Aluminium Foam and Magnesium Compound Casting Produced by High-Pressure Die Casting

This article introduces the paper 'Aluminium Foam and Magnesium Compound Casting Produced by High-Pressure Die Casting' published by 'MDPI'.

1. Overview:

• Title: Aluminium Foam and Magnesium Compound Casting Produced by High-Pressure Die Casting
• Author: Iban Vicario, Ignacio Crespo, Luis Maria Plaza, Patricia Caballero, and Ion Kepa Idoiaga
• Publication Year: 2016
• Publishing Journal/Academic Society: Metals
• Keywords: high pressure die casting (HPDC); hybrid magnesium aluminium foam cast composite; aluminium foam core; magnesium cast composite

2. Abstracts or Introduction

This research investigates the production of lightweight composite components using High-Pressure Die Casting (HPDC). The study aims to evaluate the influence of different aluminium foams as cores and HPDC injection parameters on the properties and weight of magnesium cast parts. The paper explores the feasibility of using aluminium foams to reduce weight in HPDC magnesium components, focusing on achieving a balance between component properties and weight reduction. The research varies aluminium foam type and density, metal temperature, plunger speed, and multiplication pressure to assess their impact on casting quality. The study successfully produced sound composite castings with aluminium foam cores, demonstrating the potential of HPDC for manufacturing magnesium-aluminium foam composites, particularly for applications requiring weight reduction, such as a bicycle component demonstrator.

3. Research Background:

Background of the Research Topic:

The primary driver for this research is the increasing demand for weight reduction in the transport industry, particularly in vehicle design, to reduce fuel consumption and carbon dioxide emissions. Substituting heavier materials like steel and iron with lighter alternatives such as plastics, carbon fiber, aluminium, and magnesium alloys is a key trend. In the bicycle industry, this trend is evident in the shift towards carbon fiber and away from steel, aluminium, and titanium for high-performance applications. Magnesium components produced via HPDC are already utilized in automotive and bicycle sectors, and the industry seeks further applications where the combination of lightness and mechanical properties offered by magnesium-lightened structures is advantageous. HPDC is highlighted as a high-productivity and economically viable process for large production volumes, exceeding 5000–10,000 parts per year [2].

Status of Existing Research:

Existing research acknowledges HPDC as a complex process influenced by numerous parameters affecting casting quality [6]. Key parameters include injection parameters, die temperatures, alloy castability, part geometry, cooling rate, and equipment type. Internal porosity is a known drawback of HPDC components, limiting heat treatment and mechanical properties compared to other casting methods. Strategies to mitigate porosity and improve HPDC component performance are being explored, including new HPDC variants. Employing aluminium-magnesium composites is one weight reduction strategy [13], exemplified by the BWM aluminium-magnesium block [14]. However, achieving metallurgical bonding between magnesium and aluminium core is challenging due to the alumina surface layer on aluminium [15]. Alternative methods for weight reduction include using salt cores to create hollow parts [16-18], and developing new HPDC alloys with improved properties [19]. The use of aluminium foams is recognized as a significant advancement [20], with various production methods available as shown in "Figure 1. Some of the most employed processes to produce aluminium foams." and summarized in "Table 1. Pros and cons of different foams production routes.".

Necessity of the Research:

A major challenge in producing composite castings with aluminium foam cores via HPDC is preventing core deformation or collapse due to high velocity and specific pressures, while also avoiding casting defects. The paper identifies a gap in understanding and controlling HPDC parameters specifically for aluminium foam core composite casting. Therefore, research is needed to analyze and control HPDC parameters to achieve high-quality composite parts using aluminium foam cores, addressing the challenges of core integrity and casting soundness in this specific application.

4. Research Purpose and Research Questions:

Research Purpose:

The primary research purpose is to evaluate the influence of different aluminium foams and HPDC injection parameters to produce compound castings that achieve a compromise between mechanical properties and weight reduction. The ultimate goal is to develop a HPDC process for producing a bicycle rod, currently manufactured using materials like forged aluminium, titanium, magnesium, or carbon fibers. The redesigned bicycle rod adapted for HPDC features is illustrated in "Figure 2. (a) 3D rod design; and (b) detail of the placement and example of an aluminium foam core.".

Key Research:

The key research questions focus on determining the optimal combination of:

• Aluminium foam type (Alporas ALPO-PLA-03 [27,28], Formgrip-based processed foam [29,30], and 0.4% TiB2 AlSi12 Alulight foam [31-34]) and density.
• HPDC injection parameters, including metal temperature, plunger speed, and multiplication pressure.
• Core placement and squeeze pin suitability.

These investigations aim to identify the parameters that yield sound magnesium-aluminium composite castings with aluminium foam cores, suitable for lightweight structural applications.

Research Hypotheses:

While not explicitly stated as formal hypotheses, the research operates under the premise that:

• By carefully selecting the type and density of aluminium foam and optimizing HPDC parameters, it is possible to produce sound magnesium-aluminium composite castings with significant weight reduction.
• Aluminium foams with an external skin will exhibit better performance as cores in HPDC due to enhanced robustness against high injection pressures.
• Proper core placement and controlled injection speeds are crucial to prevent foam damage and ensure casting quality.

5. Research Methodology

Research Design:

The research employs an experimental design involving systematic variation of aluminium foam types and HPDC process parameters. Castings were produced using AM60B alloy and three different aluminium foam cores. Preliminary tests using die casting, plastic injection, and HPDC processes were conducted to assess pressure effects on foam integrity. Subsequently, HPDC trials were performed on a 950-ton HPDC machine to produce magnesium-aluminium foam composite castings.

Data Collection Method:

Data collection methods included:

• Visual Inspection: To assess external casting quality and identify macroscopic defects.
• Radiographic Inspection (RX): Using a General Electric X-cube 44XL at 160 kV, as mentioned in the paper, to evaluate internal casting soundness, detect porosity, and verify aluminium foam core integrity.
• Tensile Testing: Performed according to UNE-EN ISO 6892-1 B:2010 standards at room temperature using an Instron 3369 machine with a crosshead speed of 5 mm/min to determine mechanical properties of composite rods.

Analysis Method:

The analysis involved:

• Qualitative Assessment: Visual and RX inspection results were qualitatively analyzed to evaluate casting defects, foam deformation, and core placement.
• Quantitative Analysis: Tensile test data were used to calculate tensile stress, ultimate tensile strength, and elongation, enabling quantitative comparison of mechanical properties between AM60B alloy and composite castings.

Research Subjects and Scope:

The research focused on:

• Materials: AM60B magnesium alloy and three types of closed-cell aluminium foams: Alporas ALPO-PLA-03, Formgrip-based foam, and Alulight 0.4% TiB2 AlSi12 foam.
• Process: High-Pressure Die Casting (HPDC).
• Component: A bicycle rod demonstrator part.
• Parameters: Aluminium foam type and density, metal casting temperature (680 °C and 720 °C), injection pressure (16-80 MPa, up to 200 MPa for squeeze pin), and injection speed (20-80 m/s).

6. Main Research Results:

Key Research Results:

The key findings of the research are:

• Metal Casting Temperature: A minimum pouring temperature of 720 °C for AM60B alloy was necessary to avoid short fill/cold shut defects, as observed in "Figure 8. (a) Short fill and cold shut defects; and (b) gas porosity defects.".
• Aluminium Foam Integrity: Alulight foam demonstrated superior resistance to HPDC pressures compared to Alporas and Formgrip foams, as shown in "Table 4. Integrity of aluminium foams after plastic injection.". Alulight foam with densities from 0.54 to 1.55 Kg/dm³ successfully withstood 40 MPa pressure.
• External Skin Importance: Aluminium foams with an external skin, like Alulight, prevented gas porosity and core collapse during magnesium over-injection, as illustrated in "Figure 9. Central aluminium core covered with AM60B.". Foams without a skin, or with damaged skin, resulted in gas porosity, as seen in "Figure 13. (a) Placement of a skinned foam with a non-skin area in the die; and (b) release of gas from the foam in the non-skinned area.".
• Core Placement: Vertical placement of the aluminium foam core relative to the metal flow minimized damage, while horizontal placement led to shear rupture, as shown in "Figure 15. Horizontal placement to the metal flow of the core." and "Figure 16. Horizontal core placement to metal flow.".
• Injection Speed: Reduced first phase injection speed adversely affected part quality, while standard HPDC parameters with a second phase speed of 80 m/s and 80 MPa pressure yielded sound parts, as shown in "Figure 18. Reduced second phase speed (20 m/s) HPDC cast part." and "Figure 19. Injected HPDC with core foam at standard parameters.".
• Weight Reduction: Using Alulight-type aluminium foam with a density of 0.56 Kg/dm³ resulted in a weight reduction of approximately 35% for the bicycle rod demonstrator.
• Mechanical Properties: "Table 5. Obtained properties of composite rods." presents the tensile properties of AM60B, composite, and extrapolated composite properties. While direct chemical bonding was not achieved, the composite rods exhibited reasonable mechanical properties with the weight reduction.

Analysis of presented data:

Data analysis revealed that Alulight foam, particularly due to its external skin, is the most suitable aluminium foam core for HPDC magnesium composite casting among the tested foams. The external skin provides necessary robustness to withstand injection pressures and prevents gas release into the magnesium melt. Optimal HPDC parameters, including a metal temperature of 720°C and standard injection speeds and pressures, are crucial for producing sound composite castings. Squeeze pins are not suitable for areas near the aluminium foam core due to excessive pressure. The achieved 35% weight reduction demonstrates the potential of this approach for lightweight component manufacturing. The mechanical property analysis indicates a reduction in Yield Strength (YS) and Ultimate Tensile Strength (UTS) in the composite compared to solid AM60B, but the extrapolated values suggest that with optimized design and bonding, the composite approach can offer a viable balance of weight and performance.

Figure Name List:

• Figure 1. Some of the most employed processes to produce aluminium foams.
• Figure 2. (a) 3D rod design; and (b) detail of the placement and example of an aluminium foam core.
• Figure 3. Metallic die to produce Aluminium foams with Alulight.
• Figure 4. Metallic die to die cast magnesium over the aluminium foam.
• Figure 5. The plastic injection mould with an aluminium foam.
• Figure 6. HPDC process in order to obtain the magnesium-aluminium foam core composite.
• Figure 7. Detail of fixing pins in the fixed die cavity for placing the aluminium foam.
• Figure 8. (a) Short fill and cold shut defects; and (b) gas porosity defects.
• Figure 9. Central aluminium core covered with AM60B.
• Figure 10. Different configurations for plastic injection over the aluminium foams.
• Figure 11. (a) Alpora's foam (0.25 to 0.4 Kg/dm³); and (b) Formgrip's foam (0.4 to 0.65 Kg/dm³).
• Figure 12. HPDC part with totally destroyed aluminium foam.
• Figure 13. (a) Placement of a skinned foam with a non-skin area in the die; and (b) release of gas from the foam in the non-skinned area.
• Figure 14. 1.55 Kg/dm³ Aluminium foam after squeeze pin application.
• Figure 15. Horizontal placement to the metal flow of the core.
• Figure 16. Horizontal core placement to metal flow.
• Figure 17. Rod made by magnesium HPDC with the internal core of aluminium foam.
• Figure 18. Reduced second phase speed (20 m/s) HPDC cast part.
• Figure 19. Injected HPDC with core foam at standard parameters.

7. Conclusion:

Summary of Key Findings:

This study successfully demonstrated the feasibility of producing HPDC magnesium-aluminium foam composite castings for weight reduction applications. The Alulight-type aluminium foam, characterized by its closed-cell structure and external skin, proved to be the most effective core material for HPDC magnesium over-injection. Optimizing HPDC parameters, including maintaining a magnesium melt temperature of 720°C and employing standard injection speeds and pressures, is crucial for achieving sound castings. A weight reduction of 35% was achieved in a bicycle rod demonstrator part using this composite approach.

Academic Significance of the Study:

This research contributes to the academic understanding of composite casting in HPDC, specifically using aluminium foam cores within magnesium matrices. It provides valuable insights into the influence of aluminium foam characteristics (type, density, skin presence) and HPDC process parameters on the resulting composite casting quality. The study highlights the importance of external skin on aluminium foams for core integrity during HPDC and offers guidance on core placement and injection speed control.

Practical Implications:

The findings have significant practical implications for the manufacturing industry, particularly in sectors seeking lightweighting solutions. The demonstrated HPDC process for magnesium-aluminium foam composites offers a viable route for mass production of lightweight components, potentially replacing heavier materials in automotive, aerospace, and sports equipment applications. The bicycle rod demonstrator showcases the immediate applicability in the bicycle industry.

Limitations of the Study and Areas for Future Research:

A primary limitation identified is the lack of chemical bonding between the aluminium foam core and the magnesium matrix due to the alumina layer on the foam. This absence of bonding may limit the full potential of mechanical property enhancement in the composite. Future research should focus on:

• Investigating surface treatments for aluminium foams, such as Zn-based coatings, to promote metallurgical bonding with magnesium and improve interfacial strength.
• Further optimization of HPDC parameters and component design to maximize weight reduction while maintaining or enhancing mechanical performance.
• Exploring the application of this composite casting approach to a broader range of component geometries and industrial applications.

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9. Copyright:

• This material is "Iban Vicario et al."'s paper: Based on "Aluminium Foam and Magnesium Compound Casting Produced by High-Pressure Die Casting".
• Paper Source: https://doi.org/10.3390/met6010024

This material was summarized based on the above paper, and unauthorized use for commercial purposes is prohibited.
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