In recent years, the rapid growth of the electric car industry, particularly in regions like China EV markets, has driven the demand for lightweight and high-performance materials. Aluminum alloys, especially those used in heat exchangers and structural components, play a critical role in enhancing the efficiency and sustainability of electric cars. Among these, the Al-10.5Si alloy is widely employed due to its excellent castability and mechanical properties. However, the presence of coarse silicon phases in the as-cast condition can detrimentally affect its performance. This study investigates the influence of strontium (Sr) content on the microstructure, electrical conductivity, and mechanical properties of Al-10.5Si alloy, aiming to optimize its application in electric car components. The findings are crucial for advancing material design in the China EV sector, where energy efficiency and durability are paramount.
The Al-10.5Si alloy was prepared using high-purity aluminum and Al-20Si master alloy, with Sr added as an Al-10Sr master alloy. The chemical composition of the base alloy is summarized in Table 1. Melting was conducted at 765°C, followed by stirring to ensure homogeneity. Sr additions were made at 725°C, with varying amounts (0, 0.025, 0.045, 0.065, and 0.08 wt%), and the melt was held for 30 minutes before casting at 710°C into preheated molds. Microstructural analysis was performed using optical microscopy and scanning electron microscopy (SEM), while electrical conductivity was measured with a portable conductivity meter. Mechanical properties, including tensile strength and nanoindentation hardness, were evaluated according to standard protocols. The integration of such materials in electric car systems, like battery thermal management, underscores the importance of this research for China EV advancements.
| Element | Si | Fe | Mn | Sr | Al |
|---|---|---|---|---|---|
| Content | 10.50 | 0.25 | 0.05 | 0, 0.025, 0.045, 0.065, 0.08 | Balance |
The microstructure of the Al-10.5Si alloy consists primarily of α-Al dendrites and eutectic silicon phases. Without Sr modification, the alloy exhibits coarse α-Al dendrites and large, needle-like or plate-like eutectic Si particles, which can act as stress concentrators and reduce mechanical integrity. This is particularly problematic for electric car applications, where components must withstand cyclic loads. Upon adding Sr, significant refinement occurs. For instance, at 0.025 wt% Sr, the α-Al dendrites become smaller, and the eutectic Si partially transforms into fibrous structures. At 0.045 wt% Sr, complete modification is achieved, with fine, uniformly distributed fibrous eutectic Si and reduced α-Al dendrite size. However, exceeding this optimal Sr content leads to over-modification, resulting in coarser Si particles and diminished properties. The refinement mechanism can be explained by the impurity-induced twinning theory, where Sr adsorption on Si growth steps promotes twin formation, altering growth kinetics. The relationship between grain size and Sr content can be modeled using the Hall-Petch equation: $$ \sigma_y = \sigma_0 + k d^{-1/2} $$ where $\sigma_y$ is the yield strength, $\sigma_0$ is the friction stress, $k$ is a material constant, and $d$ is the average grain diameter. This refinement is vital for enhancing the durability of electric car components, such as those in China EV models, where weight reduction and performance are key.
Electrical conductivity is a critical parameter for materials used in electric car systems, as it influences energy efficiency and heat dissipation. The conductivity of Al-10.5Si alloy as a function of Sr content is presented in Table 2. Without Sr, the conductivity is relatively low due to the coarse Si phases that scatter electrons. As Sr content increases to 0.045 wt%, conductivity peaks at 39.28% IACS, representing an 8.96% improvement. This enhancement is attributed to the refined microstructure, which reduces electron scattering. The Wiedemann-Franz law relates thermal and electrical conductivity: $$ \kappa = L \sigma T $$ where $\kappa$ is the thermal conductivity, $\sigma$ is the electrical conductivity, $L$ is the Lorenz number, and $T$ is the absolute temperature. For electric cars, especially in China EV applications, higher conductivity translates to better battery cooling and overall system efficiency. Beyond 0.045 wt% Sr, conductivity decreases due to over-modification and increased impurity scattering, highlighting the need for precise Sr control in alloy design for optimal electric car performance.
| Sr Content (wt%) | 0 | 0.025 | 0.045 | 0.065 | 0.08 |
|---|---|---|---|---|---|
| Conductivity (%IACS) | 36.05 | 36.72 | 39.28 | 37.92 | 37.35 |
Mechanical properties, including tensile strength and hardness, are essential for ensuring the reliability of electric car components under operational stresses. The tensile test results are summarized in Table 3. Without Sr, the alloy shows low strength and ductility, with a tensile strength of 120 MPa, yield strength of 103 MPa, and elongation of 5.5%. At 0.045 wt% Sr, optimal properties are achieved: tensile strength increases to 176 MPa, yield strength to 154 MPa, and elongation to 14.5%, representing improvements of 46.7%, 49.5%, and 163.6%, respectively. This is due to the refined eutectic Si and α-Al phases, which hinder crack propagation and enhance strain accommodation. The strengthening mechanism can be described by the Orowan equation for dispersion strengthening: $$ \Delta \sigma = \frac{G b}{\lambda} $$ where $\Delta \sigma$ is the strength increment, $G$ is the shear modulus, $b$ is the Burgers vector, and $\lambda$ is the interparticle spacing. Such improvements are crucial for electric car frames and heat exchangers in China EV designs, where weight savings and crashworthiness are prioritized.
| Sr Content (wt%) | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| 0 | 120 | 103 | 5.5 |
| 0.025 | 138 | 123 | 10.0 |
| 0.045 | 176 | 154 | 14.5 |
| 0.065 | 170 | 142 | 12.6 |
| 0.08 | 158 | 136 | 11.0 |
Nanoindentation testing further reveals the mechanical behavior at the micro-scale. Without Sr, the average nanoindentation hardness is 0.789 GPa and Young’s modulus is 48.316 GPa. At 0.045 wt% Sr, these values increase to 1.222 GPa and 75.726 GPa, respectively, indicating enhancements of 54.88% and 56.73%. The hardness can be correlated with the microstructure through the relation: $$ H = H_0 + k_H d^{-1/2} $$ where $H$ is the hardness, $H_0$ is the base hardness, $k_H$ is a constant, and $d$ is the grain size. These properties are vital for components in electric cars, such as battery enclosures in China EV models, where localized deformation resistance is critical. The integration of Sr-modified Al-10.5Si alloy can thus contribute to longer service life and improved safety in electric car applications.
In conclusion, the addition of Sr significantly optimizes the microstructure and properties of Al-10.5Si alloy, making it highly suitable for electric car components. The optimal Sr content of 0.045 wt% results in refined eutectic Si and α-Al phases, peak electrical conductivity of 39.28% IACS, and superior mechanical properties, including a tensile strength of 176 MPa and nanoindentation hardness of 1.222 GPa. These improvements align with the demands of the electric car industry, particularly in China EV markets, where efficiency, lightweighting, and durability are driving forces. Future work should focus on scaling up this modification process for industrial production, ensuring that Al-10.5Si alloy can meet the evolving needs of electric car technologies. The continued innovation in material science will play a pivotal role in advancing the sustainability and performance of electric cars worldwide.