Hydraulic Compound Forming Technology Applied to Tee Fitting Manufacturing

Literature Overview

This study, published in Forging & Stamping Technology (2018, Vol. 43, No. 1, pp. 72–77) by Feng Sule, Xu Yongchao, Zhao Tao, Guan Yajuan, and Xu Aijie from the Shanghai Aerospace Precision Machinery Research Institute and Harbin Institute of Technology, presents an innovative approach to manufacturing spherical tee fittings using hydraulic compound forming technology. The research addresses the manufacturing challenges of producing complex tee geometries from 1Cr18Ni9Ti stainless steel, combining liquid chamber deep drawing (LCD) and internal high-pressure bulging (IHPB) processes.

Core Technical Content

The hydraulic compound forming process integrates two sequential forming operations:

Stage 1 - Liquid Chamber Deep Drawing (LCD): A pre-machined blank is deep drawn using hydraulic pressure applied through a liquid medium, with the punch geometry defining the initial tee shape.
Stage 2 - Internal High-Pressure Bulging (IHPB): The partially formed part is then bulged internally using high-pressure hydraulic fluid to achieve the final spherical tee geometry with uniform wall thickness.

The study establishes mechanical models for both stages and analyzes the influence of critical process parameters including friction coefficient, drawing ratio, blank holder force, and bulging force on part quality.

Process Parameters and Results

Parameter	Value	Effect on Forming
Hydraulic pressure	40 MPa	Optimal for minimum thinning
Minimum wall thickness reduction rate	27.5%	After LCD stage
Location of thinnest wall	Punch fillet area	Critical zone for material flow
Final part ID	SR90 mm	Spherical radius
Minimum wall thickness at flange	1.59 mm	After complete forming
Development cycle reduction	8 days	Compared to conventional methods
Material utilization improvement	>60%	Versus traditional machining

Interpretation of Technical Points

The hydraulic compound forming approach offers several distinct advantages over conventional manufacturing methods for tee fittings:

Material flow control: The liquid medium provides uniform pressure distribution, reducing localized thinning and improving dimensional accuracy compared to conventional mechanical forming.
Process flexibility: The two-stage approach allows independent optimization of each forming operation, with the LCD stage establishing the basic geometry and the IHPB stage refining the shape and wall thickness.
Stainless steel formability: The 1Cr18Ni9Ti material (equivalent to AISI 321) presents challenges due to its high work hardening rate. The hydraulic forming process accommodates this by providing gradual, controlled deformation.

The friction coefficient analysis reveals that lower friction conditions (achieved through proper lubrication) reduce the thinning rate in the punch fillet area, which is the critical zone where material experiences maximum strain concentration. The drawing ratio directly influences the degree of thinning, with higher ratios leading to greater wall reduction.

Engineering Practice Integration

For manufacturing engineers, this study provides actionable insights for implementing hydraulic compound forming in production environments:

Tooling design: The punch fillet geometry is the most critical tooling feature, as it directly controls the wall thinning pattern. Optimization of this radius requires iterative simulation and testing.
Pressure control: The 40 MPa hydraulic pressure identified as optimal for this specific geometry must be validated for different material grades and wall thicknesses.
Quality assurance: The successful hydraulic strength and air-tightness testing of the prototype demonstrates that the formed parts meet functional requirements without subsequent machining.
Cost-benefit analysis: The 8-day reduction in development cycle and 60% improvement in material utilization translate to significant economic benefits for high-value aerospace and energy applications.

Key Questions and Reflections

The study raises important questions about process scalability and repeatability. While the prototype results are promising, the transition from laboratory-scale forming to production-scale manufacturing requires careful consideration of:

Batch-to-batch consistency in hydraulic pressure control
Material variability effects on forming outcomes
Tool wear and its impact on dimensional accuracy over extended production runs
Integration with downstream inspection and testing procedures

Additionally, the study focuses on a single material grade (1Cr18Ni9Ti). The applicability to other materials, such as carbon steels (e.g., A105, A234 WPB) or high-nickel alloys (e.g., Inconel 625), would require separate investigation due to differences in formability, work hardening behavior, and springback characteristics.

Study Insights and Implications

This research represents a meaningful advance in the manufacturing technology for complex pipe fittings. The hydraulic compound forming approach bridges the gap between traditional forging/machining methods and modern net-shape forming technologies, offering a viable alternative for producing high-quality tee fittings with reduced material waste and shorter development cycles. The systematic approach to process parameter analysis, combined with successful prototype validation, provides a solid foundation for industrial implementation. For the pipe fitting industry, this technology offers a pathway toward more sustainable manufacturing practices with significant economic and quality benefits.