Hydraulic Composite Forming Technology Applied to Tee Fittings

Technical Overview

This 2018 publication in Forging and Stamping Technology by Feng Sule, Xu Yongchao, Zhao Tao, Guan Yajuan, and Xu Aijie from Shanghai Aerospace Precision Machinery Research Institute and Harbin Institute of Technology describes the application of hydraulic composite forming technology to the manufacture of tee fittings. The work was conducted under the Shanghai Aerospace Bureau model technology research project (716A-YZ), addressing the demanding requirements of aerospace-grade tee fittings that require high dimensional accuracy, superior mechanical properties, and complex geometry formation.

Hydraulic Composite Forming Process

Hydraulic composite forming is a specialized metal forming technique that combines the advantages of hydroforming and multi-axis forming to produce complex-shaped tube fittings with high precision. The process involves the coordinated action of internal fluid pressure and external forming tools to shape a tubular blank into the desired tee geometry.

Process Sequence

Step	Operation	Key Parameters
1	Tube end forming (reduction)	End diameter reduction by 10–15%
2	Internal pressure application	Hydraulic pressure 200–400 MPa
3	External die closure	Synchronized die movement
4	Internal pressure hold	Pressure maintained at 350–450 MPa
5	Die opening and part extraction	Controlled cooling
6	Trim and deburr	Flash removal at parting lines

Material Specifications

Property	Specification	Requirement
Material grade	Inconel 718 (UNS N06625)	Aerospace-grade
Tube OD × Wall	50.8 mm × 6.35 mm	Seamless precision tube
Solution treated condition	1040°C ± 10°C, 1h, air cool	Pre-forming condition
Age hardened condition	720°C/8h + 620°C/8h + 540°C/8h	Post-forming treatment
Tensile strength (final)	≥ 1100 MPa	Per AMS 5663
Yield strength (final)	≥ 965 MPa	Per AMS 5663
Elongation (final)	≥ 10%	Per AMS 5663
Impact energy (final)	≥ 27 J at 23°C	Per ASTM A370

Forming Analysis and Technical Challenges

Stress State During Forming

The hydraulic composite forming of tee fittings involves a complex multi-axial stress state that must be carefully controlled to avoid defects while achieving the desired geometry. The following table illustrates the stress distribution:

Region	Primary Stress	Secondary Stress	Failure Risk
Tee branch opening	Hoop tension	Radial compression	Thinning
Main body wall	Axial compression	Hoop tension	Buckling
Branch-body intersection	Biaxial tension	Shear	Cracking
Tube ends (grip region)	Axial tension	Radial compression	Over-reduction

Key Process Parameters and Their Effects

Parameter	Low Value Effect	Optimal Range	High Value Effect
Internal pressure	Incomplete forming, wrinkles	350–420 MPa	Over-thinning, bursting
Die speed	Slow cycle time, material strain aging	2–5 mm/s	Impact loading, cracking
Blank temperature	High forming force, cracking	800–950°C (hot forming)	Oxidation, grain growth
Lubricant temperature	Poor lubrication, galling	500–700°C	Lubricant degradation
Forming ratio	Low forming accuracy	15–25% wall reduction	Excessive thinning

Defect Analysis and Countermeasures

Defect Type	Root Cause	Detection Method	Countermeasure
Wall thinning exceeding tolerance	Excessive internal pressure or die speed	UT thickness mapping	Pressure reduction, speed optimization
Surface cracks at branch opening	High strain concentration	MT or PT	Increased blank temperature, modified die geometry
Dimensional inaccuracy	Springback or die wear	CMM measurement	Compensation programming, die resurfacing
Internal oxide inclusions	Overheating during forming	UT or RT	Temperature control, inert atmosphere forming
Grain boundary slippage	Excessive temperature + strain	Metallographic examination	Temperature reduction, strain rate control
Residual stress exceeding limits	Non-uniform plastic deformation	X-ray diffraction or hole drilling	Stress relief annealing, optimized forming sequence

Comparison with Alternative Manufacturing Methods

Method	Cycle Time	Material Utilization	Surface Quality	Mechanical Properties	Cost Index
Hydraulic composite forming	15–25 min	85–92%	Ra 3.2–6.3 μm	Superior (controlled grain flow)	3.0
CNC machining from bar	45–90 min	20–35%	Ra 0.8–1.6 μm	Anisotropic, reduced by machining	5.0
Welding from plate	30–60 min	70–80%	Ra 6.3–12.5 μm	Variable, weld-affected zone concerns	2.5
Casting	2–4 hours	60–70%	Ra 12.5–25 μm	Lower than wrought	2.0
Powder metallurgy	3–8 hours	80–85%	Ra 6.3–12.5 μm	Variable	4.0

Quality Assurance Program

The quality assurance program for aerospace-grade tee fittings manufactured by hydraulic composite forming includes:

Incoming material inspection — Full chemical analysis per AMS 5663, heat treatment certificate verification, and ultrasonic inspection of tube blanks
In-process monitoring — Real-time pressure and temperature monitoring, die position feedback, and forming force recording
Post-forming inspection — Dimensional inspection by CMM, surface inspection by PT, internal quality by UT, and mechanical property verification by tensile and impact testing
Final qualification — Full dimensional inspection, hydrostatic pressure testing (1.5× design pressure), and material certification package compilation per NADCAP requirements

Integration with Cladding and Bimetal Considerations

For tee fittings requiring corrosion resistance in addition to mechanical performance, the hydraulic composite forming approach can be integrated with subsequent cladding operations:

Post-forming weld overlay — After forming and heat treatment, the tee surface can be overlay welded with a corrosion-resistant alloy using GTAW or plasma arc processes, with the formed grain flow providing a favorable base for weld attachment
Pre-formed clad tube blanks — Explosion-clad or roll-bonded tube blanks can be used as the starting material for hydraulic forming, provided the forming parameters are adjusted to avoid delamination at the bond line
Hybrid approach — Forming the tee from carbon steel, followed by selective weld overlay of the internal surface with a nickel-based alloy, offers a cost-effective solution for high-pressure corrosion-resistant tee fittings

Study Reflections and Engineering Implications

This literature demonstrates the maturity of hydraulic composite forming as a manufacturing technology for complex aerospace-grade fittings, offering superior mechanical properties through controlled grain flow orientation and significantly improved material utilization compared to machining. From the perspective of a cladding and bimetal engineer, the key insight is that advanced forming technologies create new opportunities for hybrid manufacturing approaches — combining the structural advantages of precision forming with the corrosion resistance benefits of overlay cladding. The challenge lies in ensuring that the subsequent cladding operation does not compromise the mechanical properties achieved through forming, and vice versa. The integration of these technologies requires careful process planning, comprehensive qualification testing, and close coordination between forming and welding specialists. As manufacturing continues to evolve toward integrated multi-process solutions, the ability to combine forming, cladding, and finishing operations in a single manufacturing chain will be a critical competency for engineers working in the bimetal products and pressure vessel fabrication industry.