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In this study, we investigated toroidal shell wrinkling in free hydroforming. We specifically focused on toroidal shells with a regular hexagonal cross-section. Membrane theory was used to examine the distribution of stress and yield load in both preform and toroidal shells. The wrinkling moment was then predicted using an empirical formula of shell buckling. In addition, the wrinkling state was investigated using a general statics method, and the free hydroforming of toroidal shells was simulated using the Riks method. Subsequently, nonlinear buckling and equilibrium paths were analyzed. A toroidal preform was manufactured, and free hydroforming experiments were conducted. Overall, the experimental results confirmed the accuracy of the theoretical predictions and numerical simulations. This indicates that the prediction method used in the study was effective. We also found that wrinkling occurs during hydroforming in the inner region of toroidal shells due to compressive stress. Consequently, we improved the structure of the toroidal shells and performed analytical calculations and numerical simulations for the analysis. Our results indicate that wrinkling can be eliminated by increasing the number of segments on the inner side of toroidal preforms, thereby improving the quality of toroidal shells.

Tube hydroforming is a manufacturing process used to produce structural components in cars and trucks, and the success of this process largely depends on the careful control of parameters such as internal pressure and end-feed force. The objective of this work was to establish a methodology, and demonstrate its effectiveness, to determine the optimal process parameters for a tube hydroformed in a die with a square cross section. The Taguchi method was used to establish a design of virtual hydroforming experiments, and numerical simulations were carried out with the finite element code LS-DYNA®. A sensitivity analysis was also carried out with analysis of variance. Multi-objective functions that consider necking/fracture, wrinkling, and thinning were formulated, and the response surface methodology was used with the most sensitive factors to obtain a defect-free part. An objective function, based on the final corner radius in the part, was also included in the optimization model. The forming severity of virtual hydroformed parts was evaluated using the forming limit stress diagram and the forming limit (strain) diagram. Finally, the normal-boundary intersection method and the L 2 norm were used to obtain the Pareto-optimal solution set and the optimal solution within this set, respectively. The hydroforming process for this part was also optimized using the commercial optimization software LS-OPT®, with two different single-objective algorithms. However, the optimum load path predicted with the proposed methodology was shown to achieve a smaller corner radius. The proposed optimization technique helped to define a process window that leads to a robust manufacturing process and improved part quality.

Abstract- Application of pulsating pressure is a new and effective method to improve the formability of the tube hydroforming process. However, the factors that cause this improvement are still unclear. In this paper, the forming mechanism of pulsating free bulge hydroforming of tubes is studied using both finite element simulation and experiment. The effects of oscillating pressure on deformation behavior, thickness distribution, strain path and friction force are examined. It is shown that for a constant pressure path, the wall thickness decreases quickly up to bursting; whereas for the pulsating pressure, the thickness decreases gradually, and thus, local thinning is prevented by oscillating internal pressure. Formability is improved due to an increase of the longitudinal compressive strain and better wrinkling behavior. Small harmonic wrinkles appear and are removed during the pulsating process, hence, by this mechanism bursting and wrinkling are prevented, causing improvement of formability. During the last decades, finite element (FEM) simulations of metal forming processes have become important tools for designing and optimizing feasible production processes [4, 5]. Mori et al. [6, 7] have simulated the axi-symmetric pulsating hydroforming of tubes by rigid-plastic finite element method. Hama et al. [8] have exhibited the effectiveness of the oscillation of internal pressure on the formability for an automotive part by the static explicit finite element method. The authors have studied the effect of oscillation of internal pressure on the formability and geometrical accuracy of the products in the pulsating hydroforming process for T-shaped parts using a dynamic explicit finite element code [9]. The authors have also proposed a new method to improve die corners filling in box-shaped tube hydroforming by controlling wrinkling and under oscillation of internal pressure [10]. Since the frequency of the ultrasonic vibration is much higher than the pulsating hydroforming, it is likely that the latter may have a different mechanism for improving the formability as compared with the former [11]. The factors that improve the formability in pulsating tube hydroforming are still unclear and fundamental studies about these factors are scarce. In this paper, the pulsating free bulging process has been further studied by the three dimensional dynamic explicit finite element method. Effects of oscillating internal pressure on the deformation behavior, thickness distribution, strain path and friction force are examined. In addition, the results of finite element simulation are compared with experimental data. It is also shown that the wall thickness of the tube decreases gradually for the pulsating pressure; whereas for the same non-pulsating peak pressure path, the wall thickness decreases more quickly up to bursting, due to different bulging deformation during the hydroforming process. In addition, the effect of pulsating pressure on the strain path of the tube center was studied by the finite element method. It was found that the bursting can be postponed by pulsating pressure, due to the increase of the longitudinal compressive strain. The increase of longitudinal compressive strain was achieved by the improvement of the material end feeding.

The complex curvature parts are studied on by the rigid-flexible hydroforming, aimed at the accuracy analysis. In this paper, the cylindrical die with convex sphere and sharp fillet at the bottom, and with small size concave characteristic at the peak of the convex sphere, was designed to analyze the effect of bulging height and hydraulic pressure on the part accuracy, including large characteristic forming and small characteristic filling. The part-mold contact gap and springback distributions were discussed by experimental and numerical method to analyze the part accuracy. The results show that free bulging height and hydraulic pressure are the main factors that affect the part accuracy, and when the hydraulic pressure is constant, the springback of the large characteristic reduces with the increase of the free bulging height; when the free bulging height is constant, higher hydraulic pressure is better for the local small characteristic filling, and the part attach the mold better from the rear position where it contacts with the mold to the first position where it contacted with the mold.

Laser-activated high-speed hydraulic microforming is a typical high-speed hydroforming technology with a closed liquid chamber that uses laser shock wave pressure as driving source and suitable for microforming of magnesium–lithium alloy foil. Free-bulging experiments of laser-activated high-speed hydraulic microforming and corresponding numerical simulations using LS-DYNA finite element analysis software were conducted. Homogenisation mechanism of liquid shock wave pressure was revealed using numerical simulations, and whipping effect during dynamic deformation of the workpiece caused by the homogenous liquid shock wave pressure was found which could improve plastic formability of the workpiece. Increasing the concentration of the liquid medium NaCl solution was effective in increasing strain rate during dynamic plastic deformation of the workpiece, which in turn increased the pulsed laser energy threshold and resulted in increased free-bulging height. Meanwhile, the maximum thinning rate tolerated by the micro free-bulging feature at the threshold pulsed laser energy also increased with the concentration of the liquid medium.

Large-dimension complex integral thin-shell components are widely used in advanced transportation equipment. However, with the dimensional limitations of raw blanks and the manufacturing process, there are inhomogeneous geometric and mechanical properties at welded joints after welding, which have a significant effect on the subsequent forming process. Therefore, in this paper, the microstructure of welded joints with a sharp property change was accurately characterized by the proposed isothermal treatment method using the BR1500HS welded tube as an example. In addition, an accurate constitutive model of welded tubes was established to predict the deformation behavior. Firstly, the heat-treated specimens were subjected to uniaxial tensile tests and the stress–strain curves under different heat treatment conditions were obtained. Then, the continuous change in flow stress in the direction of the base metal zone, the heat-affected zone and the weld zone was described by the relationship between the microhardness, flow stress and center angle of the welded tube. Using such a method, a continuous constitutive model of welded tubes has been established. Finally, the constitutive model was compiled into finite-element software as a user material subroutine (VUHARD). The reliability of the established constitutive model was verified by simulating the free hydro-bulging process of welded tubes. The results indicated that the continuous constitutive model can well describe the deformation response during the free hydro-bulging process, and accurately predicted the equivalent strain distribution and thickness thinning rate. This study provides guidance in accurately predicting the plastic deformation behavior of welded tubes and its application in practice in hydroforming industries.

Bursting in tube hydroforming is preceded by localized deformation, which is often called necking. The retardation of the initiation of necking is a means to enhance hydroformability. Since high strain gradients occur at necking sites, a decrease in local strain gradients is an effective way to retard the initiation of necking. In the current study, the expansion at potential necking sites was intentionally restricted in order to reduce the strain gradient at potential necking sites. From the strain distribution obtained from FEM, it is possible to determine strain concentrated zones, which are the potential necking sites. Prior to the hydroforming of a trailing arm, lead patch is attached to the tube where the strain concentration would occur. Due to the incompressibility of lead, the tube expansion is locally restricted, and the resultant strain extends to adjacent regions of the tube during hydroforming. After the first stage of hydroforming, the lead is removed from the tube, and the hydroforming continues to obtain the targeted shape without the local restriction. This method was successfully used to fabricate a complex shaped automotive trailing arm that had previously failed during traditional hydroforming processing.

The aim of the present paper is to evaluate existing constitutive models and to fitting hardening laws of SS304 tubes for the accurate prediction of the deformation behaviors of the tubes in hydroforming. Uniaxial tensile test (UTT) and free hydro-bugling (FHB) experiments were conducted on SS304 tubes, and a hi-speed three-dimensional (3D) digital image correlation (DIC) system was applied to obtain the deformation data of the samples. Eight constitutive relationships of the tubes were then established by fitting the equivalent stress and strain data with the four existing constitutive models of Hollomon, Ghosh, Voce and Ghosh Voce, and the fitting accuracy of the obtained constitutive relationships were analyzed and compared. The results show that Ghosh Voce model holds the highest accuracy in describing the deformation behaviors of the tubes in UTT and FHB, followed by the Ghosh model and then the Hollomon model. The Voce model holds the lowest accuracy. A distinct discrepancy between the constitutive relationships obtained using UTT and FHB experiments are observed in present research conditions.