Compound Extrusion Bulging Process for Automotive Hydraulic System Tee Fittings

Literature Overview

The paper by Gao Fengling, Wang Zhiyuan, and Zhao Wei from Henan University of Science and Technology, published in "Tractor and Farm Transport" (2007, Vol. 34, No. 5, pp. 55-57), investigates a compound bulging process to form tee fittings directly from seamless tubes in a single operation. This research is particularly relevant to automotive hydraulic system components, where tee fittings serve as branching elements for fluid distribution. The work was supported by the Youth Research Fund of Henan University of Science and Technology (Grant No. 2003QN01).

Core Technical Approach

The authors propose a compound bulging process that integrates multiple forming operations into a single deformation cycle. The fundamental concept is to simultaneously create the main bore and the branch bore in a seamless tube without requiring subsequent welding or machining operations. This approach eliminates the need for separate forging, welding, and post-weld heat treatment steps that are conventionally required for tee fitting manufacture.

The process involves the following key stages:

Initial radial expansion of the seamless tube to establish the main bore geometry
Simultaneous axial displacement of material to form the branch opening
Controlled material flow to achieve the desired wall thickness distribution at the intersection zone

Process Parameters and Forming Windows

Parameter	Typical Range	Remarks
Tube material	20 steel, 45 steel	Automotive-grade seamless tubes
Initial tube OD	12-25 mm	Depends on hydraulic system pressure rating
Wall thickness	1.5-3.0 mm	Must accommodate material flow to branch
Branch angle	90 degrees	Standard for hydraulic branching
Die temperature	20-60 degrees C	Cold forming preferred for precision
Bulging force	50-150 kN	Depends on tube diameter and material
Forming speed	2-8 m/min	Controlled to avoid material tearing

Engineering Practice Integration

From my decades of experience in pipe fitting fabrication, the compound bulging approach addresses a critical bottleneck in automotive component manufacturing. Conventional tee fittings for hydraulic systems are typically produced by:

Cutting a branch hole in the main tube and welding a branch tube
Forging from bar stock followed by machining
Seamless tube cutting and forming

Each of these methods introduces quality concerns. The welded approach requires thorough NDT (PT or RT) of the weld joints, heat treatment to relieve residual stresses, and dimensional inspection. The forged approach is economical only at high volumes and requires significant machining time.

The compound bulging process offers several advantages:

Material continuity: No weld discontinuities exist, which is critical for high-pressure hydraulic applications where fatigue life is paramount.
Production efficiency: Single-operation forming reduces cycle time significantly compared to multi-step conventional processes.
Surface integrity: The formed surface retains the original tube surface quality, reducing the need for secondary finishing.

However, there are challenges that must be addressed in practice:

Wall thinning at the branch intersection: The material flow to the branch inevitably thins the wall at the intersection zone. Finite element simulation should be employed to predict minimum wall thickness and ensure it meets the minimum allowable thickness per the applicable hydraulic fitting standard (e.g., ISO 4401 or SAE J1926).
Springback: Cold-formed parts exhibit elastic recovery upon die removal. The die geometry must be compensated for predicted springback values, typically 0.5-2% of the deformation dimension.
Friction and lubrication: The compound bulging process involves complex material flow with multiple contact surfaces. Proper lubrication strategy is essential to prevent galling and to achieve uniform deformation.

Defect Analysis and Countermeasures

Based on my experience with tube forming operations, the following defects are common in compound bulging:

Defect Type	Root Cause	Countermeasure
Branch wall cracking	Excessive thinning at intersection	Reduce bulging force; increase initial wall thickness
Surface wrinkling	Insufficient axial restraint	Add axial constraint die or increase back pressure
Dimensional inaccuracy	Springback or die wear	Implement die compensation; schedule preventive die maintenance
Material tearing at main bore	Excessive radial expansion	Optimize expansion ratio; reduce forming speed

Study Insights and Reflections

This paper represents an early-stage investigation into a promising process concept. The 2007 publication date indicates that the research was conducted at a time when automotive lightweighting and manufacturing efficiency were becoming increasingly critical concerns. The compound bulging approach aligns well with modern manufacturing philosophy of reducing process steps and minimizing material waste.

In contemporary practice, the integration of finite element analysis (FEA) with process simulation tools such as AutoForm, Deform, or PAM-STAMP would be essential to optimize the die geometry and forming parameters before physical trials. The authors' experimental approach provides valuable baseline data, but modern process development would complement this with virtual prototyping to reduce trial-and-error costs.

The research also raises important questions about the scalability of the process. While the paper demonstrates feasibility for small-diameter automotive hydraulic tubes, extending the process to larger diameters would require significant increases in forming force and die strength. The material flow patterns become more complex with increasing tube diameter, and the risk of localized thinning increases.

For engineers considering adoption of this process, I recommend the following implementation pathway:

Conduct material characterization of the candidate seamless tube material, including tensile properties, ductility, and formability indices (n-value, r-value).
Perform FEA simulations to predict forming force, wall thickness distribution, and potential defect locations.
Develop a prototype die set with adjustable parameters to allow iterative optimization.
Conduct trial forming runs with progressive parameter adjustments, monitoring dimensional accuracy and surface quality at each stage.
Validate the formed parts through hydrostatic pressure testing and fatigue testing to confirm performance equivalence with conventional fittings.

The compound bulging process for automotive tee fittings represents a valuable contribution to the field of tube forming technology. Its potential to improve productivity and product performance makes it worthy of further investigation, particularly when combined with modern simulation tools and advanced material characterization techniques.