Zhuojin Pipe Fitting Co., LtdOptimization of Internal High-Pressure Bulging Parameters for Ellipsoidal Pipe Fittings Using Orthogonal Experimental Design
Literature Overview
This paper, published in Modern Manufacturing Engineering (Vol. 2023, No. 6, pp. 89-94) by Wang Shuqiang et al. from Shenyang University of Chemical Technology and Hayward Machinery (China) Co., Ltd., addresses the manufacturing of ellipsoidal energy-absorbing boxes used in automotive crash safety systems. The authors propose using internal high-pressure bulging (also known as internal high-pressure forming or IHF) as the manufacturing process and optimize the key process parameters using orthogonal experimental design combined with finite element simulation.
The research was funded by the Liaoning Provincial Natural Science Foundation and the Liaoning Province "Xing Liao Talent Program" for young outstanding talents. The optimized parameters obtained are: wave-shaped induction structure diameter of 8 mm, bulging pressure of 25 MPa, and bilateral punch axial feed of 8 mm.
Process Description and Technical Background
Internal high-pressure bulging is a forming process in which a tubular workpiece is placed inside a die cavity, and internal hydraulic pressure (typically 20-40 MPa) is applied to force the tube to expand and conform to the die shape. The process is well-suited for producing complex-shaped tube components with uniform wall thickness, free of weld seams, and with enhanced mechanical properties due to work hardening.
For the ellipsoidal energy-absorbing box application, the process offers three key advantages over conventional manufacturing methods:
| Advantage | Explanation | Engineering Benefit |
|---|---|---|
| Lightweight | Uniform wall thickness, no excess material | Reduces vehicle weight, improves fuel efficiency |
| No weld seams | Monolithic forming, no joining required | Eliminates weld defects and stress concentrations |
| Enhanced energy absorption | Work hardening improves crash performance | Superior energy absorption compared to welded boxes |
The ellipsoidal shape is specifically chosen for energy absorption because it distributes external uniformly distributed loads efficiently. During a vehicle collision, the ellipsoidal box undergoes controlled plastic deformation, absorbing kinetic energy through progressive wall collapse. The absence of weld seams is critical because welds are potential initiation sites for fracture, which can lead to catastrophic failure rather than controlled energy absorption.
Orthogonal Experimental Design
The authors used orthogonal experimental design (OED) to systematically investigate the effects of three key process parameters on the bulging outcome. OED is a statistical method that allows the evaluation of multiple factors with a reduced number of experiments compared to full factorial design.
The three factors and their levels were:
| Factor | Level 1 | Level 2 | Level 3 | Unit |
|---|---|---|---|---|
| Wave-shaped induction structure diameter | 6 | 8 | 10 | mm |
| Bulging pressure | 20 | 25 | 30 | MPa |
| Bilateral punch axial feed | 6 | 8 | 10 | mm |
The wave-shaped induction structure is a critical feature of the process. It is a pre-formed corrugated section that serves as the pressure vessel for the internal hydraulic fluid. The diameter of this structure determines the volume of the pressure chamber and affects the pressure distribution during bulging.
The bulging pressure is the primary forming force, applied through the internal hydraulic system. It must be high enough to overcome the material's yield strength and die friction, but not so high as to cause wall thinning beyond acceptable limits or burst failure.
The bilateral punch axial feed is a supplementary forming action where punches on both sides of the tube apply axial compressive force during bulging. This axial compression helps to control the material flow and prevents excessive thinning at the pole regions of the ellipsoidal shape.
Optimization Results and Process Window
The orthogonal experimental analysis identified the optimal parameter combination as:
- Wave-shaped induction structure diameter: 8 mm
- Bulging pressure: 25 MPa
- Bilateral punch axial feed: 8 mm
These parameters represent a balance between forming quality (shape accuracy, wall thickness uniformity) and process efficiency (cycle time, equipment requirements). The optimized parameters were validated through actual bulging trials, confirming the accuracy of the FEA-based optimization.
The process window for successful forming is relatively narrow, which is typical for internal high-pressure bulging of complex shapes. Deviations from the optimal parameters can lead to:
| Deviation | Effect | Consequence |
|---|---|---|
| Pressure too high | Excessive wall thinning | Reduced mechanical strength, potential burst |
| Pressure too low | Incomplete forming | Shape defects, dimensional non-conformance |
| Induction diameter too small | Insufficient pressure volume | Uneven pressure distribution |
| Induction diameter too large | Excessive pressure volume | Overshoot, wall thinning |
| Punch feed too high | Excessive axial compression | Buckling, wrinkling |
| Punch feed too low | Insufficient material control | Pole thinning, shape distortion |
Engineering Practice Integration
For engineers implementing this process in production, several practical considerations are important:
- Material selection: The tube material must have adequate formability (high n-value, low r-value) to accommodate the complex strain distribution in ellipsoidal bulging. Cold-rolled low-carbon steel sheets with thickness in the 1.5-3.0 mm range are typically suitable.
- Die design: The die cavity must be designed with appropriate draft angles and surface finish to minimize friction and ensure uniform material flow. Surface treatments such as DLC (Diamond-Like Carbon) coating can significantly reduce friction and improve surface quality of the formed part.
- Process monitoring: Real-time monitoring of hydraulic pressure, punch force, and displacement is essential for quality control. Deviations from the expected process signature can indicate die wear, material variation, or hydraulic system issues.
- Post-forming inspection: The formed ellipsoidal box should be inspected for dimensional accuracy (using CMM or laser scanning), wall thickness uniformity (using UT or CT), and surface defects (using VT or MT).
Study Insights and Implications
This paper exemplifies the effective integration of computational methods (FEA) and experimental design (OED) for process optimization. The approach is particularly valuable when the process involves multiple interacting parameters, as is the case with internal high-pressure bulging. The FEA provides detailed physical insight into the forming mechanism, while the OED ensures that the parameter space is explored efficiently.
The elimination of weld seams through monolithic forming is a significant quality improvement for crash safety components. In the context of automotive safety standards (such as ISO 9669 for energy absorption), the absence of welds means that the energy absorption performance is governed solely by material properties and geometry, without the variability introduced by weld quality. This leads to more predictable and consistent crash performance.
The process also has implications for the broader field of tube forming. Internal high-pressure bulging is increasingly being adopted for manufacturing complex automotive components (crossmembers, subframes, battery housings) and aerospace structural elements. The key challenge remains the narrow process window and the need for precise control of process parameters. The orthogonal experimental approach provides a systematic methodology for process development that can be adapted to different shapes and materials.