From Bauxite as a Critical Material to the Required Properties of Cast Aluminum Alloys for Use in Electro Automotive Parts

From Bauxite to Battery Housings: Optimizing Cast Aluminum Alloys for Modern Electric Vehicle Parts

This technical summary is based on the academic paper "From Bauxite as a Critical Material to the Required Properties of Cast Aluminum Alloys for Use in Electro Automotive Parts" by Mile Djurdjevic, Srecko Manasijevic, Marija Mihailović, and Srecko Stopic, published in Metals (2023).

Keywords

• Primary Keyword: Cast Aluminum Alloys for EV Parts
• Secondary Keywords: electro automotive parts, HPDC, dimensional stability, corrosion resistance, electromagnetic compatibility, crashworthiness, aluminum heat treatment

Executive Summary

• The Challenge: The transition to electric vehicles (EVs) demands aluminum castings with a new set of critical properties beyond just mechanical strength, including dimensional stability, corrosion resistance, and crashworthiness.
• The Method: The paper reviews and analyzes the suitability of various aluminum-silicon alloys, casting processes (HPDC, LPDC, CPS), and heat treatments for producing high-performance electro automotive components.
• The Key Breakthrough: Specific heat treatments (T5, T6, T7) and targeted alloying can significantly enhance the properties of existing commercial alloys, enabling them to meet the stringent requirements of high-torque EV engines and structural components.
• The Bottom Line: Optimizing the selection of alloys, casting processes, and post-casting treatments is crucial for producing reliable, safe, and efficient Cast Aluminum Alloys for EV Parts, but engineers must navigate critical trade-offs, such as corrosion resistance versus electromagnetic compatibility.

The Challenge: Why This Research Matters for HPDC Professionals

The global automotive industry's rapid shift towards electrification is creating new challenges and opportunities for casting professionals. While aluminum has long been a material of choice for lightweighting, EV components like e-drive housings, battery packs, and cooling plates demand more than what was required for internal combustion engines. These parts must exhibit not only moderate mechanical strength but also exceptional dimensional stability under thermal cycling, robust corrosion resistance in condensed environments, predictable crashworthiness for safety, and specific electromagnetic compatibility (EMC). This research addresses the critical need for a comprehensive understanding of how to transform bauxite-derived aluminum into advanced cast components that meet these multifaceted demands.

The Approach: Unpacking the Methodology

This study provides a thorough review of the processes and materials used to produce electro automotive parts. The authors analyze the most common approaches to identify the most suitable combinations for achieving the required performance characteristics.

Method 1: Casting Process Selection: The paper identifies three primary casting processes for the production of electro automotive parts: High Pressure Die Casting (HPDC), Low Pressure Die Casting (LPDC), and Core Package System (CPS). Each process offers a unique combination of advantages, from HPDC's high production rates and dimensional accuracy to the high integrity and strength of parts produced via CPS.

Method 2: Alloy and Heat Treatment Analysis: The research focuses on common aluminum-silicon cast alloys such as EN AC—47000 (AlSi12(Cu1)), EN AC—43500 (AlSi10MnMg), and EN AC—42100 (AlSi7Mg). It investigates how their baseline mechanical properties can be significantly enhanced through various heat treatment processes, including T5 (controlled cooling and artificial aging), T6 (solution heat treated and artificially aged), and T7 (solution heat treated and over-aged/stabilized).

Method 3: Alloying Element Impact Review: The study summarizes the effects of both major (Si, Cu, Mg, Mn, Zn) and minor (Zr, Mo, Sc, La) alloying elements on the final mechanical properties (UTS, YS, and Elongation). This analysis provides a roadmap for fine-tuning alloy chemistry to achieve specific performance targets.

The Breakthrough: Key Findings & Data

The paper synthesizes existing data to highlight critical relationships between processing, material composition, and final part performance.

Finding 1: Heat Treatment is Key to Unlocking Higher Strength

While as-cast alloys can meet baseline EV requirements (YS ≥ 140 MPa), high-performance applications demand significantly higher strength (YS > 240 MPa). The paper reinforces that heat treatment is the most effective way to achieve this. As shown in Figure 3, selecting a T6 heat treatment for a high-magnesium alloy can increase yield strength from ~125 MPa in the as-cast (F) state to over 250 MPa. Conversely, a T7 treatment can be used to improve elongation and stability.

Finding 2: Dimensional Stability is Temperature and Composition Dependent

Irreversible growth in castings can be a significant issue for parts exposed to heat. The research highlights that this effect is highly dependent on both operating temperature and alloy composition. As shown in Figure 5, at 100°C, there is no significant difference in growth between alloys. However, at 150°C, the alloy with higher copper and magnesium content (Alloy No. 1) exhibited nearly double the irreversible growth of the lower Cu/Mg alloy (Alloy No. 3) after 400 hours. This indicates that for parts operating above 120°C, minimizing Cu and Mg is critical for dimensional stability.

Finding 3: The Copper Conundrum: Corrosion vs. Electromagnetic Shielding

The paper identifies a crucial design trade-off related to copper content. Low copper content is desirable for preventing intergranular corrosion, which is a major concern on sealing surfaces and electrical connections in the cooler, condensation-prone operating environment of EVs. However, the copper-rich Al2Cu phase is beneficial for electromagnetic compatibility, as it helps absorb electromagnetic waves and protect sensitive electronics. This forces engineers to make a critical choice based on the specific application of the part.

Practical Implications for R&D and Operations

• For Process Engineers: This study suggests that for high-strength EV housing parts, implementing a T6 heat treatment process is essential. Careful control of quenching and artificial aging parameters will be critical to achieving the target yield strength of >240 MPa without sacrificing other properties.
• For Quality Control Teams: The data in Figure 5 of the paper illustrates the effect of alloy composition on dimensional stability at elevated temperatures. This could inform new quality inspection criteria for components intended for high-heat applications, potentially involving batch testing for irreversible growth.
• For Design Engineers: The findings indicate that specifying a lower copper content for parts with critical sealing surfaces is a key consideration to mitigate corrosion risk. Conversely, for battery housings requiring maximum EMC, an alloy with controlled copper content may be necessary, along with other protective measures.

Paper Details

From Bauxite as a Critical Material to the Required Properties of Cast Aluminum Alloys for Use in Electro Automotive Parts

1. Overview:

• Title: From Bauxite as a Critical Material to the Required Properties of Cast Aluminum Alloys for Use in Electro Automotive Parts
• Author: Mile Djurdjevic, Srecko Manasijevic, Marija Mihailović and Srecko Stopic
• Year of publication: 2023
• Journal/academic society of publication: Metals
• Keywords: electro mobility parts; casting processes; dimensional stability; corrosion resistance; electromagnetic compatibility; crashworthiness

2. Abstract:

There is a long process to transform bauxite, a critical raw material, into a substance with the required properties of cast aluminum alloys for use in electro automotive parts. Thanks to its unique properties, aluminum has become the material of choice for clean technology manufacturers in applications such as use in the automotive industry, renewable energy, batteries, electrical systems, resource-saving packaging, energy efficient buildings and clean mobility. Restructuring of the economy, the oil crisis, air pollution and global warming are some of the factors that have moved the automotive industry towards electrification since the beginning of the 21st century. This paper aims to highlight the required properties of cast aluminum alloys applied to the production of electro automotive parts, such as their mechanical and thermophysical properties, dimensional stability, corrosion resistance, electromagnetic compatibility and crashworthiness. Furthermore, this paper discusses which of the cast aluminum–silicon alloys, as well as the heat treatments and casting processes, are most suitable.

3. Introduction:

The paper traces the history of aluminum production from its discovery in the 19th century to the development of the Bayer–Hall–Héroult process, which enables the refining of bauxite ore into alumina and subsequent smelting into aluminum. It establishes bauxite as a critical raw material for the aluminum industry. The introduction then pivots to the modern automotive industry, noting the drastic shift towards electrification driven by environmental regulations and economic factors. This shift has increased the use of aluminum in vehicles, with projections showing the average content rising from 174 kg in 2019 to 256 kg by 2030, largely due to applications in Battery Electric Vehicles (BEVs) such as e-drive housings and battery packs. The paper states its intention to highlight the specific properties required for these new applications, which go beyond traditional mechanical characteristics.

4. Summary of the study:

Background of the research topic:

The increasing production of electric vehicles necessitates a re-evaluation of the material properties required for automotive components. Aluminum alloys, long used in the industry for their lightweight and good mechanical properties, must now meet new criteria specific to electro mobility applications.

Status of previous research:

Previous research and industry standards, such as DIN 1706, have established the baseline properties of various cast aluminum-silicon alloys. Studies have documented the effects of different casting processes (HPDC, LPDC), heat treatments (T1, T4, T5, T6, T7), and individual alloying elements on the microstructure and mechanical performance of these alloys. The paper synthesizes this existing knowledge to apply it to the specific context of electro automotive parts.

Purpose of the study:

The study aims to identify and highlight the required properties of cast aluminum alloys for use in electro automotive parts. This includes not only mechanical and thermophysical properties but also newer, critical considerations such as dimensional stability, corrosion resistance, electromagnetic compatibility, and crashworthiness. The paper seeks to determine which alloys, casting processes, and heat treatments are most suitable to meet these combined requirements.

Core study:

The core of the study is a comprehensive review that connects material science principles to the practical demands of EV component manufacturing. It analyzes how to improve the strength and ductility of standard alloys through heat treatment and the strategic addition of alloying elements. It then examines four key performance criteria critical for EV parts:
1. Dimensional Stability: Investigating irreversible casting growth as a function of time, temperature, and alloy composition (specifically Cu and Mg).
2. Corrosion Resistance: Discussing the role of the passive oxide layer and the influence of alloying elements, particularly the negative impact of copper on intergranular corrosion.
3. Electromagnetic Compatibility: Explaining how aluminum's paramagnetic nature and the precipitation of copper-rich phases provide shielding against electromagnetic fields.
4. Crashworthiness: Relating ductility and energy absorption to microstructure, particularly the morphology of silicon particles, which can be modified through solid solution treatment and alloying.

5. Research Methodology

Research Design:

The study is designed as a literature review and synthesis. It does not present new experimental data but instead collates and analyzes existing findings from academic papers, industry standards (e.g., DIN 1706), and technical reports.

Data Collection and Analysis Methods:

The authors collected data from a wide range of published sources. The analysis involves summarizing this information and presenting it in consolidated tables and figures to illustrate key concepts, such as the effect of alloying elements on mechanical properties (Table 3) and the impact of temperature on irreversible growth (Figure 5). The methodology is qualitative and analytical, focused on drawing connections between disparate research findings to build a cohesive guide for material selection in the EV industry.

Research Topics and Scope:

The scope is focused on cast aluminum-silicon alloys for electro automotive parts. It covers the entire production chain from raw material (bauxite) to final component properties. The research topics include:
- Aluminum cast alloys and casting processes (HPDC, LPDC, CPS).
- Improvement of mechanical properties via heat treatment and alloying.
- Analysis of specific properties: dimensional stability, corrosion resistance, electromagnetic compatibility, and crashworthiness.

6. Key Results:

Key Results:

• Standard commercial cast aluminum alloys (e.g., AlSi12(Cu1), AlSi10MnMg, AlSi7Mg) can be used for mass production of EV parts using HPDC, LPDC, and CPS processes.
• The mechanical properties of these alloys can be significantly improved to meet high-performance requirements (e.g., YS > 240 MPa) through appropriate heat treatments (T5, T6, T7) and the addition of major (Mg, Mn, Zn, Cu) and minor (Zr, Mo, Sc, La) alloying elements.
• Irreversible growth (dimensional instability) is a significant concern for parts operating at temperatures above 120°C, and is exacerbated by higher concentrations of copper and magnesium.
• A key conflict exists with copper content: it is beneficial for electromagnetic compatibility but detrimental to corrosion resistance, requiring a careful balance based on the part's function.
• Crashworthiness is primarily linked to ductility, which can be improved through solid solution heat treatments and the modification of silicon particles using elements like strontium.

Figure Name List:

• Figure 1. The historical perspective of road vehicle electrification [5].
• Figure 2. Strength-ductility trade off dilemma [13].
• Figure 3. Impact of heat treatment processes on the elongation and strength of HPDC alloys [15].
• Figure 4. Increases in the yield strength through a combination of natural and artificial ageing at 200 °C; the isotherm shows the influence of natural ageing time on the maximum strength [16].
• Figure 5. The impacts of temperature and time on irreversible growth during engine working conditions for an engine block [36].
• Figure 6. The effects of aluminum’s major alloying elements on the electrolytic solution’s potential [39].

7. Conclusion:

Cast aluminum alloys are highly attractive for producing electro automotive parts due to their combination of good mechanical properties, dimensional stability, corrosion resistance, EMC, and crashworthiness. Several commercially available alloys, such as EN AC-47000, EN AC-44300, EN AC-43500, and EN AC-42100, are suitable for this market. Their performance can be further enhanced via heat treatment and alloying. The primary concerns for implementation are the potential cost of minor alloying elements and managing the trade-offs associated with certain elements, particularly copper's dual role in corrosion and EMC.

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Expert Q&A: Your Top Questions Answered

Q1: The paper mentions HPDC, LPDC, and CPS as suitable casting processes. For a complex battery housing that requires high structural integrity and ductility, which process would be most appropriate according to the paper?

A1: Based on the advantages listed in Table 1, the Core Package System (CPS) would be the most suitable choice. The paper highlights that CPS provides a "tranquil transfer of metal," leading to minimal oxides and high integrity. Most importantly, it is noted for delivering "high strength and ductility of cast products," which are essential for a structural component like a battery housing that must perform well in a crash scenario.

Q2: According to the paper, what is the primary mechanism for improving crashworthiness in these aluminum alloys?

A2: The paper states that crashworthiness is primarily dependent on the alloy's ability to deform plastically and absorb energy, which is a function of its ductility. The key mechanism for improving this is the "solid solution treatment" process. This heat treatment modifies and separates the silicon particles within the aluminum matrix, minimizing crack initiation points. Additionally, the use of modifiers like strontium can change large, sharp silicon plates into smaller, rounded particles, further enhancing ductility and thus crashworthiness.

Q3: Figure 5 shows that irreversible growth is a major concern above 120°C. Are typical EV battery and motor housings expected to operate in this range?

A3: The paper states that "irreversible growth could be a significant issue only at localized positions of electro automotive parts that are exposed to temperatures higher than 120 °C." While it doesn't specify the exact operating temperatures of all components, this implies that many parts operate below this threshold. However, for localized hot spots, such as areas near power electronics or motor windings, exceeding 120°C is a real possibility, making alloy selection (specifically lower Cu and Mg content) critical for those designs.

Q4: The paper presents a conflict with copper: it's bad for corrosion but good for EMC. Is there a recommended solution for a part that needs both good corrosion resistance and good EMC?

A4: The paper does not offer a single alloy solution to this trade-off. It presents the conflict as a critical design consideration for engineers. The implication is that a compromise must be made. An engineer might choose an alloy with a moderate copper level or prioritize one property over the other based on the component's specific environment and function. For example, a part with critical sealing surfaces might use a low-copper alloy, and EMC requirements would have to be met through other means, such as separate shielding.

Q5: Table 3 shows that adding elements like Scandium (Sc) and Zirconium (Zr) improves strength. Why aren't these used more commonly for EV parts?

A5: The paper explicitly addresses this in the text following Table 3. It states, "Additional consideration needs to be taken when using minor alloying elements. Their prices are significantly high and they are, therefore, sometimes not suitable for commercial application." While technically effective, the high cost of elements like Scandium makes them economically unviable for the mass production of automotive parts, limiting their use to more niche, high-performance applications.

Conclusion: Paving the Way for Higher Quality and Productivity

The successful transition to electric mobility hinges on the ability to produce advanced components that are safe, reliable, and efficient. This paper provides a clear roadmap for engineers, demonstrating that the key to success lies in the intelligent optimization of Cast Aluminum Alloys for EV Parts. By carefully selecting the right combination of alloy chemistry, casting process, and heat treatment, it is possible to overcome the challenges of dimensional stability, corrosion, and crashworthiness. The research underscores that a deep understanding of material science is no longer optional—it is essential for achieving the highest levels of quality and productivity in the EV era.

"At CASTMAN, we are committed to applying the latest industry research to help our customers achieve higher productivity and quality. If the challenges discussed in this paper align with your operational goals, contact our engineering team to explore how these principles can be implemented in your components."

Copyright Information

This content is a summary and analysis based on the paper "From Bauxite as a Critical Material to the Required Properties of Cast Aluminum Alloys for Use in Electro Automotive Parts" by "Mile Djurdjevic, Srecko Manasijevic, Marija Mihailović and Srecko Stopic".

Source: https://doi.org/10.3390/met13111796

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