DEVELOPMENT OF LOW-Si ALUMINUM CASTING ALLOYS WITH AN IMPROVED THERMAL CONDUCTIVITY

This introduction paper is based on the paper "DEVELOPMENT OF LOW-Si ALUMINUM CASTING ALLOYS WITH AN IMPROVED THERMAL CONDUCTIVITY" published by "Materiali in tehnologije / Materials and technology".

1. Overview:

• Title: DEVELOPMENT OF LOW-Si ALUMINUM CASTING ALLOYS WITH AN IMPROVED THERMAL CONDUCTIVITY
• Author: Jesik Shin, Sehyun Ko, Kitae Kim
• Year of publication: 2014
• Journal/academic society of publication: Materiali in tehnologije / Materials and technology
• Keywords: alloy design, low-Si aluminum casting alloy, thermal conductivity, castability

2. Abstract:

To develop an aluminum alloy that can combine a high thermal conductivity with a good castability and anodizability, low Si-containing aluminum alloys, Al-(0.5-1.5)Mg-1Fe-0.5Si and Al-(1.0-1.5)Si-1Fe-1Zn alloys were assessed as potential candidates. The developed alloys exhibited a thermal conductivity of 170-190 % level (160-180 W/(m K)), a fluidity of 60-85 % level, and an equal or higher ultimate tensile strength compared to those of an ADC12 alloy. In each developed alloy system, the thermal conductivity and the strength decreased and increased, respectively, as the content of the major alloying elements, Mg and Si, increased. The fluidity was inversely proportional to the Mg content and directly proportional to the Si content. The Al-(1.0-1.5)Si-1Fe-1Zn alloys showed better thin-wall castability due to their lower surface energy. In the experimental aluminum alloys with a low Si content, the fluidity was mainly dependent on the melt surface energy, the Al dendrite coherency point (DCP), and the first intermetallic crystallization point (FICP), rather than on the solidification interval, latent heat, or the viscosity.

3. Introduction:

The increasing heat removal demand from electric devices like LED lighting necessitates the development of efficient heat-dissipating components. Aluminum, a common heat-sink material, faces challenges: high-purity aluminum has excellent thermal conductivity but is difficult to diecast, requiring alloying elements that unfortunately reduce thermal conductivity. The ADC12 alloy, a commercial Al-Si-based aluminum alloy, is widely used for heat-sinks due to its ability to be fabricated into complex shapes via high-pressure diecasting. However, its low thermal conductivity (below 100 W/(m K)) and poor anodizing characteristics, due to high Si content, are problematic for high-power applications. Other commercial aluminum alloys also struggle with diecastability or offer insufficient conductivity for such uses.

4. Summary of the study:

Background of the research topic:

There is a growing need for aluminum alloys with high thermal conductivity, good castability, and good anodizability for heat-dissipating components in high-power electric devices. Existing commercial alloys like ADC12 have limitations in thermal conductivity and anodizability.

Status of previous research:

ADC12, a common heat-sink alloy, suffers from low thermal conductivity (<100 W/(m K)) and poor anodizing due to its high Si content. Other commercial aluminum alloys are either difficult to diecast or have inadequate thermal conductivity for high-power applications.

Purpose of the study:

The aim of this study was to develop a novel, low-Si-containing anodizable aluminum alloy that possesses both a good thermal conductivity and castability.

Core study:

The study focused on designing and evaluating two series of low-Si quaternary aluminum alloys: Al-xMg-1Fe-0.5Si and Al-xSi-1Fe-1Zn. Elements like Mg, Si, Zn, and Fe were chosen for their benefits in castability, strength, die sticking prevention, and minimal impact on resistivity. The total alloying elements were kept between 2% and 3.5% (mass fractions), with Si content below 1.5%. The thermal conductivity, fluidity, and mechanical strength of these newly designed alloys were investigated as functions of Mg and Si content and compared to those of the ADC12 alloy.

5. Research Methodology

Research Design:

Two low-Si quaternary aluminum alloy systems were designed: Al-xMg-Fe-Si (Alloy 1 series) and Al-xSi-Fe-Zn (Alloy 2 series).

• Mg and Si were chosen as major alloying elements based on their effects on electrical resistivity, energy release for solidification, and viscosity (referencing Table 1).
• Fe (1%) was included to prevent mold-sticking problems, similar to ADC12.
• In Al-xMg-Fe-Si alloys, 0.5% Si was added as a supplementary element.
• In Al-xSi-Fe-Zn alloys, 1% Zn was added as a supplementary element, noting Zn's low resistivity increment.
• Total alloying elements were kept between 2% and 3.5%, and Si content below 1.5% for good anodizability.
• Mg and Si content varied from 0.5% to 1.5%.
• The chemical compositions are detailed in Table 2.

Data Collection and Analysis Methods:

• Fluidity Test: Conducted using a ceramic-coated steel mold with multiple channels (diameters 8, 4, 2, and 1 mm; 100 mm long) on a low-pressure casting machine under inert gas. Mold temperature: 190 °C, superheat: 100 °C, pouring pressure: 15 kPa. Average flow lengths were measured (Figure 1a, 1b).
• Melting Points: Determined by a thermal analyzer TG/DTA (model SDT Q600) with a heating rate of 10 °C/min in Ar.
• Thermal Conductivity: Derived from electrical resistivities measured by an eddy-current technique, using the Wiedemann-Franz law.
• Tensile Strength: Evaluated according to ASTM B 557M using specimens from a Y-block casting.
• Comparative Material: ADC12 alloy (Al-10% Si-2.5% Cu-1% Fe-0.2% Mg).
• Thermophysical Modeling: JMatPro 5.0 software was used to obtain thermophysical properties related to castability and phase-equilibria information.
• Microstructural Analysis: Cross-sections of fluidity test channels were examined using a field-emission scanning electron microscope (FESEM), model FEI Quanta 200F, with energy-dispersive spectroscopy (EDS).
• Cooling Curve Analyses (CCA): Carried out using a two-thermocouple method (one at center TC, one near wall TW of a graphite mold) to record solidification history and determine dendrite coherency point (DCP) (Figure 2).

Research Topics and Scope:

The research investigated the thermal conductivity, fluidity, and mechanical strength of newly designed Al-xMg-1Fe-0.5Si and Al-xSi-1Fe-1Zn alloys. The study explored the effects of varying Mg and Si content on these properties and compared them with the commercial ADC12 alloy. Solidification behavior, including phase formation and characteristic solidification points (DCP, FICP), was analyzed to understand fluidity.

6. Key Results:

Key Results:

• The developed alloys (Al-xMg-1Fe-0.5Si and Al-xSi-1Fe-1Zn) exhibited thermal conductivity of 160-180 W/(m K) (70-90% higher than ADC12), fluidity of 60-85% of ADC12, and equal or higher ultimate tensile strength than ADC12 (Figure 3a, 3b, 3c).
• In both alloy systems, thermal conductivity decreased, and strength increased with increasing Mg or Si content.
• Fluidity of Al-xMg-1Fe-0.5Si alloys decreased with increasing Mg content. Fluidity of Al-xSi-1Fe-1Zn alloys increased with increasing Si content.
• For 2 mm diameter channels, Al-xMg-1Fe-0.5Si alloys showed higher fluidity; for 1 mm channels, Al-xSi-1Fe-1Zn alloys showed higher fluidity, attributed to their lower surface energy (Figure 4c).
• JMatPro calculations showed that energy release for solidification varied with alloy composition (Figure 4a), and viscosity increased with Mg and Si content (Figure 4b).
• Phase equilibria calculations (Scheil equation, JMatPro, Figure 5) indicated that in Al-xMg-1Fe-0.5Si alloys, Al3Fe phase increased with Mg, and solidification range decreased. In Al-xSi-1Fe-1Zn alloys, Al3Fe decreased with increasing Si.
• Cooling curve analysis (Figure 6) and SEM-EDS (Table 3) revealed that in Al-0.5Mg-1Fe-0.5Si, β-AlFeSi formed between dendrite arms and α-AlFeSi in final solidification regions. In Al-1.5Mg-1Fe-0.5Si, α-AlFeSi and Mg2Si were observed. In Al-xSi-1Fe-1Zn alloys, β-AlFeSi formed inter-dendritically, with α-AlFeSi and Si phases in final solidification regions. β-AlFeSi (low-temperature stable) crystallized earlier than α-AlFeSi (high-temperature stable) in inter-dendritic regions, except in high-Mg alloy.
• Characteristic solidification parameters (DCP, FICP) were determined (Table 4).
• Latent heat increased with increasing Mg and Si in both alloy systems based on CCA (Figure 7a), contrasting JMatPro results for Mg in Al-xMg-1Fe-0.5Si.
• Liquid fraction at DCP and FICP (Figure 7b) showed that in Al-xMg-1Fe-0.5Si alloys, it was higher at FICP than DCP and increased with Mg. In Al-xSi-1Fe-1Zn alloys, it was higher at DCP than FICP and decreased with Si.
• Fluidity in low-Si alloys was mainly dependent on melt surface energy, Al dendrite coherency point (DCP), and first intermetallic crystallization point (FICP), rather than solidification interval, latent heat, or viscosity.
• In Al-xMg-1Fe-0.5Si alloys, increased Mg content deteriorated fluidity by increasing viscosity (due to Mg2Si formation before dendrite coherency) and increasing DCP, despite increased latent heat.
• In Al-xSi-1Fe-1Zn alloys, increased Si content improved fluidity by increasing latent heat and lowering DCP, with minimal disturbance to secondary phases.

Figure Name List:

• Figure 1: a) Parting plane of the metal mold for fluidity test and b) fluidity test casting
• Figure 2: Cooling-curve analysis, set-up with a graphite mold and two K-type thermocouples
• Figure 3: Measured: a) thermal conductivity, b) fluidity and c) ultimate tensile strength of Al-(0.5-1.5)Mg-1Fe-0.5Si and Al-(1.0-1.5)Si-1Fe-1Zn alloys
• Figure 4: a) Heat release for solidification, b) viscosity and c) surface energy of Al-(0.5-1.5)Mg-1Fe-0.5Si and Al-(1.0-1.5)Si-1Fe-1Zn alloys calculated by JMatPro
• Figure 5: Phase equilibria calculated by JMatPro: a) Al-0.5Mg-1Fe-0.5Si, b) Al-1.5Mg-1Fe-0.5Si, c) Al-1.0Si-1Fe-1Zn and d) Al-1.5Si-1Fe-1Zn alloys
• Figure 6: Cooling-curve analysis results and SEM (BSE) microstructural images in as-cast state: a) Al-0.5Mg-1Fe-0.5Si, b) Al-1.5Mg-1Fe-0.5Si, c) Al-1.0Si-1Fe-1Zn and d) Al-1.5Si-1Fe-1Zn alloys
• Figure 7: a) Latent heat and b) liquid fraction at DCP and FICP of Al-(0.5-1.5)Mg-1Fe-0.5Si and Al-(1.0-1.5)Si-1Fe-1Zn alloys, which were obtained from a cooling-curve analysis

7. Conclusion:

1. The developed aluminum alloys Al-(0.5-1.5)Mg-1Fe-0.5Si and Al-(1.0-1.5)Si-1Fe-1Zn exhibited a thermal conductivity of 170–190 % level (160–180 W/(m K)), a fluidity of 60–85 % level, and an equal or higher ultimate tensile strength compared to those of the ADC12 alloy.
2. In each developed alloy system, the thermal conductivity decreased and the strength increased with increasing amounts of Mg and Si, the major alloying elements. The fluidity exhibited an inverse relationship with the Mg content and a direct relationship with the Si content.
3. The contradictory fluidity variation behavior in the two alloy systems with the compositions of Mg and Si was caused by the opposing tendencies of DCP and FICP and the relatively different occurring sequences of DCP and FICP.
4. In the experimental aluminum alloys with a low Si content, the prevailing Fe-containing intermetallic compound and the solidification path were observed to be mainly dependent on the Si segregation behavior and the Mg alloying level, rather than on the initial Si/Fe alloying ratio.
5. It was found that the Al-Mg-Fe-Si-based aluminum alloys that show a higher strength and good fluidity in channels greater than 2 mm in diameter are potential materials for general cast heat-dissipating components, and that the Al-Si-Fe-Zn-based aluminum alloys that possess a lower surface energy are potential materials for thin-wall cast heat-dissipating components.

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9. Copyright:

• This material is a paper by "Jesik Shin, Sehyun Ko, Kitae Kim". Based on "DEVELOPMENT OF LOW-Si ALUMINUM CASTING ALLOYS WITH AN IMPROVED THERMAL CONDUCTIVITY".
• Source of the paper: DOI URL: Not provided in the paper. (Published in Materiali in tehnologije / Materials and technology, 48 (2014) 2, 195-202)

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