Warm Hydroforming Research Articles

A Study on Warm Hydroforming of Al and Mg Sheet Materials: Mechanism and Proper Temperature Conditions

Hydroforming of lightweight materials at elevated temperature is a relatively new process with promises of increased formability at low internal pressure levels. In this study, the mechanism of warm hydroforming processes is presented in terms of its formability by comparison with warm forming, and cold hydroforming processes. Additionally, a strategy is proposed to control process parameters, such as temperature, hydraulic pressure, blank holder force, and forming speed. As a part of this strategy, the proper temperature condition is determined by adaptive-isothermal finite element analysis (FEA) and a design of experiment (DOE) approach. The adaptive-isothermal FEA determines the temperature levels of the blank material, which is selectively heated, by checking position of the blank material and adopting temperature level of the neighboring tooling. The proposed adaptive-isothermal FEA/DOE approach leads to the optimal temperature condition in a warm hydroforming system accurately and rapidly as opposed to costly and lengthy experimental trial and errors and/or fully coupled thermo-mechanical simulations. Other process parameters are also optimized in a continued study (Choi et al., 2007, “Determination of Optimal Loading Profiles in Warm Hydroforming of Lightweight Materials,” J. Mater. Process. Techn., 190(1–3), pp. 230–242.).

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.

Application of a combined heating system for the warm hydroforming of lightweight alloy tubes

A combined heating system – comprised of an induction coil plus a heating element – has been applied to the warm hydroforming of aluminum alloy tubes. For the rapid heating of tubes, a specially designed induction heating system has been developed as the main heating source. The induction heating system uses a high frequency induction coil in order to concentrate the energy into the tube and reduce energy losses. However, heat produced by the induction heating system was unequally distributed over the length and circumference of the tube, and as a result, the temperature distribution was non-uniform. An additional heating element was inserted into the inside of the tube to increase temperature uniformity by maintaining the forming temperature and reducing the heat lost to the tooling. Warm hydroforming tests with the combined heating system exhibit increased hydroformability, producing more uniform bulging in the tubes. The processing parameters such as internal pressure, axial feeding and heating conditions have also been successfully optimized.

연성파괴 이론을 적용한 튜브 온간액압성형의 성형한계 예측

Hydroformability and fracture criteria of FE analysis based on ductile fracture were investigated in warm hydroforming of A16061 tube. To evaluate the hydroformability, uni-axial tensile test and bulge test were performed at room temperature and . The measured flow stresses were used as input parameters for FE analysis. The damage values were calculated by FE analysis based on ductile fracture criteria at maximum radius of free bulged tubes. Damage values were compared of hexagonal shaped hydroformed parts. As a result, the formability by critical damage value for extruded tube is lower than that of full annealed tube up to 0.5.

Formability of Al6061 Extruded Tube in Warm Hydroforming

Formability of tube in elevated temperature is essential data to design the warm hydroforming process parameters, such as tube diameter, forming temperature and die geometries. Since the quantitative data of forming limit can be used to predict the failure on forming process, formability data available on the FE analysis is one of the very important information for the optimum design. In this study, the effect of heat treatment conditions and deformation temperature on the formability was investigated for the warm hydroforming of Al6061 tube. Full annealing and T6-treatment are applied for the heat treatment of Al6061 tubes. To evaluate the hydroformability, uni-axial tensile test and bulge test were performed at temperature ranges between room temperature and 300oC. The measured flow stresses were used as input parameters for the simulation of warm hydroforming process. The damage value and strain variation during hydroforming are analysed by FEM. A forming limit based on the ductile fracture criteria has been proposed by combining the results of experimental and FE analysis for the estimation of formability and optimization of warm hydroforming process.

Warm Hydroforming of Magnesium Alloy AZ31 Sheets

In the present study, warm hydroforming of cell phone cases with magnesium alloy AZ31 sheets was investigated. Fine-grained magnesium alloy sheets were prepared by cross rolling. And the tensile tests were first conducted in order to determine the proper forming temperature. The results showed that the most suitable temperature range appears to be 150-200°C. At last, the magnesium alloy cell phone cases characterized with the small round radius of all edges were formed successfully at 170°C with the low punch velocity and the maximum pressure not less than 5MPa.

Determination of optimal loading profiles in warm hydroforming of lightweight materials

Abstract In this study, a methodology is developed to determine proper profiles of the hydraulic pressure and the blank holder force under warm hydroforming conditions at different punch speeds. First, finite element models (2-D and 3-D) are developed and validated through comparisons with the experimental results from the literature. To determine the optimal hydraulic pressure and blank holder force profiles simultaneously, an adaptive FE analysis with fuzzy control algorithm was developed. Thinning, wrinkling, punch wall contact, die corner floating and conduction were used as criteria in the fuzzy control algorithm. The effects of the determined loading profiles are then presented in terms of thickness, strain and stress, and temperature distributions as well as success/failure of the forming process. The effect of punch speed was also investigated to reveal its impact on part formability. It was found that the optimal process conditions can be rapidly and accurately obtained with the developed adaptive FEA method coupled with fuzzy control algorithm.

Finite element analysis and experimental confirmation of warm hydroforming process for aluminum alloy

Abstract The hydroformability of an extruded aluminum alloy at elevated temperatures was investigated in this study. To properly analyze the process, it is necessary to account for the variation in the mechanical properties of the aluminum that depend on the forming temperature and the heat conduction during warm hydroforming. Simulations coupling the plastic deformation and temperature distribution in the warm hydroforming process were performed and compared with experimental data. The multi-purpose finite element code DEFORM-2D can handle the calculations, but requires significant computation time if contact heat transfer between the die, the tube, and the pressure medium occurs. Experiments were conducted with a high temperature tribometer (pin-on-disk) allowing the measurement of the friction coefficient for the aluminum alloys at several temperatures. These friction results are applied to the coupled simulation. From the simulations, the optimal process parameters, such as internal pressure and preset temperature on hydroformability, can be found. The comparison of the finite element analysis with the experimental results shows that the hydroformability trends (given by bulge height) and the temperature distribution of the tube specimen agree well with one another. The finite element results also showed that the temperature distribution did not strongly affect the hydroformability of the aluminum tubes.

A study on the analytical modeling for warm hydro-mechanical deep drawing of lightweight materials

The warm hydroforming process has become an emerging technology in recent years to reduce the weight of automotive body structure and minimize the number of process steps. In this study, analytical models were developed to investigate the effects of process conditions such as temperature, hydraulic pressure, blank holder force and forming speed. The analytical model under hydro-mechanical deep drawing (HMD) condition was developed based on experimental results in the literatures. FE models were also developed to validate the analytic models. Then, the analytic model was validated through comparisons with both existing experimental results and FE results. The analytical model provided rapid and reasonably accurate results for the design of warm hydroforming process. Based on this analytic model, several parametric studies were performed regarding to the temperature, hydraulic pressure, blank holder force, and punch speed conditions. It was demonstrated that the process windows for a successful part forming could be rapidly predicted with a reasonable accuracy by the analytic model compared to lengthy and costly thermo-mechanical FEA or experimental trial and error.

Tube and Sheet Hydroforming-Advances in Material Modeling, Tooling and Process Simulation

Tube Hydroforming is a well accepted production technology in automotive industry while sheet hydroforming is used in selected cases for prototyping and low volume production. Research in advanced methods (warm sheet and tube hydroforming, double blank sheet hydroforming, combination of hydroforming and mechanical sizing, use of multi-point and elastic blank holders) is expanding the capabilities of hydroforming technologies to produce parts from Al and Mg alloys, as well as Ultra High Strength Steels. In the development of advanced hydroforming methods, experience based knowledge is not readily available. Thus, robust process simulation is required, along with adequate material modeling and identification of friction coefficients as input to process simulation. This paper gives an overview of advanced hydroforming methods, including examples of novel machine and tooling designs. The use of reliable process simulation is illustrated with examples that demonstrate the significance of material and friction date for making accurate predictions. Advanced simulation methods for warm forming and for programming multiple-point blank holder are also discussed. This review illustrates that hydroforming continues to make advances and has the potential to make many contributions to production technology in the near future.

The Effects of the Heating Conditions on the Hydro-Formability of the Aluminum Alloys at High Temperatures

Modern automobiles are built with a steadily increasing variety of materials and semifinished products. The traditional composition of steel sheet and cast iron is being replaced with other materials such as aluminum and magnesium. But low formability of these materials has prevented the application of the automotive components. The formability can be enhanced by conducting the warm hydroforming using induction heating device which can raise the temperature of the specimen very quickly. The specimen applied to the test is AA6061 extruded tubes which belong to the age-hardenable aluminum alloys. But in the case of AA6061 age hardening occurs at room temperature or at elevated temperatures before and after the forming process. In this study the effects of the heating condition such as heating time, preset temperature, holding time during die closing and forming time on the hydroformability are analyzed to evaluate the phenomena such as dynamic strain hardening and ageing hardening at high temperatures after the hydroforming process.

Deformation Behaviors of 6061 and 7075 Aluminum Tubes at Elevated Temperatures for Warm Hydroforming

The deformation behaviors of fully annealed or T6-treated 6061 and 7075 aluminum tubes are investigated at elevated temperature using uniaxial tensile test. Fully annealed 6061 and 7075 tube, and T6-treated 7075 tube do not show sharp local necking with an elongation of 50% at tensile temperature of 300oC, accordingly, it is expected that warm hydroforming process can be applied. The increase of tensile temperature does not significantly affect the total elongation of T6-treated 6061 tube.

고강도 알루미늄 튜브의 온간 하이드로포밍 특성

Hydroformability of 6061 and 7075 aluminum tube materials was studied by warm hydroforming experiments. A special tooling and heating system was designed and manufactured in order to perform warm hydroforming between room temperature and <TEX>$300^{\circ}C$</TEX>. The control of tube temperature for warm hydroforming was made by the control of temperature of oil medium. Warm hydroformability was analyzed by tube appearances, tube elongation and hardness values. Hydroforming characteristics of 6061 and 7075 tubes showed different temperature dependence between room temperature and <TEX>$300^{\circ}C$</TEX>. The difference in hydroformabilities of 6061 and 7075 at elevated temperatures was interpreted by the different sensitivity to dynamic strain aging of both aluminum materials.

Effects of pre-treatment conditions on warm hydroformability of 7075 aluminum tubes

Warm hydroformability of 7075 tube materials was studied by hydroforming experiments between room temperature and 300°C. Warm tension test was also performed to investigate the characteristics of the warm hydroforming process of high strength aluminum alloys. Especially, the effects of pre-treatment conditions on the flow behavior and the microstructures during warm deformation of tube materials were qualitatively analyzed. The results showed that sufficient elongation properties of high strength aluminum alloys, which are necessary for successful hydroforming of complex shaped parts in a single operation, could be achieved by the selection of pertinent pre-treatment conditions and deformation temperatures for hydroforming. It was shown that the flow behavior of 7075 was mainly affected by interplay of dynamic strain aging, dynamic recovery and dynamic precipitation according to deformation temperature.