In recent years, stainless steel materials have been widely used in the manufacturing of rail vehicle bodies. Due to the adoption of surface non-coating techniques in stainless steel body construction, the precision requirements for body manufacturing are high, particularly for the forming accuracy of structural components. The length of the roof beam is large, and the cross-sectional shape changes significantly during the bending process, which poses great challenges in the design of clamps and bending dies. Defects such as wrinkling, section distortion, and inability to meet usage requirements due to springback easily occur during the forming process. Therefore, it is crucial to use the PS2F software for numerical simulation of the stretch forming process of the roof beam to ensure the forming accuracy.
Stretch Forming Process Principles
Stretch forming of profiles is a common cold bending method where tangential tensile forces are applied at both ends of the profile, causing stretching and bending simultaneously. When any rigid material is bent, there is an imaginary straight line along the longitudinal axis of the part where no deformation occurs, known as the neutral axis. In the absence of “pre-stretching,” it aligns with the geometric centerline of the part. Before bending, all fibers have the same length. However, after bending, due to the thickness of the part, fibers at different thicknesses will have different lengths. Fibers outside the neutral axis will increase in length, while fibers inside will shorten.
Upon unloading, the material retains a shape similar to the contour of the die. However, due to the presence of residual internal fiber stresses, the material tends to recover its initial shape. Therefore, there is a certain degree of springback at both ends of the die. Springback is the cumulative effect of the entire forming process and is closely related to factors such as die geometry, material properties, frictional contact conditions, and loading methods. Effective prediction and control of springback are crucial for improving the accuracy of profile bending components.
Stretch Forming Simulation Software
Numerical simulation technology is an effective method for studying stretch forming processes. Through numerical simulation, the material deformation mechanism and flow behavior during the stretch forming process can be analyzed, potential forming defects can be predicted, and the forming process can be optimized.
PS2F software is a high-performance dedicated simulation software for stretch forming based on analytical algorithms. The software consists of two parts: the PS2F concept module and the machine module. The concept module is primarily used for simulating and optimizing forming process parameters, calculating blank dimensions, and designing molds. The machine module is mainly used to generate CNC programs for driving the stretch bending machine. PS2F software can simulate the stretching arm stretch bending process of profiles, evaluate the feasibility of profile bending, automatically optimize the clamping trajectory based on the shape characteristics and frictional contact status of the profile parts, calculate springback, and compensate the mold profile accordingly. Based on the simulated optimization results and the specific profile bending equipment, PS2F software can generate corresponding CNC codes to drive the motion of the mechanism, avoiding the difficulties of relying solely on personal experience and determining processing parameters through experiments.
Material Parameter Determination
Prior to stretch forming, a tensile test was conducted on the material to determine the material parameters required for numerical simulation. The final test results are shown in Table 1.
| Parameter | Value |
|---|---|
| Rpo2 | 601 MPa |
| r | 0.81 |
| Rm | 988 MPa |
| A | 2762.867 MPa |
| E | 176250 MPa |
| m | 1.52912 |
| μ | 0.27 |
| exos | 0.279 |
Research on Stretch Forming of the Roof Beam
Numerical Simulation Results Analysis
PS2F software was used to simulate the stretch forming of the roof beam of the Dalian 202 extended stainless steel rail vehicle. The material of the component is SUS301L-ST, with a total length of 2552mm, as shown in Figure 1. The process requirements state that the gap at the two end arcs of the sample should be less than or equal to 0.5mm, and the presence of defects such as wrinkling is not allowed. To ensure a better fit between the beam surface and the sample, the covering coefficient was increased from the default 1.0 to 1.6 during the simulation. The simulation results showed a maximum springback of 6.148, and the corresponding compensation was automatically applied during the generation of the mold surface. Figure 2 shows the simulated distribution of strain along the length direction, with the minimum strain occurring at the “r” position, indicating that the material experiences tensile stress at this point, making wrinkling unlikely. The maximum strain of the material occurs on the side subjected to tensile stress near the “s” position, but it is still within the allowable range, indicating that no defects will occur.
Experimental Verification
To verify the accuracy of the simulation results, an actual forming experiment was conducted using the optimized parameters. The results showed that the actual springback value of the roof beam was 5.981, which was close to the simulated value of 6.148. The error between the two was less than 3%, indicating good consistency. The gap at the two end arcs of the sample was less than 0.5mm, meeting the process requirements. No defects such as wrinkling were found on the formed surface.
Conclusion
Through the research on three-dimensional stretch forming technology, it has been demonstrated that this method can effectively meet the requirements of one-time three-dimensional precise forming for both hollow and solid complex-axis and complex-section aluminum alloy extruded components. The use of PS2F software for numerical simulation of the stretch forming process can accurately predict the springback phenomenon and optimize the forming process parameters. This research provides a reliable technical reference for the design and manufacturing of rail vehicle body components.