Sheet Hydroforming Process Research Articles

New Developments of Hydroforming in China

This paper reviews the recent developments of hydroforming technology in China. Limited corner radius, ring hoop tension test and tube bulging test were introduced on fundamentals of hydroforming. New hydroforming and hydro-bending process of ultra-thin tubes were investigated. Ultra-thin Y-shaped parts and complex section components were manufactured and applied in aerospace and aviation industry. Applications of tube hydroforming in automotive industry in China are also presented, including the hydroforming machines and production lines, typical automotive parts and potential market. New sheet hydroforming process with controllable radial pressure was proposed to increase the formability of sheets metals. Warm tube and sheet hydroforming were analyzed and discussed. [doi:10.2320/matertrans.MF201122]

Strain Hardening Analysis Coupled to Remeshing Procedure Application to Sheet Hydroforming Processes

In this study, we present an experimental/numerical methodology which aims to improve 3D thin sheet hydroforming considering coupled constitutive equations formulated in the framework of of irreversible processes accounting for isotropic hardening as well as isotropic ductile damage. The experimental study is dedicated to the identification of stress-strain flow from the global measure of pole displacement, thickness evolution and internal pressure expansion. Or during the hydroforming processes severe mesh distortion of element occur after a few incremental steps. Hence an automatic mesh generation with remeshing capabilities is essential to carry out the FEA. The proposed technique based on geometrical criteria includes adaptive refinement and coarsening procedure is integrated in a computational environment.

Strain Hardening Law Coupled to Damage and Remeshing Procedure Application to Sheet Hydroforming Processes

In this study, we present an experimental/numerical methodology which aims to improve 3D thin sheet hydroforming considering coupled constitutive equations formulated in the framework of irreversible processes accounting for isotropic hardening as well as isotropic ductile damage. The experimental study is dedicated to the identification of stress-strain flow from the global measure of pole displacement, thickness evolution and internal pressure expansion. Indeed, Hill48 yield surface anisotropy parameters and coefficients of the Swift law coupled to ductile damage allowing to locate plastic instability zones of hydroformed sheets are identified with three dies cavities shapes. Or during the hydroforming processes severe mesh distortion of element occur after a few incremental steps. Hence an automatic mesh generation with remeshing capabilities is essential to carry out the FEA. The proposed technique based on geometrical criteria includes adaptive refinement and coarsening procedure is integrated in a computational environment.

An Optimization of the New Die Design of Sheet Hydroforming by Taguchi Method

During the last few years, several sheet hydroforming processes have been introduced. Despite the advantages of these methods, they have some limitations. Of the processes, the two main ones are the standard hydroforming and hydromechanical deep drawing. A new sheet hydroforming die set was proposed that has the advantages of both processes and eliminates their limitations. In this method, a polyurethane plate was used as a part of the die-set to control the blank holder force. This paper outlines the Taguchi optimization methodology, which is applied to optimize the effective parameters in forming cylindrical cups by the new die set of sheet hydroforming process. The process parameters evaluated in this research are polyurethane hardness, polyurethane thickness, forming pressure path and polyurethane hole diameter. The design of experiments based upon L9 orthogonal arrays by Taguchi was used and analysis of variance (ANOVA) was employed to analyze the effect of these parameters on the forming pressure. The analysis of the results showed that the optimal combination for low forming pressure is harder polyurethane, bigger diameter of polyurethane hole and thinner polyurethane. Finally, the confirmation test was derived based on the optimal combination of parameters and it was shown that the Taguchi method is suitable to examine the optimization process.

An Experimental Study on Robustness and Process Capability of the Warm Hydroforming Process

The warm sheet hydroforming process was investigated to determine the optimal process conditions of temperature, pressure, and pressurization rate for maximum formability of AA5754-O using an experimental stretch forming die shape. The optimal process conditions were evaluated to determine the robustness and process capability based on physical measurement of formed parts including thickness strain, cavity fill ratio, and radius of curvature. For the simple die shape investigated, a temperature of 268°C, a pressure of 25 MPa, and a pressurization rate of 0.22 MPa/s provided the most balanced combination of uniform thickness strain with the greatest cavity fill ratio and sharpest radius. Temperature had a greater effect on measured properties than either pressure or pressurization rate, although the effect of pressure increased as temperature decreased. The procedures demonstrated in this experimental study could be used to optimize process parameters for robust operation of production applications for more complex automotive body panels fabricated by the warm hydroforming process.

Experimental and numerical analysis of titanium/aluminum clad metal sheets in sheet hydroforming

Clad metals are becoming increasingly emphasized in sheet metal applications. In this research, sheet hydroforming process (SHF) was adopted to improve the formability of Ti/Al clad metal sheets and SUS 304 metal sheets used in computer, communication, and consumer product housings. Both finite element simulation and experimental verification were carried out to investigate the deformation of blanks. Several significant process parameters, such as holding force, friction, counter pressure history, and blank dimensions, were discussed for improving the formability of the two metal sheets. In SHF simulation, a virtual film technique was proposed to realistically approach the hydraulic loading condition during SHF. Finally, the deformed shape and thickness distribution of parts manufactured with SHF were compared with the results of simulation. Good agreements were obtained.

Novel combined standard hydromechanical sheet hydroforming process

Sheet hydroforming techniques are widely used in industries for the production of different components with specific characteristics. Despite the advantages of the conventional sheet hydroforming techniques, they have their own limitations. In this paper, to increase the application and to overcome the limitations of conventional deep drawing, a new sheet hydroforming technique is proposed that is a combination of the standard and the hydromechanical sheet hydroforming processes. The proposed method has the advantages of conventional processes but eliminates their limitations. This technique is used in forming a cylindrical cup and a complex industrial part. After applying the appropriate pressure path, the products are formed successfully in one stage without any defects. It is also shown that the required forming pressure in the proposed process is low compared with other sheet hydroforming processes. It is demonstrated that because of the low internal pressure required in the present method, pre-bulging does not have any significant effect on the thickness distribution of the product. It is concluded that in this process increasing the friction coefficient between punch and blank can reduce the sheet thinning.

Outflow Characteristics of a Pressure Medium during Sheet Hydroforming

The outflow characteristics of a pressure medium during a sheet hydroforming process have been studied experimentally by directly measuring the hydraulic pressure distribution. Initial measurements of the outflow of the pressure medium though the gap between two clamped dies were carried out in order to verify the method for measuring the hydraulic pressure distribution employed in this study and to investigate the basic properties of the outflow. The upward forces were calculated by integrating the measured hydraulic pressure during the outflow and found to be in satisfactory balance with the clamping forces, which demonstrates the validity of this measurement method. Furthermore, we suggest that the critical outflow pressure can be predicted by considering the force equilibrium. An experimental investigation of a square-cup sheet-hydroforming process was then carried out. The upward forces were calculated by integrating the measured hydraulic pressure and again found to be in satisfactory balance with the forming forces at the beginning of the process, which shows that the critical outflow pressure can also be predicted for square-cup deep-drawing processes. The hydraulic pressure distributions in the flange area and the chamber change significantly as the sheet conforms to the die shoulder, and the friction loss of hydraulic pressure at the die shoulder becomes large. These results indicate that the magnitude of the fluid-lubrication effect on the drawability of the sheet can vary with the stage of sheet deformation.

Correlation between Sheet Deformation and Hydraulic Pressure Variation during Sheet Hydroforming

In our series of studies, the outflow characteristics of a pressure medium used for a sheet hydroforming process have been investigated. The variation of hydraulic pressure during a square-cup sheet hydroforming process was examined, focusing on correlation with sheet deformation. In the initial stage, the outflow of the pressure medium remained stable because a greater extent of pre-bulging arose around the die shoulder just after the process commenced. When the pre-bulging ceased, the pressure medium was sealed in the chamber. Thus, the hydraulic pressure began to increase rapidly within the chamber, and fell in the flange area. The punch stroke at which the hydraulic pressure in the chamber began to rise was highly dependent on the blank holding force because the height of pre-bulging was determined by this force. When the sheet conformed completely to the die shoulder, the pressure medium was also sealed at the shoulder. Therefore, the hydraulic pressure on the die shoulder as well as in the flange area started to fall. The correlation between the critical punch stroke for fracture and the hydraulic pressure in the flange area was also examined.

Computer Aided Simulation as valid tool for sheet hydroforming process development

Sheet Hydroforming is considered a good opportunity when it is necessary to deal with complex shapes. However, it is common knowledge that it is quite difficult to control such a kind of technology because of an appropriate press tooling is necessary and the press tooling supplier is often the technology supplier for the given problem [1]. Within a larger research program it is demonstrated that it is possible to use traditional hydraulic press tooling having the chance to manage a high level of process variables thanks to the development of a dedicated forming tool named hydroforming cell. The architecture and the number and type of process variables are developed thanks to the extensive usage of CAE techniques. An implicit solver is used to verify the structural behaviour of the hydroforming cell in terms of maximum stress levels and components stiffness while an explicit solver has helped to define the samples shapes and their main features and thanks to them it is possible to explore the process design space. An appropriate experimental phase has demonstrated the effectiveness of the developed procedure.

Experimental and numerical analyses of sheet hydroforming process for production of an automobile body part

Abstract An approach for substituting conventional manufacturing method of a three pieces shell fender by one piece has been proposed. This approach is based on sheet hydroforming process, which has many advantages over conventional stamping processes. For production of shell fender by sheet hydroforming, two possible manufacturing procedures have been evaluated numerically, namely; pure stretching and draw-in. In addition, results of both finite element simulations and experiments on small size specimens show superiority of draw-in procedure. The advantage of draw-in over pure stretching procedure has been explained using thickness variation measurements. Furthermore, the draw-in procedure of shell fender has been optimized with respect to initial blank size, fluid pressure and required tools using an explicit commercial forward finite element program. Finally, the effects of sheet material and fluid pressure on the required deformation force in laboratory scale have also been evaluated.

Integrated Tube and Double Sheet Hydroforming Technology - Optimised Process for the Production of a Complex Part

The possibility to produce lightweight components with a complex geometry enhanced, in the last decades, the industrial application of the tube hydroforming and, more recently and restricted to specific industrial fields, of the sheet hydroforming technology. The integration in one tool of a tube and a double sheet hydroforming process represents an innovative technology which further emphasises the advantages offered by hydroforming in terms of costs reduction and complexity of the manufactured part. This paper describes the design and the construction of a complex hollow part resulting from the simultaneous hydroforming of two sheets and a tubular component in one tool. The focus is set in particular on the optimisation of the joining zone between tube and sheet pair, whose geometry allows a “metallic” sealing of the gap between the sheets and the tube, i.e. without using sealing components. The contact between tube and sheet pair allows the transmission of the axial force used to support the bulging of the tube to the sheet blanks, thus increasing their draw-in in the die and, consequently, avoiding the occurrence of tearing on the part. The paper describes the optimisation of different process parameters like the shape and the dimension of the blanks, their initial positioning in the tool, the value of the axial force applied to the tubular component and the blankholder force during the preforming and the calibrating stages.

Triangulation Based Digitizing of Tooling and Sheet Metal Part Surfaces - Measuring Technique, Analysis of Deviation to CAD and Remarks on Use of 3D-Coordinate Fields for the Finite Element Analysis

The dynamic development of highly accurate optical measuring machines within the last years pushed the introduction of digitizing techniques to many applications in the fields of quality control, reverse engineering and rapid prototyping. By projecting fringe patterns onto the object's surface and recording pictures of the curvature dependant deformation of the pattern, 3D coordinates for each camera pixel are calculated on the basis of the principle of triangulation. The generation of a polygon mesh can be used for the analysis of the deviation of a die or a formed part to the initial CAD data, i.e. by means of full field or section based comparison. This paper presents the application of the above mentioned techniques on a double sheet hydroforming process. The gathered 3D data of the clam-shell part as well as of the tooling dies served for the calculation of the deviation to the respective reference geometry. With respect to the utilization of digitized tooling data within the finite element analysis, further investigations were performed on the impact of data reduction strategies. Aiming on the minimization of the necessary number of elements, representing the tooling surface in a discrete state, and on the request for a sufficient degree of accuracy, these strategies have to be considered of high priority.

Optimized constitutive equation of material property based on inverse modeling for aluminum alloy hydroforming simulation

By using aluminum alloys, the properties of the material in sheet hydroforming were obtained based on the identification of parameters for constitutive models by inverse modeling in which the friction coefficients were also considered in 2D and 3D simulations. With consideration of identified simulation parameters by inverse modeling, some key process parameters including tool dimensions and pre-bulging on the forming processes in sheet hydroforming were investigated and optimized. Based on the optimized parameters, the sheet hydroforming process can be analyzed more accurately to improve the robust design. It proves that the results from simulation based on the identified parameters are in good agreement with those from experiments.

Finite-element simulation of the elliptical cup deep drawing process by sheet hydroforming

This paper presents the development of an elastoplastic finite-element method (FEM) program dedicated for the sheet hydroforming (SHF) process and the simulated results of the elliptical cup deep drawing process using the program. First, the development of the SHF simulation program based on the home-made static FEM code STAMP3D is described. Several new techniques to give a sheet the proper hydraulic pressure are proposed and implemented in STAMP3D. Then, simulations of the elliptical cup deep drawing process are carried out and the simulated results are compared with those of experiment. Local thinning is observed at the punch shoulder and at the wall part in the simulated thickness strain distribution, which is in good agreement with the experimental observations. The formability of the product is also carefully examined by comparing with the result of the conventional press forming in which much larger thinning is observed at the punch shoulder. It is clarified that the difference in the thickness strain distribution between the SHF and the conventional press forming is due to the friction increasing effect which is one of the major features of the SHF process. The result shows that the simulation predicts the friction increasing effect well, verifying the validity of the developed program. Moreover, it is clarified that the SHF process gives much better formability than the conventional press forming process for the present product.

Numerical/experimental investigations for enhancing the sheet hydroforming process

In this work the authors present some results of their research activity aimed to enhance the sheet hydroforming process. The main focus is the adoption of a movable inferior plate through which the control of the blank forming is realised. Tests on hemitoroidal parts using an innovative equipment with a movable die were carried on at the Institute of Metal Research (Chinese Academy of Sciences) in cooperation with the Aalborg University, putting in evidence main defects and fracture causes. Finite element simulations of the modified hydroforming process using a movable inferior plate were performed at the Centre of Excellence for Mechanical Computation (Polytechnic of Bari), with the aim of evaluating critical strain values and their location, strain paths, load curves and stress maps were analysed. Specimens with proper geometrical shape were designed to avoid possible ruptures and to reduce material thinning in the critical regions.

Applications of hydroforming processes to automobile parts

Hydroforming process recently draws attention of automotive industries due to its advantages like the increased strength, weight reduction, improved quality and reduced tooling cost. This paper summarizes some of our experiences of tube and sheet hydroforming process design by simulation technique through actual tryout. Parts included in this paper are tie bar, subframe and engine mount bracket. The simulations for the entire hydroforming processes were performed and their results were used to predict and fix the forming failures like the wrinkling and the fracture. Based on this experience, we also suggested necessary improvements in the design procedure.

Optimal Programming of Multi-point Cushion Systems for Sheet Metal Forming

Numerical optimization technique coupled with finite element analysis of the stamping/sheet hydroforming process was developed to predict four possible modes for application of blank holder force (BHF) in multiple-point cushion systems, namely a) BHF constant in space/location and time/stroke, b) BHF variable in time/stroke and constant in space/location, c) BHF variable in space/location and constant in time/stroke and d) BHF variable in space/location and time/stroke. The BHF was predicted by (a) minimizing the risk of failure by tearing (thinning) in the formed part and (b) avoiding wrinkling. The developed technique was applied to predict the BHF to form a) an automotive part (liftgate-inner) from AA6111-T4 aluminum alloy, b) an asymmetric part from aluminum alloy AA5083-H32 by sheet hydroforming process with die (SHF-D) and c) a round cup by sheet hydroforming with punch (SHF-P). Experimental results showed that the FEM based optimization methodology can reduce trial and error effort and is able to predict the blank holder force necessary to form the parts without fracture and wrinkling in the investigated stamping and sheet hydroforming operations.

Wrinkling control in aluminum sheet hydroforming

In this paper, the wrinkling behavior of 6111-T4 aluminum alloys during sheet hydroforming process was numerically and experimentally investigated. In sheet hydroforming, one or both surfaces of the sheet metal are supported with a pressurized viscous fluid, while a punch forms the part. In sheet hydroforming the use of a matching female die is not needed. The use of the pressurized fluid delays the onset of material rupture (International Journal of Mechanical Science 2003;45:1815–48) and also acts as an active blank-holding force to control wrinkling in the flange area. To form a wrinkle-free deep-drawn hemispherical cup with sheet hydroforming, a theoretical analysis based on the work of Lo et al. (Journal of Materials Processing Technology 1993;37:225–39) was initially used to predict the optimum fluid pressure profile. Simplifying geometrical assumptions and Tresca material model used in the theoretical analysis provided a fluid pressure profile that resulted in premature rupture of the sheet metal. However, an optimum fluid pressure profile generated by the finite element method, using Barlat's anisotropic yield function (Journal of Mechanical Physics and Solids 1997;45(11/12):1727–63), was successfully applied in sheet hydroforming to make the deep-drawn hemispherical cup without tearing and with minimal wrinkling in the flange area. The finite element model was also capable of accurately predicting the location of the material rupture in pure stretch, and wrinkling characteristics of the aluminum alloy sheet in the draw-in process.

Numerical/experimental analysis of the sheet hydro forming process using cylindrical, square and compound shaped cavities

In this work, the authors focus the attention on the problems involved with the production of complex shapes using the sheet hydro forming (SHF) process adopting a numerical experimental approach. A specific in-home equipment has been used to perform tests on single sheet metal blanks. SHF tests with different die cavity shapes have been performed on circular blanks in AISI 304. The finite element (FE) technique has been adopted to perform the numerical analysis; experimental data have been used to both define and validate the FE model. Simulations of the SHF process applied to the production of cylindrical, squared and compound shaped parts have been undertaken. The feasibility and the main problems related to the forming operation of such shapes have been investigated through the analysis of the equivalent plastic strain (PEEQ) distribution and the strain paths of the most critical areas.