In recent years, the rapid growth of the electric vehicle (EV) industry, particularly in China EV markets, has driven the demand for advanced energy storage systems. Hydrogen storage alloys, especially vanadium-based types, are promising candidates for EV batteries due to their high hydrogen storage capacity and room-temperature operation. However, issues such as poor corrosion resistance and inadequate charge-discharge cycle stability have hindered their commercial application. Rotary forging, a metal forming process that combines rotation and compression, has been explored as a method to enhance these properties by modifying the microstructure. In this study, I investigate the influence of rotary forging deformation amount on the corrosion resistance and cycle stability of V3TiNi0.56Cr0.3 vanadium-based hydrogen storage alloys, which are critical for improving the performance and longevity of electric vehicle batteries. The findings aim to provide insights into optimizing processing parameters for broader adoption in China EV and global markets.
The V3TiNi0.56Cr0.3 vanadium-based hydrogen storage alloy was selected as the test material due to its relevance in electric vehicle applications. The alloy was prepared using medium-frequency induction melting, followed by homogenization annealing at 850°C for 10 hours. The chemical composition, as verified through analysis, is summarized in Table 1. This composition aligns with the requirements for high-performance hydrogen storage in electric vehicle batteries, emphasizing the importance of material purity and consistency in China EV manufacturing.
| Element | Ti | Ni | Cr | Other Impurities | V |
|---|---|---|---|---|---|
| Required | 19.200 ± 0.500 | 13.200 ± 0.500 | 6.300 ± 0.500 | ≤ 0.150 | Balance |
| Measured | 19.286 | 13.174 | 6.347 | 0.108 | Balance |
Rotary forging tests were conducted on a PYT-20.1T rotary forging machine using cylindrical specimens with an initial diameter of 15 mm and length of 20 mm. The rotary forging deformation amount, a key parameter, is defined as the ratio of the cross-sectional area before and after forging, expressed by the formula: $$ A = \frac{S_0}{S_1} \times 100\% $$ where \( A \) is the deformation amount, \( S_0 \) is the initial cross-sectional area, and \( S_1 \) is the final cross-sectional area. This parameter is crucial for tailoring the properties of alloys used in electric vehicle batteries. Five different deformation amounts—10%, 15%, 20%, 25%, and 30%—were applied, while other parameters such as heating temperature (950°C), holding time (10 minutes), axial feed speed (900 mm/min), and number of passes (1) were kept constant. The process parameters are detailed in Table 2, highlighting the systematic approach to evaluating the impact on electric vehicle battery performance.
| Specimen ID | Deformation Amount (%) | Heating Temperature (°C) | Holding Time (min) | Axial Feed Speed (mm/min) | Number of Passes |
|---|---|---|---|---|---|
| 1 | 10 | 950 | 10 | 900 | 1 |
| 2 | 15 | ||||
| 3 | 20 | ||||
| 4 | 25 | ||||
| 5 | 30 |
To assess corrosion resistance, specimens were cut into disc-shaped samples (10 mm diameter × 5 mm thickness) from the forged alloys. Electrochemical corrosion tests were performed at room temperature using a CHI440B electrochemical workstation with a three-electrode system in a 6 mol/L sodium hydroxide solution, simulating harsh conditions that electric vehicle batteries might encounter. The corrosion potential was measured to evaluate resistance, and post-test surface morphology was examined using scanning electron microscopy (SEM). For charge-discharge cycle stability, electrode samples were prepared by mixing alloy powders (60–80 μm particle size) with nickel powder in a 1:1 mass ratio, adding polytetrafluoroethylene emulsion, and pressing onto nickel foam at 250 MPa. The electrodes underwent 5000 charge-discharge cycles on the same workstation, with a charge current of 120 mAh/g for 3 hours and a discharge current of 90 mAh/g down to a cutoff potential of -0.4 V. The discharge capacity attenuation rate, indicating cycle stability, was calculated using: $$ \eta = \frac{C_{\text{max}} – C_i}{C_{\text{max}}} \times 100\% $$ where \( \eta \) is the attenuation rate, \( C_{\text{max}} \) is the maximum discharge capacity, and \( C_i \) is the capacity after 5000 cycles. This metric is vital for ensuring the reliability of electric vehicle batteries in China EV applications, where long cycle life is a key requirement.
The results demonstrated that the rotary forging deformation amount significantly influenced the corrosion behavior of the V3TiNi0.56Cr0.3 alloy. As the deformation increased from 10% to 30%, the corrosion potential in sodium hydroxide solution initially shifted positively and then negatively, indicating an improvement followed by a decline in corrosion resistance. Specifically, at 10% deformation, the corrosion potential was -0.183 V, reflecting poor resistance. At 25% deformation, it reached -0.101 V, a positive shift of 82 mV, suggesting optimal performance. However, at 30% deformation, the potential shifted negatively, revealing reduced resistance. SEM observations corroborated these findings: at lower deformations, surfaces showed deep corrosion pits, whereas at 25%, only minor pitting was observed, and at 30%, corrosion intensified. This trend underscores the importance of controlled deformation for enhancing the durability of materials in electric vehicle batteries, particularly in the context of China EV advancements where environmental robustness is critical.
Similarly, the charge-discharge cycle stability was affected by the deformation amount. The discharge capacity attenuation rate decreased initially with increasing deformation and then increased, mirroring the corrosion resistance pattern. At 10% deformation, the attenuation rate was 73.4%, indicating poor stability. At 25% deformation, it dropped to 32.7%, a reduction of 40.7%, demonstrating significant improvement. At 30% deformation, the rate increased again, highlighting the detrimental effects of excessive deformation. These results are summarized in Table 3, which provides a comparative analysis of the performance metrics across different deformation levels. The data emphasize that a deformation amount of 25% yields the best balance, enhancing both corrosion resistance and cycle stability for electric vehicle battery applications. This optimization is essential for meeting the growing demands of the China EV market, where energy efficiency and longevity are paramount.
| Deformation Amount (%) | Corrosion Potential (V) | Discharge Capacity Attenuation Rate (%) |
|---|---|---|
| 10 | -0.183 | 73.4 |
| 15 | -0.162 | 58.9 |
| 20 | -0.134 | 45.2 |
| 25 | -0.101 | 32.7 |
| 30 | -0.121 | 48.5 |
The underlying mechanisms for these observations relate to microstructural changes induced by rotary forging. At optimal deformation amounts, such as 25%, the process refines grain size, improves homogeneity, and reduces internal defects and stress concentrations. This enhances resistance to corrosion and minimizes capacity fading during cycling. However, excessive deformation (e.g., 30%) can cause grain fragmentation, increasing susceptibility to corrosion and accelerating degradation. The relationship between deformation and performance can be modeled using additional formulas, such as the Hall-Petch equation for grain size strengthening: $$ \sigma_y = \sigma_0 + \frac{k}{\sqrt{d}} $$ where \( \sigma_y \) is the yield strength, \( \sigma_0 \) is the friction stress, \( k \) is a constant, and \( d \) is the grain diameter. This illustrates how finer grains from moderate deformation improve mechanical and chemical properties. For electric vehicle batteries, these microstructural benefits translate to better performance in real-world conditions, supporting the expansion of China EV technologies.
In conclusion, the rotary forging deformation amount plays a critical role in determining the corrosion resistance and charge-discharge cycle stability of V3TiNi0.56Cr0.3 vanadium-based hydrogen storage alloys. An optimal deformation of 25% results in a significant positive shift in corrosion potential (82 mV compared to 10% deformation) and a substantial reduction in discharge capacity attenuation rate (40.7% decrease). These findings provide valuable guidelines for optimizing processing parameters in the manufacturing of hydrogen storage alloys for electric vehicle batteries. Future work should focus on exploring other rotary forging parameters and their effects on hydrogen absorption-desorption properties to further advance the application of these alloys in the rapidly evolving electric vehicle sector, particularly in China EV initiatives. By leveraging such improvements, the adoption of vanadium-based alloys can contribute to more efficient and durable energy storage solutions, driving the sustainability of electric vehicles worldwide.