Hot Extrusion Tee Die Design Research

Literature Overview

The paper by Wei Xiao, published in Hot Working Technology in 2014 (Vol. 43, No. 11, pp. 145-147), presents a research study on the design of hot extrusion dies for tee fittings using finite element analysis (FEA). The work was conducted at Beijing Guodian Futong Technology Development Co., Ltd., and addresses the critical challenge of die design for complex geometries that are difficult to produce by conventional forging or machining methods. The study combines numerical simulation with practical die modification to improve production quality and die life.

Core Technical Content

Hot extrusion of tee fittings involves pushing a heated billet through a shaped die cavity to form the three-way intersection geometry in a single forming operation. The complexity of the tee geometry, with its intersecting branches at right angles, creates significant challenges for die design, including non-uniform material flow, high localized stresses at the intersection, and the risk of folding or cracking at the branch apex.

Finite Element Analysis Approach

The authors employ a rigid-plastic finite element method to simulate the material flow during hot extrusion of tee fittings. The analysis considers the billet material as a power-law viscous material with temperature-dependent flow stress, using a constitutive model of the form σ = K·ε^n·T^m, where K is the strength coefficient, n is the strain hardening exponent, and m is the temperature softening exponent.

Die Design Optimization

The study identifies several critical design features that influence the quality of extruded tees. The die cavity geometry must incorporate sufficient relief angles to facilitate material flow into the branch sections, typically in the range of 5-15 degrees depending on the branch-to-body diameter ratio. The fillet radius at the branch-body intersection should be at least 2-3 times the wall thickness to prevent excessive stress concentration and to ensure complete filling of the intersection.

A key finding from the FEA analysis is that the material flow velocity distribution is highly non-uniform, with peak velocities occurring at the branch apex and significantly lower velocities at the branch edges. This velocity gradient creates shear stresses that can lead to folding defects if not properly managed through die geometry design. The authors recommend incorporating guide channels in the die design to direct material flow more uniformly into the branch sections.

Die Strength Verification

The paper emphasizes the importance of die strength verification using FEA to ensure that the die material can withstand the forming forces without plastic deformation or fracture. The maximum equivalent stress in the die occurs at the re-entrant corners of the cavity, where stress concentrations can exceed the die material's yield strength at the operating temperature. The authors propose a safety factor of 1.5-2.0 for the die strength calculation, with the allowable stress determined from the die material's yield strength at the peak operating temperature.

Engineering Practice Implications

The practical modifications described in the paper have been validated through production trials, showing improvements in die life from approximately 500 hits to over 1200 hits for carbon steel tees. The key modifications include increasing the fillet radius at the branch-body intersection by 30%, adding a 2-degree draft angle to the branch sections, and incorporating cooling channels in the die body to reduce the peak die temperature by 50-80°C.

For engineers selecting hot extrusion as a production method for tee fittings, the economic viability depends on production volume and the complexity of the tee geometry. Hot extrusion is most cost-effective for production runs exceeding 5000 pieces, where the high die development cost is amortized over a large number of pieces. For smaller production runs or highly customized geometries, forging or machining may be more economical despite higher per-piece costs.

The paper's methodology of combining FEA with iterative die modification is directly applicable to other complex fitting geometries, including cross fittings, reducers, and custom-shaped components. The approach demonstrates that numerical simulation can significantly reduce the number of trial-and-error iterations required for die development, saving both time and cost in the die qualification process. The emphasis on die strength verification is particularly important for high-strength alloy tees where forming forces are substantially higher than for carbon steel grades.