As a researcher focused on advancing energy storage materials for the electric vehicle (EV) industry, particularly in the context of China’s rapidly growing EV market, I have investigated the impact of rotary forging deformation on the properties of vanadium-based hydrogen storage alloys. These alloys are critical for developing high-performance batteries in electric vehicles, offering advantages like room-temperature hydrogen absorption and desorption with substantial storage capacity. However, challenges such as inadequate corrosion resistance and poor charge-discharge cycle stability hinder their commercial application. In this study, I explore how varying the deformation amount during rotary forging can optimize these properties, contributing to the sustainability goals of the China EV sector. The widespread adoption of electric vehicles in China EV initiatives underscores the importance of such material innovations for reducing carbon emissions and enhancing energy efficiency.
Rotary forging is a metal forming process that combines forging and rolling principles, where rotating dies apply compressive forces to a workpiece, inducing plastic deformation. This method is known to refine microstructures, eliminate defects, and improve mechanical properties. For vanadium-based hydrogen storage alloys, which are often used in battery systems for electric vehicles, optimizing the deformation amount could lead to significant enhancements in durability and performance. In my experiments, I focused on a V3TiNi0.56Cr0.3 vanadium-based hydrogen storage alloy, commonly considered for electric vehicle batteries due to its high hydrogen storage capacity. The alloy’s composition is critical for its performance in China EV applications, where reliability and longevity are paramount. The chemical composition of the alloy is summarized in Table 1, based on mass fraction percentages.
| 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 |
The rotary forging process was conducted on a PYT-20.1T rotary forging machine, using cylindrical specimens with an initial diameter of 15 mm and length of 20 mm. These specimens underwent homogenization annealing at 850°C for 10 hours prior to forging to ensure uniform microstructure. I varied the deformation amount, defined as the ratio of the cross-sectional area before and after forging, expressed by the formula:
$$ A = \frac{S_0 – S_1}{S_0} \times 100\% $$
where \( A \) is the deformation amount, \( S_0 \) is the initial cross-sectional area, and \( S_1 \) is the final cross-sectional area after rotary forging. This parameter is crucial for tailoring the alloy’s properties for electric vehicle batteries, as it influences grain refinement and stress distribution. Five different deformation amounts were tested: 10%, 15%, 20%, 25%, and 30%, while other parameters such as heating temperature (950°C), holding time (10 minutes), axial feed speed (900 mm/min), and number of forging passes (1 pass) were kept constant. This approach allows for a systematic analysis of how deformation affects the alloy’s performance in China EV contexts. The detailed rotary forging parameters are provided in Table 2.
| 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 evaluate the corrosion resistance, which is vital for the longevity of electric vehicle batteries in harsh environments, I prepared disc-shaped specimens (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. The corrosive medium was a 6 mol/L sodium hydroxide solution, simulating alkaline conditions common in battery systems. The corrosion potential was measured to assess the alloy’s susceptibility to corrosion, with higher (more positive) values indicating better resistance. This is particularly relevant for China EV applications, where batteries must withstand varied operational conditions. After testing, the surface morphology of the corroded specimens was examined using scanning electron microscopy (SEM) to visualize the extent of damage.
For charge-discharge cycle stability tests, which directly impact the reliability of electric vehicle batteries, I extracted powder samples (particle size 60–80 μm) from the forged specimens. These were mixed with nickel powder in a 1:1 mass ratio, combined with polytetrafluoroethylene emulsion, and pressed onto nickel foam substrates at 250 MPa to form electrodes. The electrodes were then subjected to 5000 charge-discharge cycles on the CHI440B workstation, with a charging time of 3 hours, charging current of 120 mAh/g, discharging current of 90 mAh/g, and a discharge cut-off potential of -0.4 V. The discharge capacity attenuation rate was calculated to quantify stability:
$$ \eta = \frac{C_{\text{max}} – C_i}{C_{\text{max}}} \times 100\% $$
where \( \eta \) is the attenuation rate, \( C_{\text{max}} \) is the maximum discharge capacity, and \( C_i \) is the discharge capacity after 5000 cycles. A lower attenuation rate signifies better cycle stability, which is essential for the sustained performance of electric vehicles in China EV deployments.
The results revealed that the deformation amount significantly influences both corrosion resistance and cycle stability. 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, an 82 mV positive shift, demonstrating optimal performance. This trend is critical for electric vehicle batteries, as enhanced corrosion resistance can extend battery life in China EV applications. The SEM images of corroded surfaces supported these findings, showing reduced corrosion pits and more uniform surfaces at intermediate deformation levels.
Similarly, the charge-discharge cycle stability showed a non-linear relationship with deformation amount. The discharge capacity attenuation rate decreased from 73.4% at 10% deformation to a minimum of 32.7% at 25% deformation, representing a 40.7% improvement, before increasing again at 30% deformation. This suggests that moderate deformation enhances the alloy’s ability to withstand repeated cycling, a key requirement for electric vehicle batteries that undergo frequent charging and discharging in real-world China EV usage. The underlying mechanisms involve microstructural changes induced by rotary forging. At optimal deformation levels, grain refinement and homogenization occur, reducing internal defects and stress concentrations. This improves the alloy’s resistance to corrosion and cyclic degradation. However, excessive deformation can lead to grain fragmentation, exacerbating issues like intergranular corrosion and oxidation during cycling.
To further analyze the results, I derived a mathematical model to describe the relationship between deformation amount and performance metrics. For corrosion potential (\( E_{\text{corr}} \)), the data can be fitted to a quadratic equation:
$$ E_{\text{corr}} = aA^2 + bA + c $$
where \( A \) is the deformation amount, and \( a \), \( b \), and \( c \) are constants derived from experimental data. Similarly, for the discharge capacity attenuation rate (\( \eta \)), the relationship can be expressed as:
$$ \eta = dA^2 + eA + f $$
where \( d \), \( e \), and \( f \) are constants. These equations highlight the optimal deformation range for maximizing performance in electric vehicle batteries. For instance, in China EV scenarios, where cost-effectiveness and durability are prioritized, a deformation amount of 25% emerges as the sweet spot. This is summarized in Table 3, which compares the key performance indicators across different deformation amounts.
| Deformation Amount (%) | Corrosion Potential (V) | Discharge Capacity Attenuation Rate (%) | Relative Performance for Electric Vehicle Batteries |
|---|---|---|---|
| 10 | -0.183 | 73.4 | Poor |
| 15 | -0.156 | 58.2 | Moderate |
| 20 | -0.124 | 45.1 | Good |
| 25 | -0.101 | 32.7 | Excellent |
| 30 | -0.135 | 48.9 | Fair |
The discussion delves into the microstructural aspects. Rotary forging induces plastic deformation that refines the grain structure of the V3TiNi0.56Cr0.3 alloy, which is a multi-phase material with network-like grain boundaries. These boundaries are prone to corrosion and degradation due to their higher activity and structural disorder. At lower deformation amounts (e.g., 10%), the microstructure may retain coarse grains and defects, leading to pronounced corrosion and capacity fade. As deformation increases to 25%, dynamic recrystallization and grain boundary strengthening occur, enhancing density and reducing susceptibility to corrosion and cyclic fatigue. This is particularly beneficial for electric vehicle batteries in China EV markets, where environmental factors can accelerate material degradation. However, at 30% deformation, over-deformation causes grain crushing, increasing the surface area for corrosion and promoting crack initiation during charge-discharge cycles.
Moreover, the hydrogen absorption and desorption properties of the alloy are indirectly influenced by deformation. Although not directly measured in this study, improved microstructural homogeneity at optimal deformation likely facilitates better hydrogen kinetics, which is crucial for the efficiency of electric vehicle batteries. Future work should focus on quantifying this relationship, as it could further optimize the alloy for China EV applications. The role of rotary forging in mitigating issues like hydrogen embrittlement and oxidation is also worth exploring, as these are common challenges in hydrogen storage materials for electric vehicles.
In conclusion, my findings demonstrate that rotary forging deformation amount plays a pivotal role in enhancing the performance of vanadium-based hydrogen storage alloys for electric vehicle batteries. The optimal deformation amount of 25% significantly improves corrosion resistance and charge-discharge cycle stability, making it a promising parameter for industrial applications. This research contributes to the advancement of sustainable energy storage solutions for the electric vehicle industry, particularly in the context of China EV development, where material innovation is key to achieving global environmental targets. Further investigations into other rotary forging parameters, such as temperature and strain rate, could provide additional insights for tailoring these alloys to meet the specific demands of electric vehicles in diverse operating conditions.