Optimization of electrohydraulic forming process parameters using the response surface methodology

This work’s main objective is to determine the optimum process parameters in the electrohydraulic forming (EHF) of austenitic stainless steel AISI 304 of 0.25 mm thickness for macro and micro shape. A truncated cone with grooves in the apex is considered as macro-micro shape. The response surface methodology (RSM) was developed for process variables such as voltage and standoff distance to determine the optimum parameters. To validate the model, confirmation experiments have been conducted, i. e. for the optimum value of voltage (V) = 8.935 kV and standoff distance (SOD) = 40.60 mm, and from the experiments the forming depth predicted is 9.221 mm and depth from the experiments is 9.5 mm. The percentage deviation from the predicted and experimental forming depth is 3.025 %, an acceptable range of less than 5 % for the surface roughness, the predicted value is 0.2598 microns, and the experimentally measured value is 0.268. The percentage deviation is 3.156 % between the predicted and experimental values, an acceptable range of less than 5 %. This shows that the model is suitable for predicting both responses. The validation experiments also found that the sheet fills one of the grooves and partially fills the other, which shows the capability of the electrohydraulic forming process. Confirmation experiments have been conducted.

Joining of Sheets with Tubes by Electrohydraulic Forming

Electrohydraulic forming is a high-speed process, which is based on a force transmission by a working media. In this process, shock waves transmit the punching force in a very short period of time. These shock waves are applied to accelerate the workpiece towards a passive die. Besides forming and embossing of sheets and tubes, joining of sheets with tubes is enabled as a novel application presented in this contribution. Thereby, the tube is embedded temporarily in the die as a functional part. By accelerating the sheet towards the tube end, the joint is formed. This study deals with the question of how this joint is formed in sense of process kinematics and material flow. Therefore, the loading energy, the distance of sheet and tube as well as the sheet thickness was varied and the influence of these parameters and geometric conditions of the tube on the process and resulting joints was observed. Joining of EN AW-1050 aluminum alloy sheets to EN AW-6060 aluminum alloy tubes was performed. These joints were analyzed by microsections and head tension tests. The investigations introduce the new joining process regarding its process behavior and show first joining results.

Experimental and Numerical Analyses of Tubular Electrohydraulic Forming Process

Electrohydraulic forming (EHF) is a kind of high speed forming process, which deforms the metal by shock wave through instantaneous discharge of high voltage in water. Compared with the traditional forming methods, this high speed forming process can greatly improve the formability of the materials. There are many processing factors that affect the forming efficiency and performance of the electrohydraulic forming process, one of which is the discharge voltage between the electrodes. In this paper, three electrohydraulic forming experiments with various die shapes were carried out under various discharge voltage conditions. And the bulge height and axial length of the aluminum alloy A6061 tubes under different conditions were compared. Besides, finite element numerical simulation was also performed to quantitatively investigate the deformation history of the tube.

Acquisition of Dynamic Material Properties in the Electrohydraulic Forming Process Using Artificial Neural Network

Electrohydraulic forming is a high-velocity forming process that deforms sheet metals with velocities above 100 m/s and strain rates more than 100 s−1. This experiment was conducted in a closed space because of safety concerns related to the high-velocity conditions; therefore, we were not able to examine the deformation process of the sheet metal. To observe the electrohydraulic forming process in detail, we performed virtual numerical simulations using accurate material properties. Therefore, in this paper, we obtained the material property of a sheet metal from a numerical estimation by using a surrogate model based on the reduced order model and the artificial neural network. The Cowper–Symonds constitutive equation was selected for the Al 6061-T6 sheet metal, and two strain rate parameters were adopted as the unknown parameters. From the two sampling techniques, the training and test samples were extracted from the specific ranges of two unknown parameters, and a numerical simulation was performed for these samples by using the LS-DYNA program. The z-axis displacements of the deformed sheet metal were obtained from the results of the numerical simulation, and two basis vectors were extracted by using principal component analysis. In addition, to predict the weighting coefficients of the two basis vectors at the defined range of parameters, we used the artificial neural network technique as a surrogate model. By comparing the surrogate model and the experimental results and calculating the root mean square error value, we estimated the optimal parameter for Al 6061-T6. Finally, the reliability of the obtained material parameters was proved by comparing the experimental results, the surrogate model, and LS-DYNA.

Acquisition and Evaluation of Theoretical Forming Limit Diagram of Al 6061-T6 in Electrohydraulic Forming Process

The current study examines the forming limit diagram (FLD) of Al 6061-T6 during the electrohydraulic forming process based on the Marciniak–Kuczynski theory (M-K theory). To describe the work-hardening properties of the material, Hollomon’s equation—that includes strain and strain rate hardening parameters—was used. A quasi-static tensile test was performed to obtain the strain-hardening factor and the split-Hopkinson pressure bar (SHPB) test was carried out to acquire the strain rate hardening parameter. To evaluate the reliability of the stress–strain curves obtained from the SHPB test, a numerical model was performed using the LS–DYNA program. Hosford’s yield function was also employed to predict the theoretical FLD. The obtained FLD showed that the material could have improved formability at a high strain rate index condition compared with the quasi-static condition, which means that the high-speed forming process can enhance the formability of sheet metals. Finally, the FLD was compared with the experimental results from electrohydraulic forming (EHF) free-bulging test, which showed that the theoretical FLD was in good agreement with the actual forming limit in the EHF process.