Conventional Stamping Research Articles

Influence of Thermal History on Microstructure and Mechanical Properties of Steels for Hot Stamping

Hot stamping is an attractive method to produce extra high strength automotive components. In the conventional hot stamping, the furnace heating is employed and the heating rate is quite low. To improve the productivity of the hot stamping technology, the reduction of time for the heating process is required. In this study, the influence of the heating rate in a range up to 200°C/s, heating temperatures between 650°C and 950°C and cooling condition on microstructure and mechanical properties of 0.22% C -3%Mn steel has been investigated. The steel is a promising material for the highly productive new hot stamping technology because this steel transformed into martensite from austenite even at cooling in free air. The specimens heat-treated at a high heating rate and for short holding time at the heating temperature just above Ac3 show significantly fine martensite microstructure and a good strength-toughness balance. In this paper, the α→ γ transformation behavior and the γ→ α transformation behavior after inter-critical annealing are discussed to explain the evolution of the microstructures and mechanical properties.

Numerical Simulation Research on a New Spinning Process: Double-Roller Clamping Spinning Process

Thin-wall rotary parts with complex flange geometry have been widely used in many equipments, especially industrial fans and blowers. Conventional stamping and welding methods for those parts have some limitations such as low manufacturability, poor quality and low utilization ratio of material. Thus in this paper, a new spinning process, double-roller clamping spinning process (DCSP), is introduced to form the complex flange shape. During the process, the cylindrical workpiece made from sheet metal is held by the internal expanding clamp device and rotates together with the device, while two spinning rollers deforms and shapes the material of flange region according to the rollers moving paths. In order to deeply investigate deformation mechanisms of DCSP, both 2D and 3D FE simulation models are built and the spinning process for rectangular cross-section shape of flange is simulated by using explicit FE code ABAQUS/explicit. Simulation results show large flange length can induce wrinkles on the inner region of flange. After spinning process simulation, the springback of workpiece is calculated by implicit FE code ABAQUS/standard. The effect of flange length on the thickness reduction of workpiece is obtained by theoretical analysis.

플랜지 형성 액압성형시 공정변수에 따른 성형 특성

Hydroforming is the technology that utilizes hydraulic pressure to form tube or sheet materials into desired shapes inside die cavities. Tube hydroforming provides a number of advantages over the conventional stamping process, including fewer secondary operations, weight reduction, assembly simplification, adaptability to forming of complex structural components and improved structural strength. In many case, hydroformed parts have to be structurally joined at some point. Therefore it is useful if the hydroformed automotive parts can be given a localized attachment flange. In this study for the numerical process design FE analysis was performed with DYNAFORM 5.5. Die parting angle and circumferential expansion ratio was optimized. With optimized condition, bulge and hydroforming experiments to form flange were performed. Forming characteristic at various pressure conditions was analyzed and optimized internal pressure condition was evaluated. The results show that flanged parts can be successfully produced by tube hydroforming process.

Electromagnetically assisted sheet metal stamping

A new approach, electromagnetically assisted sheet metal stamping, has been developed to alter strain distribution and improve formability in sheet metal stamping. In this study, this new approach was applied to form a non-symmetric panel from Al 6111-T4. The results show that this new approach greatly increased the draw depth of the formed panel, compared with conventional stamping. A detailed analysis of strain distribution, stretching and draw-in shows that both a more homogeneous strain distribution and enhanced draw-in contribute to producing deeper pans in a single press operation. This work demonstrates the feasibility of electromagnetically assisted sheet metal stamping, which offers a much improved ability to make complex components in a single press stroke as compared to conventional stamping.

Two-step method of forming complex shapes from sheet metal

A two-step method of forming a part and a method of designing a preform shape are being discussed. The part may be formed from lightweight material to an extent that would normally exceed the forming limits of the material if the part were attempted to be formed in one-step conventional stamping die. Critical areas including deep pockets and sharp radius areas of the final part are formed from a preform or intermediate shape part. The first forming step can be conducted by variety of sheet metal forming methods; the preformed blank is further formed in a fluid pressure forming process to a final part shape wherein broad radius areas and pockets of accumulated metal of the preform are formed into deep pockets and sharp corners of the final shape. Electrohydraulic forming technology is employed for the second forming step.

Evaluation of the effects of a movable tool in the hydroforming process

In this paper, the production of stainless steel hemi-toroidal parts has been investigated using three different processes: the conventional stamping, the standard sheet hydroforming and the hydroforming adopting a movable die (MD) which can have a vertical stroke and whose shape completes the female die cavity. Tests adopting the MD were performed at Aalborg University using a specific experimental equipment; numerical simulations were performed using ABAQUS\Explicit and were aimed at investigating the effect of the friction condition of the MD tool on the blank deformation. The friction coefficient was varied on both the whole surface of the MD tool and locally (in the region of the MD corner radius). The role of the MD in the hydroforming process was thus studied by comparing results in terms of thickness and strain distributions on the final part obtained adopting the same geometry of tools, but different forming techniques. Simulations put in evidence that a moderate increase of the friction can be successfully used for reducing thinning if the MD is adopted; in addition, the friction in the region of the MD corner radius shows to play a key role in forming sound parts.

Numerical modeling and deformation analysis for electromagnetically assisted deep drawing of AA5052 sheet

Electromagnetic forming(EMF) is a high-velocity manufacturing technique which uses electromagnetic (Lorentz) body forces to shape sheet metal parts. One of the several advantages of EMF is the considerable ductility increase observed in several metals, with aluminum featuring prominently among them. Electromagnetically assisted sheet metal stamping(EMAS) is an innovative hybrid sheet metal processing technique that combines EMF into traditional stamping. To evaluate the efficiency of this technique, an experimental scheme of EMAS was established according to the conventional stamping of cylindrical parts from aluminum and the formability encountered was discussed. Furthermore, a “multi-step, loose coupling” numerical scheme was proposed to investigate the deformation behaviors based on the ANSYS Multiphysics/LS-DYNA platform through establishing user-defined subroutines. The results show that electromagnetically assisted deep drawing can remarkably improve the formability of aluminum cylindrical parts. The proposed numerical scheme can successfully simulate the related Stamping-EMF process, and the deformation characteristics of sheet metal reflect experimental results. The predicted results are also validated with the profiles of the deformed sheets in experiments.

Numerical simulation of sheet stamping process using flexible punch

Different from the conventional stamping process, rubber forming uses a rubber pad contained in a rigid box to act as a punch. The main attraction of the process is its simplicity, and it can be used for prototype development or for low-volume production. In this study, an axisymmetric rubber-pad forming operation is investigated via numerical simulations and experiments. Some key process parameters, such as rubber material, hardness of flexible punch, stamping velocity, rubber-pad thickness, and friction are studied in detail. Good correlation is achieved between the finite element predicted and experimentally measured thickness—thinning in the formed part. Stress and strain distributions in the specimen are investigated numerically during the process using ABAQUS/Standard.

Investigation into the pre-forming's effect during multi-stages of tube hydroforming of aluminum alloy tube by using useful wrinkles

Tube hydroforming is a process for manufacturing of lightweight components, especially automotive parts, with advantages of cost and weight reduction, better structural integrity and increased strength and stiffness over the conventional stamping process. Generally, the hydroformed parts can be obtained by multi-stages made up of bending, pre-forming and finishing. Based on the characters of FEM and using the useful wrinkles, the process of multi-stages was investigated by using an aluminum alloy. For avoiding the typical failure modes such as folding back, wrinkling, buckling and fracture due to unreasonable selection of the internal pressure and the axial punch feeding, the optimization strategy was created and the process parameters were optimized by using “useful” wrinkles instead of “harmful” wrinkles in the pre-forming stage and in the finishing stage. The suitable pre-forming die cavity shape was discussed. The results from simulation keep a reasonable agreement with that from experiment.

Principle and applications of multi-point matched-die forming for sheet metal

To reduce the cost and cycle time in fabrication of the conventional stamping die, a new forming technology, so-called multi-point matched-die forming (MPMDF), has been developed based on the discretization of the die. In MPMDF, the fixed shape matched-dies of conventional stamping are replaced by a pair of opposed matrices of movable punch elements. Using multi-point matched-dies generated by punch matrices, a variety of three-dimensional sheet parts of different shapes can be produced. Taking advantage of the flexibility of the multi-point die, new sheet forming techniques have been developed, which realize the formation of parts of large size and with large deformation. In this paper, the principle of MPMDF and the new forming techniques are described, new MPMDF equipment is introduced, and typical application examples are presented.

Improvement of the Drawability Based on the Surface Friction Stir Process of AA5052-H32 Automotive Sheets

Based on the imperative social demand for lighter vehicles, lightweight materials such as aluminum alloys are expected to replace conventional steels in many automotive applications. In automotive parts manufacturing, most of the components produced in conventional stamping operations are geometrically complex as the blanks are subjected to both stretching and drawing deformations. However, aluminum alloys have intrinsic drawbacks, such as the inferior formability of these materials, although the effects of the weight reduction in terms of performance are highly promising. In an effort to improve the formability of aluminum alloy sheets, the surface friction stir process is proposed in this study. This process locally modifies the surface of automotive aluminum alloy sheets via stirring and advancing on the surface of the sheet, similar to the Friction Stir Welding (FSW) process that utilizes a probe without a pin. When the surface of the sheet is modified locally by stirring, dynamic recrystallization due to the severe shear deformation along with heat resulting from the friction occur due to changes in the micro-structure and mechanical properties in the stirred zone, while the dislocation density and grain size refinement are curtailed. In this work, the drawability performance of AA5052-H32 sheets (thickness 1.5 mm) that were welded using the surface friction stir process was experimentally and numerically investigated in cylindrical cup drawing tests. When applied to AA5052-H32 automotive sheets, the surface friction stir process improved the drawability of the entire aluminum alloy sheet. For numerical simulations, the non-quadratic anisotropic yield function Yld2000-2d was employed along with isotropic hardening, while the formability was evaluated by utilizing theoretical forming limit diagrams (FLD) based on Hill's bifurcation and M-K theories.

Formability evaluation in hydroforming steel Tailored Welded Blanks

The main objective of this work was to evaluate the formability in hydroforming of Tailored Welded Blanks (TWBs) by comparing it with the formability in conventional stamping. Forming Limit Diagrams (FLD) were plotted from tests carried out with the modified Nakazima test, to determine the forming limit of the raw material. The Maximum Front of Deformation (MFD) was also determined for both processes. Results showed that the formability of TWBs is increased in hydroforming when compared to conventional stamping, and therefore it was possible to form products with higher and regular domes, lower reduction of thickness, and without any failure.

Manufacture of crown-wheel forgings for a volgar caterpillar tractor

Currently, at OAO Rosmashkom (Volgograd), crown-wheel forgings (of mass around 42 kg) are stamped in the forging shop in two successive lines (Fig. 2), with different heating in semicontinuous flame furnaces: first, in a line based on a steam‐air stamping hammer, where the weight of the falling parts is 10 t; then (after reheating), in a line based on a crank hotstamping press, with a rated force P rat = 40 MN. This technology is characterized by low productivity, considerable expenditures of effort, considerable fuel and energy consumption, and considerable losses of metal. The forgings, as a rule, are of unsatisfactory accuracy and poor surface quality. To find an optimal technology, crown-wheel manufacture by closed inverse pressing on a crank hotstamping press has been developed. This method is supported by reliable theoretical and experimental data. The new process is shown in Fig. 3. The forgings obtained in the existing and new technologies are sketched in Fig. 4. The inverse stamping in the second stage (Fig. 3) is a very energy-intensive operation, since it extends over a relatively large path of the press’s slide to the lower dead point (LDP), corresponding to more than 60 ° rotation of the eccentric shaft. In conventional stamping, the shaft rotation is no more than 15 ° ‐25 ° .

A study of forming pressure in the tube-hydroforming process

Abstract The forming pressure required to produce a desired part using the tube-hydroforming process was investigated in the present study. The relationship between hydraulic pressure, outer corner radius of the deformed tube, tube thickness and tube yield stress was established based on a proposed theoretical model. In the theoretical model, the material hardening property was taken into consideration. Since the friction in the tube-hydroforming is smaller than that in the conventional stamping process, the die-closing force was also calculated according to the forming pressure predicted by the proposed theoretical model under frictionless condition. The finite element analysis was performed to validate the proposed theoretical model. In order to confirm the accuracy of the finite element simulations, two different finite element codes were employed to conduct the analysis and the simulation results were compared. The predicted values calculated by the proposed theoretical model were found to agree well with those obtained from the finite element simulation results.

Application of electromagnetic-assisted stamping (EMAS) technique in incremental sheet metal forming

The application of high velocity electromagnetic-assisted stamping (EMAS) technique in incremental sheet metal forming has been proposed. EMAS whose principle is based on Lorenz force is a hybrid forming process that uses both quasi-static conventional stamping technique and electromagnetic forming actuators built into sharp corners and other difficult-to-form contours to form metals. The recent push to use more artificial intelligent (AI) aluminum alloys in automobile and aircraft industries as a result of increasing demand for fuel efficient cars and aircrafts, large size vehicle panels, improved formability limit of materials and weight reduction have placed EMAS as one of the best high velocity forming technique. It is believed that the end result of this vision will lead to more cost effective land and aerospace vehicles.

Flexible manufacturing of sheet metal parts based on digitized-die

Digitized-die forming (DDF) is a flexible manufacturing technology through which a variety of three-dimensional sheet metal parts can be produced in a DDF system. It eliminates the need to design and produce the conventional die. The central component of DDF system is a pair of matrices of punches, the punches are controlled by computer and the desired shape of die is constructed by changing the heights of punches. Based on the flexibility of DDF, new forming processes are designed that cannot be realized in conventional stamping. In varying deformation path DDF, a sheet part is manufactured along an optimal forming path, and large deformation can be achieved for the material with poor formability. In sectional DDF, a sheet part is formed section by section, and this technique makes it possible to manufacture large-size parts in a small DDF press. A closed-loop forming system was built by combining DDF with rapid 3D-shape measurement system. It is used to compensate for material springback and improve dimensional accuracy of the formed part. And a DDF system with multi-point blankholder control system was developed to control the material flow, thereby to prevent sheet parts from wrinkling and tearing. The DDF integrated system is described, and the detailed forming procedures are explained in the paper. Typical examples are presented showing the applicability of the DDF technology.

Effect of Heat Treatment Conditions on Tube Hydroformability

Tube hydroforming provides a number of advantages over conventional stamping process, including fewer secondary operations, weight reduction, assembly simplification, adaptability to forming of complex structural components and improved structural strength and stiffness. It can produce wide range of products such as sub-frame, engine cradle, and exhaust manifold. In this study, the effect of heat treatment conditions, such as post seam annealing(PSA) and bright annealing(BA), on the ovality and hydro-formability of steel tubes has been investigated. Hydroformability has been estimated as the bulging height obtained depending on the various process parameters such as an internal pressure, axial feeding, and heat treatment conditions. The ovality and forming height were found to strongly depend on material properties after heat treatments.

Sheet bulging experiment with a viscous pressure-carrying medium

Usually, flexible die forming technology and conventional stamping technology have their own separate and limited application range, as well as their own particular shortcomings. So a more advanced forming method is needed to meet the requirements of increasing component complexity. Viscous pressure forming (VPF), which uses a semi-solid viscous material as the pressure-carrying medium, is suitable for forming parts requiring large sheet deformation using a sequential forming method. In this paper, a sheet metal bulging experiment with three kinds of pressure-carrying media, viscous medium, polyurethane and steel, was studied. The results show that the specimens formed with VPF, compared with those formed with polyurethane and steel, have less wall-thinning and consequently a more uniform wall thickness distribution, and the shape of part is closer to the desired hemisphere. When bulged with back-pressure, the formability of sheet metal can be improved significantly. Thus VPF is a promising sheet metal forming method.

Comparison stamping and hydro-mechanical forming process for an automobile fuel tank using finite element method

Recently, the use of hydro-mechanical forming process offers a new manufacturing technology for the automotive industry. In this paper, a comparison is made between the hydro-mechanical forming of sheet metal and a conventional stamping for production of an automobile fuel tank using a commercial explicit FEM code. A special attention is paid to establish an optimal loading path for producing an automobile fuel tank by means of the hydro-mechanical forming process. The finite element model for a conventional stamping process is also verified by comparison with experiment. Finally, it is concluded that the hydro-mechanical forming could produce the part with a more uniform and sound thickness distribution than the stamping operation.

A memory application of light reflection from anisotropic micro-structured thin films

Ellipsometric memories are optical memories based on the ellipsometric principle. They were proposed in 1994 for the first time and were examined by preliminary experiments using multi-layered thin film structures. Though this idea has a great potential for new high-capacity optical memories, the multi-layered thin film structures are not suitable in consideration of a large-scale fabrication from a master disc. To overcome this difficulty we propose an ellipsometric memory with anisotropic micro-structured surfaces. In the proposed memories, the ellipsometric parameters of reflected light from each cell are measured and the information is stored in an optically anisotropic micro-structured surface instead of in a multi-layered structure. This modification makes a large-scale production possible from a master disc using a conventional stamping technique. We carried out preliminary experiments using Mg, Ag and Al thin films with striped surfaces and confirmed the feasibility of this type of ellipsometric memory.