The rapid advancement of electric vehicles (EVs) necessitates continuous improvements in key subsystems, with the EV battery pack being paramount. As driving range increases, the number of battery modules grows, demanding a robust enclosure that provides exceptional structural rigidity and protection. The enclosure must effectively resist elastic deflection and permanent deformation under various loads. A prevalent design strategy employs a cross-rib structure using longitudinal and transverse beams, which enhances overall stiffness while allowing for a thinner base plate, contributing to vehicle lightweighting.
Currently, aluminum alloys dominate the material selection for EV battery pack structures due to their favorable strength-to-weight ratio. Processes like casting and extrusion are commonly used. However, the relatively high cost of aluminum and the significant energy consumption during its processing present economic and environmental challenges, particularly within the context of global carbon neutrality goals. In contrast, advanced ultra-high-strength steels (UHSS) offer superior specific strength and energy absorption capabilities at a lower overall cost. Replacing aluminum beams with UHSS counterparts in the EV battery pack presents a compelling opportunity to enhance structural performance while reducing production costs and energy footprint.
This article explores the application of chain-die forming, a progressive manufacturing technique, for producing complex longitudinal beams for EV battery pack from QP1180 grade UHSS. The process, feasibility analysis via numerical simulation, die optimization, and experimental validation are detailed herein.
1. Introduction to Chain-Die Forming and Component Challenge
Chain-die forming is an incremental sheet forming technology particularly suitable for manufacturing long, profile-intensive components from high-strength materials. Its principle involves discretizing the forming die into individual blocks mounted on a circulating chain mechanism. As the chain moves, the die blocks follow a precisely controlled path, progressively shaping the metal sheet. Compared to traditional roll forming, chain-die forming significantly reduces redundant longitudinal strain and is adept at handling profiles with variable cross-sections and depths, making it ideal for complex automotive structural parts like those in an EV battery pack.
The specific component addressed here is a longitudinal beam for an EV battery pack. Its key challenge lies in its geometry: it features step-like, variable-height sections along its length, designed as mounting points for transverse beams. Forming such a profile from 1.2 mm thick QP1180 steel (with a tensile strength exceeding 1400 MPa) using conventional stamping is difficult due to high springback and the risk of die interference during forming or ejection. Chain-die forming, with its incremental and controlled nature, is hypothesized to overcome these challenges effectively.
2. Finite Element Model Development
2.1 Geometric and Process Model
The longitudinal beam model for the EV battery pack is 864 mm long with two variable-height features spaced 432 mm apart. The height differential is 15 mm, with target flange and web angles of 90°. A critical design requirement is high flatness of the longitudinal flanges for subsequent welding to the base plate and cross beams. To prevent tooling interference during the forming and stripping stages of the variable-height section, a novel die design with substantial clearance was conceptualized. The forming simulation utilizes a dynamic, explicit finite element approach. A half-symmetry model is employed to reduce computational cost. The sheet metal is modeled with C3D8R elements, with five elements through the thickness and local refinement in high-deformation zones. The die blocks are modeled as discrete analytical rigid bodies, each rotating about a large virtual radius (R = 40,000 mm) to simulate the chain’s path. The sheet starts stationary and is incrementally formed as the die blocks engage and disengage.
2.2 Material Constitutive Model
Accurate prediction of springback, crucial for the dimensional accuracy of the EV battery pack beam, requires a material model that captures the Bauschinger effect. The Chaboche nonlinear kinematic hardening model, superimposed on an isotropic hardening rule, is used for QP1180 steel. The flow stress is described by:
$$ \sigma_y = \sigma_0 + Q(1 – e^{-b\epsilon_{pl}}) $$
and the evolution of the backstress $\alpha$ (sum of three components) governs kinematic hardening:
$$ \dot{\alpha}_k = C_k \frac{\sigma_y – \alpha}{\sigma_y} \dot{\epsilon}_{pl} – \gamma_k \alpha_k \dot{\epsilon}_{pl}, \quad k=1,2,3 $$
$$ \alpha = \sum_{k=1}^{3} \alpha_k $$
The fitted parameters for the QP1180 steel used in the EV battery pack beam are listed in Table 1.
| Parameter | C1 (MPa) | γ1 | C2 (MPa) | γ2 | C3 (MPa) | γ3 | Q (MPa) | b |
|---|---|---|---|---|---|---|---|---|
| Value | 3490.2 | 10.4 | 64580.5 | 137 | 159.6 | 8199.8 | 228.3 | 10518.8 |
3. Simulation Results and Initial Feasibility Assessment
3.1 Stress Distribution and Forming Load
The simulation results indicated a uniform distribution of stress throughout the formed EV battery pack beam. Residual stress analysis along the web and flange angles (Figure 1) showed consistent levels with minor increases at the transition zones, but no evidence of severe stress concentration. This suggests a low probability of forming defects like cracking, wrinkling, or severe twisting during the production of the EV battery pack component. The absence of stress peaks at the step regions confirms the effectiveness of the clearance-based die design in preventing interference.
The total forming load on the machine frame was extracted by summing reaction forces from all active die blocks. The load curve (Figure 2) shows a peak force of approximately 951 kN when the first die pair reaches its deepest engagement. This load is well within the capacity of standard chain-die forming equipment, confirming the process feasibility from a tonnage perspective for manufacturing this UHSS EV battery pack part.
3.2 Dimensional Accuracy Analysis and the Need for Compensation
Despite good forming feasibility, the as-formed geometry from the initial die design showed significant deviations from the target part, primarily due to springback. Sectional measurements at six critical locations (S1-S6) revealed angular errors in both web and flange angles exceeding 5°, and in one location (S4) surpassing 30°. Furthermore, longitudinal bowing, a common challenge in profile forming, was evident with a height error of approximately ±1 mm/m. This level of inaccuracy is unacceptable for the precise assembly requirements of an EV battery pack. The results are summarized in Table 2 and underscore the necessity for systematic die face compensation.
| Cross-Section | Web Angle (Sim/Target/Δ) | Flange Angle (Sim/Target/Δ) | Web Radius (Sim/Target/Δ) | Flange Radius (Sim/Target/Δ) |
|---|---|---|---|---|
| S1 | 94.69 / 90 / +4.69 | 91.84 / 90 / +1.83 | 4.26 / 3 / +1.26 | 13.41 / 3 / +10.41 |
| S2 | 96.26 / 90 / +6.26 | 90.54 / 90 / +0.54 | 4.61 / 3 / +1.61 | 12.02 / 3 / +9.02 |
| S3 | 93.18 / 90 / +3.18 | 83.99 / 90 / -6.01 | 6.20 / 3 / +3.20 | 10.56 / 3 / +7.56 |
| S4 | 118.91 / 90 / +28.91 | 121.20 / 90 / +31.20 | 5.02 / 3 / +2.02 | 3.79 / 3 / +0.79 |
| S5 | 93.53 / 90 / +3.53 | 88.15 / 90 / -1.85 | 5.36 / 3 / +2.36 | 11.78 / 3 / +8.78 |
| S6 | 95.45 / 90 / +5.45 | 91.08 / 90 / +1.08 | 4.65 / 3 / +1.65 | 13.17 / 3 / +10.17 |
4. Die Face Compensation Strategy and Optimization
To achieve the stringent dimensional tolerances required for the EV battery pack beam, a systematic die face compensation was undertaken. The goal is to pre-deform the tooling so that the elastic springback of the part results in the correct final geometry. Four key parameters on the punch and die were identified for adjustment: the web radius (r1), web angle (β1), sidewall radius (r2), and flange angle (β2). An iterative simulation-based optimization loop was conducted. The compensation values were adjusted based on the deviation analysis from the previous simulation until the predicted formed shape met the tolerance criteria. The final compensation parameters applied to the die faces are listed in Table 3.
| Compensation Parameter | Web Radius r1 (mm) | Web Angle β1 (°) | Sidewall Radius r2 (mm) | Flange Angle β2 (°) |
|---|---|---|---|---|
| Value | 56.7 | 96.5 | 261.3 | 85.5 |
4.1 Results of Optimized Simulation
The simulation with the compensated dies showed a dramatic improvement in dimensional accuracy. The angular deviations at all measured sections were reduced to within ±1°, and the longitudinal bow was effectively controlled to less than 1 mm/m. This level of precision is fully acceptable for the assembly of an EV battery pack. The results confirm that with proper die design and compensation, chain-die forming can produce complex, variable-height UHSS components with high dimensional fidelity. The optimized simulation results are summarized in Table 4.
| Cross-Section | Web Angle (Opt. Sim/Target/Δ) | Flange Angle (Opt. Sim/Target/Δ) |
|---|---|---|
| S1 | 90.61 / 90 / +0.61 | 90.15 / 90 / +0.15 |
| S2 | 89.81 / 90 / -0.19 | 90.52 / 90 / +0.52 |
| S3 | 89.09 / 90 / -0.91 | 89.47 / 90 / -0.53 |
| S4 | 89.49 / 90 / -0.51 | 90.43 / 90 / +0.43 |
| S5 | 89.18 / 90 / -0.82 | 89.99 / 90 / -0.01 |
| S6 | 89.57 / 90 / -0.43 | 89.87 / 90 / -0.13 |
5. Experimental Validation
Based on the optimized design, physical tooling was manufactured, and trials were conducted on a chain-die forming machine. The formed QP1180 longitudinal beam for the EV battery pack was free from visible defects such as wrinkles, cracks, or twists. The part geometry was digitized using a high-precision 3D scanner (RaySCAN 711). Comparative analysis between the scanned part, the target geometry, and the final simulation predictions was performed.
5.1 Dimensional Verification
The experimental measurements, detailed in Table 5, show excellent agreement with both the target design and the optimized simulation. All measured web and flange angles on the physical EV battery pack beam fall within the ±1° tolerance. The longitudinal profile also conforms to the required flatness, with deviations consistent with the simulation predictions. A cross-sectional comparison at the critical step-overlap region further confirms the geometric fidelity achieved in the actual forming process.
| Cross-Section | Web Angle (Exp/Target/Δ) | Flange Angle (Exp/Target/Δ) |
|---|---|---|
| S1 | 90.75 / 90 / +0.75 | 90.72 / 90 / +0.72 |
| S2 | 89.16 / 90 / -0.84 | 90.90 / 90 / +0.90 |
| S3 | 89.35 / 90 / -0.65 | 89.54 / 90 / -0.46 |
| S4 | 90.82 / 90 / +0.82 | 90.13 / 90 / +0.13 |
| S5 | 89.12 / 90 / -0.88 | 89.21 / 90 / -0.79 |
| S6 | 89.55 / 90 / -0.45 | 90.82 / 90 / +0.82 |
6. Conclusion and Outlook
This study successfully demonstrates the viability of chain-die forming for manufacturing complex, variable-height longitudinal beams from QP1180 ultra-high-strength steel for EV battery pack applications. The key findings are:
- Process Feasibility: Chain-die forming can effectively shape UHSS sheets with challenging geometric features like step-height variations without die interference or major forming defects. The required forming load is manageable for industrial equipment.
- Critical Role of Simulation and Compensation: Finite Element Analysis (FEA) incorporating advanced material models (Chaboche kinematic hardening) is essential for accurately predicting springback in UHSS. An iterative die face compensation strategy, informed by simulation results, is mandatory to achieve high dimensional accuracy.
- Demonstrated Precision: Through systematic optimization, the chain-die forming process produced a longitudinal beam for an EV battery pack with angular dimensional errors controlled within ±1° and longitudinal flatness within 1 mm/m, meeting strict assembly requirements.
- Material and Cost Advantage: This approach validates a pathway to utilize advanced ultra-high-strength steels in EV battery pack structures, offering potential benefits in strength, energy absorption, and overall system cost compared to incumbent aluminum solutions.
The successful integration of numerical simulation, die design innovation, and process optimization paves the way for adopting chain-die forming as a competitive and reliable manufacturing solution for other high-strength, complex-profile components in electric vehicle architectures, ultimately contributing to the development of safer, lighter, and more cost-effective EV battery pack systems.