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Hot stamping is an advanced forming process used in automobile lightweight manufacturing, but it has not been used in the manufacture of bimetal composite parts. In this work, based on the hot stamping forming technology and the advantages of bimetal composite in strength, stiffness and corrosion resistance, the finite element simulation theoretical analysis and test on the hot stamping forming of AA6061/Q345 bimetal composite U-shaped parts were performed. The effect of initial temperature, stamping speed, die gap and pressure holding time on forming quality were investigated. The results show that the initial temperature not only affects the temperature distribution of each layer of the bimetal composite after stamping but also affects the deformation resistance during forming. The increasing stamping speed reduces the contact time between the die and plate rising the plate temperature, but the increasing stamping speed also strengthens the stamping hardening effect in the forming process leading to the stress increase. The die gap affects the contact area between the plate and the die. The increasing gap is beneficial to reduce the temperature difference between different layers. However, an excessive die gap also increases the tensile stress in the plate and affects the forming accuracy. With the pressure holding time larger, the temperature of the formed part becomes low, the residual stress releasing causes the stress to be smaller. According to the analysis of tensile and bending mechanical properties, it is indicated that the formed parts prepared by the optimal parameters obtained by numerical simulations have excellent mechanical properties and formability. [ABSTRACT FROM AUTHOR]

The design principle of an additively manufactured (AMed) lattice structural conformal cooling channel for hot stamping is investigated. AM with selective laser melting is adopted to fabricate a lab-scale rapid cooling die filled with conformal lattice structures, which provide structural stiffness, act as thermal fins, and expedite the occurrence of turbulent flow in the channel. Three different surface area densities with the same relative volume density were considered to evaluate the heat transfer and cooling performance. Computational fluid dynamics is used to analyze the flow of coolant in the lattice structures with different surface area densities. The experimental and computational results show that if the surface density of the lattice structure is selected properly, the cooling performance can be enhanced significantly while maintaining a constant relative volume density, which directly affects the weight reduction and stiffness of the cooling die. [ABSTRACT FROM AUTHOR]

In this study, 220 AA7075-T6 B-pillars were fabricated using a thoroughly instrumented hot-stamping press under varied conditions. Feature engineering work identified nineteen attributes of the hot-stamping process as impacting four characteristics of the obtained B-pillars: electrical conductivity (%IACS), mechanical strength, distortion, and the presence of visible defects. Pearson correlation suggests an important correlation between the heating phase and the mechanical strength, as well as the %IACS values. As for distortion, the influence of the stamping phase is more obvious. Finally, no correlation was obtained between the hot stamping attributes and the presence of visible cracks. This is mainly due to the pre-assembly phase, i.e., Friction Stir Welding, which will be considered in future works.

Hot forming of aluminium sheets enables the forming of complex shapes with high-strength alloys due to increased formability at elevated temperatures. A fast, uniform and accurate heating method is required to meet the narrow solution heat treatment window from 460 to 490 °C for AA7xxx alloys before forming. Heated plates and convection furnaces are commonly used in hot forming, but require large equipment and can have medium to long cycle times. Induction heating, which uses an alternating current in a coil surrounding the workpiece, could provide shorter heating times. However, induction heating requires a part-specific coil and can exhibit significant temperature gradients over the blank. Multi-physics electromagnetic and thermal simulation can be used as a design tool to help achieve the target temperature uniformity. In this study, induction heating of an irregular aluminium blank was modelled and validated with experimental data. Methods to improve the temperature uniformity were also tested numerically.

The article analyzes various ways to ensure the geometric parameters of forgings in the processes of hot volumetric stamping. The fact of high efficiency of coordinate measurements of geometric parameters of forgings has been established to ensure the identification of negative production factors affecting the geometric accuracy of forgings manufacturing. The designs of common control devices and measurement schemes of geometric parameters of the crankshaft forging do not allow performing coordinate measurements of the crankshaft forging in shop conditions. The coordinate scheme, the design of the control device and the method of determining the size of the parameters of the allowance distribution along the root and connecting rod necks of the crankshaft are considered. On the basis of the methodology given in the article for determining the parameters of the allowance distribution, a technique for controlling the geometric accuracy of hot volumetric stamping has been developed, the accuracy of controlling the output parameters of the geometric accuracy of forgings of crankshafts of heavy-duty vehicles has been improved.

In this study, hot stamping tests on continuous glass fiber (GF)-reinforced thermoplastic (PP) composites were conducted under different process parameters using a self-designed hemispherical hot stamping die with a heating system. The effects of parameters such as preheating temperature, stamping depth, and stamping speed on the formability of the fabricated parts were analyzed using optical microscopy and scanning electron microscopy (SEM). The test results show that the suitable stamping depth should be less than 15 mm, the stamping speed should be less than 150 mm/min, and the preheating temperature should be about 200 °C. From the edge of the formed parts to their pole area, a thin-thick-thin characteristic in thickness was observed. Under the same preheating temperature, the influence of stamping depth on the thickness variation of the formed parts was more significant than the stamping speed. The primary defects of the formed parts were cracking, wrinkling, delamination, and fiber exposure. Resin poverty often occurred in the defect area of the formed parts and increased with stamping depth and stamping speed. [ABSTRACT FROM AUTHOR]

Forming limit curves (FLCs), which are constructed using the limit strains at localised necking, are the most widely used tools for the evaluation of the formability of sheet metals. Fracture forming limit curves (FFLCs) are more recently developed, complementary tools for formability evaluation which are instead constructed using the limit strains at fracture. Since the formability depends strongly on forming conditions such as strain state, temperature and strain rate, models for predicting FLCs and FFLCs are essential for the optimisation and further application of hot forming processes in which these forming conditions vary significantly with both position and time. However, no model has so far been developed to predict FFLCs either alone or in conjunction with FLCs for sheet metals such as boron steel under hot stamping conditions. In this study, a set of unified viscoplastic constitutive equations for the prediction of both FLCs and FFLCs based on continuum damage mechanics (CDM) has been formulated from a set of recently developed constitutive equations for dislocation-based hardening, in combination with two novel coupled variables characterising the accumulated damage leading to localised necking and fracture. The novel variables take into account the effects of strain state, temperature and strain rate on the formability of sheet metals. The material constants in the CDM-based constitutive equations have been calibrated using experimental data comprising true stress-true strain curves and limit strains of a 22MnB5 boron steel obtained at a range of temperatures and strain rates. Investigation of the effect of varying selected parameters in the coupled damage variables on the resulting computed FLCs and FFLCs has demonstrated the flexibility of the model in enabling curves of different shapes and numerical values to be constructed. This indicates the potential of the CDM-based constitutive model for application to other materials for warm or hot stamping processes. [ABSTRACT FROM AUTHOR]

Hot stamping patchwork blanks technology can flexibly adjust the mechanical properties by designing different thicknesses and profiles of the main blank and the patched blank. Due to the spot welding connection between the main blank and the patched blank, the material flow between them is uneven and crack defects easily occur. To investigate the influence of the forming parameters, especially welding spot arrangement, on the formability, a high-temperature welding spot failure damage model was established through high-temperature tensile shear and cross-tension experiments. Subsequently, the thermal–mechanical coupling finite element model (FEM) of A-pillar patchwork blanks in hot stamping process was established. The results indicated that the strength of the welding spots satisfied the forming requirements, and the forming defects were mainly caused by the excessive thinning of the base material around the local welding spots. By analyzing the causes of the cracking and then optimizing the welding spot arrangement, the thinning rate of the base material around the welding spot decreased from 33% to less than 12%. The thickness distribution of different sections was compared with the simulated and the experimental results and the error was less than 10%. [ABSTRACT FROM AUTHOR]

Copyright of BHM Berg- und Hüttenmännische Monatshefte is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this study, a new medium manganese (medium-Mn) steel alloy is developed for the hot stamping process and required material properties were obtained. The differences of microstructural and mechanical behaviors among typical 22MnB5 steel and medium-Mn steel are investigated during the hot stamping both in uncoated condition. The commercial Finite Element software PAM-STAMP is employed for the simulation of the hot stamping. ThermoCalc and JMatPro are used for the calculation of necessary thermophysical properties. The hot ductility and fracture behaviors of medium-Mn steel are investigated by Gleeble hot tensile experiments over the deformation temperatures of 600–900 °C and strain rates of 10− 3 and 10− 4 after annealing at different temperatures during the thermomechanical process. Dilatometer tests were conducted to obtain the phase transformations. Hot tensile testing and dilatometry studies indicated that the hot tensile behavior of medium-Mn steel is influenced by the microstructural alterations occurred via cooling or heating prior to the deformation. To validate the thickness distribution and the microstructural evolution, a prototype part is manufactured on a semi-industrial scale by hot stamping for both materials. A good correlation between simulation and experiments was observed. In addition, decarburization of the part is investigated. medium-Mn steel exhibited a lower decarburization layer. It was also seen that the higher hardenability of medium-Mn steel favors the martensitic transformation. [ABSTRACT FROM AUTHOR]

The prediction of failure and optimisation of the initial blank shape of a component formed under the complex thermal and mechanical conditions could not simply rely on its thinning or strain. In contrast, the prediction of forming limit of material as a comprehensive result of temperature, strain rate and loading path is an essential indicator in a non-isothermal hot stamping process. However, it is challenging to implement an advanced forming limit model into conventional finite element (FE) software to simulate the failure of the formed component and thus optimise its initial blank shape. In this work, a unified viscoplastic-Hosford-MK model was developed to post-process the simulated evolutionary thermomechanical characteristics and then numerically predict the forming limit criterion of AA7075 in a hot stamping process, of which results were demonstrated on a dedicated visualiser that independent of FE software. Subsequently, the initial blank shape was optimised by analysing the evolutionary temperature, strain rate and loading path of failure regions. Furthermore, the developed forming limit model and optimised initial blank shape were experimentally verified with an error of less than 10%.

For the purpose of manufacturing the automotive body structure parts with tailored properties, the hot stamping process with partial heating was experimentally and numerically investigated. In this study, the deformation behaviors of the steel 22MnB5 with the fully and incompletely austenitized microstructure were studied by hot tensile testing and modeled by the modified Zerilli–Armstrong constitutive equations taking the lattice structure into account. The classical Kirkaldy–Venugopalan transformation kinetics equations were modified to be suitable for the investigated steel, and the diffusional phase transformation equations were calibrated by using the continuous cooling transformation data. By the virtue of user subroutine VUHARD in finite element program ABAQUS, the metallurgical-field coupled finite element model was established for simulating the hot stamping process with partial heating. A special fixture was developed to realize the partial heating process of circular sheet blank. The finite element simulation and experimental research were carried out for a cup-shaped part. Finally, the phase distribution, hardness variation, shape error, and thickness variation of the cup-shaped part were analyzed and discussed. The validity and accuracy of the constitutive model and the transformation kinetics model were verified by comparing the simulation results with the experimental data. Besides, the present research results also demonstrate the hot stamping parts with variable mechanical properties can be manufactured by changing the austenitization degree of sheet blank during the heating stage.

With increasingly severe environmental problems, energy saving and environmental protection have become two important issues to be solved in the automobile industry. Patchwork blank hot-stamping technology can be used to obtain light-weight and high-strength parts and is thus increasingly used in the manufacture of autobody parts. Because the main blank and the patched blank need to be connected through spot welding before forming, the welding spots’ arrangement has a great influence on the formability of the part. In this study, a thermal–mechanical coupling finite element analysis model of A-pillar patchwork blanks was established. With the thickness of the patched blank, the distance between the welding spot and the external contour of the patched blank, and the number of welding spots as optimization variables, together with the maximum thinning rate and the maximum welding spot force as objectives, the influence of welding spot arrangement on forming quality was analyzed, and the welding spots’ arrangement was optimized using a central composite design (CCD), the response surface method (RSM), and the genetic algorithm (GA). The results showed that when the initial welding spot was located close to the contour of the patched blank, the bending moment was greater when the weld spot passed through the die corner, leading to the rupture of the welding spot or its surrounding base material due to the greater thinning rate. When the patched blank was thicker than the main blank, the main blank cracked during the forming process due to a greater increase in the thinning rate. The optimal solution of the weld spot arrangement on the A-pillar patchwork blanks was a 1.2 mm thick main blank, 0.8 mm thick patched blank, a distance of 29 mm between the weld spot and the contour line of the patched blank, and 16 weld spots. Hot-stamping experiments were conducted using the optimized weld spots’ arrangement, and high-quality parts were obtained.

The strength of panel sheets can be increased by 2–3 times after hot stamping, and for high quality of hot stamped sheets, uniform cooling rate on the die face to quench the panel is needed. However, it is difficult for cooling water channels, commonly used for hot stamping dies, to dissipate heat evenly. In addition, dies are easily corroded by contact with water at high temperature. Thus, this study considers air cooling for the quenching process. The airflow can be applied to the sheet directly during the forming stage to cool and quench, eliminating difficulties with cooling channel design and providing more uniform cooling. Simulations and experiments were used to test the feasibility of air cooling for hot stamping with CSC-15B22 steel. A simple hot stamping tool with flat die face and either water or air cooling channels was used for testing, and results show that the both direct and indirect airflow can be used for hot stamping. With indirect air cooling, the cooling rate is sufficient for complete martensite transition, though the cooling rate is 40% lower than with indirect water cooling. To improve cooling efficiency of the air cooling system and obtain more uniform cooling, an air jet impingement cooling system (direct airflow cooling) was also tested. Results show that a similar cooling rate can be obtained in both indirect water cooling and the air jet impingement cooling. Thus, it is possible for air jet impingement cooling to provide hot stamping superior to water cooling. These results can be a reference for the design of new cooling systems for hot stamping dies. [ABSTRACT FROM AUTHOR]

Hot stamping is a hot metal forming technology increasingly in demand that produces ultra-high strength parts with complex shapes. A major concern in these systems is how to shorten production times to improve production Key Performance Indicators. In this work, we present a Reinforcement Learning approach that can obtain an optimal behavior strategy for dynamically managing the cycle time in hot stamping to optimize manufacturing production while maintaining the quality of the final product. Results are compared with the business-as-usual cycle time control approach and the optimal solution obtained by the execution of a dynamic programming algorithm. Reinforcement Learning control outperforms the business-as-usual behavior by reducing the cycle time and the total batch time in non-stable temperature phases. [ABSTRACT FROM AUTHOR]

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

Diamond cutting tools used in natural stone cutting processes are also matrix design; it is very important in terms of cutting performance of the tools. In this study, nickel-based superalloys with commercial code 9062 and 9045 were added to Fe-Ni-Cu metal powders in different weight ratios (10-20-30%) by powder metallurgy method. Microstructure and some mechanical properties of the produced samples were investigated. The test samples were hot pressed at 920 °C under 35 MPa pressure for 4 min. Hardness, density, three-point bending tests of the obtained samples were evaluated. SEM (scanning electron microscope) and XRD were used for microstructure investigations. As a result; in the SEM images, it was seen that uniform spherical powders reinforced with Ni based alloy with commercial code 9045 and 9062 were homogeneously dispersed in the structure and the porosity decreased in the 30% 9062 doped sample. In both groups, the highest hardness values were obtained at 30% additive rates. In the three-point bending test, it was observed that the 10% added 9062 sample had the highest strength value. [ABSTRACT FROM AUTHOR]

Hot stamping process is a thermo-mechanical process in which blank is heated to austenitization temperature and then subsequent forming and quenching are done simultaneously. Due to complex process cycle in hot stamping conditions, like high temperature and cooling etc., determination of formability is difficult. Therefore, in this work a series of experiment was performed on thermomechanical simulator (Gleeble-3800) at different temperatures (750oC, 800 oC, 850 oC) and strain rates (0.01/s, 0.1/s, 1/s) to obtain flow behavior and formability of 22MnB5 Steel. Due to limitation of the uniaxial deformation in Gleeble-3800, modified tensile specimen was used to obtain plain strain conditions. Thickness gradient-based necking criterion was validated for high temperature forming at different strain rates. This criterion was further used to determine the formability of 22MnB5 steel. Thickness distribution and thickness gradient for modified specimen and simple tensile specimen is observed under various conditions. It was found that with increase in temperature and strain rate formability of 22MnB5 steel increased as well as uniform deformation occurs. Thickness gradient necking criteria indicates that at point of necking thickness gradient ratio falls below the critical value of 0.92.. [ABSTRACT FROM AUTHOR]

The analysis of the development of the microstructure of deep drawn automotive B pillar, as well as the analysis of deformation based on numerical simulation and experiment, was performed. The microstructure of steel sheet as well as Al-Si coating after various stages of B pillar production was investigated. It was found that the obtained microstructure of the B pillar was significantly different from that described in many studies as a proper one. The microstructure of the investigated material consisted of martensite, bainite, and a small amount of ferrite. Al-Si coating, despite its morphological changes, remained on the surface of B pillar and, in spite of this, did not fully eliminate oxidation and decarburization of B pillar material. The analysis of the state of strain allowed to evaluate the deformation safety of the process, as well as to verify the simulation results through measurements of sheet thickness variations.

The simultaneous occurrence of phase transformation and deformation for titanium alloy sheets is an important feature in hot stamping process where components are formed in room-temperature tools by hot blanks. This work takes a research on the constitutive modeling of TA15 alloy by coupling the effect of phase transformation under hot stamping conditions. Based on Gleeble testing system, the continuous cooling tests and interrupted tensile tests are conducted to study the effects of cooling rate and deformation on phase transformation. Subsequently, a diffusion controlled phase transformation model is extended to describe the phase transformation kinetics. The flow behaviors of TA15 alloy are found to be not only sensitive to temperature and strain rate, but also cooling rate dependent, which can be ascribed to the phase transformation. Hence, a physically based model coupling the effect of phase transformation is proposed. Compared with experimental stress–strain values, the average relative error of predicted results by the proposed model is 2.29%. Finally, hot stamping simulations and experiments of a U-shape part are performed. The simulation results of the proposed model show a better agreement with the experiments.

Spring-back of sheet metal affects the dimensional accuracy of the components after stamping and has to be eliminated or corrected. Advanced high strength steels (AHSS) are prone to spring-back while forming at ambient temperatures. Hot stamping process is an effective technique to reduce the spring-back upon forming. The forming temperatures and the phase transformations associated with quenching are known to influence the spring-back. However, the detailed experimental analysis on spring-back behaviour is limited due to the difficulty in physically simulating the thermal-mechanical conditions prevailing in a typical hot forming process. In the present work, a novel V-bending technique is presented for effectively measuring the spring-back using a thermal-mechanical simulator, Gleeble™. V-bending is carried out by applying a compressive force along the longitudinal plane of AHSS steel strips at forming temperatures. Upon V-bending, samples were cooled at different cooling rates while maintaining the forming loads to simulate die quenching. Results indicate that the spring-back decreases and becomes close to zero (at a cooling rate of 20 K s−1) with an increase in cooling rate for a hot stamping steel. Further, an increase in cooling rate results in spring-forward owing to the formation of an increased volume fraction of martensite.

A self-developed strip-type high-temperature friction and wear test device was used to simulate the high-temperature friction process of uncoated 22MnB5 boron steel under actual hot stamping conditions. The mold was preheated to simulate the temperature increase of the die in the hot stamping process. The effects of mold temperature on the friction behavior and mechanism of uncoated boron steel were studied by using the friction coefficient test, surface wear morphology observation, and cross-section and matrix structure chart of hot stamping boron steel. The results show that the friction coefficient between the uncoated boron steel and the H13 steel is basically stable at 0.5 when the temperature of the mold is low, and the wear mechanism is mainly classified to abrasive wear and adhesive wear. Besides, when the mold temperature exceeds 100 ℃,the friction coefficient of the uncoated boron steel decreases from 0.474 to 0.414 with an increase of temperature from 150 ℃ to 200 ℃,inferring that the adhesive wear is weakened. The Vickers hardness of the boron steel matrix is approximately close to 430 from room temperature to 100 ℃. Moreover, with the temperature further rising to 150 ℃ and 200 ℃,the hardness decreases to 413.5 and 399.7 respectively, which indicates that the mold temperature has a significant effect on the mechanical behavior of formed parts.

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 production of ultra-high strength automotive components requires a multi-directional approach. Hot stamping combines both forming and heat treatment processes to obtain a usually martensitic structure of complicated shaped automotive parts. The preparation for production using hot stamping must involve the latest methods of numerical analysis of both temperature changes and forming, which are applied for an increasing range of materials used. In this paper, the current state of knowledge about the basics of hot stamping, used technological lines, and the current state of material used with applied heat treatments and possible coatings have been reviewed. Moreover, the numerical modeling process has been described. The most important aspects of process automation, including the use of digital twins for simulation and optimization of operational kinetics of the robots accomplishing the production process, analysis and minimization of time of production cycles, and searching for weak operational points of the control systems and for real time visualization of operation of complete line, are considered. The digital twins and corresponding numerical models enable the symmetrical design of real production lines. The details of heat treatment profiles with so called tailored zone heat treatment are provided. Hot stamping is a dynamically developing technology as evidenced by the increasing range of materials used, also from the 3rd generation of advanced high strength (AHSS) steels. It starts to combine forming of symmetric or asymmetric elements with more complex heat treatment processes as required for dual phase (DP) stainless steels or the newest generation of high-strength and ductile medium-manganese steels. [ABSTRACT FROM AUTHOR]

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In hot stamping severe tool wear occurs. The main tool wear mechanism is the so-called compaction galling, in which fractured particles from the coating of the part (blank) being build-up and become compacted upon the tools. Since fractured particles play an important role in the wear mechanism, fracture of the AlSi coating is investigated. For this purpose, simplified B-pillar parts are pressed at several temperatures to investigate the effect of temperature, strain and tool contact on the fracture behavior of the AlSi coating. Three positions which exhibit different strain modes are analysed by means of optical topography and optical microscopy measurements. The results indicate a strong correlation between the AlSi coating fracture, strain and temperature. For instance, at relatively low forming temperatures (i.e. 650 ºC), mode I fracture as well as spallation of the coating and at higher temperatures (750 and 800 ºC) mainly mode I coating fractures are observed. Interestingly, this type of mode I coating fractures are found to widen, eventually giving rise to mode II coating fractures along the coating-substrate interface. The onset strain of coating fracture is low and approximately at strains of 0.017, coating cracks are observed at all start temperatures. At contact-dominated regions, additional coating cracks are observed mainly due to a drop in blank temperature and added shear stresses due to the relative sliding between the blank and tools. [ABSTRACT FROM AUTHOR]

Herein, the effect of process parameters on the formability of Zn‐coated stamping steel is analyzed, including heating temperature, soaking time, forming temperature, deformation level, and strain rate. The studied heating temperature ranges from 850 to 930 °C; the soaking time ranges from 3 to 7 min; the deformation level ranges from 10% to the break; and the strain rate ranges from 0.1 to 10 s−1. The thermal mechanical property is analyzed and fitted using the power model of allometric equation. The length of microcrack is measured using scanning electron microscope (SEM). It can be concluded that the true stress decreases with the increase of the heating temperature and soaking time. The decrease of the forming temperature and the increase of the strain rate lead to the increase of the true stress. The critical strain in the thermal tension process is about 0.35. The length of microcrack is below 10 μm for most of the cases. The optimal process parameters are proposed. The heat temperature should be at the range of 850−900 °C. The soaking time should not be larger than 5 min. The fracture is typical ductile fracture with dimples, resulting from the growth and emerging of microvoids. [ABSTRACT FROM AUTHOR]

Designing materials for targeted materials properties is the key to tackle the demands for personalized consumer products. The deficiency in the existing linear and nonlinear correlation methods attributed to simplifying assumptions and idealizations, nondeterministic simulations, and limited experimental data due to heavy computational time and cost, necessitates a design method that provides sufficient confidence to designers in decision making. To address this requirement, we propose, in this paper, an inverse goal-oriented materials design method supported by the design space exploration framework (DSEF). Keeping in view the accuracy and precision in the prediction confidence of machine learning-based methods, we developed an Artificial Neural Network based prediction model that supports DSEF. The proposed method for materials design can help designers to (1) explore PSPP spaces starting from end property requirements, (2) adjust the errors being propagated in the PSPP chain as well as in the predictions made by the model, and (3) timely adjust model parameters of the prediction model for accurate predictions. The efficacy of the method is illustrated for the hot stamping process to produce structural components from ultrahigh-strength steels (UHSS). The proposed method and prediction model are generic and applicable to any sequential manufacturing process to realize an end product. [ABSTRACT FROM AUTHOR]

The effect of copper content in the range of 0.8–1.4% on mechanical properties and cold resistance of low-alloy precipitation-hardening steel for fittings after heat treatment is investigated. It is established that steel containing 1.4% copper provides the maximum level of strength properties after normalizing and tempering and in this case the steel has an acceptable level of impact strength and cold resistance. The microstructure of steel with 1.4% Cu is investigated by scanning and transmission electron microscopy after tempering at different temperatures. Structure formation in steel with a high copper content during heat treatment by regimes simulating fitting preparation by hot stamping with subsequent tempering, including changes occurring with a secondary structural component, carbonitride particles, and copper particle strengthening precipitates, is studied in detail. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Aluminum alloys are lightweight materials widely used in the automobile industry because of their high specific strength. As the aluminum alloy with the highest strength at room temperature, 7075 aluminum alloy has great potential for usage in the manufacturing of structural parts. However, its formability at room temperature is poor and its springback is large. Although both good formability and high strength in aluminum alloys can be realized by hot forming, 7075 aluminum alloy has high susceptibility to adhesive wear, which means its tribological properties are poor during hot forming. Exploration of the influence of process parameters on the friction behavior and wear mechanism of 7075 aluminum alloy has great significance for the numerical simulation of the hot-stamping process and lubrication engineering. The high-temperature friction process of 7075 aluminum alloy during the actual hot forming–quenching integrated process was simulated by a self-made high-temperature strip friction tester. The upper and lower friction components were cooled and heated, respectively, to simulate the cooling and heating of the die (blank holder) in the actual hot-stamping process. The effects of the preheating temperature of the lower die, normal load, and sliding speed on the friction behavior and wear mechanism of 7075 aluminum alloy were analyzed. The results show that the friction coefficient of aluminum alloy increases with increase in the preheating temperature, and the wear mechanism changes from adhesive wear at 300 °C to adhesive, abrasive, and oxidative wear at 500 °C. The larger the normal load applied, the larger is the friction coefficient. The wear mechanism under different loads was determined to be adhesive wear with slight abrasive wear, with the degree of adhesive wear increasing with the increase in load. A high sliding speed leads to the formation of local oxides on the surface, which makes the friction coefficient decrease with an increase in the sliding speed. The main wear mechanisms are oxidative, abrasive, and adhesive wear when the sliding speed is 30 mm·s−1.

The accurate description of friction is critical in the finite element (FE) simulation of the sheet metal forming process. Usually, friction is oversimplified through the use of a constant Coulomb friction coefficient. In this study, the application of an existing multiscale friction model is extended to the hot stamping process. The model accounts for the effects of tool and sheet metal surface topography as well as the evolution of contact pressure, temperature, and bulk strain during hot stamping. Normal load flattening and strip drawing experiments are performed to calibrate the model. The results show that the model can relatively well predict friction in strip draw experiments when the tool surface evolution due to wear is incorporated. Finally, the application of the formulated multiscale friction model was demonstrated in the FE simulation of a hot-stamped part. [ABSTRACT FROM AUTHOR]

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

7000 series high strength aluminum alloys are increasingly used in manufacturing automobile body parts to meet the more stringent demands for automobile lightweight. Hot stamping of 7000 series high strength aluminum alloys is a complex thermal-mechanical coupling process and precise simulation is needed to predict material fracture. To obtain damage model of 7075 aluminum alloy in hot stamping, five different stress triaxiality specimens were designed. The fracture strain, critical strain and average stress triaxiality of different specimens were obtained by the hybrid finite element simulation and experiment (FE-EXP) method. GISSMO model of 7075 aluminum alloy at 400 °C was established. Compared with the experimental results of U-shaped part hot stamping under different lubrication conditions, the calibrated GISSMO model was demonstrated to predict the damage behavior of 7075 aluminum alloy during high temperature deformation accurately. [ABSTRACT FROM AUTHOR]

A joining and forming integrated process for high-strength steel/CFRP multi-material parts was developed, where the hot stamping technology was used to join the high-strength steel and CFRP prepreg. The preparation processes of multi-material composite parts under different quenching temperatures during hot stamping were studied and microscopic observation of the steel sheets was conducted. Then the three-point bending tests for the multi-material composite parts were carried out. The microscopic observation results show that the microstructure of the steel sheets is full martensite. The three-point bending tests indicate that the bending angles of the samples with no CFRP, with two layers CFRP, and with four layers CFRP are as 130°, 110° and 104°, respectively. Besides, the force-displacement curves of the bending tests were integrated to calculate the energy absorption of the samples. The results show that the energy absorption of the samples with no CFRP, with two layers CFRP, and with four layers CFRP are as 9410 J, 9692 J, and 10050 J. The energy absorption of the samples with two-layer CFRP and the samples with four-layer CFRP increase by 2.9% and 6.8% compared with the samples without CFRP. [ABSTRACT FROM AUTHOR]

We developed a hot stamping method for panel parts with a step-shaped wall. The die-set for the developed method divides the punch at a step-shaped wall into an outer punch and an inner punch. The outer punch is placed first. In addition, an opening is added to the blank of the developed method. The developed method makes it possible to change the main forming type from draw forming to cylindrical stretch flange forming in the middle of a forming stroke. The stretch flange forming can prevent a fall in temperature at a hole edge, because the hole edge does not come into contact with the die-set during forming and because the high ductility of high-temperature materials can be used, making it advantageous for hot stamping. If the preceding amount of the outer punch is large with the developed method, the effect of suppressing wrinkles is large but the sheet thickness reduction that can cause cracking rises. If the preceding amount of the outer punch is small, the rate of sheet thickness reduction ratio is greatly restrained, but the wrinkles become larger. By setting an appropriate preceding amount of the outer punch with the developed method, forming without cracks or wrinkles is possible. [ABSTRACT FROM AUTHOR]

In this paper, the feasibility of a newly proposed forming method, being the warm forming of as-quenched 22MnB5 boron steels, was studied through a series of proof of concept experiments. To assess the material thermo-mechanical behaviours under the proposed forming conditions, first, the as-received 22MnB5 boron steel was austenized and quenched to below the martensite transformation finish temperature to obtain a martensitic microstructure; second, uniaxial tensile tests of the as-quenched steel were conducted under proposed warm forming conditions on a Gleeble 3800 materials simulator. To evaluate the material post-form properties, first, tempering treatments on the as-quenched steel samples were performed to simulate the heat-treating conditions in the proposed warm forming process; second, the mechanical properties (hardness, strength, and ductility) of as-tempered samples were measured and a microstructure analysis was conducted. From the experimental results, it was found that, under the proposed warm-forming process conditions (420 °C–620 °C), the material showed significant strain softening, which would increase the tendency of necking during stamping and adversely affect its drawability. In addition, it was found that the heating of martensite in a 22MnB5 boron steel to higher temperatures (>400 °C) adversely affected its post-form strength and ductility due to the tempering effect. Therefore, according to the results obtained in this study, the warm forming of as-quenched 22MnB5 boron steel may reduce the strength of formed parts by more than 50% in comparison to the possible strength the material could achieve under the investigated process.

In this study, the microstructure and mechanical properties of the 22MnB5 steel sheet were investigated in different heat treatment conditions. The steel sheet heated to 700 °C, 800 °C, and 900 °C was subjected to air cooling, water quenching, and water quenching + tempering procedures. Microstructural properties post-heat treatment were analyzed under an optical microscope. To determine the mechanical properties, uniaxial tensile tests, and hardness measurements were performed. The highest mechanical properties were achieved with the water quenching from 800 °C and 900 °C. The elongation value was ranged of 2-4% with the water quenching. Also, the tempering process slightly increased the elongation value.

PIG (Pin-Injection-Gate) die-casting and hot stamping was developed for fabrication of small-sized and thin-walled aluminum alloy parts from the recycled feedstock. The pure aluminum and aluminum alloy granules were utilized as a feedstock model of recycled materials. The measured mass of granules with the estimated weight from 3D-CAD (Computer Aided Design) of products was poured into the PIG-nozzles before injection. After quickly melting by induction heating inside the PIG-nozzle units, the aluminum melts were injected into a die cavity through the PIG-nozzle. No furnaces and no crucibles were needed to store the melt aluminum stock in different from the conventional die-casting system. No clamping mechanism with huge loading machine was also needed to significantly reduce the energy consumption in casting. Much less wastes were yielded in these processes; the ratio of product to waste, or, the materials efficiency was nearly 100%. Nitrogen supersaturation and TiAlN coating were used to protect the PIG-nozzle and the stamping die surfaces from severe adhesion from aluminum melt. The pure aluminum gears and thin-walled mobile phone case were fabricated by this process. X-ray tomography proved that both products had no cavities, pores and shrinkages in their inside. Using the hot stamping unit, the micro-pillared pure aluminum heatsink was fabricated to investigate the holding temperature effect on the aspect ratio of micro-pillar height to width.

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]