Die-less Hydroforming Research Articles

Evaluation of dieless hydroforming process for manufacturing of variable cross section tubular structures

The dieless hydroforming is a type of manufacturing method that forms materials using pressurised hydraulic fluid rather than dies and presses as in traditional hydroforming, resulting in significant cost savings. The dieless hydroforming of tubular structures with variable cross section has received little attention, indicating that more research is needed in this area. Therefore, this article investigated dieless hydroforming of low carbon steel material with fluid pressures ranging from 10 to 50 bar and sheet thicknesses ranging from 1 to 2.5 mm to produce tubular structures with variable cross section. The main goal is to comprehend the deformation characteristics in relation to the parameters, as well as the impact of deformation on the microstructural and mechanical behaviour. The inflation and contraction patterns of hydroformed structures are numerically determined using the finite element method at various parameters and then compared to experimental results which show a good correlation. Plastic deformation induced grain refinement in all deformed samples, as compared to as-received material. The sample hydroformed at 30 bar and with a thickness of 2.5 mm showed higher yield strength (291 MPa), tensile strength (404 MPa) and elongation (48%) than the other samples. Hydroformed tubular structures with variable cross section are created and used in electric poles to hold solar panels after the process repeatability is tested and found to be highly repeatable with greater than 97% accuracy.

Fundamentals and Processes of Fluid Pressure Forming Technology for Complex Thin-Walled Components

• New fluid pressure forming processes for tubes, sheets and shells were developed. • Methodology of adjusting stress state was invented to solve defects of forming thin-walled structures. • Outstanding applications in aerospace and automobile industry are presented. A new generation of fluid pressure forming technology has been developed for the three typical structures of tubes, sheets, and shells, and hard-to-deform material components that are urgently needed for aerospace, aircraft, automobile, and high-speed train industries. In this paper, an overall review is introduced on the state of the art in fundamentals and processes for lower-pressure hydroforming of tubular components, double-sided pressure hydroforming of sheet components, die-less hydroforming of ellipsoidal shells, and dual hardening hot medium forming of hard-to-deform materials. Particular attention is paid to deformation behavior, stress state adjustment, defect prevention, and typical applications. In addition, future development directions of fluid pressure forming technology are discussed, including hyper lower-loading forming for ultra-large non-uniform components, precision forming for intermetallic compound and high-entropy alloy components, intelligent process and equipment, and precise finite element simulation of inhomogeneous and strong anisotropic thin shells.

Joining by die-less hydroforming with outer pressurization

Joining by hydraulic expansion is a well-investigated process to create force- and form-fit joints. In order to extend the application possibilities of this technology, the process of joining by die-less hydroforming with outer pressurization is developed. In this process, the force-fit is a result of an elastic springback restriction of the inner joining partner, caused by an elastic-plastic compression of the outer joining partner. The influence of the chosen materials on the establishment of a force-fit connection is investigated both numerically and experimentally. In this context, the ratio of the maximum elastic strains λ is introduced. For values of λ close to 1, the interference pressure is rather a result of the difference in the hardening gradient than of a restricted elastic springback. For values of λ close to 0 in turn, the interference pressure is a result of a restricted elastic springback. The obtained numerical data is validated with the experimental results for tube-to-tube connections of the aluminum alloys EN AW-2007 with EN AW-6060 and EN AW-6060 O, respectively.

Analytical process design for interference-fit joining of rectangular profiles

Abstract The classical joining of round tubes by die-less hydroforming is extended to joining of rectangular profiles. A new analytical approach for the calculation of required process parameters is developed. It allows the calculation of the interference pressure p and the required fluid pressure range given by a lower and upper limit ( p i , m i n and p i , m a x ). These limits are the result of the deformation state of the inner and outer joining partner. The minimum pressure, where plastic deformation of the inner joining partner occurs first, characterizes the value at which the joint formation starts. The maximum pressure is the value, at which plastic deformation of the outer joining partner occurs. The analytical model is validated by experimental studies where two rectangular aluminum profiles (EN AW-6060) with different heat treatment conditions were joined. Above p i , m a x , the increase of the achievable joint strength with respect to the internal pressure is not significant anymore. The maximum deviation between the analytically and experimentally determined maximum fluid pressure p i , m a x is about 12% in case of a wall thickness of 3 mm and 11% in case of a wall thickness of 5 mm.

Joining by Die-Less Hydroforming of Profiles with Oval Cross Section

Joining by die-less hydroforming is used to produce overlap joints by means of hydraulic expansion. Due to a difference in the elastic recovery of the two joining partners an interference pressure p remains at their contact area. Due to the possibility to produce multi-material joints without relying on heat, the process has great potential for joining parts in lightweight applications. Therefore, the process limits were extended so that profiles with non-rotationally symmetric cross-sections can be joined. For this purpose a new tool for profiles with oval cross-section was developed. The inner and the outer joining partner ware made of aluminum 6060 and aluminum 6082 respectively. The influence of the overlap length and different wall thicknesses of the outer joining partner were investigated by numerical simulations and validated by experiments. An upper limit in interference pressure was observed which was also found previously for profiles with circular cross sections. The fluid pressure limit is compared with the analytically calculated value for a configuration with circular tubes under equivalent conditions. The analytical model underestimated the pressure limit. In contrast to circular tubes, the strain distribution of profiles with oval cross sectional shapes is not uniform, which results in superposed bending stresses. Also a difference in stiffness of the inner and outer joining partner leads to a pressure depended contact area which is assumed constant in the analytical model.

Analytical Model to Determine the Strength of Form-Fit Connection Joined by Die-Less Hydroforming

Modern lightweight concept structures are increasingly composed of several dissimilar materials. Due to the different material properties of the joining partners, conventional and widely used joining techniques often reach their technological limits when applied in the manufacturing of such multimaterial structures. This leads to an increasing demand for appropriate joining technologies, like joining by die-less hydroforming (DHF) for connecting tubular workpieces. The present work introduces an analytical model to determine the achievable strength of form-fit connections. This approach, taking into account the material parameters as well as the groove and tube geometry, is based on a membrane analysis assuming constant wall thicknesses. Besides a fundamental understanding of the load transfer mechanism, this analytic approach allows a reliable joining zone design. To validate the model, experimental investigations using aluminum specimens are performed. A mean deviation between the calculated and the measured joint strength of about 19% was found. This denotes a good suitability of the analytical approach for the design process of the joining zone.

Research on deformation and stress in hydroforming process of an ellipsoidal shell without constraint

In order to the study the deformation of die-less hydroforming of ellipsoidal shell under the condition of without restricting the elongation of short axis, experimental research on hydroforming of an ellipsoidal shell with the initial axis length ratio 1.8 was carried out to analyze the effect of internal pressure on the variation of shell shape, volume, and wrinkling. The results show that as the internal pressure increases at the early stage of deformation, the short axis elongates; correspondingly, the volume varies slightly; wrinkling occurs on the equatorial plane when the internal pressure is up to 1.0 MPa, and it becomes severe when the internal pressure is 2.2 MPa; as the internal pressure continuously increases, the long axis begins to shrink rapidly, but the shell volume has a very tiny change within the range of internal pressure 2.2–3.5 MPa; when the internal pressure is 3.8 MPa, the wrinkling is eliminated completely, and the shell volume suddenly increases; when the internal pressure is up to 5.5 MPa, the equatorial plane is no longer shrinking but expanding and the out-of-roundness approaches zero; finally, when the internal pressure is 6.8 MPa, the ellipsoidal shell with the axis length ratio 1.06 is obtained, and the variation of long axis, short axis, and volume are −2.9, 64.6, and 52.7 %, respectively. Simulation was also carried out simultaneously to analyze the stress locus of typical points. It has indicated that the compressive stress in the circumferential direction is the reason of wrinkling occurring; accordingly, the tensile stress in the circumferential direction is the reason of wrinkling disappearing.

Die-less hydroforming of multi-lobe tubular structures

The process capability of die-less hydroforming for producing tubular structures of complex geometries was investigated. Multi-lobe tubular structures were chosen for this study as they are capable of carrying higher loads than normal tubes of the same weight. The forming characteristics of three variants of tubular geometry with longitudinal lobes, circumferential lobes, and helical lobes were studied through numerical analysis. The parameters that were investigated were tube wall thickness, tube diameter, tube length-to-diameter ratio, pressure loading paths, and lobe-forming patterns. The finite element analysis showed that the length of the tube does not influence the lobe formation for all three tube variants. The finite element analysis results also demonstrated that lobe wall thinning varies linearly with hydroforming pressure for all multi-lobe tube patterns studied. The strength-to-weight benefit of the tubular structures was also verified through finite element analysis for annealed stainless steel tube sample of 200 mm length, 40 mm diameter, and 2 mm wall thickness. The longitudinal lobed geometry, circumferential lobed geometry, and the helical lobed geometry all were able to carry significantly larger loads as compared to a blank tube of the same mass under compressive, flexural, and torsional loading conditions. To test the viability of the die-less hydroforming process, a longitudinal lobed tubular structure was fabricated and formed. The results from this study indicate that a die-less hydroforming manufacturing process is viable and capable of producing strong, lightweight parts of complex geometries. Besides being capable of producing complex tubular structures, the costs associated with die-less hydroforming are significantly lower due to the absence of a press and dies. However, preparation of tubular blanks requires reliable weld seams and rolling operations.

Many-to-Many Regression Analysis of Loading Characteristic in Dieless Tube Hydroforming

Based on uniform design, finite element simulation of dieless tube hydroforming and double-screening stepwise regression method, a many-to-many polynomial regression equation set was established, whose independent variables are time t1 at which axial compression displacement from 0 reaches 25mm and time t2 at which internal pressure from 0 achieves 65MPa in 60 seconds, while dependent variables are hoop plastic strain s11 and radial plastic strain s22 in the midmost position of hydroformed tube. Analysis of partial correlation coefficient indicates that the effect of t2 on s11 is a little stronger than t1 and there is no interaction between t1 and t2 on s11, while s22 is influenced by t22 and t1t22. The many-to-many regression model not only quantificationally describes the action of loading factors on plastic strains but also reflects the inherent relevance between plastic strains, which is more useful than several one-to-many regression models to solve problems such as loading characteristic analysis, loading parameters design and optimization, etc.

Groove Filling Characteristics and Strength of Form-Fit Joints Produced by Die-Less Hydroforming

The manufacturing of modern lightweight structures and the implementation of multi material concepts, for example in automotive engineering, entails appropriate joining technologies. The absence of additional connection elements or filling materials as well as the possibility to join dissimilar metals are basic requirements in this field of application to reach the aspired weight reduction. In case of tubular joints the die-less hydroforming process meets these demands and thus makes it an interesting alternative to conventional welding and riveting processes. The present work focuses on form fit joints produced by die-less hydroforming. It provides a verification of a previously presented analytical approach that allows the calculation of the working fluid pressure required to bulge the tube material into the groove of the outer joining partner. For that purpose, the groove filling characteristics of joined specimens with different groove geometries are analyzed. Here both joining partners were made of the aluminum alloy EN AW-6060. Additionally the connection strength of the joined specimens are determined using tensile tests. The results prove that the groove angle is the main influencing factor on the connection strength and that it can be used for an ordinal comparison of different groove geometries.

Analytic Prediction of the Process Parameters for Form-Fit Joining by Die-Less Hydroforming

The Commission of the European Communities aims for a reduction of new car CO2 emissions of 120 grams per kilometer in 2012. As a result of the omnipresent efforts of the automotive industry to hit these tighter emission standards innovative lightweight strategies, e.g. the use of lightweight materials are developed. This entails new joining techniques that are appropriated to the new lightweight materials. The die-less hydroforming process is a joining method for tubular joints which meets the new demands of lightweight strategies. Since there is no need for any additional connection elements or filling material, it is an interesting alternative to conventional welding and riveting processes. The present paper describes the basic principle of the die-less hydroforming joining technology with a special focus on form-fit connections. An analytical model, based on the membrane theory with an additional local consideration of bending stresses is developed. This analytic approach can be used to calculate the working fluid pressure, required to bulge the tube material into the groove of the outer joining partner. Taking into account the material parameters as well as the groove and tube geometry, this model allows a reliable process design. Additionally, validation of the model by experimental investigations will be provided.

Joining of lightweight frame structures by die-less hydroforming

A successful approach to achieve a reduction of a car’s total weight is the implementation of lightweight strategies in the design process, e.g. using lightweight materials. An interesting alternative to conventional welding and riveting processes is joining by die-less hydroforming. This work describes an analytical model which can be used to calculate the strengths of these joints, taking into account the material parameters, joint geometry and process parameters. Additionally, validation of the model by both finite element simulations and experiments will be provided. Furthermore, investigations were carried out to implement the described methodology for a multi-joint used in a space frame structure.

Manufacturing of lightweight frame structures by innovative joining by forming processes

The majority of the materials used in modern lightweight frame structures are aluminium alloys. But some aluminium alloys, e.g. EN-AW6060, are not or hardly joinable using conventional joining techniques. In this case innovative joining methods like friction stir welding; joining by dieless hydroforming or by electromagnetic compression lead to good joint’s quality. Friction stir welding has been widely investigated for joining sheet metal, but joining tubular workpieces creates new challenges in handling and processing, which will be described here. Joining by Dieless Hydroforming has been introduced in both plant and automotive engineering in the 1970ies. This joining method was used to join heat exchangers or camshafts made of steel. Recently, joining of aluminium has been targeted and is addressed below. Joining by Electromagnetic Compression offers the opportunity to manufacture high strength joints by e.g. form-fit. Groove’s design has been adopted targeting failure of the basic tube material, which is described here.

New Aspects of Joining by Compression and Expansion of Tubular Workpieces

Joints which are solely produced by forming processes are capable of transmitting e.g. axial loads by means of dominating force- (alternatively named interference) or form-fit. Furthermore, a transmission of load could be directed by a combination of both and with supporting adhesives. Joining by forming has been done by compressing tubes using external rolling and electromagnetic forming; joining by expansion has been performed by dieless hydroforming and rolling. Moreover, a problematic constraint for joining by expansion or compression, namely joining of identical materials, which is a topic of a current research project within the scope of SFB/Transregio 10, has to be taken into account. Consequently, the article focuses on the feasibility of combining joining by forming processes and adhesives for joining of tubular workpieces made of identical material (EN-AW6060F22). Resisting loads of form- and force-fit joints have been taken as a reference.

Research into the dieless hydro-forming of non-uniform thickness spherical vessels

On the basis of the practical load-supporting conditions of large-scale spherical vessels for water or oil storage, the integral hydro-bulge forming technology of non-uniform thickness spherical vessels and a new method for measuring the strain of the inner surface of spherical vessels are advanced in this paper, and they are proved to be successful by means of experiment. The feasibility of the technology is investigated; the design of the structure and the choice of technological parameters are also studied in this paper. The strain, thickness distribution and diameter variations are analysed, together with the deformation process of the shell, which is analysed by using the elastoplastic finite-element method (FEM). The results of FEM simulation are very similar to those of the experiment.

Investigation of the hydroforming of spherical vessels from a new type of flat polyhedral shell

The technology of the die-less hydroforming of spherical vessels from a new type of flat polyhedral shell (combination frustum shell) is advanced and the forming process is researched by means of the elastoplastic finite-element method and experimentation. The research results show that the new technology is feasible not only in theory but also in practice. Inner concave deformation and wavy line deformation occur near to the weld seams or crest lines (the latter are the intersection lines of two petals and may be a weld seam or just a straight line formed by the bent petals). Severe deformation takes place on the horizontal weld seams, with less deformation on the vertical weld seams. High tensile stress normal to the weld seams exists on the inner surface near to the weld seams. Proper shell structure design should ensure the successful forming of the spherical shells. The number of the frustum bands and the side number of the polar blanks should be selected properly to ensure that the minimum dihedral angle is greater than 135° and the horizontal angular weld seams where severe deformation occurs should be replaced by crst lines in the roll-bent blanks.

Research into the dieless hydroforming of double layer spherical vessels

The principle, the technical procedure and features of the dieless hydroforming technology of double layer spherical vessels are introduced. The structures and materials of the shells before bulging are discussed, as well as the technical problems associated with bulging of the shells, taking account of experiments on a few double layer spherical shells. The deformation process, the thickness distribution and the diameter variation are also analysed, together with the deformation process of the shell at the initial deformation stage which is analysed by using elastoplastic finite-element method. Finally the industrial prospect is pointed out.