Multi-Pass Draw Spinning of Inclined Pipe Fittings Numerical Simulation and Experimental Validation

Literature Overview

This paper by Xia Qinxia, Shang Yue, Zhang Shuaibin, and Ruan Feng from South China University of Technology was published in the Journal of Mechanical Engineering in 2008 (Vol. 44, No. 8, pp. 78-84). The study investigates the multi-pass draw spinning process for inclined (non-axisymmetric) pipe fittings using 6061-T1 aluminum alloy in the annealed condition. The research employs finite element analysis using MSC.MARC software and validates the simulation results through experimental observations of strain distribution using grid patterns and microstructural analysis. The work was supported by the National Natural Science Foundation of China (50275054), Guangdong Provincial Natural Science Foundation (020923), and Guangdong Provincial Industrial Science and Technology Program (2003C102013).

Core Technical Content

Process Description

Multi-pass draw spinning involves progressively reducing the diameter of a pipe blank while simultaneously controlling the axial length to achieve the desired geometry. For inclined pipe fittings, the die profile is non-axisymmetric, creating complex strain states that differ significantly from conventional axisymmetric spinning. The process involves multiple passes, each with a specific reduction ratio, to achieve the final geometry without exceeding the forming limits of the material.

Simulation Results

The finite element analysis revealed several key characteristics of the stress and strain distribution:

Experimental Validation

The experimental validation used two methods:

• Grid circle analysis: Circular grids drawn on the blank surface were observed after spinning to determine strain distribution patterns
• Microstructural analysis: Metallographic examination of cross-sections revealed grain deformation and orientation changes consistent with simulation predictions

The relative error between experimental and simulated wall thickness changes in the intermediate deformation zone was not greater than 10%, confirming the validity of the finite element model.

Key Technical Points and Interpretation

Inclination Effect on Strain Distribution

The inclination of the die introduces non-uniformity in the stress and strain state throughout the workpiece. Unlike axisymmetric spinning where strain is uniform around the circumference, inclined spinning creates a gradient from the 0° domain (where the spinning tool contacts the inclined surface) to the 180° domain (the opposite side). This non-uniformity has several important implications:

• The 0° domain experiences higher strain, which increases the risk of thinning and potential rupture
• The 180° domain experiences lower strain, which may result in insufficient forming in that region
• The layered stress distribution along the axial direction indicates that different regions of the workpiece are at different stages of deformation

Multi-Pass Process Design

The multi-pass approach is essential for inclined pipe fittings because:

• A single pass would result in excessive strain concentration at the 0° domain, likely causing failure
• Multiple passes with controlled reduction ratios distribute the total strain more evenly
• The return pass (reverse spinning) creates a thickening effect in the intermediate deformation zone, compensating for thinning

The wall thickness behavior during multi-pass spinning shows an interesting phenomenon: the intermediate deformation zone experiences thickening due to the return pass effect. This is counterintuitive to the typical thinning expected in spinning operations and represents an important process characteristic that can be exploited for process optimization.

Process Parameter Optimization

Based on the simulation and experimental results, the following process parameters are critical:

Integration with Engineering Practice

Application to Automotive Exhaust Manifolds

The paper reports the successful production of inclined automotive exhaust manifold samples. This application is significant because exhaust manifolds require complex geometries with multiple inclined ports, and traditional manufacturing methods such as casting or welding result in heavier and less efficient components. Draw spinning produces lighter, stronger manifolds with better fatigue performance due to the beneficial grain flow alignment.

Manufacturing Process Flow

The typical manufacturing process for inclined pipe fittings by multi-pass draw spinning includes:

1. Selection of appropriate aluminum alloy blank with correct initial dimensions
2. Annealing of the blank to achieve desired forming properties (6061-T1 annealed condition)
3. Setup of spinning machine with appropriate die and spinning tool geometry
4. First pass spinning with controlled reduction ratio
5. Intermediate inspection of wall thickness and dimensions
6. Subsequent passes with progressive reduction
7. Final heat treatment (if required) to achieve target mechanical properties
8. Dimensional inspection and surface quality verification

Quality Control Considerations

Key quality control checkpoints include:

• Wall thickness measurement at multiple circumferential positions (0°, 90°, 180°, 270°) to detect non-uniform thinning
• Surface inspection for wrinkles, scratches, or other defects
• Mechanical property testing including tensile strength, elongation, and hardness
• Dimensional verification using coordinate measuring machines or specialized gauges
• Microstructural examination to verify grain flow and absence of cracking

Study Insights and Reflections

The paper makes a valuable contribution to the understanding of non-axisymmetric spinning processes. The finding that inclination creates a systematic strain gradient from 0° to 180° is fundamental to process design. Engineers must account for this non-uniformity when selecting reduction ratios and the number of passes.

The thickening effect in the intermediate deformation zone during multi-pass spinning is particularly interesting. This phenomenon suggests that the return pass creates a compressive strain state that partially compensates for thinning. This insight could be leveraged in process design to minimize the number of passes while maintaining acceptable wall thickness uniformity.

The experimental validation using grid analysis and microstructural examination demonstrates a rigorous approach to verifying simulation results. The agreement within 10% relative error confirms that the finite element model captures the essential physics of the process. However, the study would benefit from additional analysis of residual stresses, which significantly affect the fatigue performance of spun components.

A limitation of the study is the focus on 6061 aluminum alloy. The findings may not directly transfer to other materials such as steel, titanium, or magnesium alloys, which have different forming limits, strain rate sensitivities, and work hardening behaviors. Future research should extend the investigation to other materials and explore the effects of temperature on the spinning process.

Summary

This paper provides comprehensive insights into the multi-pass draw spinning of inclined pipe fittings through combined numerical simulation and experimental validation. The key findings include the non-uniform strain distribution caused by inclination, the layered stress pattern along the axial direction, and the thickening effect in the intermediate deformation zone during multi-pass forming. The successful production of automotive exhaust manifold samples demonstrates the practical viability of the process. Engineers should carefully consider the inclination-induced strain non-uniformity when designing spinning processes for non-axisymmetric pipe fittings, and the multi-pass approach with controlled reduction ratios offers a reliable path to achieving complex geometries with acceptable quality.