sheet blank by conducting finite element analysis. The fractional factor method was also employed to establish empirical equations for describing the cooling performance of the flat plate model. The empirical equations established in the present study can efficiently predict the cooling rate and cooling uniformity of sheet blanks with various cooling channel designs, and are of much help in providing an initial cooling system design for hot stamping tools. To validate the accuracy of the finite element analysis, experiments on sheet blank cooling process were conducted using the proposed flat plate tool design. The cooling processes were cyclically repeated for the validation of blank temperature distribution. The temperature evolution of sheet blanks obtained from the finite element simulations agree well with the experimental results and the validity of the finite element model is confirmed, and thus the effectiveness of the empirical equations is established.

Investigation on basic friction and wear mechanisms within hot stamping considering the influence of tool steel and hardness

Hot stamping is a well-established technology for producing safety relevant car components. The use of hot stamped components in modern car bodies offers the possibility of improving crash performance due to their high strength while simultaneously decreasing the fuel consumption by reducing sheet thicknesses and thus weight. Hot stamped components are mainly produced using the boron-manganese-alloyed steel 22MnB5. To prevent formation of oxide layer during heat treatment and subsequent forming process, AlSi-coatings are applied on the workpiece surface. Since hot stamped parts are formed at temperatures between 600 °C and 800 °C, no suitable lubricants have been found yet. Thus, severe wear and high friction occur during the forming process affecting final part quality as well as life-time of hot stamping tools. Consequently, measures for reducing tribological load during the forming operation have to be found in order to improve part quality and increase efficiency of industrial hot stamping applications. Within this study, the impact of the tool material on friction and wear is analyzed by comparing the tribological behavior of the newly developed high thermal conductivity hot work tool steel Thermodur 2383 Supercool with the reference material 1.2367. Both are characterized by means of flat strip drawing experiments under hot stamping conditions. The experiments are performed with different hardness values for each of the two tool materials. Furthermore, wear behavior is analyzed using scanning electron microscope and confocal microscope measurements of workpiece and tool. Hereby, fundamental wear and friction mechanisms within hot stamping applications are identified. The results of this study help to increase the process understanding regarding the tribological conditions during hot stamping. In future research work tool-side measures for increasing the life-time of hot stamping tools will be developed.

A pre-design method for drilled cooling pipes in hot stamping tool based on pipe parameter window

The ‘pre-design + optimization’ is one of the main methods to obtain appropriate cooling pipe parameters in hot stamping tools for satisfied quenching quality of hot stamping parts. Since good ‘pre-design’ can effectively reduce the workload of optimization, a novel pre-design method of cooling pipe parameters was put forward and applied in this paper. Firstly, the concept of pipe parameter window and its related building method were proposed, by which the domain of satisfied cooling pipe parameters, under certain quenching process and quality demand, could be obtained. Secondly, the pipe parameter window of drilled cooling pipes, suitable for flat die surface, was built based on the above method, and then used to pre-design the cooling pipes in flat die for application. Finally, the satisfied cooling performance of pre-designed cooling pipes demonstrated the validity of proposed method in guiding the pre-designing of cooling pipe parameters in hot stamping tools.

Tribological and Thermal Investigation of Modified Hot Stamping Tools

Tribological and Thermal Investigation of Modified Hot Stamping Tools

Topology optimization of channel cooling structures considering thermomechanical behavior

A topology optimization method is presented to design straight channel cooling structures for efficient heat transfer and load carrying capabilities. The optimization is performed on the structural cross section that consists of solid, void, and fluid coolant. A simplified convective heat transfer model is used to simulate the flow characteristic in the channels with a low computational cost. Besides, a continuous design-dependent surface-based penalty approach is proposed to ensure a meaningful inlet fluid temperature during the continuous process of the fluid topology alteration. Coupled thermomechanical problems are solved to account for the engineering requirements on the uniformity of temperature and structural deformation tolerance. Furthermore, a phase-interface constraint is implemented to prevent unrealistic boundaries that adjoin the liquid to the void or to the outer boundaries of a design domain. Numerical examples of designing a lightweight cooling support frame and a hot stamping tool structure subject to uniform or non-uniform thermomechanical loads are given to demonstrate its applicability. Verification results of 3D structures by a full-blown turbulent fluid simulation show that the proposed approach is effective in yielding channel cooling structures with optimized heat transfer capabilities and well-controlled structural deformation.

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.

Effect of Deep Cryogenic Treatment on Mechanical Properties and Microstructure of the Tool Steel CR7V for Hot Stamping

To solve the failure problem of hot stamping tool, the CR7V steel was deep cryogenically treated at different dwelling times with multiple tempering. The hardness, impact toughness and wear resistance of the CR7V steel were examined, and the effects of deep cryogenic treatment on the microstructure of the CR7V steel were studied by means of optical microscope, scanning electron microscope and energy-dispersive spectroscopy. The results show that deep cryogenic treatment has little effect on the hardness of the tempered CR7V steel, but improves its impact toughness and wear resistance. The improvement in mechanical properties is a result of the combination of dissolution of large carbide particles, formation of small-sized and uniformly distributed carbide particles and fine tempered martensite structure. The cryogenic treatment is optimized as follows: vacuum gas quenching, deep cryogenic treatment at − 196 °C for 6 h and triple tempering at 560 °C for 2 h, which improves the wear resistance by 68% and the impact toughness by 58% compared to the conventional heat treatment.

Increasing the Adhesive Wear Resistance of Hot Stamping Tools by Modifying the Surfaces

Over the last few years lightweight construction became increasingly important in modern cars. Motivated by reducing greenhouse gas emission the car industry is currently working on different approaches to decrease the weight of structural body parts. In this regard, a reduced sheet thickness of these components and therefore a reduced overall weight can be achieved by using high-strength steels. Hot stamping has been established as a suitable manufacturing process for these steel grades, in which a hot austenitic blank is formed and quenched simultaneously. The high strength of the formed parts is realized by the phase transformation of an austenitic to a martensitic structure during hot stamping. Due to the alternating thermo-mechanical loads, which occur during forming and quenching, the hot stamping tools are highly stressed. In addition, when the blank slides over the surface of its counterpart, a substantial adhesive wear occurs, which is the predominant wear mechanism in hot stamping. The aim of this study is, to increase the wear resistance of the tools by modifying the surface. In this context, the chemical affinity between the interacting components need to be reduced in order to decrease the adhesive wear on the hot stamping tool, which is possible by alloying the base material. For this reason, the wear development is investigated for samples alloyed with different materials with a modified pin-on-disc test. This experimental setup enables a continuous contact of the tool with the blank and thermal alternating stress of the pin. The contact area is investigated with a laser-scanning microscope to qualify the tool surface before and after the experiments by measuring the tool topographies. The results of an unalloyed and alloyed tool will be compared with each other to evaluate the wear behavior. In order to quantify the amount of wear the wear volume will be calculated with an algorithm of the software WinSam. The experiments will be carried out under process like conditions to ensure transferability to the real hot forming process.

A Tailored Tempering Process for CSC-15B22 Steel Sheet

The simulation and experimental methods are used to determine the parameters of a tailored tempering process for a lab-scale B-pillar that is made from CSC-15B22 high-strength steel. The finite element software, DEFORM-3D, is used to simulate the tailored tempering process. A segmented hot stamping tool is developed for testing. Results demonstrate that the cooling and heating systems are successful. On the cooled side of the tooling, the cooling rate for the sheet is more than 30 °C/s. The material structure of the sheets is entirely a martensite structure, which results in an ultra-high strength material. The average hardness is measured as HV423, which translates to a tensile strength of 1350 MPa. On the heated side of the tooling, the cooling rate for the sheet is less than the critical cooling rate, the microstructure of the material is ferrite and pearlite, and the average hardness is measured at HV205, which translates to a tensile strength of approximately 660 MPa. The study demonstrates that a tailored tempering process allows production using integrated tooling and produces sheets that have different mechanical properties.

High Thermal Conductivity and High Wear Resistance Tool Steels for cost-effective Hot Stamping Tools

In hot stamping/press hardening, in addition to its shaping function, the tool controls the cycle time, the quality of the stamped components through determining the cooling rate of the stamped blank, the production costs and the feasibility frontier for stamping a given component. During the stamping, heat is extracted from the stamped blank and transported through the tool to the cooling medium in the cooling lines. Hence, the tools’ thermal properties determine the cooling rate of the blank, the heat transport mechanism, stamping times and temperature distribution. The tool’s surface resistance to adhesive and abrasive wear is also an important cost factor, as it determines the tool durability and maintenance costs. Wear is influenced by many tool material parameters, such as the microstructure, composition, hardness level and distribution of strengthening phases, as well as the tool’s working temperature. A decade ago, Rovalma developed a hot work tool steel for hot stamping that features a thermal conductivity of more than double that of any conventional hot work tool steel. Since that time, many complimentary grades have been developed in order to provide tailored material solutions as a function of the production volume, degree of blank cooling and wear resistance requirements, tool geometries, tool manufacturing method, type and thickness of the blank material, etc. Recently, Rovalma has developed a new generation of high thermal conductivity, high wear resistance tool steel grades that enable the manufacture of cost effective tools for hot stamping to increase process productivity and reduce tool manufacturing costs and lead times. Both of these novel grades feature high wear resistance and high thermal conductivity to enhance tool durability and cut cycle times in the production process of hot stamped components. Furthermore, one of these new grades reduces tool manufacturing costs through low tool material cost and hardening through readily available gas-quenching, whereas the other new grade enables a faster manufacturing of the tool at reduced cost by eliminating the time and money consuming high temperature hardening altogether. The latter newly developed grade can be hardened from a soft delivery state for easy machining to 52 HRc by way of a simple low temperature precipitation hardening. In this work, these new grades and the role of the tool material’s thermal, mechanical and tribological properties as well as their processing features will be discussed in light of enabling the manufacture of intelligent hot stamping tools.

Tribological performances of new steel grades for hot stamping tools

In the last years, the use of High Strength Steels (HSS) as structural parts in car body-in-white manufacturing has rapidly increased thanks to their favourable strength-to-weight ratio and stiffness, which allow a reduction of the fuel consumption to accommodate the new restricted regulations for CO2 emissions control. The survey of the technical and scientific literature shows a large interest in the development of different coatings for the blanks from the traditional Al-Si up to new Zn-based coatings and on the analysis of hard PVD, CVD coatings and plasma nitriding applied on the tools. By contrast, fewer investigations have been focused on the development and test of new tools steels grades capable to improve the wear resistance and the thermal properties that are required for the in-die quenching during forming. On this base, the paper deals with the analysis and comparison the tribological performances in terms of wear, friction and heat transfer of new tool steel grades for high-temperature applications, characterized by a higher thermal conductivity than the commonly used tools. Testing equipment, procedures as well as measurements analyses to evaluate the friction coefficient, the wear and heat transfer phenomena are presented. Emphasis is given on the physical simulation techniques that were specifically developed to reproduce the thermal and mechanical cycles on the metal sheets and dies as in the industrial practice. The reference industrial process is the direct hot stamping of the 22MnB5 HSS coated with the common Al-Si coating for automotive applications.

Effect of Heating and Mold Temperature on the Mechanical Properties and Microstructure of B1500HS Boron Steel

Abstract To know the effect of the heating temperature and the temperature of mold surface on the mechanical properties and microstructure of B1500HS steel, and supply some practical guides for the components with the distributed microstructure and mechanical properties, B1500HS steel sheets, heated at different austenitization temperatures (870°C, 900°C, 930°C, and 960°C), were formed in hot stamping tools with the different mold temperatures (100°C, 200°C, 300°C, and 400°C). The strength of hot stamping parts was tested by the uniaxial tensile experiment. Moreover, the microstructure, fractography, and micro-hardness of hot stamping parts were measured to evaluate the effect of austenitization temperature and mold temperature on the microstructure and mechanical performance of UHSS steel. The results show that when the heating temperature is 900°C or above, the original microstructure of B1500HS steel can be fully austenitized. Variation of austenitization temperature in the range of 900°C ∼ 960°C has no obvious effect on the mechanical properties of hot stamping parts cooled at the same mold temperature. The mold temperature has a significant effect on the mechanical properties; the structural components with distributed properties can be manufactured by controlling the mold temperature. The austenitization and mold temperatures have no obvious effect on the elongation of parts except the part with the subcritical quenching. The austenitization temperature of 900°C and mold temperature of 100°C are the better parameters of hot stamping for the parts of B1500HS steel required with higher hardness and strength.

Evaluation of Models for Cooling System Design in Hot Stamping Tools

The hot stamping process is beneficial in the production of parts with high strength and light weight properties. In hot stamping, the sheet metal is firstly heated and rapidly formed and cooled in a closed tool. One of the critical goals in research linked to hot stamping involves the reduction in cycle time. This is mainly because the cycle time has an effect on the economics of the process and it also affects the resultant quality characteristics of the formed part. Hence recent studies have focused on improving the design of cooling systems and use of thermally conductive tool materials so as to increase the cooling performance of the tools. This enables the manufacture of a sound product and the reduction of the overall cycle time. Researchers have developed models which guide in the selection of optimum cooling system parameters. However, the models have some limitations and some of them are difficult to implement. The purpose of this paper is to evaluate the developed models.

Virtual method for the determination of an optimum thermal design of hot stamping tools

This work presents a new virtual method for the optimised thermal design of hot stamping tools. It provides optimal positions of the tool's tempering ducts with respect to the average working temperature and its homogeneous distribution on the surface of a tool. It consists of a specific procedure for hot stamping tool design and a software framework in order to interconnect three domains: (I) a parametrised CAD tool model, (II) a linear thermal solver using a fast boundary element method and (III) an optimisation algorithm. This enables the automated set-up, simulation and optimisation of a duct topology. The boundary conditions for the simulations are derived from a reduced model of the thermal loading of the tool. The virtual method proposed is demonstrated on simplified tool segment geometries. The results are transferred to complex tool designs used in industry. For a selected use case, the number of ducts could be reduced by 50% through the application of the proposed method. These results are validated virtually based on an existing design. Hence, the new virtual method contributes to a CAE-driven tool design and a more efficient tool manufacturing.

Optimal design of longitudinal conformal cooling channels in hot stamping tools

A new longitudinal CCC (conformal cooling channels) design in a B-pillar tool is proposed in this study. Three kinds of design parameters, the radius (Rad) of cooling channels, the distance (H) from channel center to tool work surface and the ratio (rat) of each channel center, are defined in a parameterized CAD model with the new design. For optimizing the layout of the longitudinal CCC which is determined by the above design parameters, an integrated process established by multiple software platforms based on the response surface methodology and multi-objective optimization is put forward. A design matrix with 17 factors and 50 levels is generated through OLH (optimal Latin hypercube) method. The average temperature and temperature deviation of work surface are used to evaluate the cooling performance. Quadratic response surface models are established to describe the relationship between design parameters and evaluation objectives. The accuracy of fitting models is checked by error analysis. The layout of the longitudinal CCC is optimized to find the Pareto-optimal frontier which consists of some optimal combinations of design parameters. Besides, the influence of initial design is discussed and a novel manufacture method of hot stamping tools with longitudinal CCC is briefly introduced.

Experimental Validation for Hot Stamping Process by Using Taguchi Method

Due to the demand for reduction in gas emissions, energy saving and producing safer vehicles has driven the development of Ultra High Strength Steel (UHSS) material. To strengthen UHSS material such as boron steel, it needed to undergo a process of hot stamping for heating at certain temperature and time. In this paper, Taguchi method is applied to determine the appropriate parameter of thickness, heating temperature and heating time to achieve optimum strength of boron steel. The experiment is conducted by using flat square shape of hot stamping tool with tensile dog bone as a blank product. Then, the value of tensile strength and hardness is measured as response. The results showed that the lower thickness, higher heating temperature and heating time give the higher strength and hardness for the final product. In conclusion, boron steel blank are able to achieve up to 1200 MPa tensile strength and 650 HV of hardness.

Tribological behavior of high thermal conductivity steels for hot stamping tools

To improve the performances of new hot stamping processes for automotive industry, as well as to overcome their numerous drawbacks-such as oxidation, dies wear, severe thermal and mechanical cycles-new dies materials are being developed to obtain shorter heating and cooling cycles as well as improved strength and service-life.The paper investigates the tribological behavior of two new high thermal conductivity steel grades, by applying thermo-mechanical cyclic loads in the temperature range 600–800°C. The results show that a material microstructure characterized by agglomerates of Mo allows better performances than the case of finer elements of Mo homogeneously distributed in the metal matrix, with reduced friction and abrasion wear more relevant at the lowest testing temperatures.

Research on optimization design of conformal cooling channels in hot stamping tool based on response surface methodology and multi-objective optimization

In order to optimize the layout of the conformal cooling channels in hot stamping tools, a response surface methodology and multi-objective optimization technique are proposed. By means of an Optimal Latin Hypercube experimental design method, a design matrix with 17 factors and 50 levels is generated. Three kinds of design variables, the radius Rad of the cooling channel, the distance H from the channel center to tool work surface and the ratio rat of each channel center, are optimized to determine the layout of cooling channels. The average temperature and temperature deviation of work surface are used to evaluate the cooling performance of hot stamping tools. On the basis of the experimental design results, quadratic response surface models are established to describe the relationship between the design variables and the evaluation objectives. The error analysis is performed to ensure the accuracy of response surface models. Then the layout of the conformal cooling channels is optimized in accordance with a multi-objective optimization method to find the Pareto optimal frontier which consists of some optimal combinations of design variables that can lead to an acceptable cooling performance.