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Al-Si alloys have been widely used in automotive, aerospace, electronics and communication industries due to their excellent castability, low thermal expansion, and good wear and corrosion resistance. However, the presence of coarse eutectic Si often results in relatively low thermal conductivity. With the rapid development of the electronics and communication industries, the requirements for thermal conductivity and mechanical properties of materials are increasing. In this study, the effects of heat treatment and minor Mg addition on the microstructure, mechanical properties, and thermal conductivity of Al-7Si alloys are systematically investigated. The results indicate that heat treatment at 300 ℃ after solution treatment promotes the spheroidization of eutectic Si and reduces the solid solubility of solute atoms in the aluminum matrix, thereby enhancing the thermal conductivity and reducing the hardness of the Al-7Si alloy. The three-step heat treatment process (solution treatment+300 ℃ treatment+180 ℃ treatment) not only facilitates the spheroidization of eutectic Si, but also induces the precipitation of nanoscale (Mg, Si) strengthening phases, further reducing the solid solubility of solute elements in the Al-7Si alloy with 0.4%Mg addition. After the three-step heat treatment, the Al-7Si-0.4Mg alloy reaches to 189 W/(m·K) in thermal conductivity and 73.5 HV in microhardness, respectively, which are increased by 11.2% and 62.6% respectively, compared with the as-cast Al-7Si alloy. According to the Wiedemann-Franz law and the Matthiessen-Fleming rule, the primary factors influencing the thermal conductivity of alloys are solute atoms in solid solution and secondary phases. In this study, a three-step heat treatment process is used to transform the plate-like eutectic silicon in the Al-7Si-0.4Mg alloy into fine spherical particles. Additionally, micrometer-sized silicon particles and nanoscale (Mg, Si) precipitates are induced within the alloy matrix. This microstructural modification simultaneously enhances the thermal conductivity and mechanical properties of the alloy. Our work is expected to inspire the design of Al-Si alloy with high strength and high conductivity. -
Keywords:
- Al-Si alloy /
- microstructure /
- thermal conductivity /
- mechanical property
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图 1 铸态Al-7Si合金的二次电子图像和能谱元素分布 (a) 二次电子图像; (b) 铝; (c) 硅; (d) 铁; (e) 镧; (f) 锶
Figure 1. Secondary electron imaging and energy dispersive spectroscopy elemental distribution of as-cast Al-7Si alloy: (a) SEI; (b) Al; (c) Si; (d) Fe; (e) La; (f) Sr.
图 2 Al-7Si合金的SEM图像 (a) 铸态; (b) 530 ℃固溶1 h; (c) 530 ℃固溶2 h; (d) 铸态, 300 ℃保温80 min; (e) 530 ℃固溶1 h后, 300 ℃保温80 min; (f) 530 ℃固溶2 h后, 300 ℃保温80 min
Figure 2. SEM images of the Al-7Si alloy: (a) As-cast; (b) after solution treatment at 530 ℃ for 1 hour; (c) after solution treatment at 530 ℃ for 2 hours; (d) as-cast, then held at 300 ℃ for 80 minutes; (e) after solution treatment at 530 ℃ for 1 hour followed by holding at 300 ℃ for 80 minutes; (f) after solution treatment at 530 ℃ for 2 hours followed by holding at 300 ℃ for 80 minutes.
图 3 Al-7Si合金在300 ℃下保温80 min的二次电子图像和能谱元素分布 (a) 二次电子图像; (b) 铝; (c) 硅; (d) 二次电子图像; (e) 铝; (f) 硅
Figure 3. Secondary Electron Imaging and Energy Dispersive Spectroscopy elemental distribution of Al-7Si alloy at 300 ℃ for 80 minutes: (a) SEI (b) Al; (c) Si; (d) SEI (e) Al; (f) Si.
图 4 Al-7Si合金热处理过程中热导率及硬度随时间的变化曲线 (a) 固溶过程中的热导率变化; (b) 300 ℃保温热处理过程中的热导率变化; (c) 固溶过程中的硬度变化; (d) 300 ℃保温热处理过程中的硬度变化
Figure 4. Variations in thermal conductivity and hardness of Al-7Si alloy during heat treatment as a function of time: (a) Thermal conductivity variation during solution treatment; (b) thermal conductivity variation during 300 ℃ isothermal heat treatment; (c) hardness variation during solution treatment; (d) hardness variation during 300 ℃ isothermal heat treatment.
图 5 铸态Al-7Si-0.4Mg合金的二次电子图像和能谱元素分布 (a) 二次电子图像; (b) 铝; (c) 硅; (d) 铁; (e) 镧; (f) 锶; (g) 镁
Figure 5. Secondary electron imaging and energy dispersive spectroscopy elemental distribution of as-cast Al-7Si-0.4Mg alloy: (a) SEI; (b) Al; (c) Si; (d) Fe; (e) La; (f) Sr; (g) Mg.
图 6 Al-7Si-0.4Mg合金的SEM图像 (a) 铸态; (b) 530 ℃固溶1 h; (c) 530 ℃固溶2 h; (d) 铸态, 300 ℃保温80 min; (e) 530 ℃固溶1 h后, 300℃保温80 min; (f) 530 ℃固溶2 h后, 300 ℃保温80 min
Figure 6. SEM images of the Al-7Si-0.4Mg alloy: (a) As-cast; (b) after solution treatment at 530 ℃ for 1 hour; (c) after solution treatment at 530 ℃ for 2 hours; (d) as-cast, then held at 300 ℃ for 80 minutes; (e) after solution treatment at 530 ℃ for 1 hour followed by holding at 300 ℃ for 80 minutes; (f) after solution treatment at 530 ℃ for 2 hours followed by holding at 300 ℃ for 80 minutes.
图 7 Al-7Si-0.4Mg合金热处理过程中热导率及硬度随时间的变化 (a) 固溶过程中的热导率变化; (b) 高温保温热处理过程中的热导率变化; (c) 固溶过程中的硬度变化; (d) 高温保温热处理过程中的硬度变化
Figure 7. Variations in thermal conductivity and hardness of Al-7Si-0.4Mg alloy during heat treatment as a function of time: (a) Thermal conductivity variation during solution treatment; (b) thermal conductivity variation during high-temperature isothermal heat treatment; (c) hardness variation during solution treatment; (d) hardness variation during high-temperature isothermal heat treatment.
图 8 不同热处理工艺后的Al-7Si-0.4Mg合金的室温拉伸性能
Figure 8. Mechanical properties at room temperature of Al-7Si-0.4Mg alloy after different heat treatment processes.
图 9 经历不同热处理后Al-7Si-0.4Mg合金的TEM分析结果 (a) 双级热处理(530 ℃固溶1.5 h+300 ℃保温60 min)样品的明场像及Mg, Si元素分布; (b) 三级热处理(530 ℃固溶1.5 h+300 ℃保温60 min+180 ℃×6 h)样品的明场像及Mg, Si元素分布; (c) β''及β'相的高分辨图像及傅里叶变换花样
Figure 9. Representative TEM images of Al-7Si-0.4Mg alloy after different heat treatments: (a) Bright-field images, Mg element distribution and Si element distribution of the double-step heat treatment samples (solution at 530 ℃ for 1.5 h+holding at 300 ℃ for 60 min); (b) bright-field image, Mg element distribution, Si element distribution of the triple-step heat treatment samples (530 ℃×1.5 h solution +300 ℃×60 min+180 ℃×6 h); (c) high resolution TEM images and corresponding FFT images of β'' and β' phase.
表 1 实验合金的化学成分(质量分数)(单位: %)
Table 1. Chemical compositions (weight percent) of the experimental alloys (Unit: %).
Alloy Mg B La Sr Fe Si Al Al-7Si 0 0.024 0.04 0.02 0.25 7 Bal. Al-7Si-0.4Mg 0.4 0.024 0.04 0.02 0.25 7 Bal. 
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表 2 Al-7 Si和Al-7 Si-0.4 Mg合金的热处理工艺参数
Table 2. Heat treatment process parameters for the Al-7 Si and Al-7 Si-0.4 Mg alloys
Alloy Process Temperature/℃ Time Al-7Si SS 530 0 h, 0.5 h, 1 h, 1.5 h, 2 h HT 300 0 min, 20 min, 40 min,
60 min, 80 min, 100 minAl-7Si-
0.4MgSS 530 0 h, 0.5 h, 1 h, 1.5 h, 2 h HT 300 0 min, 20 min, 40 min,
60 min, 80 min, 100 minLT 180 0 h, 6 h, 12 h
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表 3 Al-7Si-0.4Mg试样的热处理工艺及性能
Table 3. Heat treatment process and properties of the Al-7Si-0.4Mg samples.
Heat treatment process Thermal conductivity
/(W·m–1·K–1)Hardness
/HVUltra tensile stress/MPa Elongation/% As-cast 162 60.9 169 6.25 SS1.5 h+HT60 min 185 62.9 176 8 SS1.5 h+HT60 min+LT6 h 183 79.5 206 6 SS1.5 h+HT60 min+LT12 h 189 73.5 186 5.75
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