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Non-isothermal hot forming processes are of great scientific and industrial interest to produce e.g. high-strength aluminum components for lightweight applications. The interfacial heat transfer coefficient (IHTC) is a key factor in the design of these forming processes. However, the IHTC values reported in the literature vary significantly, even for the same material combinations and load conditions. The present work reveals the causes of these variations by a combined experimental and model-based approach. It is shown that the measurement of dynamically changing blank temperatures and the determination of the die surface temperature are two core aspects of non-isothermal experimental IHTC analysis. [ABSTRACT FROM AUTHOR]

To achieve lubrication and quenching during hot stamping of aluminum alloys (AAs), a combination of volatile medium (liquid CO 2) and additive manufactured (AMed) die is suggested. The liquid CO 2 as a lubricant and coolant is directly applied to the heated AA blank through the AMed die with confusor microholes. While the continuously supplied liquid CO 2 reduces the friction between the heated AA blank and AMed die, the cooling rate of the liquid CO 2 successfully induces the status of solid solution treatment in the formed blank for a following artificial aging process. [ABSTRACT FROM AUTHOR]

A novel quick model predicting the thinning distribution in hot stretch forming of hat-shaped profiles as a building block for a closed-loop control of a multi-stage forming process operated at a stroke rate of up to 12 strokes/min (5 s per stroke) is presented. The key characteristic of this new model is that it considers both a spatially as well as time-variant temperature distribution within the blank during forming. This is achieved by using temperature sensors and applying the information as input for the model, which is based on force equilibria and the theory of plasticity. The model predicts the geometry of the formed part which can be used as crucial input information to the process control. For the quick model, an element-based time-discrete approach was chosen. By assuming a plane strain condition and a decoupling of thermal and mechanical computation as well as by adapting further assumptions, the calculation time on a current desktop PC is 4 s. The achieved calculation time is sufficiently short to realize the control at the given stroke rates. The model is validated by experiments using a hot stretch forming setup designed to simulate the multi-stage process. The model successfully replicates the influence of various stroke rates on the final thinning distribution and predicts the effects of diverse pre-cooling scenarios. [Display omitted] • The developed quick model computes thinning in hot stretch forming in 4 s. • Spatially and time-variant temperature distributions in the blank are considered. • The model facilitates closed-loop control in multi-stage hot sheet metal forming. • Setting thinning using actuators in multi-stage hot sheet metal forming is shown. [ABSTRACT FROM AUTHOR]

The combination of hot-stamped steels and fibre-reinforced thermoplastics (FRTP) offers great potential for lightweight design. The process steps of hot stamping and thermoforming for the manufacturing of such hybrid structures are usually carried out in two separate steps. After hot stamping, the blank has to be reheated for the application of FRTP in order to enable the bonding of FRTP to the blank. Process integration of the individual processes means that reheating of the blank during thermoforming is no longer necessary. Due to non-uniform contact conditions during hot stamping between blank and tool, an inhomogeneous temperature distribution in the blank occurs, which would lead to non-uniform bonding properties between blank and FRTP after thermoforming. The aim of this work is to determine this non-uniform contact and the resulting heat transfer by a numerical model. By segmenting the tool in the model, a possibility is shown to control the non-uniform contact and to achieve a homogeneous temperature distribution in the blank after hot stamping. Furthermore, the temperature adaptability of the used tool and the resulting temperature distribution in the blank is investigated. Finally, the manufactured structures are tested in a numerical three-point bending test in order to be able to make a statement about their performances. [ABSTRACT FROM AUTHOR]

The effects of hydrogen gas pressure and prior austenite grain size (PAGS) on the susceptibility of a 22MnB5 press-hardened martensitic steel (PHS) to hydrogen embrittlement were studied. The hydrogen test apparatus at NIST-Boulder was modified for tensile testing of plate-type and sheet-type specimens in gaseous hydrogen. This modification made it possible to evaluate the slow strain rate tensile (SSRT) properties of the PHS with three different PAGS at various hydrogen pressures (0.21 MPa–5.5 MPa). SSRT testing in gaseous hydrogen resulted in significant reductions of both the tensile strength and ductility, as compared to those measured in air. In addition, the presence of gaseous hydrogen resulted in a transition in fracture morphology from the near-45° slant fracture to a more brittle fracture along a plane perpendicular to the tensile axis. The hydrogen-affected fracture zones were connected to the sheet specimen free surfaces, signifying the effect of external hydrogen. The fracture surfaces of the hydrogen-embrittled specimens contained relatively flat, "cleavage-like" facets, the size of which depended on the PAGS or packet size. The PHS having the largest PAGS represented generally larger secondary cracks and straighter crack paths in addition to a greater area fraction of the "cleavage-like" facets, likely indicative of a lower frequency of crack deflections. Compared to the largest PAGS condition, the two PHS with smaller PAGS were more resistant to the hydrogen-induced fracture especially at relatively low hydrogen gas pressures (<0.52 MPa). In contrast, with an increase in hydrogen pressure, all PHS specimens exhibited significant decreases in tensile strength and ductility. The positive effect of refining martensitic microstructure, at the low hydrogen pressures, is likely associated with improved toughness of the smaller grain-sized specimens. • Gaseous H embrittlement (HE) in ultra-high strength martensitic steel is studied. • Test apparatus for sheet-type specimen testing in gaseous H is developed. • Martensitic constituent sizes affect HE susceptibility and fracture morphologies. • Martensite with smaller prior austenite grain size is more resistant to gaseous HE. • Various metallurgical factors related to martensite HE sensitivity are discussed. [ABSTRACT FROM AUTHOR]

We investigated the influence of hydrogen on a 1500 MPa hot-stamped steel 22MnB5 with different prior austenite grain size (PAGS, ranging from 14.0 μm to 22.2 μm), which were produced by precise control of the austenitizing temperature and time. All steel specimen exhibited a significant reduction of ductility (e.g., by 70%) and a small decrease in ultimate tensile strength (≤15%), measured using the linearly increasing stress test. The effect of PAGS on hydrogen embrittlement of the hot-stamped steel was not significant for these test conditions. Martensite with smaller PAGS exhibited a slightly lower hydrogen diffusion coefficient. Fracture for all steel variants after yielding occurred for hydrogen contents greater than those that actually occur during service. • The influence of hydrogen on the press-hardened steel (PHS) 22MnB5 was investigated. • The prior austenite grain size (PAGS) of the PHS increased with increasing austenitizing temperature and with increasing soaking duration. • Fracture for all steel variants after yielding occurred for hydrogen contents greater than those that actually occur during service. • Martensite with smaller PAGS showed slightly greater resistance to hydrogen embrittlement at low hydrogen fugacities. [ABSTRACT FROM AUTHOR]

Hydrogen embrittlement (HE) characteristics of 22MnB5 steel (U-bent specimen) manufactured using hot-stamping process at various temperatures were experimentally and numerically investigated. Steel resistance to HE was examined through delayed failure tests under static and cyclic loading during hydrogen charging. First, the low cyclic loading caused severe HE, in which a clear difference in the extent of HE was obtained depending on the hot-stamped sample, which directly affected the microstructural characteristics and stress–strain distribution. The hot-stamped samples with large martensite phase showed low resistance to HE compared with those with small martensite phase because of the high concentration of hydrogen trapped in the phase boundaries. Moreover, the dual phase (ferrite and martensite) of the hot-stamped samples reduced their resistance to HE, which is caused by the hydrogen trapped in the laminar-shaped pearlite phase. The resistance to HE was improved by low-temperature heating at 200 °C for 1 h because of the generation of ε -carbides as trap sites as they render the hydrogen non-diffusible. Furthermore, the internal strain in the U-bent sample could accelerate HE because of the high concentration of hydrogen. These results were verified by experimental and numerical analyses. Thus, the hydrogen trapping mechanism was proposed as a valid mechanism for HE in 22MnB5. [Display omitted] • Hydrogen embrittlement characteristics of hot-stamped steel were clarified. • Effects of microstructure and internal strain on hydrogen embrittlement were examined. • Improvement of resistance of hydrogen embrittlement was proposed. [ABSTRACT FROM AUTHOR]

A ferrite layer is formed between the outer intermetallics layers and the martensite substrate during the hot stamping process of Al-Si coated press-hardened steel (PHS). Unexpectedly, it is found here that the ferrite layer does not effectively prevent brittle cracks propagating continuously from the intermetallics to the martensite substrate. This leads to a high stress intensity factor (SIF) at the crack tip, therefore initiating highly localized shear deformation in the martensite substrate, degrading the bendability of the steel. A thinner Al-Si coating produces thinner brittle intermetallics and ferrite layers, and therefore shorter coating cracks and smaller SIF at the crack tip, thus improving the bendability. In addition, a thinner Al-Si coating also has a lower material cost while keeping similar paintability and corrosion resistance. The thin Al-Si coating could potentially change the current practice of Al-Si coating, impacting the global automotive industry. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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 (TiB 2 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. [ABSTRACT FROM AUTHOR]

Hot stamping of Boron steel is a thermal-mechanical and phase transformation coupled process. The reliable testing and parameter identification method of the constitutive behavior is crucial for accurate simulation on hot stamping. The imprecise strain and strain rate calibration due to the inevitable temperature gradient along the gauge length in the Gleeble tensile test has been a great challenge to the accurate description on constitutive behavior at elevated temperatures. In the present work, a novel design of grips is developed to flexibly regulate the length of Uniform Temperature Zone (UTZ) along the specimen gauge length. A system with new speckle preparation method and great optical anti-interference ability is developed for the accurate strain calibration at elevated temperature and high-speed deformation based on digital image correlation (DIC). The influence of the length of UTZ and the strain measurement methods on the accuracy of parameters identification is further investigated. The DIC based calibration method on full-field strain and strain rate makes the parameter identification more robust on the length variation of UTZ, that is, the identified parameters are nearly the same using different UTZ length of specimen. The appropriate UTZ length should be satisfied to prevent the ends of gauge length from blue brittleness and the undesirable end failure. The identified constitutive parameters are verified by comparison between simulation and experiments of the hot stamping hat-type parts. It shows that the newly developed DIC-based method is the most precise within the context of the tested approaches to determine constitutive parameters of hot stamping process. [ABSTRACT FROM AUTHOR]

The environmental regulations and safety requirements have enhanced the requirement of advanced and ultra-high strength steels. The hot forming process is known to deliver ultra-high strength steels. The boron added steels are heated beyond the austenitization temperature and subsequently deformed and quenched to form deformed components having fully martensitic structure. However, the bare steel suffers from oxidation and decarburization if no coating is applied. The conventional zinc based coatings like, galvanised, galvannealed are not suitable as zinc or iron-zinc phases melt at high temperature of austenitization and give rise to microcrakcs. The present work deals with a new zinc based coating system formed by a combination of electrodeposition and galvanising incorporating nickel in the coating. The coating microstructure is comprised of different nickel-zinc phases having difference in melting points as well. The evolution of coating microstructure at different soaking temperature and time is studied. The coating microstructure and compositions are observed by optical and confocal laser scanning microscope along with Secondary Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). The different coating phases undergo melting and there is homogeneous nucleation of iron-rich solids in the coating. The composition and phase fractions of iron and zinc-rich phases are verified from ThermoCalc calculations. The coating transforms to complete iron-rich solid after 300 s of soaking at the austenitizing temperature. However, the initial zinc content largely determines the final fraction of zinc-rich liquid phases. The hot deformation study shows microcrack free regions at different areas of the hat sections. • Nickel-zinc alloy coating by galvanising pre-nickel coated press hardenable steels. • The coating phases undergo multiple transformations during heat-treatment. • The zinc and iron-rich phases are formed with different morphology. • The equilibrium phase fractions are calculated for different nickel content. • The coating after hot deformation shows occasional microcracks. [ABSTRACT FROM AUTHOR]

• IHTC were characterised using a dedicated IHTC test facility. • The coating thickness and thermal conductivity ratio determined the IHTC. • The new IHTC model can predict IHTC as a function of tool and coating materials. • A 3-stage variable contact pressure was identified using a data-driven approach. • Abrupt changes in IHTC were captured experimentally following data-driven approach. The interfacial heat transfer coefficient (IHTC) is an essential parameter in hot stamping for determining the optimal process parameters necessary to achieve the critical cooling rate and thus the desired post-form strength in a formed component. Aluminium chromium nitride, chromium nitride and titanium nitride are currently being widely studied in the hot stamping industry as tool coatings due to their excellent wear performance, although their effects on the IHTC have yet to be determined. In practical hot stamping processes, the contact pressure may also change over extremely short periods of time, leading to abrupt changes in the IHTC and consequently the temperature evolutions of the workpiece. These abrupt changes in contact pressure and their effects on the IHTC have yet to be studied either. In this paper, to address those challenges, the IHTCs between an AA7075 aluminium alloy and different coated tools with a 3-stage evolutionary variable contact pressure were quantitatively determined, and a mechanism-based model developed to accurately predict their values. [ABSTRACT FROM AUTHOR]

Abstract The quenching and partitioning process has proven to be an efficient way to improve the ductility of high-strength steel parts. In this study, two-step quenching and partitioning combined with the hot stamping process was investigated. A multi-scale coupling simulation was performed, in which the micro-scale carbon diffusion, interface migration and austenite domain were coupled into the macro-scale thermomechanical simulation by Abaqus user-defined subroutines. The effect of stress on martensite start temperature (M s) was taken into account for calculating the retained austenite volume fraction after the first die quenching. The non-uniformity results caused by non-synchronous cooling and the effect of stress were evaluated with the coupled simulation. The carbon diffusion and interface migration were greatly affected by the loading history during the partitioning process. A careful comparison between quasi-static partitioning and dynamic partitioning results indicates that the heating history during the partitioning process cannot be neglected. Finally, a U–shape part was produced by hot stamping quenching and partitioning process to verify the multi-scale simulation. Initial martensite and fresh martensite were distinguished using scanning electron microscopy. The retained austenite volume fractions were quantitatively measured using X-ray diffraction and compared with predictions. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract In recent years, the use of Al-Si-coated ultra-high strength steels has grown rapidly in hot stamping applications in order to protect the workpiece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperatures can lead to unstable friction, increased wear and damage both the tool and the workpiece surfaces. In the present study, self-lubricating coatings incorporating silver and molybdenum disulfide have been prepared by means of laser cladding, aiming at controlling friction and wear in hot stamping. The friction and wear characteristics of these coatings have been investigated at high temperatures against Al-Si-treated boron steel using two different test configurations featuring open and closed tribosystems respectively. A significant reduction in friction as well as decreased material transfer from the Al-Si-coated workpiece to the tool surface have been observed for the self-lubricating claddings. The closed configuration tribological tests have been found to underestimate friction and wear of the tool/workpiece system, and may be less suited for the lab-scale simulation of hot metal forming compared to an open test configuration. These findings are considered relevant as most of the published results in the available literature have been obtained by using closed configurations tests. [ABSTRACT FROM AUTHOR]

Maximum strengths of up to 2,000 MPa can be achieved by hot stamping high strength boron-alloyed quenched and tempered steels to an austenitised state at 950 °C followed by forming and quenching. The interruption of the quenching process at a temperature range of 150 – 250 °C allows a complete transformation of the steel structure into martensite and allows for an adhesive material bond of fibre-reinforced thermoplastic in a suitable temperature range. In this paper, a model of the heat transfer mechanism of an integrated hot stamping of steel and thermoforming of fibre-reinforced plastic process is built up and investigated experimentally. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

The paper presents the results of recent experimental and numerical investigations on the adhesive onset in AA7075 hot stamping, in order to evaluate the influence of the material thermal treatment and the main process parameters. Solid graphite lubrication was investigated for both the solubilized and the T6 material conditions by means of strip drawing tests in the temperature range 200–450 °C varying the normal contact pressure up to 10 MPa and the sliding velocity up to 50 mm/s. The results show that the friction coefficient presents an initial decrease as the temperature increases, followed by adhesion phenomena at the highest temperatures with material transfer to the dies. Such behaviour was explained by using a micro-scale FE model showing that the local temperature in proximity of the roughness peaks may locally influence the material strength causing the adhesive wear onset. • AA7075 solubilized presents lower values of coefficient of friction and less adhesive phenomena than AA7075-T6. • Both AA7075-T6 and solubilized has similar trends of the coefficient of friction values vs. the temperature. • Sliding velocity appears having more influence than the normal pressure on the material adhesion. • At high temperatures and pressures the precipitates in the AA7075 T6 can be considered responsible for the adhesive wear. • The material adhesion appears weakly linked to the normal pressure, whose main effect is the extension of the worn surface. [ABSTRACT FROM AUTHOR]

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. • Improved resistance against adhesive wear with increasing tool hardness revealed. • Material transfer, ploughing and flattening identified as main friction mechanisms. • New hot stamping tool steel with high thermal conductivity has been presented. [ABSTRACT FROM AUTHOR]

Abstract The automotive industry's desire for weight reduction while maintaining crashworthiness demands development of materials and material properties within the economic framework of consumers. The industrial process of hot stamping provides a technique to utilize steel in an efficient way. In hot stamping, microstructural characteristics of a steel blank are influenced by controlling the cooling rate. Hot stamping has become a prevalent method for lightweight solutions in car bodies without sacrificing passenger safety. The process of hot stamping applies sequential forming and quenching in a single production step. During the cooling of the blank, various microstructures can be formed depending on the cooling rate or holding temperature. Special tooling allows the application of different cooling rates within the same blank. Thus, the microstructure and mechanical properties can be influenced in designated areas of a blank. Fracture toughness properties of sheet metal are necessary to better understand fracture initiation and crack propagation during crash loading as well as improve crashworthiness predictions. This paper focus on fracture toughness of low-alloyed boron steel sheet common in the automotive industry. A heat treatment process is used to form different microstructures, predominately consisting of one single phase or mixed microstructures with two distinct phases. The fracture toughness of the present microstructures is evaluated using the Essential Work of Fracture methodology. Results are discussed in terms of the different microstructures obtained and the consequent part performance. Results show a strong connection between microstructure and fracture toughness. The bainitic grade shows favorable fracture toughness while a mixed microstructure of bainite and martensite shows a very brittle fracture behavior. A post heat treatment in the form of paint bake curing shows a negligible effect on fracture toughness of martensite. [ABSTRACT FROM AUTHOR]

Abstract A CAE-based process design method was developed to improve formability in hot stamping by controlling the temperature distribution during the forming step. Specifically, areas where deformation is not desired should be cooled to make them harder, and areas where deformation is desired should be kept at higher temperature. In this study, a tailored initial temperature distribution was calculated through three-step analysis of the deformation behavior by a thermo-mechanical forming simulation. First, the deformation severity was evaluated from the damage value for each finite element by using a forming limit diagram (FLD) at elevated temperature. The cautionary damage value was then defined to determine whether the element should be cooled. Finally, the tailored initial temperature was calculated based on the temperature-dependent flow stress. This procedure was applied to a high-rigidity pillar model that could not be formed by the conventional hot stamping process without cracking. The improved formability was confirmed experimentally. Thus, controlling the temperature distribution increases the diversity of shapes achievable by hot stamping. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The process of Hot Form and Quench of aluminum alloys, called Direct HFQ®, has been developed and applied to manufacture high-strength panel components, in which aluminum alloy sheet is heated to solution heat treatment temperature, quickly transferred to cold press dies, simultaneously formed and quenched, and subsequently artificially aged. For Direct HFQ, however, forming occurs at high temperatures, which results in high workpiece/die friction and wear, and hence high tooling and maintenance costs. In the present study, a novel Indirect HFQ for aluminum alloys has been proposed, in which alloy sheet in the O temper is formed at room temperature, then heated to solution heat treatment temperature, and quickly transferred to cold press dies for shape calibration and quenching, followed by artificial aging. In order to compare Indirect HFQ with Direct HFQ, AA6082 sheet specimens have been deformed uniaxially using the two HFQ techniques to a given strain or fracture. Mechanical properties of the deformed specimens have been measured, and differences in mechanical properties after the two HFQ processes have been quantified. Their microstructures have also been characterized to explain those differences. In addition, both HFQ techniques have been applied to form a B-pillar sectional component. It has been found that grain growth occurs in alloy deformed uniaxially to a strain higher than or equal to 10% during Indirect HFQ process, and the degree of grain growth decreases with increasing deformation. The grain growth during Indirect HFQ leads to a lower yield strength (up to ∼8%) and tensile strength (up to ∼12%) than that of the alloy processed using Direct HFQ. In addition, the alloy has a lower ductility and formability during Indirect HFQ than Direct HFQ. [Display omitted] • A novel Indirect Hot Form and Quench (HFQ) combines cold stamping and age hardening. • Indirect HFQ forms fully annealed aluminum alloys in cold stamping for highest ductility. • Grain growth occurs in solution treatment for cold-formed parts due to high dislocations. • Indirect HFQ results in reduced formability for aluminum alloys compared to Direct HFQ. • Strength of parts formed via Indirect HFQ is lower than Direct HFQ due to grain growth. [ABSTRACT FROM AUTHOR]

The effect of furnace heating rate on the intermetallic growth of Al Si coatings for 22MnB5 hot stamping steel was characterized and quantified for two different commercial coating weights, AS80 and AS150. Characterizing and quantifying the formation of the Al Fe and Al-Fe-Si intermetallic layers is crucial to the optimization of the austenitization process, which continually strives to reduce furnace time while maintaining acceptable intermetallic development. Initial nominal heating rates up to the Al Si melting temperature (577 °C) varied from Low (7 °C/s) to Very High (23 °C/s) and then proceed to reduce which resulted in the nominal heating rates to vary from Low (1 °C/s) to Very High (7 °C/s) within a temperature range of 600 to 900 °C. The coating composition was similar at 600 °C for all the heating rate tests and suggests that there is potential to save furnace time by imposing high initial heating rates. SEM and EDS analysis was conducted on the coating at 50 °C intervals. The overall intermetallic transformation occurred significantly faster at the higher heating rates due to the accelerated diffusion of Fe into the coating. The intermetallic transformation also occurred faster for the thinner AS80 coating due to the shorter diffusion path of Fe. A simple empirical model was developed to predict the coating transformation percentage for any temperature and time from 600 °C. The formation of Fe Al and Fe-Al-Si intermetallics was characterized. For each coating weight, the evolution of each intermetallic species was consistent for each heating rate, but at a delayed rate for each subsequently higher heating rate. Additionally, the tests conducted in this work showed that full austenitization of the base metal can be achieved during the heating phase (72 s for Very High heating rate). • The coating composition was similar at 600 °C for all the heating rate tests. • The overall intermetallic transformation occurred faster at the higher heating rates. • The intermetallic transformation occurred faster for the thinner AS80 coating. • The evolution of each phase was consistent for each heating rate, but at a delayed rate for higher heating rate. • Full austenitization of the base metal can be achieved during the heating phase. [ABSTRACT FROM AUTHOR]

In order to reduce the cost of hot stamping of aluminum alloys and develop high-strength aluminum alloy parts for light-weight car body, short-time solid solution (SS) followed by shot-time artificial aging process was implemented for 6061 aluminum alloy with salt bath furnace and silicone oil bath. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM-EDS) and high-resolution transmission electron microscope (HR-TEM) were used to characterize the microstructural evolution in details. The holding time for SS can be reduced by 80 % with short-time SS treatment because of the decreased vacancy generation energy and effective boundary areas. Peak aging can be obtained at 200 °C for only 60 min, and the microstructure was characterized by a lot amount of acicular nano-size β" phase in the coherent with matrix, while precipitate-free-zone (PFZ) also appeared along the grain boundaries. All tensile properties such as 327 MPa yield strength, 362 MPa tensile strength and 13.1 % elongation meet the 6061 T6-standard. The micro stress introduced by superheated degree along with short-time treatment should be attributed to the accelerated diffusion of magnesium and silicon atoms, which greatly affects the diffusion hermodynamics and kinetics, especially nearby the grain boundaries. • The microstructural evolution during short-time solid solution followed by shot-time artificial aging was investigated. • Holding time for SS can be reduced by 80 % with short-time SS treatment. • Peak aging was obtained at 200 °C for only 60 min. The micro stress was considered to be the main reason. [ABSTRACT FROM AUTHOR]

A commercial 5 wt% Al-Zn coated steel was employed to detail the microstructural evolution, oxidation, and the related corrosion behavior changes during the austenitization heat treatment. During the early stages of austenitization, Fe 2 Al 5 Zn x phase forms between the coating and the steel substrate. With continued austenitization, the inner part of the coating transforms to the Γ-Fe 3 Zn 10 phase and the outer part of the coating to the FeAl phase, accompanying with the formation of a thin surface oxide layer mainly composed of Al 2 O 3 with minor ZnO. Subsequently, the coating transforms to the Γ-Fe 3 Zn 10 phase and α-Fe(Zn), and finally to the single α-Fe(Zn) phase. The Al in the coating is distributed mainly on the surface of the alloy layer and suppresses the oxidation of the alloy layer. The austenitization treatment enriches the coating with Fe, which, in turn, ennobles the corrosion potential and reduces the corrosion current density of the coating. Conversely, the potential difference between the coating and the steel substrate is reduced after austenitization. [ABSTRACT FROM AUTHOR]

Carbon-steel filler wire was used to solve the coating problem in laser welded blanks of Al-Si-coated 22MnB5 without a decoating procedure. The effect of the filler wire on the fusion-zone microstructure and mechanical properties was investigated. The Al content was diluted by the filler wire. The δ-ferrite phase was also reduced after the reduction in Al and homogenization. The joint strength was ∼1210 MPa, and the elongation was 1.1% without filler wire. When filler wire was used, the joint strength increased to ∼1550 MPa, and the elongation was greater than 3%. The fracture surface of the joint with the filler wire was ductile. The effects of welding speed and feeding speed were also studied. A reduction in the Al content at the higher feeding speed or the use of a slower welding speed was found to be most effective because either approach enabled more filler material to be added to lower the ratio of the Al-Si coating in the fusion zone. In the absence of the filler wire, the δ-ferrite phases were large and long, connecting to a bulk zone. When the filler wire was used, the δ-ferrite phases were broken up and became narrow and small with a uniform distribution in the martensite matrix. [ABSTRACT FROM AUTHOR]

Hot stamping of 22MnB5 sheets by partition heating may represent an effective method to produce parts of the car body-in-white with improved collision performances thanks to proper tailored microstructural characteristics. In accordance with the heating and forming thermo-mechanical features of hot stamping using partition heating, uniaxial tensile tests at varying temperature and strain rate were carried out on a Gleeble™ 3500 thermo-mechanical simulator applying different heating temperatures ranging from 700 °C to 900 °C. The effect of the heating temperature, deformation temperature, and strain rate on the flow stress was analysed, showing that the peak flow stress increased at increasing volume fraction of austenite. An improved unified viscoplastic constitutive model taking into account the austenitization degree was developed and the related material constants identified making use of a genetic algorithm-based optimization tool. The established constitutive model was implemented into the FE-based software Abaqus™ to simulate the hot stamping process of a cup-shaped part using partition heating. The comparison between the numerical and experimental outcomes in terms of thickness distribution proved the accuracy of the 22MnB5 constitutive model taking into account the austenitization degree. [ABSTRACT FROM AUTHOR]

Direct press hardening is a non-isothermal sheet metal forming method which combines forming and heat treatment in a single process. However, due to the high temperatures during the forming phase, tool wear is severe and friction is high. In this paper, hot strip draw tests are utilised to assess the influence of the forming temperature on the coefficient of friction (COF) and active wear mechanisms during sliding of Al-Si coated press hardening steel (PHS) strip in contact with uncoated tools under typical hot forming process conditions. The COF is found to be temperature dependent during initial sliding against a virgin tool surface. Whereas, for 10 consecutive strip draws, COF is only temperature dependent for the first samples over the temperature range from 400 °C to 750 °C. This would be due to the tribolayers which form in the tool-sheet contact during the test series. Conversely, the wear mechanisms active in this temperature range are temperature dependent: at higher temperatures (> 600 °C) an area of severe abrasive wear is found that precedes a thick layer of compaction galling while at lower temperatures, (< 600 °C) adhesive wear is dominant. Furthermore, the results show that particles leading to compaction galling are predominantly generated from the Al-Si coating and their size depends on temperature and are related to the fracture of the Al-Si coating. [ABSTRACT FROM AUTHOR]

A novel sheet metal forming process, by manufacturing parts in a single sheet with varying thickness, has been employed in this work. It combines hot forming and cold-die quenching, also known as HFQ®, and the use of aluminium tailor welded blanks (TWBs) into a hybrid process. A series of hot stamping tests on the AA6082 TWBs were performed to investigate the deformation behaviour and failure features. Two failure modes, i.e. circumferential necking and parallel weld necking have been observed in the formed parts depending on the forming speed and thickness ratio of the TWBs. An advanced forming limit prediction model has been developed and further integrated into finite element simulation via a cloud-based multi-objective platform 1 1 All the models presented in this paper are available as open access numerical modules at www.smartforming.com, an online knowledge-based cloud platform. to investigate the failure/necking features of AA6082 TWBs. The model incorporates the theories of Hosford yield function, the anisotropic nature of plastic deformation in sheet metals and the Marciniak-Kaczynski (M-K) theory. According to the theories, the incremental work per unit volume ratio ( d ε ¯ B ∙ σ ¯ B / d ε ¯ A ∙ σ ¯ A ) between Zone B (thickness imperfect zone) and Zone A (the remainder of the material) is a key parameter determining the formability, by which the complex failure features have been fundamentally studied. The transition of failure mode in a TWB was attributed to the joint effects of temperature, strain rate and loading path changes. Strain rate could accelerate the development of localised necking in the TWBs when the failure mode was in transition from the circumferential mode to parallel mode. [ABSTRACT FROM AUTHOR]

In this paper, a study on the innovative hot stamping process and the typical microstructures is presented, aiming to clarify insights into the influence of processing variables (asymmetric rolling temperature, rolling reduction, pre-heating method, and so on) on the evolution of microstructure, in order to evaluate the feasibility of the new process to improve properties of 22MnB5 steels. TEM investigations of asymmetrically rolled samples indicate different morphologies of dislocation density and pearlite with altering the rolling reduction, velocity ratio (VR) and rolling temperature. And the study demonstrates that asymmetric rolling before hot stamping process greatly promotes the austenite transformation during the pre-heating stage, especially when the rolling temperature is lowered to liquid nitrogen temperature (LNT) and the velocity ratio is higher than 1.5. And two-step pre-heating with induction heater and roller hearth furnace contributes positively to the austenite grain refinement in pre-deformed 22MnB5 steel. Another relevant conclusion is that the cooling microstructures can also be adjusted by changing the pre-deformed temperature of the samples. [ABSTRACT FROM AUTHOR]

A novel hot stamping process for Ti6Al4V alloy using cold forming tools and a hot blank was presented in this paper. The formability of the material was studied through uniaxial tensile tests at temperatures ranging from 600 to 900°C and strain rates ranging from 0.1 to 5 s −1 . An elongation ranging from 30% to 60% could be achieved at temperatures ranging from 750 to 900°C respectively. The main microstructure evolution mechanisms varied with the deformation temperature, including recovery, phase transformation and recrystallization. The hardness of the material after deformation first decreased with the temperature due to recovery, and subsequently increased mainly due to the phase transformation. During the hot stamping tests, qualified parts could be formed successfully at heating temperatures ranging from 750 to 850°C. The forming failed at lower temperatures due to the limited ductility of the material. At temperatures higher than 900 °C, extensive phase transformation of α to β occurred during the heating. During the transfer and forming, the temperature dropped significantly which led to the formation of transformed β, reduction of the formability and subsequent failure. The post-form hardness distribution demonstrated the same tendency as that after uniaxial tensile tests. [ABSTRACT FROM AUTHOR]

The mechanical properties and grain orientation evolution have been investigated in a TRIP-assisted steel in search of enhanced strength and plasticity. A novel combined process (HSQ&P) of Hot Stamping (HS) and Quenching and Partitioning (Q&P) treatment with different initial straining temperatures has been conducted by thermo-mechanical simulation. The microstructure was analyzed in detail using scanning electron microscopy and electron backscattered diffraction. The results revealed that carbon partitioning from saturated martensite into retained austenite led to less distortion in the lattice due to less martensite tetragonality, which decreases stored energy. In addition, the predominance of {011} and {111} crystallographic textures, which are parallel to the normal direction of retained austenite and martensite grains formed in HSQ&P sample, enhanced ductility as it facilitates displacive phase transformation. Thus, it improves the tensile mechanical properties as verified by subsized tensile test. A novel thermo-mechanical process is being proposed for TRIP-assisted steels Based on these findings. [ABSTRACT FROM AUTHOR]

The recently developed multi-step sheet forming technology with inductive in-situ heating serves to increase the productivity and to reduce costs compared to hot stamping with furnace heating. The paper reveals the mechanisms for the flexible setting of geometric and mechanical properties such as the bending angle and strength in terms of a closed loop control. The air and die bending as representative forming processes are analyzed with respect to the mechanisms overbending, springback and thermal distortion by experimental, numerical, and analytical investigations. The mechanisms for controlling strength in carbon steels by grain refinement, grain growth and cooling rate are described. [ABSTRACT FROM AUTHOR]

A novel hot stamping technique known as ‘Solution Heat treatment, Forming and in-die Quenching (HFQ ® )’ was employed to manufacture lightweight structural components from AA6082 tailor-welded blanks (TWBs) of different thickness combinations: 1.5–1.5 and 2.0–1.0 mm. A finite element (FE) model was built to study the deformation characteristics during the hot stamping process. The FE model was successfully validated by comparing simulation results with experimental ones. Subsequently, the verified simulation results were analysed through a novel multi-objective FE platform known as ‘Knowledge-Based Cloud – Finite Element (KBC-FE)’. KBC-FE operates in a cloud environment and offers various advanced unique functions via functional modules. The ‘formability’ module was implemented in the current study to predict the limiting dome height and failure mode during the hot stamping process. Good agreements were achieved between the predicted and experimental results, from which studies were extended to predict the forming features of 2.0–1.5 mm TWBs. The ‘formability’ module has successfully captured the complex nature of a hot stamping process, featuring a non-isothermal and non-linear loading path. The formability of TWBs was found to be dependent on forming speed and blank thickness, out of which the latter has a dominant effect. [ABSTRACT FROM AUTHOR]

Sheet stamping is a widely adopted manufacturing technique for producing complex structural components with high stiffness-to-weight ratios. However, designing such components is a non-trivial task that requires careful consideration of manufacturing constraints to avoid introducing defects in the final product. To address this challenge, this research introduces a novel deep-learning-based platform that optimises 3D component designs by considering manufacturing capabilities. This platform was realised by developing a methodology to combine two neural networks that handle non-parametric geometry representations, namely a geometry generator based on Signed Distance Fields (SDFs) and an image-based manufacturability surrogate model. This combination enables the optimisation of complex geometries that can be represented using various parameterisation schemes. The optimisation approach implemented in the platform utilises gradient-based techniques to update the inputs to the geometry generator based on manufacturability information from the surrogate model. The platform is demonstrated using two geometry classes, Corners and Bulkheads, each having three geometry subclasses, with four diverse case studies conducted to optimise these geometries under post-stamped thinning constraints. The case studies demonstrate how the platform enables free morphing of complex geometries, while also guiding manufacturability-driven geometric changes in a direction that leads to significant improvements in component quality. For instance, one of the cases shows that optimising the complex component geometry can reduce the maximum thinning from 45% to satisfy the thinning constraint of 10%. By utilising the proposed platform, designers can identify optimal component geometries that ensure manufacturing feasibility for sheet stamping, reducing design development time and design costs. • Deep-learning-based platform for optimising sheet stamping geometries developed. • Platform consisted of two neural networks: geometry generator and surrogate model. • Generator outputs were optimised using surrogate predicted stamping performance. • Platform implemented to optimise complex 3D geometries with small-scale features. • Expressive geometry changes to improve stamping performance achieved. [ABSTRACT FROM AUTHOR]

Dimensional deviations of channel features affect the assembly accuracy of bipolar plates, thereby influencing the efficiency and performance of a proton exchange membrane fuel cell. A challenge faced by the stamping process is how to further improve the dimensional accuracy of bipolar plates. This work proposes the hot stamping process for stainless steel bipolar plates, where a lab-scale hot stamping platform with an on-site resistance heating device is built. Influences of forming temperature, lubrication condition, grain size on the formability, and dimensional accuracy of ultra-thin stainless steel bipolar plate are investigated by employing this established hot stamping platform. It is found that within the investigated forming temperature range, the dimensional accuracy of hot-stamped ultra-thin SS316L bipolar plates increases as forming temperature increases, and a forming temperature of 1000 ℃ results in the highest dimensional accuracy and lowest dimensional variations of channel depth and rib width. BN lubricant increases the formability of ultra-thin SS316L bipolar plates, in particular when the forming temperature is higher than 600 ℃. In addition, grain size exhibits the greatest influence on the formability of ultra-thin SS316L during hot stamping, and larger grain size (>24.8 µm) increases the thinning rate and consequently the cracking risk of hot-stamped ultra-thin SS316L bipolar plates. • Hot stamping of reduced-size stainless steel BPP is reported for the first time. • BPP hot stamping system integrated with on-site resistance heating was developed. • Hot stamping improves dimensional accuracy and forming consistency of SS316L BPP. • Lubrication significantly affects formability of SS316L BPP during hot stamping. • Grain size has an effect on formability of SS316L BPP at all tested temperatures. [ABSTRACT FROM AUTHOR]

A novel hydraulic bulge test device was developed to evaluate high temperature biaxial stress–strain curves of quenchable boron steel sheets. The work mainly focuses on the resistance heating designed to assure a homogeneous temperature field in the dome area of the circular blank where the hydraulic pressure will be applied. The practical hot stamping conditions, including the heating and cooling steps, were reproduced to perform hydraulic bulge tests on the Usibor ®1500P steel for a temperature range between 700 to 900 ° C, after an austenization step at 900 °C. Stress–strain curves were obtained from these expansion tests using the data extracted with a laser profilometer, due to the difficulties associated with the use of Digital Image Correlation at such high temperatures. Although the profilometer is a compromise solution, the comparisons between tensile and biaxial stress–strain results enable to verify the feasibility of the new device for the evaluation of the stress–strain curves at a high temperature in a biaxial state. The results point-out that, besides the difficulties with the acquisition of the strain fields and in the strain-rate control, there are also challenges in the interpretation of the metallurgical evolutions that can occur during the tests, which can affect the biaxial flow curves. [Display omitted] • High temperature bulge test device using resistance heating was developed. • Homogeneous temperature at the cap top validated numerically and experimentally. • DIC constrains bypassed by profilometer to obtain biaxial stress–strain curves. • Similar hardening behaviour in uniaxial tension and bulge tests at high temperature. [ABSTRACT FROM AUTHOR]

Tailored blanks having a thickness distribution without welds were produced by successive forging, and the length of the transient region in which the thickness gradually decreases was reduced by an infinitesimal feed. The effect of the infinitesimal feed on the reduction of the length of the transient region was evaluated. A tailored blank having a desired thickness was successive-forged, and was hot-stamped into a roof bow. The produced roof bow had sufficient hardness and high paintability. [ABSTRACT FROM AUTHOR]

In order to improve the drawability of boron steel and also to increase the productivity of hot stamping process, a new hot stamping process with pre-cooling has been proposed. Stress-strain behavior at various temperatures was investigated and compared with that in traditional hot stamping processes. Detailed studies were carried out on the strain hardening parameter, n , at different temperatures and deformation rates. To evaluate this concept, hot stamping experiments were performed with both conventional (without pre-cooling) and new process (with pre-cooling) for a scaled down B-Pillar automotive component. The new hot stamping process with pre-cooling was able to produce the B-Pillar at low temperature (500 °C) with less thinning than the hot stamping carried out without precooling at high temperature (765 °C). Also the in-die quenching time was reduced by about 60%, by adopting the new hot stamping process with pre-cooling, which would increase the productivity significantly for automotive mass production without compromising the part quality. [ABSTRACT FROM AUTHOR]

Hot stamping is an advanced manufacturing process in automobile manufacturing. Different from the traditional forming process in room temperature, the strain hardening, temperature, strain rate and microstructure of the material have great influence on the mechanics and failure behavior of the high strength steel. The traditional theoretical forming limit prediction method based on room temperature is not effective enough to predict the formability of parts under hot stamping condition. A forming limit prediction model for the high temperature forming process of 22MnB5 is obtained based on Norton-Hoff constitutive model, BBC 2005 yield criterion, and M-K model. The accuracy of the prediction model is verified by comparing with the experimental data in the literature. This model can be used as the forming limit prediction criterion for high strength steel in hot stamping process. The characteristics of forming limit curves under different conditions are obtained and the effect of yield criterion, temperature, and strain rate are summarized. [ABSTRACT FROM AUTHOR]

Friction stir welded (FSWed) AA6082 tailor welded blanks (TWBs), with gauge combinations of 2.0-2.5 and 3.0-5.0 mm, have been prepared and successfully formed into automotive panel components. Experimental results indicated that the post-form strength, in terms of hardness, varied from location to location on the final parts. The strength is highly dependent on the blank gauges, with the average hardness values being HV 110 and HV 98 for the 2.0-2.5 and 3.0-5.0 mm TWB parts, respectively. Conventional FE simulation was built in PAM-STAMP and the prediction results were validated from experimental data in terms of strain distribution and temperature evolution. A typical continuous cooling precipitation (CCP) diagram for AA6082 was implemented into the verified simulation data to explain the strength variations. It is deemed that the temperature history during the stamping and quenching stages has played a major role on the post-form strength of the final parts. [ABSTRACT FROM AUTHOR]

The austenitizing process of ultra-high strength steel by radiation heating occupied most of the cycle time, which would affect the application of hot stamping in automotive industry. The conduction heating had the advantages of rapid heating, high energy efficiency and good flexibility, which solved the deficiencies of radiation heating. However, the steady temperature distribution of the blanks by conduction heating in the regions closed to the copper electrodes was lower than that of the middle regions, which would affect the mechanical properties of the hot stamping parts. According to the principle of heat transfer, the unequal radiation heat transfer technology was proposed to solve the problem of the inhomogeneous temperature distribution of the blanks. The tests were carried out on the conduction heating test line to investigate the influence of the technology on the steady temperature distribution of the blanks. The results shown that the homogeneous temperature distribution of the blanks was achieved by the local and continuous unequal radiation heat transfer technologies, satisfying the requirement of hot stamping. And the material utilization ratio of the blanks was also improved. The tensile strength, elongation and the balance of strength and ductility of the high temperature regions exceeded 1490 MPa, 6% and 11 GPa•%, respectively. The technology solved the application deficiencies of conduction heating in hot stamping. [ABSTRACT FROM AUTHOR]

Hot stamping is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production step. Within the field of hot stamping the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. The mechanical properties in a blank are modified by the formation of different microstructures. Martensite is a microstructure with high strength but low ductility, ferrite has lower strength but higher ductility. Using special tooling tough martensite and soft ferrite can be placed in adjacent sections in a blank. Between those sections a transition zone consisting of a mixed microstructure exists with mechanical properties between martensite and ferrite. Transition zones possess intermediate cooling rates, hence formation of bainite and composites of bainite and another phase can from. This paper presents an approach of modelling the complete process from austenitized blank to fracture. The method presented relies on the prediction of phases formed during cooling using an austenite decomposition model. In the course of ferrite formation the carbon content in the remaining austenite increases, the carbon content in austenite influences formation of additional daughter phases. The estimated phase composition is used in a homogenization scheme to predict the hardening of the material during plastic deformation. Fracture in the different microstructural phases is predicted using the strain decomposition provided by the homogenization and a fracture criteria. The homogenization scheme and the fracture criteria use measured data from single phase microstructures, i.e. ferrite, bainite and martensite. A heat treatment process for tensile test specimens is used to produce samples with different volume fractions of the microstructures ferrite, bainite and martensite. The pre-cut specimens are austenitized, ferrite is formed in a second furnace with lower temperature, bainite and martensite are formed by the use of a temperature controlled plane tool. [ABSTRACT FROM AUTHOR]

Aluminum alloy sheets have drawn more and more attention in recent years in the automobile industry for the purpose of weight loss and energy saving. Hot forming-quenching (HFQ) integrated process (also called hot stamping) has been developed recently for heat-treatable aluminum alloy forming to improve the formability, overcome the springback of the part. In this paper, in order to investigate the formability and lubricationof a B-pillar in hot stamping with 6061 and 7075 aluminum alloy sheets, different solution heat treatment (SHT) temperatures, SHT time and lubricants were used in the stamping experiments.The forming precision detections were also carried out after trimming. The results showed that the B-pillar wrinkled badly and cracked or even broke into pieces in cold stamping with or without lubricants.The B-pillar also cracked in hot stamping without lubricants. With the help of properlubricants, B-pillars with no crackand better surface quality could be obtained. The failure mechanismswere also explained in the present work. Besides,forming precision of most regions ranged from -1mm to 1 mm and there are still large errors in the local regions of the both ends of the B-pillar. [ABSTRACT FROM AUTHOR]

High Strength Steel (HSS) sheets formed by hot stamping is more and more widely used in automotive industry. Compared to cold formed components, hot stamped parts show significantly improved material formability as well as the desired microstructure and mechanical property distribution. However, the description of the material formability by means of Forming Limit Diagrams (FLDs), which are usually implemented in FEM dedicated codes, appears strongly limiting when applied to hot stamping, as the material behavior is dependent on temperature and strain rate. A general Lemaitre based damage model was developed and implemented in the FE model of the hot stamping process to describe the fracture onset of 22MnB5 sheets. Tensile tests were carried out at elevated temperatures, in a range between 550 ºC and 750 ºC, at different strain rates, to identify the necessary parameters for the calibration of the Lemaitre damage model. Nakajima-type experiments were conducted and numerically simulated to test the validity of the proposed criterion. The comparison between numerical and experimental results in terms of critical load at crack growth initiation shows that the proposed criterion is able to predict the ductile fracture onset in hot stamping. [ABSTRACT FROM AUTHOR]

Hot stamping is a well-established process for the production of structural automotive parts with an excellent strength-to-weight ratio. The complexity of the thermo-mechanical history in hot stamping requires a precise virtual design of hot stamping processes. The thermal and mechanical characteristics of the most common press hardening steel 22MnB5 were investigated comprehensively and are well represented by numerical models. For new steel compositions and parts involving tailored tempering, numerous experiments, e.g., on a deformation dilatometer or thermo-mechanical simulator are needed to characterise the microstructure evolution and the resulting properties. If appropriate material models were available, new steel compositions and thermo-mechanical histories could be partly explored virtually by simulated deformation dilatometer tests. This paper presents a comparison of experimental and virtual deformation dilatometer tests for the hot stamping steel 22MnB5. The virtual dilatometer is implemented as finite element model in the software LS-Dyna based on the material model *MAT248. With this model, virtual and real dilatometer curves correspond well only for conditions of full martensitic hardening, while pre-strains cause larger differences between model and experiments. When the most sensitive model parameters are formulated as a function of pre-strain, all experimental dilatometer curves can be reproduced by the model. Such a data-driven extension of the model seems to be effective for creating a virtual dilatometer model of high accuracy. [ABSTRACT FROM AUTHOR]

Hot stamping technology has been widely used to produce high strength steel parts, but the elongation of the production is limited which leads to low ductility. In this paper, the quenching & partitioning treatment (Q&P) has been introduced into the existed hot stamping process to develop the Q&P hot stamping technology. Moreover, a fractional cooling strategy was also introduced, which improves the formability and helps to reduce the cooling requirement of the tool as well. By the presented process, the specimen was firstly quenched quickly to a suitable forming temperature by water spray method, then it was stamped and quenched inside the tool, and subsequently partitioned and secondly quenched to room temperature. The presented process can improve the elongation effectively and maintain the strength in a high level, which was proved by a series of Gleeble tests and real forming tests. OM and SEM observations were also provided to study the mechanisms of the process. [ABSTRACT FROM AUTHOR]

This paper presents the lubrication performance of WC: C coated tool surfaces used in hot stamping process of AA6082. A range of cylindrical deep drawing experiments were performed to determine the required minimal lubricant amount under coated and uncoated tooling conditions. In addition, effects of process variables, forming temperature and speed, on the lubrication performance were investigated. The results have shown that the advanced WC: C Diamond-like Carbon (DLC) coating could significantly reduce the use of lubricant for hot stamping. The required amount of lubricant for successful forming decreased with increasing forming speed and decreasing forming temperature. The lubrication mechanisms under different forming conditions were discussed also. [ABSTRACT FROM AUTHOR]

A 2-stage progressive-die hot stamping process using partial resistance heating was developed to produce a small-sized ultra-high strength steel parts without additional heat treatments. In the 1st stage, a uncoiled long plate was successively heated by partial resistance heating, and then is simultaneously blanked and punched. The blank is inserted in the blanked plate to be conveyed. In the 2nd stage, the blank is bent and die-quenched. The resistance heating and die quenching operations are synchronised to double the productivity. An automobile seat belt buckle having 1500 MPa in tensile strength was successfully produced without springback. [ABSTRACT FROM AUTHOR]

The hot stamping market will be growing in the next years, but also the requirements of the OEMs and the competition between the suppliers are rising. To be successful in the hot stamping market part manufacturers need equipment with high performance, flexibility and a highly reliable process. In conventional hot stamping presses an uncontrolled process is used for forming and quenching. Deviations of the blank thickness, tool wear, polishing of classical tools is impairing the quenching condition and therefore the part quality over the time. This paper presents a method how to improve the production rates over the whole life cycle of a hot forming part. By using a hydraulic cushion in the press table, the performance of the hot stamping process is increased dramatically. The cushion force leads to a controlled process with improved heat transfer rates and a constant and reliable cooling time. Also the part quality is rising because of the reliable process. In the future, the importance of hot stamping systems with a fast, reliable and stable process and an optimized interchange between hot stamping equipment and quality assurance will increase and become the most important competition advantage for part manufacturers. [ABSTRACT FROM AUTHOR]