Free Bulge Test Research Articles

Investigation of process parameters in superplastic free bulge forming

PurposeAerospace industry was pioneered in the use of superplastic forming (SPF) process. Weight saving is the most important need in this industry. For this reason, there is special attention paid to this method. Blow forming is a common method for SPF process. Process parameters such as temperature and pressure have significant effects on part accuracy, quality and desired characteristics. The purpose of this paper is to present a numerical and experimental investigation of process parameters in superplastic free bulge forming.Design/methodology/approachIn this paper, superplastic free bulge forming of Al‐5083 has been studied. First, free bulge tests have been done at two different pressures. Bulge height variations were recorded for different pressure and temperature. The forming time was determined according to the forming pressure and temperature. Then, simulation of free bulge process has been carried out using creep behavior model at high temperature. Bulge height and thickness distribution are obtained at two different pressure settings. These results have been compared with experimental results presenting a good agreement. Also the effects of temperatures and pressure on the required process time are compared for a certain bulge height. Finally, thickness distribution profile for different temperatures, pressures and initial thicknesses have been studied.FindingsA numerical and experimental investigation has been presented that can be used to study the process parameters. These findings show the effects of temperatures, pressure and initial thicknesses on sheet forming.Originality/valueThe results of this work show that higher temperature and forming pressure will reduce the required process time for a certain bulge height. Reduction of these parameters can improve thickness distribution. Also, by considering the effects of both pressure and temperature, it is shown that using lower forming pressure at higher temperature is more suitable for forming. The findings of this work can provide more understanding of the process for aircraft part designers and manufacturing process planners.

Inverse method for flow stress parameters identification of tube bulge hydroforming considering anisotropy

The objective of this paper is to determine the behaviour parameters of hydroformed tubular materials of annealed 25CrMo4 steel seamless tubes. An inverse identification procedure has been developed to determine the flow stress parameters from experimental free bulge and simple tensile tests. Two inverse identification strategies are proposed. In the first one, the tubular material behaviour is assumed isotropic, and then the parameters of the effective stress-strain relationship are derived from experimental bulge height versus the forming pressure. In the second one, the planar anisotropy of Hill'48 yield criterion is considered. Firstly, the simple tensile test is used to fit the isotropic strain hardening law. Secondly, the anisotropic parameter is identified by inverse technique from the experimental bulge test. The obtained results are compared with experimental measurements to validate the proposed strategies. It is proven that inverse identification methods could be a better alternative to analytical methods for flow stress parameters determination. Moreover, the anisotropy parameter has shown a large effect on the hydroformed tube responses, particularly on the wall thickness. Consequently, this material anisotropy should be considered within the flow stress identification of tube bulge tests.

Evaluation of effect of flow stress characteristics of tubular material on forming limit in tube hydroforming process

The material properties for the analytical and numerical simulation in sheet metal processes, especially in tube hydroforming process, are generally obtained from the uniaxial tensile test of raw sheet material. However, the validation of the formability and reliability of the numerical simulation for the tube hydroforming process arises from the fact that the material characteristics of tubes are different from those of the raw sheet materials. In order to determine the most suitable material property of the tubular material for the evaluation of forming limit on the THF process, the uniaxial tensile test for the specimens of the raw sheet metal and the roll-formed tube and the free bulge test for the roll-formed tubular material are carried out in this paper. The forming limit curves are also derived using plastic instability based on three kinds of necking criteria, which are Hill’s local necking criterion for sheet and Swift’s diffuse necking criteria for sheet and tube, to describe and explain the forming limits for the roll-formed tubular material in the THF process. In order to acquire the informative data on the forming limit curves in the THF process, the loading condition of the free bulge test is controlled. The proper band from nearly necking initiation to nearly bursting initiation has been defined for the roll-formed tubular material in the THF process. It can be concluded that the flow stress of the tubular material should be determined from the actual free bulge test to find the practically valuable forming limit curve for the THF process.

Finite Element Simulation on Superplastic Forming of Outer Jacket of Combustion Chamber

In this paper, mould configurations are studied by finite element simulation for superplastic blow forming of combustion chamber outer jacket. One concave and two convex mould configurations are basically considered to determine which type would be advantageous in terms of forming time and thickness distribution. For the simulation, the flow stress equation of duplex stainless steel was determined from free bulging test. The simulation results showed that the concave type was quit a bit different from the other two cases. The concave mould configuration produced shorter forming time and thicker thickness distribution than the others, and it seems to be more effective for superplastic forming of outer jacket. The obtained pressure profile for the concave mould configuration was employed in actual forming and the thickness distribution was compared for the verification.

Finite Element Simulation on Superplastic Forming of Outer Jacket of Combustion Chamber

In this paper, mould configurations are studied by finite element simulation for superplastic blow forming of combustion chamber outer jacket. One concave and two convex mould configurations are basically considered to determine which type would be advantageous in terms of forming time and thickness distribution. For the simulation, the flow stress equation of duplex stainless steel was determined from free bulging test. The simulation results showed that the concave type was quit a bit different from the other two cases. The concave mould configuration produced shorter forming time and thicker thickness distribution than the others, and it seems to be more effective for superplastic forming of outer jacket. The obtained pressure profile for the concave mould configuration was employed in actual forming and the thickness distribution was compared for the verification.

Blow forming of AZ31 magnesium alloy at elevated temperatures

In this work, the forming behaviour of a commercial sheet of AZ31B magnesium alloy at elevated temperatures is investigated and reported. The experimental activity is performed in two phases. The first phase consists in free bulging test and the second one in analysing the ability of the sheet in filling a closed die. Different pressure and temperature levels are applied. In free bulging tests, the specimen dome height is used as characterizing parameter; in the same test, the strain rate sensitivity index is calculated using an analytical approach. Thus, appropriate forming parameters, such as temperature and pressure, are individuated and used for subsequent forming tests. In the second phase, forming tests in closed die with a prismatic shape cavity are performed. The influence of relevant process parameters concerning forming results in terms of cavity filling, fillet radii on the final specimen profile are analysed. Closed die forming tests put in evidence how the examined commercial magnesium sheet can successfully be formed in complicated geometries if process parameters are adequately chosen.

알루미늄 튜브를 이용한 액압성형품 특성연구

In this study, the characteristics of aluminum tube hydroformed products at different extrusion type and heat treatment conditions were investigated. For the investigation, as-extruded, full annealed and T6-treated Al 6061 tubes at different extrusion type were prepared. To evaluate the hydroformability, free bulge test was performed at room temperature to 300℃. Also mechanical properties of hydroformed products at various pre- and post-heat treatments were estimated by hexagonal prototype hydroforming test at 250℃. And the tensile test specimens were obtained from hexagonal prototype hydroformed tube. As a results, hydroformability of full annealed tube is 5~8% higher than that of extruded and T6-treated tube. The tensile strength and elongation of T6-post heat treated indirect extrusion tube were more than 330㎫ and 12%, respectively. However, T6 pre treated hydroformed product represents high strength, 330㎫ and low elongation, 8%. Therefore, Hydroformability of Al6061 tube showed similar value for both extrusion types. However flow stress of direct tube showed 20~50㎫ lower value than indirect tube.

The determination of material parameters from superplastic free-bulging tests at constant pressure

Superplastic forming (SPF) of sheets is a very interesting and useful process when there is a need to produce items with complex geometry and integrated structures characterized by high specific resistance. To accurately model SPF processes it is important to know the constitutive equation of the material. With the help of the finite element method (FEM) this study proposes an innovative method for determining the characteristic parameters of superplastic materials. This method requires the material to undergo free-forming tests in which the stress state induced is biaxial (nearer the real stress conditions to which the material is subject during the SPF processes that are of interest to industry). The dimensions of the sheet and the forming die do not place any constraint on the application of this type of characterization procedure. The method adopted to characterize superplastic materials was validated by means of experimental activity carried out on an AZ31 magnesium-based alloy.

Investigation on the Hydroforming Characteristics of Double-layered Tubes

Double-layered tubes consist of an inner tube and an outer tube. Both are similar in material, contact closely and deform simultaneously when subjected to external force. Hydroforming assembly technology has several advantages in the manufacturing of double-layered tubes. In this study, the hydroforming characteristics of double-layered tube are investigated. Free bulging tests are performed to produce formability diagrams of double-layered tubes at various forming pressures and feeding amounts. In addition, the hexagonal-shape hydroforming test is performed to estimate the dimensional accuracy of double-layered tubes through the corner filling ratio and the gap between the inner and outer tubes. Besides experimental analyses, an analytical model that can predict internal pressure for the hydroforming of double-layered tubes is proposed and experimentally validated in this study.

On the optimisation of superplastic free forming test of an AZ31 magnesium alloy sheet

In this work, an original algorithm, proposed in [1–2], has been used to predict the variable pressure-time load profile during a superplastic forming process, in order to get best performances, in terms of ductility, on the material. The so obtained curve has been used in experimental free bulging tests on an AZ31 magnesium alloy sheet. Tests results, in particular the maximum bulge height achieved by the material, have been compared with numerical results and with constant-pressure experimental tests.

이중튜브의 액압 성형특성 연구

Double layered tube is assembled with an inner tube and an outer tube, similar in material or not, contacting closely and deforming simultaneously when subjected to external force. For the manufacturing of double layered tube, the hydroforming assembly technology has several advantages. Therefore in this study, hydroforming characteristics of double layered tube was investigated. The free bulge test was performed to produce formability diagrams of double layered tubes at various forming pressure and feeding amounts. The hexagonal shape hydroforming test was also performed to estimate the dimensional accuracies of double layered tube through the corner filling ratio and the gap between inner and outer tube. Besides experimental analyses, the analytical model that can predict internal pressure for the hydroforming of double-layered tube was proposed and experimentally validated.

Warm Hydroformability and Mechanical Properties of Pre- and Post- Heat Treated Al6061 Tubes

Warm hydroformability and mechanical properties of pre- and post- heat treated Al6061 tubes were investigated in this study. For the investigation, as-extruded, fully annealed and T6- treated Al 6061 seamless tubes were prepared. To evaluate the hydroformability, uni-axial tensile test and free bulge test were performed at room temperature and 200ÓC. Also mechanical properties of hydroformed part at various pre- and post-heat treatments were evaluated by tensile test. The tensile test specimens were obtained from hexagonal shaped tube hydroformed at 200ÓC forming temperature. As a result, hydroformability of fully annealed tube is 25% higher than that of extruded tube. The tensile strength and elongation were more than 330MPa and 12%, respectively, when hydroformed part was T6 treated after warm hydroforming. However, hydroformed part using T6 pre treated tube represents low elongation, 8%. Therefore, the T6 treatment after hydroforming for as-extruded tube is proved to be the most cost-effective among various processing conditions.

Experimental and analytical evaluation on flow stress of tubular material for tube hydroforming simulation

Abstract For the finite element simulation in tube hydroforming process, the flow stress must express the bi-axial state of stresses similar to that of the actual hydroforming operation. In this work, experimental and analytical methods to acquire the flow stress of tubular materials for hydroforming application were performed. Conventional tensile tests on specimens extracted from both a flat sheet and a roll-formed tubular material were conducted. And then, in order to consider the bi-axial state of stress, free bulge test of a simple tubular material was carried out and analysis results using flow stresses obtained from both test methods were compared and evaluated with the free bulge test. Therefore, it is concluded that the flow stress obtained from the free bulge test should be used for input material property for the numerical analysis of the tube hydroforming process to enhance the accuracy of those.

알루미늄 튜브 하이드로포밍 성형 해석 및 성형성 평가

A tube hydroformability testing system was designed and manufactured to observe the forming steps and to provide arbitrary combination of internal pressure and axial fred. The forming limit diagram of an aluminum tube was obtained from the free bulge test and the T-shape forming test using this system, giving the criteria for predicting failure in the hydroforming process. The hydroformability of aluminum tube according to different conditions of a prebending process was discussed, based on the finite element analysis and the forming limit test. The effects of 2D and 3D pretending on the tube hydroforming process of an automotive trailing arm were evaluated and compared with each other.

액압성형 시험을 통한 알루미늄 튜브 재료의 성형한계도

A tube hydroformability testing system was designed and fabricated enabling to apply the forming condition along arbitrarily pre-programmed internal pressure-axial feed path. The free-bulging and T-forming tests were carried out on the extruded aluminum (A6063) tube specimens with 40.6 mm outer diameter and 2.25 mm thickness. Nine different combinations of internal pressure and axial feed, yielding different strain paths from one another, were taken into consideration in order to induce bursting at various deformation modes. Major and minor strains were automatically measured from deformed grids around the fracture using a stereo-vision-based surface strain measurement system, named ASIAS. The forming limit diagram of the A6063 tube material was successfully obtained. Most of the data points acquired from free bulging and T-forming tests appeared in the range of negative minor strain on the FLD and are mostly located near the strain paths calculated from explicit finite element simulations. The forming limit obtained from tests after pre-tension was considerably lower than that from tests without pre-tension, which showed the strain path-dependency of the forming limit as well known in the sheet forming fold.

Experimental Analysis for the Tubular Hydroformability of Aluminum Alloys at Elevated Temperatures

Aluminum alloys have a high potential for weight reduction in automotive and other applications, but they have a relatively low tubular hydroformability at room temperature. Hot working processes are commonly used for bulk metal forming, such as forging and rolling, but rare in sheet metal forming like hydroforming. The hydroformability of aluminum alloys can however be enhanced significantly at elevated temperatures. In this study, the hydroformability of aluminum alloys at elevated temperatures has therefore been investigated by using a specially designed induction heating system. The formability characteristics at high temperatures were obtained by a T-fitting forming test as well as a free bulge test. The effects of the process parameters such as an internal pressure and temperature on tubular forming limits have mainly been investigated and the results are presented in this paper

Evaluation of the formability of plastic/Zn22Al/plastic sandwiched structures by gas blowing

AbstractFor the manufacturing of electromagnetic interference (EMI) shielding enclosures for portable 3C (computer, communication, and consumer electronics) products, a superplastic Zn22Al thin sheet is inserted in between plastic plates and formed by in‐mold blowing with gas pressure. In order to evaluate the formability of a plastic/Zn22Al/plastic sandwiched structure, free bulging tests are conducted in the temperature range between 135°C and 200°C in an infrared furnace. The results show that the ABS/Zn22Al/ABS sandwiched structure can be free bulged at 150°C with 0.21 MPa of gas pressure at a forming rate similar to that of the monolithic Zn22Al sheet. The PC/Zn22Al/PC sandwiched structure cannot be effectively bulged until the forming temperature is raised to 185°C. The optimized forming condition for PC/Zn22Al/ABS is either at 150°C with a gas pressure of 0.35 MPa or at 165°C and a gas pressure setting of 0.21 MPa. Also, the thickness distributions for Zn22Al and a variety of plastic/Zn22Al/plastic specimens after free bulging at various temperatures with various gas pressures are placed in comparison in this study.

Technological analysis of the superplastic bulging of a metal sheet

The application of the superplastic bulging of metal sheet in industry has been shown to be a very good method for forming thin shell workpieces that are complicated in shape; however, it has also posed some particular problems which can be solved by mechanical theory. In this paper, superplastic bulging is divided roughly into two stages: free bulging and fill forming. On the basis of previous work, the forming limit is analysed theoretically. Furthermore, the optimum loading conditions and the method of determining technical parameters are discussed so that the forming limit of superplastic bulging can be increased and the bulging time decreased. Free-bulging tests and forming tests on Zn-22wt.%Al metal sheet are in satisfactory agreement with the analytical conclusions.