Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

The continuous needs of improve performances in the automotive sector in terms of dynamic behaviour, fuel consumption and safety of passengers, have raised the interest for lightweight alloys as well as for the optimization of the design of the structural components of the car chassis. With this twofold aim, many researches are focused in the evaluation of new car designs, materials, and processes to manufacture even more complex components with increased stiffness-to-weight ratio. From that standpoint, the use of shaped hollow parts in the car body in white appears one of the most promising solutions, due to the elevated stiffness of tubular structures and the reduced weight. However, hydroforming processes that have been traditionally used to shape such components have shown several limitations with lightweight alloys, suffering their reduced formability, the temperature limitations of the forming liquids as well as long process time and complex machines. In this paper the recently introduced technology of Hot Metal Gas Forming (HMGF) has been considered, in order to investigate the influence of the process parameters on the formability of AA6060 tubes. The semi-finished tubes were produced through direct hot extrusion, with different temperatures and feed rates process, and tested by HMGF at elevated temperatures. The properties of the final products are investigated through analyses of the microstructure, micro hardness and thickness measurements.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Liu et al. [19] studied wall thickness distribution uniformity and formability of parts in the process of deformation by using different bulging pressure in the temperature range of 350~450 • C, and the results showed that temperature uniformity has a great influence on the formability and microstructure of parts. Michieletto et al. [20] studied the HMGF process bulging behavior of AA6060 aluminum alloy tube used for automobiles with different forming rates and temperatures, and the research showed that the microstructure and thickness distribution of parts formed at different forming rates were not significantly different, and the increase in forming temperature would increase the sticking rate of parts. Moreover, the length of the initial blank has little influence on the forming of the parts. ...
Article
Full-text available
The hot metal gas forming process can significantly improve the formability of a tube and is suitable for the manufacturing of parts with complex shapes. In this paper, a double wave tube component is studied. The effects of different temperatures (400 °C, 425 °C, 450 °C and 475 °C) and different pressures (1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa and 3 MPa) on the formability of 6063 aluminum alloy tubes were studied. The influence of hot metal gas forming process parameters on the microstructure was analyzed. The optimal hot metal gas forming process parameters of 6063 aluminum alloy tubes were explored. The results show that the expansion rate increases with the increase in pressure. The pressure affects the deformation of the tube, which in turn has an effect on the dynamic softening of the material. The expansion rate of parts also increases with the increase in forming temperature. The increased deformation temperature is beneficial to the dynamic recrystallization of 6063, resulting in softening of the material and enhanced deformation uniformity between grains, so that the formability of the material is improved. The optimum hot metal gas forming process parameters of 6063 aluminum alloy tubes are the temperature of 475 °C and the pressure of 2.5 MPa; the maximum expansion ratio is 41.6%.
... Sealing devices are also necessary to avoid the risk of gas leaking during the forming process. Michieletto et al. determined the optimal extrusion parameters during AA6060 tube hot metal gas forming process [14]. Maeno et al. improved die filling by preventing the temperature drop in aluminum alloy tube's gas forming process using air-filled into a sealed tube and resistance heating [15]. ...
Article
Full-text available
In this paper, a magnetorheological elastomer (MRE) forming process is developed to form a complicated hollow part of GH4169 superalloy. The novel forming process can realize the differential loading of the forming pressure and avoid liquid leaking in the conventional hydraulic forming process. The principle of the MRE forming process can be illustrated as that the MRE will bulge under the effect of the applied magnetic field. Hence, the local forming pressure in the deformation zone of the part will increase during the forming process. To estimate the range of current and the axial displacement of the punch, a power equation based on power approach and electromagnetic theory is established. Numerical simulations and experiments study the deformation process and the die filling of tube blank under different magnetic field intensities. Response surface methodology (RSM) is used to determine the optimal process parameters. RSM also models the predicted equations of the maximum thinning ratio and the average bulging diameter. Besides, the fitted RSM model’s accuracy is quantified by a statistical method of analysis of variance. Finally, the predicted equations’ reliability and the optimal process parameters obtained by RSM are proved by a validated experiment.
Article
Full-text available
A hot gas bulging process of an aluminium alloy tube using resistance heating, set into a forming machine, was developed. The tube was rapidly heated by the electrifying to increase the formability and to decrease the flow stress. The tube was bulged by thermally expanding the air sealed in the tube without control of internal pressure during the forming. Hot gas bulging of an aluminium alloy tube without and with axial feeding was performed. The effects of the initial internal pressure and the current on the expansion ratio of the tube were examined. The decrease in temperature around the contact with the electrode was prevented by inserting a stainless steel ring having low thermal conductivity and high heat generation between the copper electrode and tube, and thus the bulging length was increased. It was found that the hot gas bulging is effective in heightening the formability of the aluminium alloy tubes.
Article
The paper reports some preliminary results about the forming of aluminium alloy sheets conducted at elevated temperatures. The investigated range of temperature varies from room temperature to 400°C, thus exploring also the hot forming range. The influence of temperature on the material flow strength and ductility was investigated by means of tensile tests carried out on a testing machine equipped with induction heating to heat up the specimen. The ductility enhancement was evidenced also by SEM observations, underlying the specimen fracture mode at increasing temperature. Nakajima tests were then carried out on a specifically developed testing machine for material form-ability evaluation in order to draw the material forming limit curves at varying temperature and finally assess the temperature effect on the material formability.
Article
Tube hydroforming (THF) is a relatively new but established technology among metal tube forming processes. It is the technology of forming closed sections, hollow parts with different cross-sections by applying an internal hydraulic pressure and sometimes additional axial compressive loads to force a tubular blank to conform to the shape of a given die cavity. Material properties have a significant influence on the process stability. Often roll-formed, non-heat treated tubular materials made of steel with longitudinally oriented welding lines are used in tube hydroforming. Different production processes involve a change of the material properties from the initial flat sheet to the hydroformable tube. Testing methods such as tensile tests and conventional forming limit diagrams do not accurately reflect the state of stress and strain conditions seen in the tubular blank during the hydroforming process. Thus, inaccuracies in FEA predictions and design failures occur. Test methods were developed to characterize the relevant geometrical and mechanical properties of tubular semi-finished products.
Article
A hot gas bulging process of an aluminium alloy tube using pressure of sealed air and resistance heating set Into a die was developed. In this process, the tube was heated during the forming to prevent the drop In temperature. The tube was bulged by thermal expansion of the air sealed in the tube without control of internal pressure during the forming. The resistance heating of tubes was performed for different thicknesses and different lengths. The temperature of the tube was controlled by adjusting the current density and heating time. The effects of the thickness and the deviation of the tube were examined. The temperature of the tube Inside the die during the forming was measured by an infrared thermography r through small holes in the die.
Article
A new gas forming process of ultra-high strength steel hollow parts using air filled into sealed tubes and resistance heating was developed to omit the subsequent heat treatment. In this process, a sealed quenchable steel tube was rapidly resistance-heated to improve the formability. By applying die-quenching for holding at the bottom dead centre of a press, the formed part had very high strength, a hardness of 450 HV10 equivalents to a tensile strength of 1500 MPa. In addition, the dimensional accuracy of the formed part was improved by the increase in internal pressure for heating and compression of air filled into the sealed tube. To increase the hardness, the formed tube was cooled with air blowing during holding at the bottom dead centre and the corner of the die was optimised as to be in contact with the tube. The oxidation on the outer surface of the formed part was prevented by forming in a case filled with CO2 gas.
Article
A hot gas bulging process of an aluminium alloy tube using resistance heating set into a die was developed. In the developed process, the tube was heated during the forming, and thus the drop in temperature was prevented. The control of the hot gas bulging was simplified by sealing air in the tube. The tube was bulged by thermal expansion of the air sealed in the tube without control of internal pressure during the forming. Hot gas bulging of an aluminium alloy tube without and with the axial feeding was performed. The deformation behaviour of the tube in the die was observed by a heatproof glass plate inserted in the die. The timing of the axial feeding, the feeding velocity and the amount of the axial feeding were optimised.
Article
In the last ten years, the automotive sector presents large interest for light alloys tubes for structural and body car parts to reduce CO2 emissions. Tubes hydroforming is one of the most popular processes to obtain complex parts by using liquids as active part of the dies (i.e. water-or oil-based emulsions) with reduced costs of equipment and machines. However, when elevated temperatures should be used to increase the material formability, hydroforming processes are strongly limited due to the boiling point of liquids. The use of gas at elevated temperature in the so-called Hot Metal Gas Forming process (HMGF) has shown promising capabilities thanks to the increased formability and the possibility to form parts with lower pressures. The paper focuses on a novel experimental set-up to evaluate the tubes formability at high temperatures. Tubes are heated by electric current and air in pressure is used to form the material. Aluminium alloy AA6060 tubes specimens were used to test the experimental equipment and evaluate temperature and pressure ranges able to shape the material.
Article
Tube hydroforming (THF) is a relative new but established technology among metal tube forming processes. It is the technology of forming closed sections, hollow parts with different cross-sections by applying an internal hydraulic pressure and sometimes additional axial compressive loads to force a tubular blank to conform to the shape of a given die cavity. Material properties have a significant influence on the process stability. Often roll-formed, non-heat treated tubular materials made of steel with longitudinally oriented welding lines are used in tube hydroforming. Different production processes involve a change of the material properties from the initial flat sheet to the hydroformable tube. Testing methods such as tensile tests and conventional forming limit diagrams do not accurately reflect the state of stress and strain conditions seen in the tubular blank during the hydroforming process. Thus,inaccuracies in FEM predictions and design failures occur. Test methods were developed to characterize the relevant geometrical and mechanical properties of tubular semi-finished products. Additional FE analysis of the roll forming process aim to improve the hydro-formability of tubular semi-finished products.
Article
Free bulging test was carried out at different temperatures ranging from 350 °C to 500 °C to evaluate the formability of AA6061 extruded tube, which can provide technology foundation for complex structures forming in hot metal gas forming (HMGF) process. Maximum expansion ratio (MER) and bursting pressure were obtained to evaluate directly the formability at heated conditions. Vickers hardness at different positions was measured. The fracture surface after bursting was observed with scanning electron microscope (SEM), and the microstructure change along axial and hoop directions was analyzed by electron backscattering diffraction (EBSD). The results show that the largest MER value is 86% at 425 °C. Bursting pressure decreases from 4.4 MPa to 1.5 MPa with temperature increasing. The Vickers hardness of fracture position is a little higher than other positions after gas bulging. The fracture mechanism is still the micro-pore aggregation fracture at elevated temperature, while overheated structure appears seriously at 500 °C. The initial fine equiaxial grain grows as temperature increases, which is elongated simultaneously in both axial and hoop directions.
Article
The influence of temperature on the mechanical properties of 5A02 aluminum tube was investigated by uni-axial tensile test. The hydrobulge test was also carried out to characterize the formability of 5A02 aluminum tube at elevated temperature. The results of both uni-axial tensile tests and hydrobulge tests show the same changing tendency of the formability of aluminum tube at elevated temperature. The appropriate hydroforming temperature of 5A02 aluminum tube is about 200–230°C. The reasons for the considerable difference between the formability characterized by the two different test methods are discussed.
Article
Lightweight materials like aluminium and magnesium alloys offer a high potential for weight reduction in automotive and other transportation vehicle construction. The high alloy percentages in aluminium alloys and the hexagonal structure of magnesium, however, lead to a relatively low formability of the sheet materials which can be enhanced by conducting the forming processes at elevated temperatures up to the recrystallisation temperature. This strategy is commonly used in rolling and forging, but still rises problems in sheet forming. In a research project at the chair of manufacturing technology the process of hydroforming of aluminium and magnesium sheet material shall be enhanced to be conducted at elevated temperatures.Based on material tests to determine the behaviour of different aluminium and magnesium alloys at elevated temperatures, appropriate hydroforming strategies for these materials have to be developed. Laboratory systems for the uniaxial tension test, the free hydraulic bulging of sheet material and for strip drawing, all at elevated temperatures, were built up and used as models for the design of a production system for hydroforming with a warm pressure fluid. The tests were also simulated with an FEM-system to show the restrictions and the potentials of simulation tools for this production strategy, aiming at the subsequent simulation of the complete hydroforming process.
Article
New requirements of the automotive industry, concerning lightweight and non-corroding construction, demand new production methods. Due to this the hydroforming process of aluminium alloys are of special interest. The disadvantage of aluminium alloys is the poorer formability compared to steel. A method to increase the formability of the aluminium alloys during the hydroforming process is the enhancement of the forming temperature.The following work starts with the description of the hydroforming process at room temperature. Afterwards a concept for the thermal hydroforming is developed and a forming tool for sheet metals is realised. With this tool, experiments are executed which investigate the formability, the wall-thickness distribution, the microstructure before and after the forming and the strain distribution of the aluminium alloys at enhanced temperatures. With this knowledge, a thermal hydroforming tube part will be developed and prototypes will be produced. Simultaneous to the practical experiments a Finite-Element Model will be developed and used for a parameter study as well as for the design of the thermal hydroforming part.