Aluminum Alloys for Electrical Engineering: A Review

Table of Contents

1. Introduction
2. The Role of Aluminum in Electrical Engineering
3. Electrical Conductivity and Microstructure
4. Aluminum vs. Copper
5. Applications of Aluminum Conductors
   • Power Transmission and Distribution
   • Automotive Wiring
   • Aerospace Applications
   • Electric Vehicle Components
   • Other Applications
6. Strategies for Improving Aluminum Conductivity and Strength
   • Alloying Concepts
   • Rare Earth Elements
   • Transition Metals
   • Severe Plastic Deformation
   • Nanostructured Conductors
   • Metal Matrix Composites
7. Specific Aluminum Alloys for Electrical Applications
   • AA1350
   • AA6201
   • AA8000 series
   • Aluminum-Rare Earth Alloys
   • Aluminum-Transition Metal Alloys
8. Advanced Materials: Composites and Nanomaterials
   • Clad Composite Conductors
   • Metal-Metal Composites
   • Aluminum-Carbon Composites
   • Aluminum-Oxide Composites
9. Thermal Stability of Aluminum Alloys
10. Corrosion Resistance
11. The Future of Aluminum Conductors
12. Conclusion
13. Sources

1. Introduction

The demand for electricity is rapidly increasing. This is fueled by the need for road transport electrification and green hydrogen production. High-performance conductors are essential for the efficient transfer of electricity. They support modern infrastructure, manufacturing, and transportation, including electric vehicles. Aluminum, a versatile metal, plays a crucial role in this domain. This article examines aluminum conductors. It will discuss their fundamentals, classification, and use in various markets. It will focus on the metallurgical characteristics of current commercial solutions. It will also look at future development strategies. The properties of aluminum, both good and bad, for electrical engineering are emphasized. Alloying concepts that speed up the decomposition of matrix solid solutions are explored. These concepts help minimize electron scattering. The development of new aluminum conductors also uses novel processing techniques. These include ultra-fast crystallization, severe plastic deformation, and complex thermomechanical treatments. These techniques aim to reduce grain size to the nanometer scale. They also aim to control crystallographic texture and engineer grain boundaries. Transition metals and rare earths are being studied as promising alloying candidates for high-strength conductors with superior thermal stability. The focus is on immiscible systems like Al–Ce, Al–La, and Al–Y, along with multiple additions for synergistic effects. Composites with cladding and particulate reinforcement are also examined. Carbon-type strengtheners are included. The article will also look at various strategies to overcome the strength-conductivity trade-off in conductor materials.

Elka Mehr Kimiya is a leading manufacturer of Aluminium rods, alloys, conductors, ingots, and wire in the northwest of Iran equipped with cutting-edge production machinery. Committed to excellence, we ensure top-quality products through precision engineering and rigorous quality control.

2. The Role of Aluminum in Electrical Engineering

Aluminum is a key material in electrical engineering. It is used for various applications, particularly in electricity transmission and distribution. The global consumption of aluminum for these purposes reached 4.2 and 5.0 million metric tons, respectively, in 2021. This accounts for over 14% of the total global aluminum consumption. Additionally, there is a need to replace over 320,000 km of transmission lines in North America. Upgrading these existing conductors increases grid resilience, transmission capacity, and enables cost-effective integration of clean energy.

Aluminum is more abundant and less expensive than copper. Specifically, aluminum is two to three times less expensive and three orders of magnitude more abundant than copper. It has 61% of copper’s conductivity but only 30% of its specific weight. This means an aluminum conductor is about 50% lighter than a copper conductor with the same electrical resistance. This makes it suitable for applications where weight is important. For example, lightweighting is crucial for next-generation wiring in electric vehicles. The automotive wiring harness market was worth USD 50.2 billion in 2021. It is expected to grow at a rate of 5.7% annually until 2029. The move from heavy copper to lighter aluminum in electric vehicles could reduce wiring weight from 25 to 10 kg per typical car. Aluminum is also used in electric vehicle components like traction motors, rotors, and inverters. These require high conductivity aluminum castings for performance.

3. Electrical Conductivity and Microstructure

Understanding how charge carriers move and interact with a conductor’s structure is key to improving conductor materials. The main goal is to maximize electrical conductivity, but in practical engineering, reaching an optimum between conductivity and mechanical properties is the actual target. Electrical conductivity is a material’s ability to carry a current. It is the ratio of current density to electric field strength. The unit is Siemens per meter (S/m), but it is often reported as %IACS. This stands for the International Annealed Copper Standard. According to this standard, commercially pure annealed copper has a conductivity of 100% IACS at 20 °C. The conductivity of a material is temperature-dependent.

• Factors Affecting Conductivity:
  • Temperature: Conductivity decreases as temperature increases.
  • Impurities: The presence of impurities reduces conductivity by increasing electron scattering.
  • Grain Boundaries: Grain boundaries impede electron flow.
  • Defects: Dislocations and other crystal defects increase electron scattering.
  • Alloying elements: Alloying elements also reduce conductivity.

4. Aluminum vs. Copper

Copper has been a long-standing choice for electrical conductors. However, aluminum presents several advantages.

• Cost and Abundance: Aluminum is two to three times less expensive and three orders of magnitude more abundant than copper.
• Weight: Aluminum is significantly lighter. It is about 50% lighter than copper for the same electrical resistance.
• Conductivity: While aluminum’s conductivity is lower, its lighter weight makes it efficient for many applications.

5. Applications of Aluminum Conductors

Aluminum is essential in various electrical applications. These include power transmission, automotive, and aerospace industries.

• Power Transmission and Distribution:
  • Overhead Conductors: Aluminum is widely used in overhead transmission lines. These include All Aluminum Conductors (AAC), All Aluminum Alloy Conductors (AAAC), and Aluminum Conductor Steel-Reinforced (ACSR).
    • AAC is a stranded conductor with a minimum purity of 99.7% aluminum, used where spacing is short.
    • AAAC is used for circuits requiring higher mechanical strength than AAC.
    • ACSR has steel reinforcement for added strength. This is ideal for long spans.
  • Underground Cables: Aluminum cables are also used in underground power distribution systems.
• Automotive Wiring:
  • Wire Harnesses: Aluminum is being adopted in automotive wire harnesses. This reduces weight and cost in electric vehicles.
  • Battery Cables: Aluminum is used for starter and battery cables in internal combustion engine vehicles, especially in Europe.
  • High-Voltage Wiring: Aluminum is being used in high-voltage wiring between batteries and inverters in electric vehicles.
• Aerospace Applications:
  • Limited Use: Aluminum wiring is mostly avoided in aircraft due to safety concerns. However, ongoing research may improve its use in the aerospace industry.
  • Weight Reduction: The potential for weight savings is a key motivator.
• Electric Vehicle Components:
  • Traction Motors: High conductivity aluminum alloys are required for traction motors, rotors, and inverters.
  • Cast Components: Aluminum castings are being developed for electric vehicle motor components.
• Other Applications:
  • Transformers: Aluminum is used in transformer windings, strips, and heat sinks.
  • Capacitors: Aluminum is used in electrolytic capacitors for energy storage.
  • Electronic Devices: Aluminum alloys are used in base panels and parts for smartphones and other devices.
  • Renewable Energy: Aluminum is used in housings for solar panels and wind turbines.
  • Busbars: Aluminum busbars are used in battery packs and other high-current applications.

6. Strategies for Improving Aluminum Conductivity and Strength

The challenge with aluminum is the trade-off between strength and electrical conductivity. Many strategies are used to enhance both properties:

• Alloying Concepts:
  • Solid Solution Decomposition: Alloying elements are added to speed up the decomposition of the aluminum solid solution, minimizing electron scattering.
  • Precipitate Formation: The formation of nanoscale precipitates can increase strength without a large decrease in conductivity.
• Rare Earth Elements:
  • Purification: Rare earth elements like cerium (Ce) can purify the aluminum solid solution and decrease the solubility of impurities, thereby increasing conductivity.
  • Grain Refinement: Rare earths can also help refine grain structure, improving strength and thermal stability.
  • Synergistic Effects: Combining different rare earth elements like La and Ce can enhance both strength and conductivity.
• Transition Metals:
  • Precipitation Hardening: Transition metals such as zirconium (Zr) can form precipitates that increase strength and thermal stability.
  • Improved Conductivity: Some transition metals, when used correctly, can improve conductivity by reducing the solute content in the aluminum matrix.
• Severe Plastic Deformation (SPD):
  • Grain Refinement: SPD techniques such as equal channel angular pressing (ECAP) reduce grain size to the nanometer scale.
  • Improved Strength and Conductivity: Ultra-fine grains increase strength, while the movement of alloying elements into precipitates can maintain good conductivity.
• Nanostructured Conductors:
  • Ultra-Fine Grains: Refining grain size to the nanometer scale improves strength.
  • Texture Control: Controlling the crystallographic texture can minimize electron scattering and improve conductivity.
  • Grain Boundary Engineering: Reducing grain boundary resistance is another goal.
• Metal Matrix Composites:
  • Cladding: Cladding aluminum with another metal like copper can improve conductivity while maintaining the structural benefits of aluminum.
  • Reinforcement: Adding reinforcement particles like carbon nanotubes or graphene can enhance mechanical and electrical properties.

7. Specific Aluminum Alloys for Electrical Applications

Several aluminum alloys are designed for electrical applications, each with specific properties.

• AA1350:
  • Composition: A refined aluminum with a minimum purity of 99.7%.
  • Applications: Primarily used for overhead transmission and distribution.
  • Properties: Good electrical conductivity but lower strength.
• AA6201:
  • Composition: An aluminum alloy with magnesium and silicon.
  • Applications: Used for aerial circuits requiring higher strength than AA1350.
  • Properties: Higher strength-to-weight ratio and better corrosion resistance than ACSR.
• AA8000 series:
  • Composition: Aluminum alloys with various additions to enhance creep resistance and thermal stability.
  • Applications: Used in overhead conductors, automotive, and other electrical applications.
  • Properties: Improved thermal stability and creep resistance.
  Table 2: Selected Standard Specifications of ASTM International, Sect. 02.03: Electrical Conductors

• Aluminum-Rare Earth Alloys:
  • Al-Ce Alloys: Cerium additions can improve conductivity and thermal stability. For example, adding Ce to Al alloys with 0.55 to 1.2 wt.% Fe and 0.2 to 1.5 wt.% Ce improved electrical conductivity by reducing Fe and Si in the aluminum solution.
  • Al-La Alloys: Lanthanum additions can also improve conductivity, though cerium is more effective in purifying the solid solution.
  • Al-Sc Alloys: Scandium can significantly improve tensile strength per atomic % added to aluminum.
  • Multiple RE Additions: Combinations of rare earth elements such as Sc and Y can lead to good thermal stability. For instance, Al0.2Y0.2Sc alloys showed good thermal stability and high conductivity.
• Aluminum-Transition Metal Alloys:
  • Al-Zr Alloys: Zirconium additions can create thermally stable conductors. A nanostructuring strategy can result in Al-Zr conductors with high conductivity and strength.
  • Al-Mn Alloys: Adding copper and zirconium can simultaneously increase the electrical conductivity and hardness of Al-Mn alloys.
  • Al-Fe Alloys: Iron is often added, and deformation processing is used to improve strength without compromising conductivity.

8. Advanced Materials: Composites and Nanomaterials

Advanced materials such as composites and nanomaterials are being developed to further improve the performance of aluminum conductors.

• Clad Composite Conductors:
  • Copper-Clad Aluminum (CCA): A composite wire with an aluminum core and a copper exterior layer, providing high conductivity and strength. For example, a 99.6% reduction ratio resulted in conductivity over 66.8% IACS.
  • Aluminum-Aluminum Composites: Different aluminum alloys can be combined to optimize strength and conductivity.
• Metal-Metal Composites:
  • Al-Ti Composites: These composites provide high strength with reasonable electrical conductivity.
  • Al-Mg Composites: These achieve high strength with a slight increase in electrical resistivity compared to pure aluminum.
  • Al-Ca Composites: Calcium can enhance conductivity and reduce density. An Al/Ca nanofilamentary composite reached an ultimate tensile strength of 476 MPa with resistivity in the 305–518 × 10^-4 μΩ m range.
  • Al-Nb Composites: Niobium can significantly enhance the strength of aluminum composites.
• Aluminum-Carbon Composites:
  • Graphene-Reinforced Aluminum: Graphene reinforcement can simultaneously improve strength and electrical conductivity. For instance, graphene/Al nanocomposites showed a 2.1% increase in electrical conductivity.
  • Carbon Nanotube (CNT)-Reinforced Aluminum: CNTs can enhance mechanical and electrical properties.
  • Aluminum Covetics: A class of materials where carbon is infused into metal, potentially increasing strength and conductivity.
• Aluminum-Oxide Composites:
  • Al-Al2O3 Composites: These in-situ composites, made using powder metallurgy, can achieve a good balance of strength and conductivity. For example, a low volume fraction of γ-Al2O3 nanoparticles stabilized a wire to reach 172 MPa strength and 60.2% IACS conductivity.

9. Thermal Stability of Aluminum Alloys

Thermal stability is a crucial design factor for conductor materials, especially for aluminum alloys. Aluminum alloys are generally thermally unstable, with their properties being affected by heat. Research is ongoing to develop heat-resistant aluminum alloys. Some strategies include:

• Rare Earth Additions: Rare earths such as cerium help in maintaining high-temperature properties.
• Transition Metal Additions: Transition metals such as zirconium can form stable precipitates that inhibit thermal degradation.
• Nanostructuring: Fine-grain structures can improve high-temperature stability.

10. Corrosion Resistance

Aluminum naturally forms a protective oxide layer, but corrosion can still be an issue. Factors affecting corrosion include:

• Atmospheric Conditions: Exposure to moisture, salts, and pollutants can accelerate corrosion.
• Electrochemical Corrosion: Contact with dissimilar metals can cause galvanic corrosion.
• Crevice Corrosion: Corrosion can occur in gaps and crevices.

Strategies to improve corrosion resistance include:

• Alloying: Adding elements such as chromium can improve corrosion resistance.
• Surface Treatments: Applying coatings can provide an extra layer of protection.
• Cladding: Cladding with a more corrosion-resistant metal like copper can prevent corrosion.

11. The Future of Aluminum Conductors

The future of aluminum conductors looks promising with intensive research and development. Key areas of focus include:

• Novel Alloying: Exploring new alloying combinations to maximize strength, conductivity, and thermal stability.
• Advanced Processing: Developing more effective processing techniques to achieve fine-grain structures and controlled microstructures.
• Composite Materials: Creating advanced composites with superior mechanical and electrical properties.
• Nanomaterials: Applying nanomaterials to enhance conductivity, strength, and other performance characteristics.
• Cost Reduction: Developing cost-effective solutions to expand the use of aluminum conductors.
• Sustainable Practices: Exploring more sustainable manufacturing processes.

12. Conclusion

Aluminum alloys are increasingly important in electrical engineering due to their unique combination of properties. Their light weight, good conductivity, and cost-effectiveness make them an ideal material for a wide variety of applications. Ongoing research and development are overcoming the limitations of aluminum by exploring novel alloying concepts, advanced processing techniques, and composite materials. The future looks bright for aluminum conductors, with continued improvements set to transform power transmission and other energy technologies.