Thermodynamic Design and Experiment on High Conductivity Die-casting Al-Si-Fe Aluminum Alloy for 5G Base Station Heat Sink

This paper introduction was written based on the 'Thermodynamic Design and Experiment on High Conductivity Die-casting Al-Si-Fe Aluminum Alloy for 5G Base Station Heat Sink' published by 'Special Casting & Nonferrous Alloys'.

1. Overview:

• Title: Thermodynamic Design and Experiment on High Conductivity Die-casting Al-Si-Fe Aluminum Alloy for 5G Base Station Heat Sink
• Author: YANG Shuang, WU Junjie, WANG Mengmeng, HUANG Zhongjia, LIU Tong, ZHAO Yu, HONG Ronghui, YAO Yingwu, WANG Jun
• Publication Year: 2024
• Publishing Journal/Academic Society: Special Casting & Nonferrous Alloys (特种铸造及有色合金)
• Keywords: Die-cast Aluminum Alloy, Heat Sinks, Thermal Conductivity, Conductivity, Thermodynamic Calculation

2. Abstracts or Introduction

Abstract: In view of the inversion problem between strength and electrical (thermal) conductivity of die-cast aluminum heat sinks in 5G base stations, effects of aging treatment on microstructure and conductivity of die-cast Al-7.5Si-0.8Fe aluminum alloys were investigated through a combination of thermodynamic calculations and experimental research by PANDAT thermodynamic calculations, metallographic microscopy, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results indicate that the conductivity of alloy is significantly improved after heat aging treatment at 320 °C for 1 h. Al-Fe-Si ternary phases and Si phase are precipitated along the grain boundaries and interior, respectively, thus reducing the solid solubility of Fe and Si in Al matrix. In addition, the continuity of eutectic Si network is deteriorated after aging, accompanied with the increased connectivity of the aluminum matrix, which is the main reason for the enhancement of conductivity.
Key Words: Die-cast Aluminum Alloy, Heat Sinks, Thermal Conductivity, Conductivity, Thermodynamic Calculation

3. Research Background:

Background of the Research Topic:

The advent of the 5G communication era propels electronic communication equipment and products towards higher integration. Consequently, the demand for enhanced thermal conductivity in materials for these devices is continuously escalating to ensure operational longevity [1]. Communication filters, critical components in 5G base stations, are characterized by high power and integration, necessitating intricate housing designs with numerous thin-walled heat dissipation fins to augment cooling capacity. High-pressure die casting, owing to its elevated production efficiency and cost-effectiveness, emerges as the predominant forming technique for mass-producing heat sink housings. Aluminum alloys, distinguished by their low density, high specific strength, and exceptional corrosion resistance, are the primary materials employed in fabricating communication filters [2]. While pure aluminum exhibits a thermal conductivity of approximately 237 W/(m·K) at room temperature, its strength is limited. Alloying can enhance the mechanical properties of pure aluminum, albeit often at the expense of its thermal conductivity [3-5].

Status of Existing Research:

Current materials for high thermal conductivity aluminum heat sinks are primarily developed from Al-Si alloys, notably the Al-8Si system. To mitigate die sticking during casting and prolong die lifespan, approximately 0.8% to 1.0% Fe is typically incorporated into high thermal conductivity die-casting aluminum alloys. Conversely, to maintain optimal electrical and thermal conductivity, stringent control over impurity element concentrations is imperative to minimize the adverse effects of solute elements on conductivity. Research indicates that transition metal elements such as Cr, Mn, V, and Ti exert the most pronounced detrimental influence on electrical and thermal conductivity [6]. Consequently, high thermal conductivity aluminum alloys intended for die casting are generally based on the Al-Si-Fe system, with Si content ranging from 6% to 9%, Fe content between 0.6% and 1.0%, and other impurity elements meticulously controlled below 0.01% to concurrently satisfy both formability and performance prerequisites [7].

Necessity of the Research:

However, the relative proportions of Si and Fe significantly influence the volume fraction of eutectic Si phase, the morphology and volume fraction of Fe-containing phases, the solidification temperature range of the alloy, and the solid solubility of Fe and Si in the aluminum matrix. These microstructural attributes, in turn, directly impact the alloy's strength, ductility, and electrical (thermal) conductivity [8-10]. High thermal conductivity Al-Si-Fe alloys commonly undergo aging treatment at temperatures between 300°C and 350°C to further augment their electrical conductivity. Nevertheless, the dynamic evolution of precipitated phases and the kinetics of precipitation during aging in these high conductivity alloys remain incompletely understood [11]. Furthermore, the intricate interplay between Fe and Si elements often necessitates a trial-and-error approach in developing high thermal conductivity die-casting materials, thereby impeding research efficiency and escalating development costs. In recent years, the domain of materials science has witnessed the burgeoning utilization of phase diagram calculation software, including Thermo-Calc, FactSage, PANDAT, and JMATPro, to guide aluminum alloy design, thereby transcending the limitations of solely relying on experimental exploration and enhancing product development efficiency while conserving resources and energy [12-13].

4. Research Purpose and Research Questions:

Research Purpose:

This research endeavors to investigate the influence of aging treatment on the microstructure and conductivity of die-cast Al-7.5Si-0.8Fe aluminum alloys through a synergistic approach encompassing thermodynamic calculations and experimental investigations. The ultimate aim is to furnish valuable insights for the design of high thermal conductivity materials.

Key Research:

• To elucidate the effects of aging treatment on the microstructure and electrical conductivity of die-cast Al-7.5Si-0.8Fe alloy.
• To unravel the interaction mechanisms between Fe and Si elements within the alloy system.
• To characterize the phase evolution and precipitation behavior during the aging process.

Research Hypotheses:

• Aging treatment at 320°C for 1 hour can effectively enhance the electrical conductivity of the die-cast aluminum alloy.
• During aging, the precipitation of Al-Fe-Si ternary phases and Si phase along grain boundaries and within the grain interior reduces the solid solubility of Fe and Si in the Al matrix.
• The deterioration of the eutectic Si network continuity, coupled with the enhanced connectivity of the aluminum matrix, constitutes the primary mechanism for the observed conductivity improvement.

5. Research Methodology

Research Design:

This study employed a research design that integrates thermodynamic calculations with experimental validation to comprehensively investigate the target alloy system.

Data Collection Method:

• PANDAT Thermodynamic Calculations: Employed to predict phase equilibria, phase fractions, and solute solubility.
• Metallographic Microscopy (OM): Utilized to observe the general microstructure of the alloy.
• Scanning Electron Microscopy (SEM): Employed to characterize the microstructure at higher magnifications and to perform elemental mapping.
• X-ray Diffraction (XRD): Used for phase identification in the alloy samples.
• Transmission Electron Microscopy (TEM): Utilized for detailed microstructural analysis, particularly of precipitates.
• Conductivity Measurement: Four-point probe method used to quantify the electrical conductivity of the alloy.
• Mechanical Property Testing: Tensile tests conducted to evaluate the mechanical performance of the alloy.

Analysis Method:

• Thermodynamic Analysis: PANDAT software was used to calculate phase diagrams, phase fractions, and solid solubility limits for the Al-Si-Fe system.
• Microstructural Characterization: OM and SEM were used to analyze grain morphology, phase distribution, and eutectic Si network. Image analysis software was used for quantitative analysis of microstructural features.
• Phase Identification: XRD was used to identify the crystalline phases present in the as-cast and aged samples.
• Precipitation Analysis: TEM was used to characterize the morphology, size, and distribution of precipitates formed during aging. EDS analysis in TEM was used to determine the composition of precipitates.
• Correlation Analysis: The relationship between aging parameters, microstructure, conductivity, and mechanical properties was analyzed to establish structure-property correlations.

Research Subjects and Scope:

The research focused on a die-cast Al-7.5Si-0.8Fe aluminum alloy specifically designed for 5G base station heat sink applications. The scope of the study included:

• Alloy Composition: Al-7.5Si-0.8Fe alloy with minor Sr addition (0.04%) as a modifier.
• Processing: High-pressure die casting.
• Aging Treatment: Isothermal aging at 320°C for durations of 1 hour and 2 hours.
• Characterization: Microstructure, phase composition, electrical conductivity, and mechanical properties were evaluated for as-cast and aged conditions.

6. Main Research Results:

Key Research Results:

• Enhanced Conductivity through Aging: Aging treatment at 320°C for 1 hour significantly improved the electrical conductivity of the die-cast Al-7.5Si-0.8Fe alloy.
• Precipitation of Intermetallic Phases: During aging, Al-Fe-Si ternary phases (β-AlFeSi) and Si phase precipitated along grain boundaries and within the grain interior.
• Reduced Solid Solubility: Aging led to a reduction in the solid solubility of both Fe and Si within the aluminum matrix.
• Eutectic Si Network Modification: The continuity of the eutectic Si network deteriorated upon aging, while the connectivity of the aluminum matrix increased.

Analysis of presented data:

Thermodynamic calculations, illustrated in Fig.3, predicted the influence of Fe content on phase fractions and solid solubility within the Al-Si-Fe system. A Fe content of 0.8% was strategically selected to achieve an optimal balance of phase constituents. Microstructural analyses, depicted in Fig.4 and Fig.6, corroborated the precipitation of Al-Fe-Si and Si phases during the aging process. XRD analysis (Fig.5) confirmed the presence of α-Al, Si, and β-AlFeSi phases in the alloy. TEM observations (Fig.8) further revealed the formation of nanoscale precipitates along grain boundaries and within the grains. Conductivity measurements indicated a notable increase after 1 hour of aging, followed by a slight decrease after 2 hours, suggesting an optimal aging duration of 1 hour. Concurrently, mechanical strength exhibited a reduction post-aging, attributed to Si spheroidization and aluminum matrix recovery.

Figure Name List:

• Fig.1 Design route of high thermal conductivity die-casting aluminum alloys through thermodynamic calculation
• Fig.2 Dimension diagram of casting test bar
• Fig.3 Thermodynamic calculation results
• Fig.4 Microstructure and element surface distribution of as-cast alloys
• Fig.5 XRD patterns of as-cast and as-aged alloy specimens
• Fig.6 SEM images of as-cast and as-aged alloy specimens
• Fig.7 EDS mapping of precipitated phase
• Fig.8 TEM images of alloy aged at 300 °C for 1 h
• Fig.9 Relationship between calculated equilibrium solid solubility of Fe and Si elements and temperature
• Fig.10 Engineering stress-strain curves and mechanical properties of as-cast and as-aged alloys
• Fig.11 Die-cast products and conductivity under different conditions

7. Conclusion:

Summary of Key Findings:

The study conclusively demonstrates that aging treatment at 320°C for 1 hour effectively enhances the electrical conductivity of die-cast Al-7.5Si-0.8Fe alloy. This improvement is attributed to the precipitation of Al-Fe-Si and Si phases during aging, which reduces the solid solubility of Fe and Si in the aluminum matrix. Furthermore, the deterioration of the eutectic Si network continuity and the concurrent increase in aluminum matrix connectivity are identified as pivotal factors contributing to the enhanced conductivity.

Academic Significance of the Study:

This research provides valuable academic contributions by elucidating the microstructure evolution and conductivity enhancement mechanisms in die-cast Al-Si-Fe alloys during aging. It experimentally validates the efficacy of thermodynamic calculations in guiding alloy design and optimizing processing parameters for enhanced performance.

Practical Implications:

The findings of this study offer practical implications for the manufacturing of high-performance heat sinks for 5G base stations. The optimized aging parameters (320°C for 1 hour) identified in this research can be directly implemented in industrial production to achieve superior electrical conductivity in die-cast Al-Si-Fe alloys. Moreover, the study underscores the utility of thermodynamic calculations as a powerful tool for designing high thermal conductivity aluminum alloys, potentially reducing development time and costs.

Limitations of the Study and Areas for Future Research:

The scope of this study was primarily focused on the Al-7.5Si-0.8Fe alloy composition and aging treatment at 320°C. Future research endeavors should explore a broader range of alloy compositions, aging temperatures, and extended aging durations to further optimize the alloy's properties. A more in-depth investigation into the kinetics of precipitation and spheroidization processes during aging is also warranted to gain a more comprehensive understanding of the underlying mechanisms.

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9. Copyright:

• This material is "YANG Shuang et al."'s paper: Based on "Thermodynamic Design and Experiment on High Conductivity Die-casting Al-Si-Fe Aluminum Alloy for 5G Base Station Heat Sink".
• Paper Source: DOI: 10.15980/j.tzzz.2024.07.016

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