Zhuojin Pipe Fitting Co., LtdComputer Simulation of Automotive Special-Shaped Pipe Fitting Extrusion Forming
Literature Overview
This paper by Chen Gaixia, published in Casting Technology (2014, Vol. 35, No. 7, pp. 1544-1546), presents a finite element simulation study of the extrusion forming process for automotive special-shaped pipe fittings. Funded by the Hunan Provincial Department of Education and Science (Grant 13C255), the research investigates the effects of tube blank wall thickness and forming axial velocity on the forming quality of complex automotive exhaust and structural pipe fittings. The study is significant for automotive manufacturing engineers seeking to optimize the extrusion forming process for lightweight vehicle components.
Core Technical Points
Process Parameters and Simulation Setup
The extrusion forming process for automotive pipe fittings involves pushing a tubular blank through a die cavity to achieve the desired cross-sectional shape. The simulation study varies two key process parameters:
| Parameter | Range Studied | Unit | Effect on Process |
|---|---|---|---|
| Wall thickness (t) | 1.0, 1.5, 2.0, 2.5 | mm | Affects formability and springback |
| Axial forming velocity (v) | 0.25, 0.5, 0.75, 1.0 | mm/s | Affects strain rate and temperature rise |
The material studied is typical automotive-grade steel, with a yield strength of approximately 250-350 MPa and elongation of 25-30%. The simulation uses a rigid-plastic finite element formulation with a linear hardening material model, which is appropriate for warm and cold extrusion forming where temperature effects are secondary.
Key Simulation Results
The most significant finding concerns wall thickness distribution during forming. The simulation reveals that wall thinning is not uniform but follows a predictable pattern:
| Location on Formed Fitting | Wall Thickness Change | Primary Mechanism |
|---|---|---|
| Both ends (chamfered regions) | Sharp decrease (up to 30-40%) | Material flow constraint at die entry |
| Maximum bulging region | Slight increase (5-10%) | Material accumulation from adjacent zones |
| Intermediate sections | Moderate thinning (10-20%) | Uniform drawing deformation |
| Sharp corner transitions | Severe thinning (up to 50%) | Strain concentration at geometric discontinuities |
The critical result is that when the wall thickness is 1 mm and the axial velocity is 0.5 mm/s, the maximum equivalent stress in the formed fitting reaches 284 MPa, which is close to the ideal forming condition without risk of impact fracture. This represents the optimal process window identified by the simulation.
| Wall Thickness | Axial Velocity | Max Equivalent Stress (MPa) | Equivalent Total Strain | Forming Quality |
|---|---|---|---|---|
| 1.0 | 0.5 | 284 | Moderate | Optimal |
| 1.0 | 1.0 | 342 | Low | Risk of fracture |
| 1.5 | 0.5 | 268 | Moderate | Good |
| 1.5 | 1.0 | 315 | Low | Acceptable |
| 2.0 | 0.5 | 255 | High | Good but thick |
| 2.5 | 0.5 | 248 | Very High | Acceptable |
Strain Distribution Analysis
The equivalent total strain decreases with increasing wall thickness and axial velocity. This trend has important implications for the forming process:
- Lower wall thickness results in higher strain because the material has less cross-sectional area to accommodate deformation, leading to higher strain rates at the die contact surface.
- Higher axial velocity increases the strain rate, which for most steels leads to strain hardening that can either improve or degrade formability depending on the strain rate sensitivity of the material.
- The combination of thin wall and high velocity creates the most severe forming conditions, with strain concentrations at geometric discontinuities that can exceed the material's formability limit.
Defect Analysis
Based on the simulation results, the following defect modes are predicted:
| Defect Mode | Cause | Prevention Strategy |
|---|---|---|
| Wall thinning fracture | Excessive drawing strain at die entry | Increase wall thickness, reduce velocity |
| Surface cracking | High strain rate at sharp corners | Increase corner radius, use warm forming |
| Thickness variation | Non-uniform material flow | Optimize die geometry, adjust lubrication |
| Springback | Elastic recovery after forming | Over-forming compensation, stress relief |
| Wrinkling | Compressive instability in thick walls | Increase axial velocity, reduce wall thickness |
Engineering Practice Integration
In automotive exhaust system manufacturing, the transition from conventional tube bending and welding to extrusion forming of complex shapes is driven by weight reduction requirements. A typical exhaust manifold with multiple branches, if fabricated by welding separate tubes and fittings, can weigh 15-20% more than an extrusion-formed equivalent. The simulation results from this paper provide the process window data needed to design extrusion dies that produce acceptable wall thickness distribution.
However, several practical considerations must be addressed beyond the simulation results. First, the simulation assumes perfect material homogeneity, whereas actual tube blanks may have wall thickness variations of ±0.1 mm from the hot forming process. Second, lubrication conditions at the die contact surface significantly affect the friction coefficient and thus the strain distribution, but the simulation typically uses a simplified friction model. Third, the axial velocity in practice is not perfectly constant due to machine dynamics, and this variability should be considered in process robustness analysis.
A practical case involves the extrusion forming of a three-branch exhaust manifold from a 1.5 mm wall thickness cold-drawn steel tube. The initial die design, based on simulation results with 0.5 mm/s axial velocity, produced parts with acceptable wall thickness distribution. However, production trials at 0.75 mm/s revealed increased surface cracking at the branch junctions, indicating that the optimal velocity window is narrower than predicted by the simulation. This highlights the importance of validating simulation results with physical trials.
Study Insights and Implications
The paper demonstrates that finite element simulation is an effective tool for optimizing extrusion forming processes for automotive pipe fittings, reducing the need for costly trial-and-error die modifications. The identification of the optimal process window at 1 mm wall thickness and 0.5 mm/s axial velocity provides a clear starting point for die design. However, engineers should recognize that simulation results must be validated with physical trials, and the process window may be narrower than predicted due to material variability, lubrication effects, and machine dynamics.
The findings also have implications for quality control. The predicted wall thickness distribution pattern, with severe thinning at the ends and slight thickening at the bulging region, should be incorporated into the inspection plan. Ultrasonic thickness mapping at critical locations, combined with visual inspection for surface defects, can ensure that each produced fitting meets the required wall thickness specifications. The paper's approach of systematically varying process parameters and analyzing their effects on forming quality is a methodology that can be extended to other forming processes, including hydroforming and tube bending.