Effect of Friction Coefficient on Wall Thickness Distribution in Liquid-Assisted Forming of Aviation Aluminum Alloy Tee Fittings

Literature Overview

Published in "Forging & Stamping Technology" in 2021, this paper by Wang Ling, Zhang Jianmin, Sun Jin, Zhang Peng, and Lang Lihui from Shenyang Aircraft Corporation, Tianjin Tianduan Aviation Technology, and Beihang University investigates the influence of friction coefficient on wall thickness distribution in liquid-assisted forming (hydroforming) of aviation aluminum alloy tee fittings. The study combines finite element simulation with experimental validation, including friction coefficient determination through tribological testing.

Core Technical Content

The researchers established a finite element model of the tee fitting hydroforming process and systematically varied the friction coefficient while maintaining constant hydroforming pressure and axial feed parameters. The study identifies friction coefficient as a critical factor affecting both wall thickness distribution and branch height in the formed tee fitting.

Key Technical Findings

Friction Coefficient	Wall Thinning Rate	Wall Thickness Distribution	Branch Height	Assessment
0.025	Lower	Poor uniformity	Lower	Insufficient material flow
0.050	Moderate	Moderate uniformity	Moderate	Acceptable but suboptimal
0.075	Low	Good uniformity	Good	Optimal balance
0.100	Higher	Moderate uniformity	Higher	Excessive thinning risk
0.150	High	Poor uniformity	High	Risk of failure

The optimal friction coefficient of 0.075 provides the best balance between low thinning rate and good wall thickness distribution. This finding is significant because it demonstrates that friction is not simply detrimental to forming quality but plays a constructive role in controlling material flow.

Friction Coefficient Determination

The researchers used a friction and wear testing machine to determine the friction coefficients corresponding to different lubrication media. Based on the simulation results, a lubricant with a friction coefficient close to 0.075 was selected for experimental validation. The experimental results confirmed that the selected lubricant produced tee fittings meeting both wall thickness and thinning requirements.

Process Analysis and Standards

Liquid-assisted forming is a complex process involving multiple interacting parameters:

Hydroforming pressure (internal fluid pressure)
Axial feed (end displacement control)
Friction conditions (lubricant selection and application)
Material properties (yield strength, strain hardening, formability)

Relevant standards for aviation aluminum alloy fittings include:

ASTM B209 for wrought aluminum and aluminum alloy forgings
AMS 2750 for aluminum alloy tubing
MIL-H-5551 for aluminum alloy tubing
AS9100 for quality management in aerospace

Critical Process Considerations

The friction coefficient in hydroforming is influenced by:

Lubricant type and viscosity
Surface finish of the die and tube
Contact pressure distribution
Strain rate during forming
Temperature effects (if warm forming is employed)

The optimal friction coefficient of 0.075 represents a balance between:

Sufficient friction to prevent material slippage and ensure controlled flow
Low friction to minimize wall thinning at critical locations
Appropriate friction to maintain contact between tube and die

Integration with Engineering Practice

For aerospace manufacturing engineers, the findings have several practical implications:

Lubricant selection is a critical process parameter that must be carefully controlled
Friction coefficient should be verified through tribological testing before production
Process monitoring should include wall thickness measurement at critical locations
Quality assurance should verify that formed fittings meet specification requirements

Process Optimization Strategy

A systematic approach to process optimization should include:

Material characterization (yield strength, strain hardening exponent, formability limits)
Finite element simulation with parameter sensitivity analysis
Tribological testing to determine friction coefficients for candidate lubricants
Small-scale experimental validation
Full-scale production trials with quality verification

Key Questions and Reflections

The study provides valuable insights into friction effects in hydroforming, but several aspects merit further consideration. The effect of friction coefficient variation during the forming process (due to lubricant breakdown, temperature changes, or surface wear) is not addressed. In practice, friction conditions may evolve during forming, and the optimal initial friction coefficient may not remain optimal throughout the process.

Additionally, the study focuses on a specific aluminum alloy grade, but different alloys may have different optimal friction coefficients due to variations in material properties. The interaction between friction coefficient and other process parameters (pressure, feed rate, temperature) should be investigated through multi-factor optimization studies.

The effect of die surface roughness and texture on friction and forming quality is another important consideration. Surface engineering techniques such as texturing or coating could potentially improve forming quality by controlling friction at specific locations.

Study Insights and Implications

This research demonstrates the critical importance of friction control in liquid-assisted forming of aerospace components. The identification of an optimal friction coefficient of 0.075 provides a concrete target for process development and quality control. For manufacturing engineers, the key insight is that friction is not merely a nuisance parameter but a controllable variable that can be optimized to achieve specific forming quality objectives. The integration of simulation, tribological testing, and experimental validation exemplifies best practices in advanced manufacturing process development.