Numerical Simulation and Experimental Study of 45 Degree Oblique Tee Extrusion Forming Based on AFDEX

Literature Overview

This paper, published in 2016 in Forging and Stamping Technology, presents a comprehensive study combining finite element numerical simulation with experimental validation for the extrusion forming of 45-degree oblique tee fittings. The research was conducted by a team from the China Academy of Machinery Science and Technology, Hebei Hongrun Heavy Industry Co., Ltd., and the Beijing Institute of Mechanical and Electrical Engineering, supported by the National Science and Technology Major Project (2014ZX04014-051) and the International Science and Technology Cooperation Project (2015DFA51740). The study employed the AFDEX finite element software to establish a thermo-mechanically coupled model for the extrusion forming process, revealing the distribution patterns of equivalent strain, load-time curves, and die stress.

Core Technical Content

The paper presents a multi-step forming process for 45-degree oblique tee fittings that includes upsetting, reverse extrusion, and piercing operations. The key technical findings include:

1. Equivalent strain distribution: High strain values are concentrated in the region where billet metal flows past the vertical punch, indicating the critical deformation zone in the forming process.
2. Load-time characteristics: The upsetting operation shows a gradual initial load increase followed by rapid escalation, reaching a forming load of 42,410 kN. The reverse extrusion operation shows rapid load increase to approximately 23,000 kN followed by a relatively stable plateau, reaching a final forming load of 34,920 kN.
3. Experimental validation: The upsetting and piercing forming loads measured experimentally were 41,520 kN and 34,500 kN respectively, showing excellent agreement with the simulation results and validating the process scheme and die design.

Technical Points Interpretation

AFDEX Finite Element Simulation

AFDEX is a specialized finite element software developed for metal forming simulation, particularly for forging and extrusion processes. The thermo-mechanically coupled model accounts for the interaction between mechanical deformation and thermal effects, which is critical for accurate prediction of forming loads, strain distributions, and material flow patterns.

Key aspects of the AFDEX simulation include:

• Material model: Appropriate constitutive models for the steel grade used in the tee fitting, accounting for strain rate sensitivity and temperature dependence
• Contact modeling: Accurate representation of the interaction between the billet and the die surfaces, including friction conditions
• Thermal analysis: Heat generation from plastic deformation and heat transfer to the dies and environment
• Mesh quality: Adequate mesh density in regions of high strain gradient to capture deformation details

Multi-Step Forming Process Analysis

The multi-step forming process for the 45-degree oblique tee involves several critical operations:

1. Upsetting: The initial operation that reduces the billet height and increases its diameter, creating the bulk of material needed for the tee geometry. The gradual-to-rapid load increase pattern is characteristic of upsetting operations where the material initially deforms uniformly before reaching the die walls.
2. Reverse extrusion: This operation forces material to flow upward (against the direction of punch travel) to form the branch of the tee. The rapid initial load increase followed by a plateau is typical of extrusion operations where the material reaches a steady-state flow condition.
3. Piercing: This operation creates the hollow interior of the tee, requiring high loads to penetrate the material. The experimental load of 34,500 kN indicates the significant force required for this operation.

Die Stress Analysis

The die stress distribution is a critical output of the simulation, as it directly relates to die life and the risk of die failure. High stress concentrations typically occur at:

• Die corners and edges where material flow is constrained
• Regions of high friction where material slides along die surfaces
• Areas where the die geometry changes abruptly

Understanding the die stress distribution allows engineers to optimize the die geometry to reduce peak stresses, thereby extending die life and reducing production costs.

Standards and Engineering Practice Connection

The 45-degree oblique tee fitting is a specialized pipe fitting used in pipeline systems where directional changes are required at 45-degree angles. The extrusion forming process described in this paper is relevant to several standards:

• ASME B16.9: Wrought Butt-Welding Fittings - specifies requirements for fittings including tees, with dimensional tolerances and pressure-temperature ratings
• ASTM A234: Carbon and Alloy Steel Fittings for Piping and Piping Components - specifies material requirements
• ASTM A403: Chromium and Chromium-Nickel Steel Fittings for Piping and Piping Components - specifies stainless steel fitting materials
• ISO 15590: General-Purpose Fittings - specifies requirements for butt-weld fittings
• GB/T 12459: Steel Forged Butt-Welding Fittings - Chinese standard for forged fittings

For the steel pipe and fitting industry, the extrusion forming process offers several advantages over traditional manufacturing methods:

• Material efficiency: Extrusion forming minimizes material waste compared to machining or forging
• Complex geometry capability: The process can produce complex tee geometries that would be difficult or impossible to achieve by other methods
• Consistent quality: The controlled process conditions lead to consistent product quality
• Cost effectiveness: For high-volume production, extrusion forming can be more economical than machining or investment casting

Key Questions and Reflections

Several aspects of this study merit further consideration:

1. Material flow analysis: While the paper reports strain and load data, a detailed analysis of material flow patterns (metal flow lines) would provide insight into potential internal defects such as laps, folds, or inclusions that could affect fitting quality.
2. Die life prediction: The die stress data should be used to predict die life and identify critical wear locations. This information is essential for production planning and maintenance scheduling.
3. Microstructure evolution: The high strain values reported suggest significant microstructural changes in the formed fitting. Metallographic analysis of the formed tee would reveal grain structure, deformation bands, and potential recrystallization patterns that affect mechanical properties.
4. Process window optimization: The study provides data for one specific specification, but a systematic study of process parameters (temperature, speed, lubrication) would establish the optimal process window for consistent quality.
5. Welding considerations: If the extruded tee requires subsequent welding operations (e.g., for branch connection), the residual stress state from extrusion must be considered in welding process design to avoid distortion and cracking.

Study Insights and Implications

This study demonstrates the power of combining finite element simulation with experimental validation in the development of metal forming processes for pipe fittings. The excellent agreement between simulated and experimental loads (deviations of only 1.2-2.1%) validates the AFDEX simulation methodology and provides confidence in using simulation for process design and optimization.

For the steel pipe and fitting industry, this research has several important implications:

• Process development: The methodology can be applied to develop forming processes for other specialized tee geometries (e.g., 30-degree, 60-degree oblique tees) without extensive trial-and-error experimentation
• Equipment selection: The predicted forming loads enable appropriate press capacity selection, avoiding over-specification (cost) or under-specification (quality risk)
• Die design optimization: The stress distribution data can be used to optimize die geometry for extended die life and improved product quality
• Quality assurance: Understanding the strain distribution enables prediction of mechanical properties at different locations within the fitting, supporting quality assurance and product certification

The multi-step forming approach (upsetting, reverse extrusion, piercing) represents an efficient manufacturing strategy for complex tee geometries, combining the advantages of different forming operations to achieve the desired shape with minimal material waste and controlled strain states.

In conclusion, this research provides a robust framework for the development and validation of extrusion forming processes for 45-degree oblique tee fittings. The combination of AFDEX simulation with experimental verification establishes a reliable methodology that can be extended to other fitting geometries and production scenarios. The findings have direct practical value for fitting manufacturers seeking to optimize their production processes while maintaining product quality and cost competitiveness in the global steel pipe and fitting market.