Conventional Stamping Process Research Articles

Enhanced Yield Strength and Total Elongation of an Ultrahigh Strength Hot‐Stamped Steel via Tempering Treatment

A 2460 MPa ultimate tensile strength (UTS) has been achieved in this experimental steel, which was processed by a conventional hot stamping process with water quenching to room temperature. Additionally, comparing the microstructure evolution and strengthening mechanism before and after tempering, the microstructure has changed from fresh martensite transformed into tempered martensite with acicular carbides, and the fracture mode has also changed from brittle fracture transformed into ductile fracture mainly. Meanwhile, the main strengthening mechanism has changed from dislocation strengthening to precipitation and dislocation strengthening. Overall, the tempering process reduces dislocation density and produces acicular carbides, which decrease UTS. However, the tempering process reduces the quenching residual stress and changes the morphology of twin martensite, resulting in an increase in ductility. Meanwhile, it enhances the effect of carbide pinning dislocations, significantly enhances precipitation strengthening, and improves yield strength.

Numerical Modelling for Efficient Analysis of Large Size Multi-Stage Incremental Sheet Forming

Incremental sheet forming (ISF) is an advanced flexible manufacturing process to produce complex 3D products. Unlike the conventional stamping process, ISF does not require any high cost dedicated dies. However, numerical computation for large-size ISF processes is time-consuming, and its accuracy for spring back due to unclamping tools after ISF cannot satisfy industrial demand. In this paper, an advanced numerical model considering complicated forming tool paths, trimming, and spring back was developed to efficiently simulate the multi-stage deformation phenomena of incremental sheet forming processes. Numerical modeling accuracy and efficiency are investigated considering the influence of tool path, material properties of the blank, mesh size, and boundary conditions. Through a series of case studies and comparisons with experimental results, it is observed that the numerical model with kinematics material properties and a moderate element size (5 mm) may reproduce the deformation characteristics of ISF with good accuracy and can obtain practical efficiency for a large-size ISF part.

Effect of evolutionary anisotropic hardening on the prediction of deformation and forming load in incremental sheet forming simulation

The incremental sheet forming (ISF) process typically accompanies large plastic strain without fracture exceeding uniform elongation or ultimate tensile strength (UTS) in the uniaxial tensile test. Therefore, ISF features improved formability compared to the conventional stamping process. Meanwhile, numerous studies have focused on modeling and simulation of formability in ISF where anisotropic yield function is one of the key constitutive laws for predicting sheet deformation. However, the conventional yield function is defined based on anisotropy of initial yielding, while the ISF modeling requires the deformation behavior at large strains beyond the UTS. In this study, an evolutionary anisotropic plasticity model is investigated based on Hill's 48 yield function combined with the non-associated flow rule; i.e., e-NAFR Hill's 48. The e-NAFR Hill's 48 model is implemented to the vectorized user-defined material subroutine in the commercial finite element software ABAQUS/Explicit. Then, the proposed FE model is applied to the forming of truncated cone, pyramid, and clover shaped single point incremental forming (SPIF) of an aluminum alloy 6014-T4 sheet. The simulated part profile, thickness variation, and forming force are compared with those of experiments. The evolutionary constitutive model shows an average accuracy of 99.15% in thickness prediction and 94.22% accuracy in forming force, demonstrating the importance of evolutionary sheet anisotropy in the SPIF process.

Manufacturing energy and environmental evaluation of metallic sheets reshaping based on theoretical models and cradle-to-gate assessment

Origami sheet metal folding (OSM) is a cutting-edge method for forming a 3D sheet metal product employing a series of folding procedures from a 2-D flat metal sheet rather than using the conventional stamping process. OSM eliminates the need for presses and considerable energy usage during manufacturing by solely using a sequence of bends to shape the necessary geometry. OSM has the potential to supplant the current stamping method and be a disruptive fabrication process for metal manufacturing. Despite OSM's potential, there is not enough analysis to assess the OSM approach in terms of its energy efficiency and emission. Therefore, the added value of this study is to highlight the application of OSM in sheet metal folding and present the key distinctions between this method and stamping in the various production phases, including primary production, processing, manufacturing, and assembly. In order to assess the applicability of this approach and its merits compared to the conventional methods, an extensive analysis of its environmental impacts (energy consumption and CO2 emission) using theoretical models and the Life Cycle Analysis (LCA) was also carried out. A case study was presented for analyzing a floor panel of the vehicle with the derived theoretical model and LCA inventory data. The theoretical model for manufacturing energy of OSM was adopted from mathematical models for the CO2 laser cutting process to estimate the energy for laser cutting. Alternatively, the model considered a mathematical approximation developed for coated sheet steel to estimate the total energy consumed for OSM. Finite element analysis (FEA) was also used to generate a more precise energy estimate using a vehicle floor panel as a case study. The highlighted environmental impacts of OSM, including energy consumption and carbon emission, were finally compared to stamping. The results of the investigation revealed that the primary distinction between OSM and stamping methods is at the manufacturing phase. The use of the OSM approach significantly reduced energy consumption and carbon dioxide emissions during the production of metal-based components by ∼94% and ∼92%, respectively. It can be concluded that OSM saves significant energy and produces fewer emissions than the stamping process and can be a potential candidate for sheet metal folding.

Investigation on Mechanical Properties and Oxidation Behavior of 1.2 and 1.7 GPa Grades Coating-Free Press-Hardened Steels

Al-Si-coated boron-alloyed steels are the most widely used press-hardened steels (PHSs), which offers good oxidation resistance during hot forming due to the presence of the near eutectic Al-Si coating. In this study, a recently developed novel un-coated oxidation resistant PHS, called coating-free PHS (CF-PHS), is introduced as an alternative to the commercial Al-Si coated PHSs. With tailored additions of Cr, Mn, and Si, the new steel demonstrates superior oxidation resistance with a sub-micron oxide layer after the conventional hot stamping process. Hence, it does not require shot blasting before the subsequent welding and E-coating process. Two CF-PHS grades have been developed with ultimate tensile strengths of approximately 1.2 and 1.7 GPa, respectively. Both grades have a total elongation of 8–9%, exceeding the corresponding Al-Si-coated PHS grades (1.0 GPa/6–7%, 1.5 GPa/6–7%). Furthermore, the bendability of CF-PHS was similar to the corresponding Al-Si PHS grades. On the other hand, performance evaluations relevant to automotive applications, such as weldability, the E-coat adhesion, and tailor-welded hot stamp door ring, were also conducted on the CF-PHS steel to satisfy the requirements of manufacturing.

Investigation of deformation mechanics and forming limit of thin-walled metallic bipolar plates

The present study is conducted on forming of the metallic bipolar plates made of 316 stainless steel sheet with a parallel serpentine flow field. The plastic deformation of straight and curved microchannels, forming limit criteria, and deformation mechanics during the process are investigated partially to present a reliable model for estimating fracture initiation. For this purpose, experimental stamping tests are employed to fabricate metallic bipolar plates and the process is simulated by finite element software. The validity of simulation results is examined by comparing thickness distribution and force-displacement curves reflecting 4.76% and 3.85% error rates, respectively. According to experimental observations, fracture starts at a channel depth of 0.610 mm. Hence, for determining the forming limit and predicting the fracture during the process, the deformation mechanic is studied at different points of the microchannels. Results of stress states analysis indicate that the stress state of plane-strain tension up to biaxial tension governs this process. Despite the presence of different loading paths during the process, the critical element in each channel is deformed under plane-strain tension. Therefore, a fracture model is developed based on thinning percentage and equivalent strain to predict the instability of metallic bipolar plates. According to the results, both the equivalent strain and thinning percentage criteria with critical limits of 0.56 and 33.45%, respectively, are considered as an allowable range of plastic deformation during the conventional stamping process of bipolar plates. Results indicate that maximum thinning in all directions is lower than 33.45% by using the modified stamping process.

Improve bendability of a Cr-alloyed press-hardening steel through an in-line quenching and non-isothermal partitioning process

A new hot stamping process consisting of an in-line quenching and simultaneous non-isothermal partitioning was applied to improve the bendability of a Cr-alloyed press-hardening steel (Fe-0.23C-1.1Mn-(Cr + Si) < 5.0, wt%). The quenching temperature was controlled by the dwell time during die quench, and the non-isothermal partitioning was achieved through the subsequent air cooling after die opening. The feasibility of carbon partitioning during the air cooling stage was verified by kinetics calculation. Compared with the conventional hot stamping (HS) sample, around 6 vol% nano-sized metastable austenite was obtained in the sample processed by hot stamping with the in-line quenching and non-isothermal partitioning process (HS&QP). As a result, the average bending angle of the HS&QP samples increased by 10 % from 51.2° to 56.2°, and the total absorbed energy increased by 11 % from 28.7 J to 31.9 J compared with conventional HS samples, respectively. Meanwhile, the average ultimate tensile strength (1590 MPa) and total elongation (7.6 %) of the HS&QP samples are still better than that of the commercial-grade AlSi-coated 22MnB5. The proposed in-line quenching and non-isothermal partitioning process can significantly improve the bending properties of the newly developed Cr-alloyed PHS, increase press utilization, while balancing the requirements for strength, ductility and dimensional accuracy. It is a technically promising alternative to the conventional hot stamping process with economic value suitable for the newly developed Cr-alloyed PHS.

Toward better metal flow control in electrohydraulic sheet forming by combining with electromagnetic approach

Electrohydraulic sheet forming is a high-velocity manufacturing process, which utilizes a pressure pulse induced by underwater electrical discharge to plastically shape metals. It has attracted wide interest for its potential to shape metal materials with poor formability. Such advantages can be exploited by combining this process with the conventional stamping process or by using it solely. In both cases, the dominant deformation mode is stretching, implying substantial thinning of sheet metal. The goal of this paper is to improve the material flow control in electrohydraulic sheet forming. A controllable drawing deformation mode is introduced into the electrohydraulic sheet forming process by using a radial inward pulsed Lorentz force at sheet edge induced by an electromagnetic coil, which can substantially enhance the draw-in of the sheet flange. The flexible combination of the stretching deformation and the introduced drawing deformation allows active control of the high-velocity material flow behavior, thus enabling much better control of the final forming quality. About 20 % improvement of the forming height limit is observed from the experimental results. Furthermore, a numerical model is established to better understand the process mechanism, and the critical roles of the amplitude and the action timing of the radial inward Lorentz force are identified.

Microstructure and mechanical properties evaluation of friction stir welded boron steel

The microstructure and mechanical properties of friction stir welded boron steel in butt joint configuration are experimentally studied. Two different friction stir welding (FSW) parameter combinations are used to successfully fabricate butt joints. Microstructural analysis exhibites that the stir zone (SZ) primarily consists of fine lath martensite, while the thermo-mechanically affected zone (TMAZ) comprises bainitic ferrite and granular bainite with a small amount of martensite. The presence of granular bainite in TMAZ suggests that alloying composition affects the phase transformation. The formation of recrystallized structures with lath martensites and high dislocation density in the SZ significantly enhance the hardness of the joints compared to that of the base metal. The results of the present study suggest that FSW can be used as a method for local hardening of structural components made of boron steels, without complicated heating and rapid cooling of a conventional hot stamping process.

Experimental and Numerical Investigation of Residual Stresses in Incremental Forming

Double Sided Incremental Forming (DSIF) is gaining importance over Single Point Incremental Forming (SPIF) due to its ability to form complex geometries and the capability to obtain better accuracies. In the present work, residual stresses are measured in pyramidal components formed using SPIF, DSIF using X-ray diffraction technique. Residual stress development mechanism during SPIF and DSIF is studied using Finite Element Analysis (FEA). Stress development along circumferential and meridional directions are explained using bending and unbending of sheet material taking place around forming tool. It is observed that the residual stresses are compressive on the outer surface and tensile on the inner surface of sheet in both circumferential and meridional directions. In DSIF, supporting tool restricts the unbending of sheet causing the residual stresses to be less compressive on the outer surface and less tensile on the inner surface compared to SPIF. It is also observed that with an increase in tool diameter, spring back increased, hence, meridional residual stress on the outer surface became more compressive and circumferential residual stress on the inner surface became more tensile. Residual stresses in ISF are compared with FEA predictions of conventional stamping process.

The effect of process parameters on forming forces in single point incremental forming

Abstract Recently, single point incremental forming has caught the attention of automotive and aerospace industry as an alternative to conventional stamping process as an economical process capable of manufacturing sheet metal prototypes devoid of expensive dies. The first objective of the study was to ascertain the nature of cutting forces expected during the single point incremental forming process. The second objective is to study the effect of different process parameters on these forming forces. Detailed experiments were conducted based on the Taguchi’s robust design approach. Forming forces were measured for different working conditions (sheet thickness, wall angle, tool diameter, and step down). Based on the experimental results, several conclusions were made on the effect of process parameters on the forming forces. Analysis of variance (ANOVA) was used to identify the most significant control factors and their interactions. From the experimental findings, an attempt was also made to find the optimal combination of the process parameters on the basis of a proposed predictive mathematical model. Finally, the study proposed guidelines for forming thick sheets and improving production rate of SPIF process.

Comparative Study of Conventional and Quick Die Change Stamping Process: The Issue of Setup Time and Storage

Metal stamping industry has Quick Die Change (QDC) as a form of Single Minutes Exchange of Dies (SMED) as an efficient production technique where the implementation depends on operator’s activities, clamping system, accessories type and position, etc. This QDC could apply to improve the process efficiency, control of inventory and reducing the cost. This research compared the traditional metal stamping process and QDC System (QDCS) of metal stamping. This research was using Focused Group Discussion (FGD), direct observation and experiments, conducted in a private metal stamping company in wider Jakarta region of Indonesia. It was observed that the QDCS significantly reduces setup time, storage space and the cost. The setup time and cost reduced to one third of the conventional and the requirement of storage decreased by 70%. In addition, QDCS reduces the waste significantly.

Electromagnetic-assisted calibration for surface part of aluminum alloy with a dedicated uniform pressure coil

Springback is an inevitable defect of aluminum alloy sheet after conventional stamping process, which directly has an influence on the forming accuracy of part. For a surface part of aluminum alloy, the stamping process was used to complete the preforming process, and then the electromagnetic forming was adopted to calibrate the shape and improve the forming accuracy. A dedicated uniform pressure coil was proposed to quasi-uniformly impose the impulse magnetic pressure on the whole part. The effect of discharge voltage on the springback phenomenon was experimentally and numerically investigated. It was found that the springback defect of surface part was significantly reduced with the discharge voltage increasing. The forming accuracy of surface part was remarkably improved under the discharge voltage of 5 kV. It can be attributed to two aspects: on the one hand, the effective plastic strain level at the bottom of surface part was greatly elevated, and on the other hand, the gap between the tensile stress on the exterior surface and the compressive stress on the interior surface was well decreased.

CAE-based process design for improving formability in hot stamping with partial cooling

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.

Dynamic compression response of self-reinforced polypropylene composite structures fabricated through ex-situ consolidation process

In case of Self-reinforced polymer composites (SRCs) the fibres and matrix belong to the same family of polymers that can ensure 100% recyclability as compared to traditional fibre reinforced epoxy matrix based composites. Most of the SRCs were studied only on material bases not structural bases, although some structures were fabricated but most of their fabrication methods can hardly be applied for mass production. We fabricated self-reinforced polypropylene (SrPP) structures by ex-situ consolidation process that can be potentially applied for mass production. Instead of conventional stamping process that involves complex formability phenomenon; a mould was designed to fabricate corrugated core from the pre-consolidated SrPP sheets and then face sheets were joined by same family of polymer adhesives to achieve maximum recyclability. Optimal bending temperature was investigated to maintain proper fibre matrix contents of corrugated SrPP sheets. Interface between core and face sheet (FS) was enhanced through phenomenon of mechanical lock by creating various abrasion levels. Dynamic strengthening effect and collapse behaviour of structures during out of plane dynamic compression tests was investigated and compared with theoretical predictions.

Modelling and demonstration of electromagnetically assisted stamping system using an interactive mapping method

Electromagnetically assisted stamping (EMAS) process is an innovative hybrid sheet metal forming technique that combines electromagnetic forming (EMF) into quasi-static conventional stamping. This paper presents a numerical modelling based on a novel triangular prism solid and shell interactive mapping element for EMAS process. In the present work, the triangular prism solid and shell interactive mapping element builds a bridge between conventional stamping and electromagnetic forming (EMF) process, in which shell element is employed in conventional stamping process. A linear six-node prism element is developed for transient eddy current analysis in electromagnetic field of EMF process. An interactive mapping rule is established to transfer information between different type elements, accordingly. That is to say, one can take full advantages of shell element solving stamping problems with contact condition in EMAS process. Electromagnetically assisted stamping example is used for validation of the present modelling. The proposed numerical modelling can successfully simulate the combined quasi-static-dynamic process, and the numerical results predict the experimental ones. The present modelling produces potential for further study in EMAS process.

Deformation Type in Forming of Horn Tubes

The purpose of this research is the development of technology to make complex-shape closed-section parts directly from sheet blanks (direct sheet forming). It is expected to form closed-section parts with large expansion of the circumferential length by direct sheet forming. In this paper, the deformation type of horn tubes, which consists of circular, conical and transient portions, is studied. The horn tube is one of the typical shapes for automotive parts. With reference to deformation types in the conventional stamping process, those in the forming process of horn tubes are discussed on the basis of the results of FEM analysis. The validity of FEM analysis is confirmed by comparison with experimental results. It is clarified that the forming process can be broken down into several stages and deformation types (uniform bending of sheet, stretch flanging, axial bending of U-section, deformation into double curved surface and plane-strain compression).

Hydroformability study of seamless tube using Gurson-Tvergaard-Needleman (GTN) fracture model

Tube hydroforming process is an advanced manufacturing process in which tube acting as blank is placed in between the dies and deformed with the help of hydraulic pressure. It has several advantages over conventional stamping process such as high strength to weight ratio, higher reliability, less tooling cost etc. Fracture surface investigation of tube hydroformed samples reveal dimple formation in the form of void coalescence which is a characteristic feature of ductile fracture. Hence, in order to accurately predict the limiting strains at fracture it is important to model the process using ductile damage criteria. Fracture criteria are broadly classified into two, microscopic and macroscopic. In the present work Gurson-Tvergaard-Neeedleman (GTN) model, which is a microscopic based ductile damage criteria, was used for predicting the limiting strains at fracture for seamless steel tubes and implemented in explicit finite element software, ABAQUS, for variety of strain path and boundary conditions to obtain fracture based forming limit diagram. The original void porosity, the critical porosity and fracture porosity of the Gurson-Tvergaard-Needleman model were determined by image analysis of scanning electron micrographs of the specimen at different testing conditions of the uniaxial tensile test. The other parameters of the model were determined by using inverse approach combined with uniaxial tensile test and simulation. Predicted FLD is found to be in good agreement with the experimental FLD. Furthermore, numerical simulation based parametric study was carried out to understand the impact of various GTN parameters on different aspects of formability parameters such as bursting pressure, bulge height, principal strains and strain path to develop the understanding of deformation and fracture behaviour at the micro-level during tube hydroforming process.

A Comparative Study of Failure with Incremental Forming

Incremental forming (ISF) is an innovative flexible sheet metal forming process which can be used to manufacture complex shapes from various materials. Due to its flexibility, it has attracted more and more attention over recent decades. Localized deformation and shear through the thickness are essential characteristics of ISF. These lead to specific failure modes and formability of ISF that are different from the conventional stamping process. In this contribution, three continuum damage models (Lemaitre, Gurson, extended GTN models) are formulated and fully coupled with the finite element simulation in a commercial software ABAQUS to predict failure in incremental forming. A comparative investigation of these three damage models has been carried out to analyze both the deformation behavior and failure mechanisms.

ANALISIS TEÓRICO-NUMÉRICO-EXPERIMENTAL DE UN PROCESO DE HIDROCONFORMADO USANDO DIÁMETRO EQUIVALENTE

The main goal of this work is to propose the application of the concept of equivalent diameter in the calculation of the hydraulic pressure required to conduct an hydroforming process in non-circulars pieces. Although this approach was carried out initially for the conventional stamping process, this paper demonstrates that it can also be applied in hydroforming. In a first phase experimental tests were carried out to form a part using this methodology, measuring the reduction in resulting thickness. It was found that difference between the calculated pressure and the experimental required pressure was 2.6%. To perform a numerical-experimental comparison, the simulation of the process was carried out using several material models by explicit finite element method in a commercial software. It was observed that using the material model HILL’90 deviation between experimental and numerical results for the thickness reduction was lower than 5.5%.