Analysis of Axial Compression Bulging Process of Tee Pipe Under Elastic-Plastic Force Transmission Medium

Literature Overview

This paper by Zhang Yan Min, Song Zhi Zhen, and Sun Ai Xue was published in the Journal of Plasticity Engineering (2008, Vol. 15, No. 2, pp. 136-139). The research was conducted at the School of Materials Science and Engineering, Henan University of Science and Technology. The study investigates the use of alternative force transmission media in the axial compression bulging process of tee pipes, addressing the limitations of conventional internal high-pressure liquid bulging.

Core Technical Content

The axial compression bulging process is a well-established method for forming tee pipes from seamless tubes. A critical factor in this process is the force transmission medium filled inside the initial tube, which transmits the bulging pressure to the tube wall. The conventional approach uses internal high-pressure liquid bulging, which suffers from several drawbacks including complex tooling structure, high cost, and stringent sealing requirements.

The authors explored two alternative force transmission media: polyurethane rubber as an elastic material and a mixture of paraffin wax and graphite as a plastic material. Experimental trials were conducted on a self-developed 30-ton axial compression bulging test platform using copper tee pipes and hard aluminum tee pipes as specimens.

Experimental Results and Analysis

The experimental results clearly demonstrated that polyurethane rubber, while offering elastic recovery and uniform pressure distribution, was unsuitable for forming branches with a height-to-diameter ratio greater than 1. This limitation arises because the elastic medium cannot provide sufficient sustained pressure during the plastic deformation of the tube wall, leading to incomplete forming and wall thinning.

In contrast, the paraffin wax and graphite mixture at a 4:1 ratio provided the most ideal forming quality. The plastic nature of this medium allows it to flow and conform to the developing geometry during the bulging process, maintaining effective pressure transmission throughout the deformation. The graphite content improves the lubricity and reduces friction between the medium and the tube wall.

Engineering Practice Integration

The findings of this study have direct implications for the selection of force transmission media in industrial tee pipe manufacturing. The paraffin wax-graphite mixture at a 4:1 ratio offers a cost-effective and technically viable alternative to liquid bulging, particularly for small to medium production runs where the investment in high-pressure hydraulic systems is not justified.

The 30-ton test platform represents a practical scale for laboratory and pilot production. For industrial-scale production of larger tees, the press capacity requirements would scale accordingly, and the thermal management of the paraffin-graphite mixture would become a critical process parameter. The melting point of paraffin wax is approximately 50-70 degrees Celsius, and the mixture must be maintained in a molten state during the hot forming process.

Key Insights and Reflections

The study raises an important consideration regarding the recyclability and environmental impact of the force transmission medium. The paraffin wax-graphite mixture can be recycled after use, but the graphite content may degrade over multiple cycles. The polyurethane rubber, while potentially reusable, has a limited lifespan due to thermal degradation and mechanical fatigue.

Another practical consideration is the dimensional accuracy achievable with each medium type. The elastic medium provides more uniform pressure distribution, which may yield better dimensional accuracy for simple geometries. The plastic medium, while offering better forming capability for complex geometries, may introduce slight asymmetry due to its flow behavior during deformation.

Study Value and Outlook

This paper provides a practical solution to the tooling complexity problem in tee pipe bulging processes. The paraffin wax-graphite mixture at a 4:1 ratio represents a simple, low-cost, and effective force transmission medium that can be adopted in production environments where high-pressure hydraulic systems are impractical. Future research should investigate the long-term performance of this medium under repeated use cycles and explore the optimization of the mixture ratio for different steel grades and forming temperatures.