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To develop medium-Mn steels with an ultimate tensile strength (UTS) exceeding 2 GPa and excellent ductility, we created a highly accurate UTS prediction machine learning (ML) model using a boosted decision tree model and 1520 dataset of tensile properties of medium-Mn steels with micro-alloying elements. We also optimized the hyper-parameters of a genetic algorithm (GA) using the Shannon diversity index to enhance search efficiency while retaining diversity. In a high-dimensional search space with millions of potential combinations, the ML-GA approach efficiently identified diverse chemical compositions and austenitizing conditions to achieve UTSs above 2 GPa. The k-means clustering method then grouped them into five distinct specimens based on similarities. These five specimens, fabricated using inversly designed chemical compositions and austenitizing temperatures, successfully exhibited UTSs exceeding 2 GPa and greater ductility compared to hot-stamped C steels. These excellent tensile properties were attributed to grain refinement resulting from the low austenitizing temperature and the pinning effect of micro-alloying element carbides, such as TiC and VC.

Hot stamping involves deforming a heated blank to form components with increased mechanical strength. More recently, warm stamping procedures have been researched. The forming occurs at lower temperatures to improve process efficiency. The process is non-linear and inefficient to solve using finite element simulations and surrogate models. This paper presents the use of dimension-reduced neural networks (DR-NNs) for predicting temperature distribution in FEM warm stamping simulations. Dimensionality reduction methods transformed the input space, consisting of assembly, material, and thermal features, to a set of principal components used as input to the neural networks. The DR-NNs are compared against a standalone neural network and show improvements in terms of lower computational time, error, and prediction uncertainty.

Warm stamping of boron containing hot stamping grade steels is generally carried out at temperatures below austenitization temperature to reduce the forming load and spring-back. It also minimizes surface oxidation of steels which is prevalent during hot stamping process (forming at temperatures above austenitization temperatures). However, there is a possibility of occurrence of dynamic strain aging (DSA) at warm stamping conditions due to the pinning of dislocations by solute atoms such as boron which in turn can increase forming loads. This work investigates the influence of boron on the flow behavior in hot stamping steels under various warm working temperatures (300-773 K) and strain rates (10–2-10–4 s–1). Results indicate the occurrence of DSA in the temperature range of 473-573 K. The ultimate tensile strength (UTS) is found to increase and then decrease with an increase in the temperature due to a combined effect of dynamic strain aging and precipitation strengthening. The increase in the UTS by DSA is higher for steel with higher boron content (35 ppm) than steel with lower boron content (29 ppm). The formation of CrFeMn(CN) during warm stamping along with solid solution boron contributes to the dynamic strain aging. [ABSTRACT FROM AUTHOR]

Hot stamping involves deforming a heated blank to form components with increased mechanical strength. More recently, warm stamping procedures have been researched. The forming occurs at lower temperatures to improve process efficiency. The process is non-linear and inefficient to solve using finite element simulations and surrogate models. This paper presents the use of dimension-reduced neural networks (DR-NNs) for predicting temperature distribution in FEM warm stamping simulations. Dimensionality reduction methods transformed the input space, consisting of assembly, material, and thermal features, to a set of principal components used as input to the neural networks. The DR-NNs are compared against a standalone neural network and show improvements in terms of lower computational time, error, and prediction uncertainty. [ABSTRACT FROM AUTHOR]

The article covers experimental research on the forming of products made of 7075 aluminum alloy. This aluminum alloy grade is characterized by high strength, but due to its low formability in T6 temper, its use in the stamping processes of complex structural elements is limited. The authors have manufactured a U-shaped element at an elevated temperature and determined the optimal parameters of the process. Conventional heating of the sheet and shaping it at the temperature of 100 and 150 °C allowed to obtain a product of high strength similar to the T6 state, above 540 MPa. Due to the excessive springback of the sheet during forming, these products were characterized by a large deviation of the shape geometry, exceeding the allowable values of + / − 1 mm. Only the use of an alternative method of heating the sheet to temperatures of 200 and 240 °C (between plates at 350 °C, heating time 2 min, heating rate 1.8 °C/s) allowed to obtain a product that meets both the strength and geometric requirements. The determined optimal process' parameters were later transferred to the stamping process of elements of a more complex shape (lower part of the B-pillar). The sheet was heated up and formed in the previously pre-heated tools. In the subsequent series of tests, the heating method and the blank's temperature were being analyzed. In the case of the foot of the B-pillar, it was necessary to lower the initial blank temperature to 200 °C (heating in a furnace with a temperature of 340 °C, heating speed 0.5 °C/s). The appropriate combination of the process parameters resulted in the satisfactory shape deviation and reaching the product's strength comparable to the strength of the material in as-delivered T6 temper. Using electron microscopy, it was verified that the structure of the finished product contained particles MgZn2 that strongly strengthen the alloy. The obtained results complement the data on the possibility of using 7075 aluminum alloy to produce energy-absorbing elements of motor vehicles. [ABSTRACT FROM AUTHOR]

A titanium bipolar plate for fuel cell not only has excellent corrosion resistance but also has a lower density than stainless steel bipolar plates. However, titanium exhibits low formability at room temperature that requires warm forming as a prominent solution to overcome this drawback. The present study investigates the forming of bipolar plates for the proton exchange membrane fuel cell out of commercially pure titanium ultra-thin sheet. In this regard, the formability of the titanium sheet with a thickness of 0.1 mm was first evaluated through the tensile test at different deformation temperatures and speeds. Also, V-die bending experiments were carried out at room and warm temperatures to examine springback of the sheet. Afterward, a warm stamping setup was conducted to fabricate the titanium bipolar plates in the range from room temperature to 200 °C with stamping speed varying between 0.6 and 4.8 mm/min. Accordingly, the formed samples were analyzed and their forming rate, thinning rate, and springback were discussed. The results indicated that the formability of the titanium sheet depends on both forming temperature and speed. The most elongation of the sheet was obtained at a temperature of 100 °C and a deformation speed of 0.6 mm/min. The maximum channel depth was obtained equal to 0.494 mm using a warm stamping process that implies an improvement by 40% in the filling rate of microchannels compared with forming at the room temperature. Likewise, the dimensional error of the titanium bipolar plate due to the springback was decreased by stamping at 100 °C compared with forming at room temperature. [ABSTRACT FROM AUTHOR]

A warm stamping process of 1 and 1.2 GPa ultra-high strength steel sheets at comparatively low temperatures using rapid resistance heating was investigated to produce high strength parts. The effects of the heating temperature and rate on mechanical and surface properties of warm-stamped parts were examined. For a rapid heat rate of 100 °C/s around a heating temperature of 300 °C, the hardness of the formed parts increased from that of the as-received sheets, and the hardness for the 1.2 GPa sheet approached that of conventional hot-stamped parts from 22MnB5 steel sheets. The increase in hardness appeared only for rapid heating, and not for slow heating using a furnace. The increase in hardness is due to the transformation of retained austenite into martensite at comparatively low temperatures using rapid resistance heating and cooling. For the comparative low heating temperature, the non-coated sheet hardly oxidised, and the galvannealed sheet having high corrosion resistance did not exhibit the exfoliation of the coating layer, whereas the springback did not improve. [ABSTRACT FROM AUTHOR]

Due to their excellent mechanical, electrical and thermal properties and ease of production, metallic bipolar plates are a suitable replacement for graphite and composite plates. Stamping is one of the most applicable processes to produce theses plates from a manufacturing cost point of view. Due to its excellent corrosion resistance and low density, titanium rises as a potential option for the manufacturing of the bipolar plates. In this paper, the formability of titanium bipolar plates having a thickness of 0.1mm with a parallel flow field has been experimentally investigated. The formability of the sheet was evaluated at warm temperatures using different forming speed and lubricants. After the experimental implementation of the designed tests based on the Taguchi method, the fracture depth of the microchannel of stamped samples was extracted. The results showed that the most elongation of the sheet will be achieved at 100℃. Likewise, the forming speed and temperature are the most effective parameters on the forming depth, respectively. On the other hand, the effect of the lubricant is not tangible compared to the other mentioned parameters. The maximum forming depth equal to 0.494mm was obtained using an experiment with a forming temperature of 100℃, speed of 4.8mm/min, and lubrication with MoS2. [ABSTRACT FROM AUTHOR]

A novel application of quenching and partitioning (Q&P) treatment to warm stamping of a warm-rolled medium Mn steel was investigated. The results show that Q&P could improve yield strength of auto-parts from the formation of carbides and twinned martensite, and reduce the yield point elongation for carbon partitioning during Q&P process and a higher dislocation density. Regardless of stamping temperature either the single austenite or dual-phase region, an extraordinary product of strength and ductility (≥24 GPa·%) was achieved, which is two or three times higher than those of hot-stamped boron steels. The combined warm rolling and warm stamping process with Q&P treatment may have potential to implement the application of medium Mn steels for ultrahigh strength-ductile auto-parts. [ABSTRACT FROM AUTHOR]

Highlights • IHTC of sheet AA5083 in warm forming process is calculated. • IHTC during transportation is 25–28 W/(m2⋅K), during stamping is 600–1300 W/(m2⋅K). • IHTC is a compound power function of contact pressure and initial temperature. • Lubricant has heat insulation effect beneficial to forming and cooling. • Influence of one-side contact on IHTC in die relief areas should be considered. Abstract The interfacial heat transfer coefficient (IHTC) between die and blank is an important boundary condition needed for numerical simulation of warm stamping of sheet aluminum alloys. The heat transfer behaviors of sheet aluminum alloy AA5083 are investigated through cylindrical-die stamping experiments. The methods for calculating IHTC such as the heat balance method and Beck's nonlinear estimation method are introduced. The IHTC during blank transportation and stamping-cooling stages are calculated and the influencing factors are analyzed. Both contact pressure and initial temperature positively influence the IHTC and the relationship can be expressed by a compound power function. Lubricant is found to have heat insulation effect beneficial to stamping and cooling in addition to its main function of lubrication. Finally, the heat transfer behaviors under one side contact the other side free and one side contact the other side insulated situations are investigated. [ABSTRACT FROM AUTHOR]

Improvement of the hot stamping process is important for reducing processing costs and improving the productivity and tensile properties of final components. One major approach to this has been to conduct all or part of the process at lower temperatures. The present paper reviews the state of the art of hot stamping techniques and their applications, considering the following aspects: (1) conventional hot stamping and its advanced developments; (2) warm stamping approaches in which complete austenitisation is not attained during heating; (3) hot stamping with a lower forming temperature, i.e., low-temperature hot stamping (LTHS); (4) advanced medium-Mn steels with lower austenitisation temperatures and their applicability in LTHS. Prospects for the further development of LTHS technology and the work required to achieve this are discussed.

A kind of hot stamping technology with lower heating temperature, as named as warm stamping, is proposed based on the medium manganese steel. The differences on the microstructure and mechanical properties among 22MnB5 steel, Al–Si coated 22MnB5 steel, and medium manganese steel are studied before and after hot/warm stamping process based on industrial trail. The results show that the tensile strength of 1500 MPa is obtained for medium manganese steel when the austenitizing temperature is only 800 °C, much lower than 950 °C for 22MnB5 steel. Finer microstructure of medium manganese steel is obtained by warm stamping and there is no surface decarburization under atmosphere environment. The total elongation of warm‐stamped medium manganese steel reaches 10%, which is higher than that of 7.5% for 22MnB5 steel. The excellent hardenability makes warm‐stamped medium manganese steel more suitable for the production of large‐size components with martensitic microstructure. The results of spot welding test between medium manganese steel and H340LAD + Z show that the welding performance meets the requirement of automobile plant. [ABSTRACT FROM AUTHOR]

A combined low-temperature rolling and warm stamping process was proposed for a Fe-5.6Mn-0.19C-1.2Al-0.05Nb-0.22Mo alloy to improve the tensile ductility and surface quality of the stamped parts. The results show that the present warm-stamped U-shaped parts exhibited the relatively uniform ultrafine multiphase microstructure and desirable mechanical properties, e.g. the average products of ultimate tensile strength and total elongation (PSE) values reached 27.5–31.3 GPa·%, which are more than double or triple times higher than those of the hot-stamped boron steels with similar strength level. As the stamping temperature decreased from 760 to 700 °C, both the fractions of ferrite and austenite increased, resulting in the enhanced tensile ductility (up to 24.5%) with sacrificing tensile strength from 1717 ± 117 to 1424 ± 53 MPa. The difference in stamping temperature greatly affected the tensile deformation behavior of the stamped parts: at 700 °C, the tensile curves exhibited Lüders band propagation (LBP) behavior, and both Portevin-Le Chatelier (PLC) and transformation induced plasticity (TRIP) effects happened, while at 760 °C, the tensile curves exhibited continuous yielding behavior, and only TRIP effect happened over the entire plastic deformation. [ABSTRACT FROM AUTHOR]

In this paper, the springback of the aluminium alloy AA5754 under hot stamping conditions was characterised under stretch and pure bending conditions. It was found that elevated temperature stamping was beneficial for springback reduction, particularly when using hot dies. Using cold dies, the flange springback angle decreased by 9.7 % when the blank temperature was increased from 20 to 450 °C, compared to the 44.1 % springback reduction when hot dies were used. Various other forming conditions were also tested, the results of which were used to verify finite element (FE) simulations of the processes in order to consolidate the knowledge of springback. By analysing the tangential stress distributions along the formed part in the FE models, it was found that the springback angle is a linear function of the average through-thickness stress gradient, regardless of the forming conditions used. [ABSTRACT FROM AUTHOR]

The application of high strength steels (HSS) for automotive structural parts is an effective way to realize lightweight and enhance safety. Therefore, improvements in mechanical properties of HSS are needed. In the present study, the warm stamping process of the third generation automotive medium-Mn steel was discussed, the characteristics of martensitic transformation were investigated, as well as the microstructure and mechanical properties were analyzed, compared to the popular hot-stamped 22MnB5 steel in the automotive industry. The results are indicated as follows. Firstly, the quenching rate of the medium-Mn steel can be selected in a wide range based on its CCT curves, which is beneficial to the control of forming process. Secondly, the influence of stamping temperature and pressure on the M s temperature of the medium-Mn steel is not obvious and can be neglected, which is favorable to the even distribution of martensitic microstructure and mechanical properties. Thirdly, the phenomenon of decarbonization is hardly found on the surface of the warm-stamped medium-Mn steel, and the ultra-fine-grained microstructure is found inside the medium-Mn steel after warm stamping. Besides, the warm-stamped medium-Mn steel holds the better comprehensive properties, such as a lower yield ratio, higher total elongation and higher tear toughness than the hot-stamped 22MnB5 steel. Furthermore, an actual warm-stamped B-pillar of medium-Mn steel is stamped and ultra-fine-grained martensitic microstructure is obtained. The mechanical properties are evenly distributed. As a result, this paper proves that the warm-stamped medium-Mn steel part can meet the requirements of lightweight and crash safety, and is promising for the industrial production of automotive structural parts. [ABSTRACT FROM AUTHOR]

In this study, a combined experimental-numerical investigation on the failure of AZ31 Mg alloy sheet in the warm stamping process was carried out based on modified GTN damage model which integrated Yld2000 anisotropic yield criterion. The constitutive equations of material were implemented into a VUMAT subroutine for solver ABAQUS/Explicit and applied to the formability analysis of mobile phone shell. The morphology near the crack area was observed using SEM, and the anisotropic damage evolution at various temperatures was simulated. The distributions of plastic strain, damage evolution, thickness, and fracture initiation obtained from FE simulation were analyzed. The corresponding forming limit diagrams were worked out, and the comparison with the experimental data showed a good agreement. [ABSTRACT FROM AUTHOR]

The medium-Mn steel is a promising third-generation automobile steel. Its chemical composition, microstructure, and thermal and mechanical properties are introduced and a warm-stamping process for the medium-Mn steel is proposed. The optimal process parameters are identified through the design of experiments (DOE) and range analysis. The evaluated experimental indexes include tensile strength, elongation, and hardness. The optimal forming process consists of an austenitization temperature of 840° C, a soaking time of 4 min, and an initial stamping temperature of 500°C. The proposed process was applied to the warm stamping of an automotive B-pillar. The microstructure of ultrafine, uniform, and complete martensite laths was obtained. The formed part exhibits approximately 1420MPa tensile strength, over 11% elongation and 460 HV hardness. The optimal warm-stamping process has proved effective and applicable for forming medium-Mn steel parts. It will help promote the application of the third-generation automotive steels. [ABSTRACT FROM AUTHOR]

The medium-Mn steel with M 3 characteristics (multi-phase, multi-scale, meta-stable) is a promising third-generation automotive steel. Its chemical composition, microstructure, thermal and mechanical properties are introduced and a warm stamping process for the medium-Mn steel is proposed. The optimal process parameters are identified through mechanical testing and microscopic analysis to achieve balanced properties of hardenability, hardness, strength, elongation and fracture behavior. The optimal forming process consists of an austenitization temperature of 790–840 °C, a soaking time of 4–7 min, an initial stamping temperature of 450–500 °C, and a cooling rate of 10–60 °C/s. A typical automotive structural part B-pillar was stamped using the proposed process and the final part exhibits ultrafine martensite-lath microstructure and desired mechanical properties for intrusion prevention and energy absorption. [ABSTRACT FROM AUTHOR]

Based on the frame work of continuum damage mechanics, a research work of anisotropic damage evolution in warm stamping process of magnesium alloy sheets has been carried out by means of a combined experimental-numerical method. The aim was to predict formability of warm stamping of AZ31 Mg alloy sheets by taking the thermal and damage effects into account. In the presented work, a temperature-dependent anisotropic yield function suitable for cold rolling sheet metals together with an anisotropic damage model was implemented into the a VUMAT subroutine for ABAQUS/EXPLICIT. The evolution of internal damage in the form of void growth and coalescence in AZ31 Mg alloy sheet was observed by means of scanning electron microscopy (SEM). Moreover, a coupled thermo-mechanical simulation of the stamping process was performed using the implemented code at different temperatures. The parameters employed in the simulation were determined by the standard tensile tests and algebraic manipulation. The overall anisotropic damage process from crack initiation to final propagation in local area of blank was simulated. Numerical results show that the prediction of the site of crack initiation and the orientation of crack propagation are consistent with the data observed in warm stamping experiments. [ABSTRACT FROM AUTHOR]

Improvement of the hot stamping process is important for reducing processing costs and improving the productivity and tensile properties of final components. One major approach to this has been to conduct all or part of the process at lower temperatures. The present paper reviews the state of the art of hot stamping techniques and their applications, considering the following aspects: (1) conventional hot stamping and its advanced developments; (2) warm stamping approaches in which complete austenitisation is not attained during heating; (3) hot stamping with a lower forming temperature, i.e., low-temperature hot stamping (LTHS); (4) advanced medium-Mn steels with lower austenitisation temperatures and their applicability in LTHS. Prospects for the further development of LTHS technology and the work required to achieve this are discussed. [ABSTRACT FROM AUTHOR]

In the present study, tensile and springback properties of medium (3–7 wt%) Mn steels for warm stamping were investigated as functions of Mn content, austenitizing temperature (T aus), V-bending temperature (T b) and time (t b). For comparison, the tensile and springback properties of 22MnB5 steel used for hot stamping were also evaluated. Whereas medium-Mn steels were air-cooled to room temperature after austenitizing at various T aus , the 22MnB5 steel was water-quenched. The 5Mn steel exhibited the highest tensile properties due to the martensitic matrix with inactive auto-tempering and negligible amount of retained austenite. The springback of medium-Mn specimens was increased by the addition of Mn at a given T b when t b was 0 s. At the T b of 300–500 °C, the fraction of martensitic transformation was raised by increasing the Mn content, resulting in high springback. At the T b of 600 °C, the rise in transformation strain by the addition of Mn raised springback. When T b was fixed to be 600 °C, the springback of medium-Mn specimens was increased by the addition of Mn due to the increases in transformation strain at the t b of 0 s and the fraction of martensitic transformation during air cooling at the t b of 2 s and over. • 5Mn steel revealed superior tensile properties to 3Mn, 7Mn and 22MnB5 steels. • Active auto-tempering and retained austenite undermined tensile properties. • Springback of medium-Mn steels was increased by the addition of Mn. • Springback depended on the fraction of martensite formed during air cooling. • Drop in bending temperature and prolongation of holding time reduced springback. [ABSTRACT FROM AUTHOR]