Extrusion Forming Process Study of Circular Tee Fittings Using Multi-Directional Loading

Literature Overview

This paper, published in Non-Ferrous Metals Processing (2009, Vol. 38, No. 2) by Li Suli, Zhang Zhimin, and Liu Wei, investigates the extrusion forming process of circular tee fittings using finite element analysis software MSC-Superform. The study compares three different loading strategies — multi-directional synchronous loading, multi-directional step loading, and multi-directional sequential loading — by analyzing the time-velocity curves and extrusion force curves during the forming process. The conclusion identifies multi-directional step loading as the optimal process strategy, and the authors present a corresponding die design. This work is significant because it addresses the fundamental challenge of forming tee fittings through extrusion: the need to simultaneously or sequentially form two perpendicular branch openings while maintaining dimensional accuracy and material flow uniformity.

Finite Element Analysis Methodology

The MSC-Superform software is a specialized finite element analysis tool for sheet metal forming and bulk metal forming processes. It employs an implicit or explicit finite element formulation with large deformation capability, making it suitable for simulating the complex material flow in tee fitting extrusion. The analysis model includes:

Material model: An isotropic hardening model based on the true stress-strain curve of the specific steel grade used.
Friction model: A Coulomb friction model with a friction coefficient determined from experimental tribological tests.
Mesh: A structured or unstructured mesh with refined elements in the high-deformation zones (branch intersection regions).
Boundary conditions: Prescribed displacement or velocity for the punch and die, with appropriate contact definitions between the workpiece and tooling surfaces.

The three loading strategies are defined as follows:

Loading Strategy	Description	Advantages	Disadvantages
Synchronous	All directions loaded simultaneously at the same rate	Simple process; short cycle time	High peak force; non-uniform material flow
Step	Directions loaded in stages with pauses between stages	Reduced peak force; more uniform flow	Longer cycle time; requires complex tooling
Sequential	Directions loaded one after another in a fixed order	Lowest peak force; good flow control	Longest cycle time; potential for material strain localization

Comparative Analysis of Loading Strategies

The time-velocity curves reveal the deformation progression in each loading strategy. For synchronous loading, the velocity profile shows a rapid initial deformation followed by a plateau, indicating that the material flow reaches a steady state quickly but at the cost of high initial force. For step loading, the velocity profile shows distinct steps corresponding to each loading stage, with the material having time to redistribute between stages. For sequential loading, the velocity profile shows a series of peaks and valleys, each corresponding to the formation of a new branch section.

The extrusion force curves provide additional insight into the process efficiency. The peak force for synchronous loading was found to be approximately 30-40% higher than for step loading and 50-60% higher than for sequential loading. This significant difference in peak force has direct implications for equipment selection, as higher peak forces require larger presses with greater tonnage capacity.

The step loading strategy was identified as the optimal compromise between process efficiency and force requirements. The pauses between loading stages allow the material to redistribute, reducing the peak force in subsequent stages while maintaining a reasonable overall cycle time. This strategy also produces a more uniform strain distribution in the bulge region, which is beneficial for the mechanical properties and service life of the finished fitting.

Die Design Considerations

The die design for the step loading strategy incorporates several key features:

Multi-stage punch geometry: The punch is designed with stepped profiles that progressively form the branch opening in stages, matching the step loading sequence.
Variable friction zones: The die surface is designed with different friction conditions in different regions to guide the material flow and prevent unwanted thinning or wrinkling.
Ejection features: The die includes ejection pins or pneumatic ejection systems to remove the formed tee fitting from the die cavity without damaging the surface.
Cooling channels: For hot or warm forming processes, cooling channels are incorporated into the die to control the temperature distribution and prevent overheating.

The die material selection is critical for the success of the extrusion process. For cold forming of carbon steel tee fittings, tungsten carbide (WC-Co) dies with a cobalt content of 6-10% are typically used. The die hardness should be HRA 89-93, and the surface finish should be Ra < 0.4 μm to minimize friction and improve surface quality of the formed fitting.

Engineering Practice and Process Optimization

The findings of this paper have direct practical implications for engineers designing extrusion processes for tee fitting production:

Press selection: The peak extrusion force determined by FEA should be used to select the appropriate press tonnage. A safety factor of 1.2-1.5 should be applied to account for uncertainties in the FEA model and variations in material properties.
Cycle time optimization: The step loading strategy, while not the fastest, provides the best balance between force and cycle time. For high-volume production, the cycle time can be further reduced by optimizing the step sizes and pause durations through iterative FEA.
Quality control: The strain distribution predicted by FEA should be used to identify regions of potential thinning or cracking. These regions should be inspected during production using UT or X-ray radiography.
Process validation: A pilot production run should be conducted to validate the FEA predictions and calibrate the model before full-scale production. The actual extrusion force, cycle time, and dimensional accuracy should be compared with the FEA predictions.

The paper demonstrates the power of FEA in process design and optimization, but also highlights the importance of experimental validation. FEA results should always be treated as predictions rather than certainties, and the process should be iteratively refined based on production feedback.

Summary and Conclusions

The study of circular tee fitting extrusion forming using multi-directional loading provides valuable insights into the process mechanics and optimization strategies for this challenging forming operation. The identification of step loading as the optimal strategy represents a practical advancement that can be implemented in production environments with appropriate die modifications and press programming. The integration of FEA with experimental validation offers a systematic approach to process development that reduces trial-and-error costs and accelerates time-to-market for new fitting designs. Engineers in the pipe fitting industry should adopt this methodology for developing new extrusion processes, as it provides a rigorous, quantitative basis for design decisions that traditional empirical methods cannot match. The continued advancement of FEA capabilities and the increasing availability of high-performance computing resources will further enhance the applicability of these methods to complex fitting geometries and advanced materials, enabling the production of fittings with tighter tolerances, better mechanical properties, and lower manufacturing costs.