Optimization of Internal High-Pressure Bulging Parameters for Ellipsoidal Energy-Absorbing Pipe Fittings via Orthogonal Experiments

Literature Overview

This paper by Wang Shuqiang et al., published in Modern Manufacturing Engineering (2023, Issue 6, pp. 89-94), addresses a critical manufacturing challenge in automotive crashworthiness engineering: the efficient production of ellipsoidal energy-absorbing boxes (EABs) using internal high-pressure bulging (IHPB). The authors, affiliated with Shenyang University of Chemical Technology and Haiwo Machinery (China) Co., Ltd., propose a novel forming route that eliminates the adverse effect of weld seams on energy absorption performance, while simultaneously achieving lightweight design. The research was supported by the Liaoning Provincial Natural Science Foundation (20170540722) and the Liaoning "Xingliao Talents Program" Young Top Talent Project (XLYC1907083).

Core Technical Content and Methodology

The ellipsoidal EAB functions as a key component in automotive collision systems, absorbing a majority of impact energy through controlled plastic deformation. The inherent advantage of this geometry lies in its ability to distribute uniformly distributed external loads across the shell, providing superior mechanical performance compared to conventional rectangular or cylindrical energy absorbers. The authors adopt a two-stage methodology: finite element simulation (FEM) coupled with orthogonal experimental design to systematically optimize the IHPB process parameters.

Three key process variables were investigated: the diameter of the wave-shaped induction structure, the bulging pressure, and the axial feed of the two-side punches. The wave-shaped induction structure is a critical geometric feature that initiates and guides the bulging deformation pattern, ensuring uniform wall expansion without localized thinning or wrinkling. The bulging pressure governs the intensity of outward radial deformation, while the axial feed of the punches controls the longitudinal material flow and final dimension accuracy.

Key Technical Parameters and Optimization Results

The orthogonal experimental design enabled the authors to efficiently identify the optimal parameter combination from a limited number of simulation runs, avoiding the computational cost of a full factorial study. The optimized parameters were subsequently validated through physical bulging experiments, confirming the accuracy of the FEM-based orthogonal approach. The authors emphasize that this method provides a practical reference for the manufacturing of ellipsoidal EABs.

Interpretation of Technical Points

From a forming metallurgy perspective, the IHPB process for ellipsoidal geometries presents unique challenges. The ellipsoidal shape requires non-uniform strain distribution across the blank, with higher strains expected at the equatorial region and lower strains near the poles. The wave-shaped induction structure serves as a geometric constraint that channels material flow preferentially along the desired deformation path, reducing the risk of wall thinning exceeding the forming limit curve (FLC). A diameter of 8 mm represents a balance between sufficient constraint effect and manageable manufacturing tolerance.

The bulging pressure of 25 MPa is notably moderate compared to typical IHPB pressures for cylindrical tubes, which often range from 30 to 60 MPa. This lower pressure requirement is attributed to the geometric advantage of the ellipsoidal shape, which provides inherent stiffness against radial expansion. The two-side punch axial feed of 8 mm ensures adequate longitudinal material supply to the bulging region, preventing excessive thinning at the equator.

Engineering Practice and Quality Control Considerations

In practical manufacturing, several quality control measures must be implemented. First, the initial blank geometry must be precisely controlled, as any deviation in the wave-shaped induction structure diameter will propagate into the final bulged shape. Second, the hydraulic system must maintain pressure stability within ±1 MPa to ensure repeatable bulging results. Third, post-forming inspection should include wall thickness measurement at critical locations (equator, poles, and transition zones) to verify that thinning remains within acceptable limits, typically not exceeding 15-20% of the original thickness.

The elimination of weld seams through IHPB is a significant advantage from a structural integrity standpoint. Traditional EABs manufactured by welding flat plates or bent tubes introduce weld heat-affected zones (HAZs) that can become preferential crack initiation sites under cyclic loading. The seamless nature of the IHPB process ensures homogeneous material properties throughout the component, directly translating to more predictable and reliable energy absorption behavior.

Study Insights and Implications

This research demonstrates the effectiveness of combining numerical simulation with statistical experimental design for process optimization in complex forming operations. The orthogonal experimental approach reduces the number of required simulation runs from a full factorial (which would be impractical for three factors with multiple levels) to a manageable subset while still capturing the main effects and two-factor interactions. For engineers working on similar forming challenges, this methodology offers a cost-effective pathway to process development.

A notable implication is the potential for extending this approach to other complex geometries beyond ellipsoidal EABs, such as multi-lobed cross-sections or variable-curvature tubes used in crash structures. The key prerequisite is the availability of a validated FEM model that accurately captures the material behavior, friction conditions, and boundary constraints of the forming process. Future work should address the interaction between forming parameters and the subsequent crashworthiness performance, establishing a direct link between manufacturing quality and functional safety.