S.J. Yuan

Experimental and numerical investigation into useful wrinkling during aluminium alloy internal high-pressure forming

Abstract Internal high-pressure forming is a process for manufacturing lightweight components, especially automotive parts, with advantages of lower cost and weight reduction, better structural integrity and increased strength and stiffness over the conventional stamping process. One of the typical failure modes, including wrinkling, buckling and splitting, will occur through an unreasonable combination of the control parameters: the internal pressure and the axial punch feeding. In most previous papers, wrinkling is considered to be a failure mode. However, not all wrinkles are defects. The collection of materials in an expanding area by the formation of wrinkles is an alternative method for obtaining a preformed shape in the hydroforming die. In this case, the key point is to obtain ‘useful’ wrinkles instead of ‘bad’ wrinkles. In this paper, an investigation will be conducted on how to control the shape of the wrinkle waves and its effect on the thickness distribution after hydroforming by using finite element simulation. LS-DYNA finite element software is used in this paper. An experiment has been carried out and the results obtained from experiment and simulation are in good agreement.

Analysis of corner filling behavior during tube hydro-forming of rectangular section based on Gurson–Tvergaard–Needleman ductile damage model

Friction plays an important role in the corner filling behavior during hydro-forming of rectangular section. In order to study the variation in corner filling behavior with internal pressure under the condition of different coefficients of friction, Gurson–Tvergaard–Needleman ductile damage model was used to describe material deformation involving damage evolution. First, an experimental–numerical hybrid method was applied to determine the parameters of critical porosity and failure porosity in Gurson–Tvergaard–Needleman ductile damage model. Second, a model describing the deformation behavior was established, in which the deformation zone was divided into three parts: sticking zone, sliding zone and corner zone. Third, the effect of coefficient of friction on the corner filling process was studied through simulation and experiments. The results show that as the internal pressure increases, the sticking zone starts at certain pressure and increases rapidly until taking over the whole contact length. Sticking friction will lead to greater thinning, and bursting occurs rapidly at the transition zone. Furthermore, the increase in the coefficient of friction will lead to greater variation in thinning and corner radius. For µ = 0.05 in experiments, corner filling is acceptable without bursting, correspondingly the final internal pressure is 67 MPa.

Effect of the initial structure on the hydro-forming of toroidal shells

Abstract In order to avoid the occurrence of wrinkling in the hydro-forming process for manufacturing elbow pipes, the effect of the initial structure on the forming of the toroidal shells has been investigated both theoretically and experimentally. The research results show that wrinkles can be prevented by selecting an appropriate initial structure.

Mechanical analysis on the cylinder-crown integrated hydraulic press with a hemispherical cylinder

As a new press, cylinder-crown integrated hydraulic press (CCIHP) is analyzed firstly in this paper. The hydraulic cylinder consists of a hemispherical shell, which also functions as a main proportion of press crown. Compared with the conventional hydraulic press which has a hydraulic cylinder mounted in the press crown, the structure and force distribution of CCIHP are significantly different. First, the section modulus of the hemispherical crown is much larger than that of the conventional rectangle crown because the hydraulic cylinder and press crown are manufactured into a solid part and have a higher geometry centroid. Therefore, the hemispherical crown has lower tensile bending stress and a higher safety factor from the strength point of view when the same bending moment is applied. Second, under the conditions of same value in terms of the allowable stress, internal pressure and internal radius, the wall thickness of hemispherical hydraulic cylinder could be much thinner than that of the conventional hydraulic cylinder. Both of the reasons mentioned above can result in a weight reduction of the press. Also, the cylinder and the press crown can be reinforced by each another, as a result, the material strength capacity is better utilized. As a prototype, the 6300 kN CCIHP with hemispherical hydraulic cylinder, which is the first one of this kind press designed and manufactured by Harbin Institute of Technology (HIT) is introduced in this paper. Finite element analysis for CCIHP has shown that both the stress and displacements on the press during the loading process are allowable, even if eccentric force is loaded. Finally the priority application range of CCIHP has also been proposed.

Research on hydroforming tubular part with large perimeter difference

The tube hydroforming process for manufacturing the tubular part with large perimeter difference was studied by means of experiments and FE simulation. It is an asymmetrical one and the expansion ratio is 70 percent. The part is successfully hydroformed by applying the useful wrinkles to accumulate sufficient metal in the area with large expansion ratio. The thickness distribution is analyzed, and the maximum thinning ratio of the part is 21.6 percent. It is shown from the experiment and simulation results that the tubular part with large perimeter difference, long expansion area and asymmetrical shape can be obtained in one step, applying the useful wrinkles. Typical failure modes were analyzed.

Complex Stress-Strain Relations of Tubular Materials Studied With a Flexible Hydroforming System

Extruded aluminum alloy tubes present strong anisotropic properties, making its deformation very sensitive to stress states. To better characterize a proper constitutive relationship, the use of experimental stress-strain response with biaxial stress states would be more suitable than that of a common uniaxial test. To achieve this purpose, an attractive testing method and homologous experimental system (i.e., hydro-bulging system) were developed. Different proportional and non-proportional loading conditions could be performed with this experimental system. Hydro-bulging tests under three typical proportional loading paths were done. A fourth-order hardening model proposed by the authors was used to reproduce these experimental stress-strain relations and compared them with the results predicted using a power law relationship. Regression results predicted by the fourth-order hardening model had a good consistency with the distribution of experimental data. Deviations of the fourth-order hardening model are much less than that obtained by a power law. The maximum deviation introduced by the power function is at least 2.6 times greater than that introduced by the fourth-order function. Therefore, the use of a new regression model to treat experimental data would improve the predicting accuracy of a related constitutive relation efficiently and further ensure the predicting accuracy regarding a hydroforming simulation.

Effect of Post-Weld Annealing Treatment on Plastic Deformation of 2024 Friction-Stir-Welded Joints

Post-weld annealing treatment (PWAT) process was developed to improve the plasticity of friction-stir-welded 2024 aluminum alloy. The effect of the PWAT on plastic deformation behavior and microstructure of the joints were studied using tensile test, the ASAME® automatic strain measuring system, and the electron backscattered diffraction (EBSD). It is found that the elongation of the as-welded joint can be improved by PWAT and increases with the decreasing PWAT temperature. The maximum elongation of the PWAT joints can reach up to 160% of that of the as-welded joint, and the joints exhibit no decrease in the tensile strength. The deformation inhomogeneity of the as-welded joint is significantly improved by large plastic strain occurring in the thermo-mechanically affected zone (TMAZ) when the PWAT temperature is lower than 250°. As the PWAT temperature increases, the deformation in the weld nugget is found to be more beneficial than that in the TMAZ for improving the plasticity of the joint. The high plasticity of the joint is attributed to the presence of the fine-equiaxed grains in the weld nugget during PWAT.

Further assessment of the plastic instability of thin-walled tubes in double-sided hydro-bulging process with verification experiments

An interesting process of double-sided tube hydro-bulging was proposed to provide a beneficial three-dimensional stress state in the deformation zone of the tube, so as to delay the occurrence of bursting on the tube. Previously proposed forming limit theory models with consideration of through-thickness normal stress found that the external pressure could lead to a deferred occurrence of plastic instability, but it has not been confirmed experimentally. In this article, the tensile plastic instability of tubes under double-sided pressures is further assessed based on the classical plastic instability theory. However, it is seen that the occurrence of plastic instability has not found delay when high external pressure was exerted on the outside surface of tube simultaneously, which is opposite to the previous forming limit models. In addition, an experiment investigation about double-sided tube hydro-bulging is conducted to verify the theoretical results, and the experimental results show that the external pressure has hardly any influence on forming limit strain of the 2A12 aluminum alloy tube before the occurrence of necking. Moreover, the contradiction between our results and the previously proposed forming limit theory models is that the previous models ignored the thickness item in the equilibrium equation.