Effects of Si and Ni Content and Preparation Process on Microstructure and Thermal Properties of Al-Si-Ni Alloy
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摘要:
利用普通凝固、水冷铜模亚快速凝固和水冷铜模亚快速凝固复合热处理（520 ℃×6 h）3种工艺,分别制备了Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金,研究了不同合金的显微组织和热学性能。结果表明：Al-11Si-5Ni共晶合金在凝固过程中仅发生Al-Si-Al3Ni三元共晶反应,形成由α-Al相、共晶硅相和共晶Al3N相组成的三元共晶组织；Al-22Si-10Ni合金与Al-33Si-15Ni合金为过共晶合金,组织由初生硅相、初生Al3N相和三元共晶组织组成,其凝固过程分为初生硅相析出、初生硅相与Al3Ni相的共同析出以及Al-Si-Al3Ni三元共晶反应3个阶段；随着镍、硅含量的同步增加,初生硅相与Al3Ni相粗化；与普通凝固工艺相比,水冷铜模亚快速凝固工艺可以细化合金组织,而热处理使共晶硅相与共晶Al3Ni相发生球化。随着硅、镍含量的同步增加,合金的热导率与热膨胀系数均下降,而水冷铜模亚快速凝固复合热处理可以有效提升合金的热导率。水冷铜模亚快速凝固复合热处理工艺制备的Al-22Si-10Ni合金具有优异的综合性能,其室温热导率为129.9 W·m−1·K−1,100 ℃热膨胀系数为13.8×10−6 K−1,25~100 ℃平均热膨胀系数为12.9×10−6 K−1。
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关键词:
- Al-Si-Ni合金 /
- 镍和硅含量 /
- 亚快速凝固 /
- 显微组织 /
- 热学性能
Abstract:Three kinds of alloys, Al-11Si-5Ni, Al-22Si-10Ni and Al-33Si-15Ni, were prepared by ordinary solidification, water-cooled copper mold sub-rapid solidification and water-cooled copper mold sub-rapid solidification combined with heat treatment (520 ℃×6 h). The microstructures and thermal properties of the alloys were studied. The results show that only Al-Si-Al3Ni ternary eutectic reaction occurred in the solidification process of Al-11Si-5Ni eutectic alloy, and the microstructure was ternary eutectic structure composed of α-Al, eutectic silicon and eutectic Al3N phases. The Al-22Si-10Ni and Al-33Si-15Ni alloys were hypereutectic alloys, the microstructure consisted of primary silicor phase, primary Al3Ni phase and ternary eutectic structure, and the solidification process was divided into three stages: the primary silicon phase precipitation, the primary silicon and Al3Ni phase co-precipitation, and Al-Si-Al3Ni ternary eutectic reaction. With the simultaneous increase of Ni and Si content, primary silicon and Al3Ni phases were coarsened. Compared with the ordinary solidification process, the water-cooled copper mold sub-rapid solidification process could refine the alloy structure, and the heat treatment made the eutectic silicon phase and the eutectic Al3Ni phase spheroidize. With the simultaneous increase of the content of Ni and Si, the alloy's thermal conductivity and the thermal expansion coefficient decreased. The water-cooled copper mold sub-rapid solidification combined with heat treatment could effectively improve the thermal conductivity of the alloy. Al-22Si-10Ni alloy prepared by the water-cooled copper mold subrapid solidification combined with heat treatment had the best comprehensive performance with the room temperature thermal conductivity of 129.9 W · m−1 · K−1, the thermal expansion coefficient at 100 ℃ of 13.8×10−6 K−1, and the average thermal expansion coefficient in the temperature range of 25–100 ℃ of 12.9×10−6 K−1.
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Keywords:
- Al-Si-Ni alloy /
- Ni and Si content /
- sub-rapid solidification /
- microstructure /
- thermal property
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0. 引言
高铝含量（质量分数50%~90%）的Al-Si合金具有导热性好、热膨胀系数低、密度低、成本低等优点,已经应用于微波功率器件、集成功率模块、收发模块等电子功率器件的封装基座等方面。然而,通过铸造工艺制备的高铝含量Al-Si合金的热物理性能无法满足电子封装用材料的要求[1],需要采用喷射沉积法复合热压工艺进行制备[2],但这种复杂的制备工艺限制了其在电子封装材料方面的应用。在铸造时,改进凝固工艺可以改善Al-Si合金的组织,进而提高其热物理性能。水冷铜模亚快速凝固集合了快速凝固与普通凝固的优点,既保证了合金熔体有较快的凝固速率,以产生较多的形核位置,从而获得细小的组织,同时解决了快速凝固无法通过模具成型的问题[3]。此外,在Al-Si基合金中加入镍可以提升合金熔体流动性,同时几乎不影响其导热性能[4-6]。用镍替代Al-Si合金中的部分硅元素,则可以降低合金熔点、提升铸造性能与力学性能[7]。目前有关水冷铜模亚快速凝固工艺制备Al-Si-Ni合金热学性能的研究报道较少,同时在实际生产过程中通常需要对凝固后的合金进行热处理以提高其导热性和抗拉强度。作者针对传统铸造工艺制备的高铝含量Al-Si合金热学性能不足的问题,在Al-11Si-5Ni共晶合金的基础上,通过成倍增加镍、硅含量,采用普通凝固、水冷铜模亚快速凝固和水冷铜模亚快速凝固复合热处理3种工艺制备Al-Si-Ni合金,研究了镍、硅含量及制备工艺对合金显微组织和热学性能的影响,以期为其在电子封装领域的应用提供试验参考。
1. 试样制备与试验方法
试验原料包括工业纯铝（纯度99.7%）、高纯镍（纯度99.99%）、Al-50Si中间合金。按照名义成分（质量分数/%）为Al-11Si-5Ni、Al-22Si-10Ni和Al-33Si-15Ni称取原料,在SG2-7.5-10型熔炼炉中进行熔炼,熔炼温度为900 ℃,随后分别采用普通凝固、水冷铜模亚快速凝固以及水冷铜模亚快速凝固复合热处理3种工艺制备合金。其中：普通凝固工艺采用尺寸为100 mm×60 mm×5 mm的钢模具；水冷铜模亚快速凝固工艺采用尺寸为80 mm×40 mm×5 mm的带水冷系统的铜模具；水冷铜模亚快速凝固复合热处理工艺在水冷铜模亚快速凝固工艺基础上增加了520 ℃×6 h炉冷的热处理。
在铸锭中心位置截取尺寸为15 mm×15 mm× 5 mm的金相试样,经过砂纸打磨、抛光膏抛光后,采用体积分数0.5% HF溶液腐蚀10 s,再依次用去离子水与无水乙醇冲洗,冷风干燥；采用质量分数10% NaOH溶液深腐蚀30 min后,在DMI3000-M型光学显微镜上观察显微组织,并用Merlin Compact型场发射扫描电镜（SEM）观察微观结构。采用Oxford X-MaxN型X射线衍射仪（XRD）进行物相分析,采用铜靶,Kα射线,扫描速率为0.013 （°）·min−1,扫描范围为10°~90°。合金的热导率由热扩散系数、密度、比热容三者求积得到,其中：热扩散系数通过闪光法用Netzsch LFA457型激光导热仪测得,测试参数为温度25 ℃、电压304 V、脉冲0.06 ms、放大器增益50 020；密度通过XHB-3000Z II型密度天平称取试样的质量,再除以体积获得；比热容通过JMatPro软件计算获得。采用TA-Q400型热机械分析仪（TMA）测试材料的热膨胀系数,试样尺寸为4 mm×4 mm×15 mm,测试温度范围为25~210 ℃,压力为0.2 N。采用National Instruments分析设备,通过NI 9212型传感器和NI cDAQ-9171型温度采集器收集信号,通过计算机辅助热分析法获得合金的凝固曲线。
2. 试验结果与讨论
2.1 显微组织与物相组成
由图1可以看出：普通凝固工艺和水冷铜模亚快速凝固工艺制备的Al-11Si-5Ni合金组织由白色α-Al基体及密集分布的黑色细小Al3Ni共晶相与灰色共晶硅相组成,该组织为Al-Si-Al3Ni三元共晶组织[8],与普通凝固工艺制备的合金相比,水冷铜模亚快速凝固工艺制备的合金组织显著细化,再经过热处理后,共晶硅相与共晶Al3Ni相均变为粒状,分布于α-Al基体中。普通凝固工艺和水冷铜模亚快速凝固工艺制备的Al-22Si-10Ni合金由不规则块状初生硅相、条状初生Al3Ni相以及Al-Si-Al3Ni三元共晶组织构成,与普通凝固工艺制备的合金相比,水冷铜模亚快速凝固工艺制备的合金中初生硅相尺寸显著细化,再经过热处理后,共晶硅相与共晶Al3Ni相发生球化,块状的初生硅相尺寸无明显变化,但其边缘发生钝化。普通凝固工艺和水冷铜模亚快速凝固工艺制备的Al-33Si-15Ni合金组织同样由初生硅相、初生Al3Ni相和Al-Si-Al3Ni三元共晶组织构成,普通凝固工艺下的初生硅相呈粗大的板状,初生Al3Ni相呈粗大的块状或条状,水冷铜模亚快速凝固工艺下的组织显著细化,初生硅相变为较小的块状,初生Al3Ni相变为细长的条状,再经过热处理后初生硅相与初生Al3Ni相尺寸不变,共晶硅相与共晶Al3Ni相发生球化。对比合金组织发现：随着硅、镍含量的同步增加,Al-Si-Ni合金中出现初生硅相与初生Al3Ni相,且初生相粗化；当合金成分相同时,水冷铜模亚快速凝固下的合金组织相比于普通凝固工艺下显著细化,再经热处理后共晶相发生球化。
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图 1 不同工艺制备Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金的显微组织Figure 1. Microstructures of Al-11Si-5Ni alloy (a–c), Al-22Si-10Ni alloy (d–f) and Al-33Si-15Ni alloy (g–i) prepared by different processes: (a, d, g) normal solidification; (b, e, h) water-cooled copper mold sub-rapid solidification and (c, f, i) water-cooled copper mold sub-rapid solidification combined with heat treatment由图2可见：水冷铜模亚快速凝固工艺制备的Al-11Si-5Ni合金中共晶硅相呈珊瑚状,共晶Al3Ni相呈纤维状；经过热处理后,共晶硅相与Al3Ni相均发生球化与粗化。在热处理过程中,共晶硅相的分叉处因缺陷较多、能量较高而出现颈缩、熔断现象,由于第二相总倾向于表现出最小的表面能,因此熔断后的共晶硅枝晶和一些尺寸较小的共晶硅相发生聚集球化[9]；而随着热处理时间延长,由于奥斯特瓦尔德熟化机制,小颗粒硅相溶解,大颗粒硅相持续长大,因此共晶硅相长大[10]。水冷铜模亚快速凝固工艺制备的Al-22Si-10Ni合金中的初生Al3Ni相呈鱼骨状,热处理后其形态与尺寸无明显变化。
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图 2 水冷铜模亚快速凝固和水冷铜模亚快速凝固复合热处理工艺制备不同合金的SEM形貌Figure 2. SEM morphology of different alloys prepared by water-cooled copper mold sub-rapid solidification (a, c) and water-cooled copper mold sub-rapid solidification combined with heat treatment (b, d): (a–b) Al-11Si-5Ni alloy and (c–d) Al-22Si-10Ni alloy由图3可知,水冷铜模亚快速凝固复合热处理工艺制备的3种合金中均只存在α-Al、Al3Ni和硅3种相,无其他二元相或三元相生成。Al-11Si-5Ni合金的α-Al相衍射峰最强,随着硅、镍含量的同步增加,硅相与Al3Ni相的衍射峰增强,说明硅相与Al3Ni相含量增加,α-Al相衍射峰强度降低。
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图 3 水冷铜模亚快速凝固复合热处理工艺制备的3种合金的XRD谱Figure 3. XRD patterns of three kinds of alloys prepared by water-cooled copper mold sub-rapid solidification combined with heat treatment2.2 凝固过程
由于不同凝固条件下合金的物相组成相同,因此仅测定普通凝固条件下的凝固曲线。由图4可见,Al-11Si-5Ni合金的凝固曲线中仅存在Al-Si-Al3Ni三元共晶平台,共晶反应温度为567 ℃,该合金是共晶合金。Al-22Si-10Ni合金的凝固曲线可以分为3个阶段,结合三元相图[11]可以判断这3个阶段分别对应初生硅相的析出、初生Al3Ni相与硅相的共同析出以及最后的Al-Si-Al3Ni三元共晶反应。初生硅相析出峰对应的温度为760 ℃,初生Al3Ni相析出峰对应的温度为637 ℃,三元共晶反应温度不变,但共晶反应时间显著缩短,凝固过程加快。Al-33Si-15Ni合金的凝固曲线也可以观察到上述3个阶段,初生硅相析出峰与初生Al3Ni相析出峰增强,对应温度分别上升至950,695 ℃,Al-Si-Al3Ni三元共晶反应温度不变,但共晶反应时间进一步缩短。Al-22Si-10Ni合金和Al-33Si-15Ni合金为过共晶合金。
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图 4 普通凝固条件下Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金的凝固曲线Figure 4. Solidification curves of Al-11Si-5Ni alloy (a), Al-22Si-10Ni alloy (b) and Al-33Si-15Ni alloy (c) under normal solidification condition2.3 热学性能
由图5可见,随着硅、镍含量的同步增加,相同工艺制备的Al-Si-Ni合金的热导率降低,这是因为更高硅镍含量下合金中析出了更多的初生相,阻碍了热传导过程自由电子的传输[12]。水冷铜模亚快速凝固工艺制备的Al-11Si-5Ni合金热导率相比普通凝固工艺下有所下降,但再经热处理后热导率大幅提升。这是因为亚快速凝固导致部分硅元素固溶于α-Al基体中,增加了自由电子的散射,导致热导率较低,但是在热处理过程中,共晶硅相由原来的二维片层状结构向三维粒状结构转变[13],同时基体中的固溶原子析出,基体中的晶格缺陷减少,自由电子传输过程中的散射作用减少,传热电子自由程增加,因此热导率显著提升[12]。水冷铜模亚快速凝固工艺制备的Al-22Si-10Ni和Al-33Si-15Ni过共晶合金的热导率相比普通凝固工艺下显著提升,并且经过热处理后,热导率进一步提升。这是由于初生硅相形貌与尺寸是影响过共晶合金热导率的主要因素,亚快速凝固可以有效细化合金中的块状初生硅相,配合热处理可以实现共晶相的球化,从而进一步提升合金的热导率。水冷铜模亚快速凝固复合热处理工艺制备的Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金在100 ℃下的线膨胀系数分别为18.1×10−6,13.8×10−6,10.5×10−6 K−1,25~100 ℃下的平均热膨胀系数分别为17.0×10−6,12.9×10−6,9.6×10−6 K−1。可见,随着硅、镍含量的同步增加,合金的100 ℃热膨胀系数和25~100 ℃平均热膨胀系数均降低。热膨胀系数主要取决于各相的热膨胀系数及其体积分数,对组织形貌不敏感[14]。Al3Ni相与硅相均为低膨胀相[15],随着镍、硅含量的增加,硅相与Al3Ni相的体积分数增加,因此合金的热膨胀系数下降。综合考虑,水冷铜模亚快速凝固复合热处理工艺制备的Al-22Si-10Ni合金具有优异的综合性能,其室温热导率为129.9 W·m−1·K−1,100 ℃热膨胀系数为13.8×10−6 K−1,25~100 ℃平均热膨胀系数为12.9×10−6 K−1。
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图 5 不同工艺制备3种合金的热导率Figure 5. Thermal conductivity of three kinds of alloys prepared by different processes3. 结论
（1）Al-11Si-5Ni合金组织为Al-Si-Al3Ni三元共晶组织,同步增加镍、硅含量后合金中出现初生硅相与Al3Ni相,两相随镍、硅含量增加发生粗化；与普通凝固工艺相比,水冷铜模亚快速凝固工艺可以细化合金组织,再进行热处理后共晶硅相与共晶Al3Ni相发生球化。
（2）Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金在水冷铜模亚快速凝固过程中均只生成α-Al、硅与Al3Ni 3种相。Al-11Si-5Ni共晶合金在凝固过程中仅发生Al-Si-Al3Ni三元共晶反应；Al-22Si-10Ni合金与Al-33Si-15Ni合金为过共晶合金,凝固过程分为初生硅相析出、初生硅相与Al3Ni相的共同析出以及Al-Si-Al3Ni三元共晶反应3个阶段,随着镍、硅含量的同步增加,初生相的析出温度升高,共晶反应时间缩短。
（3）随着硅、镍含量的同步增加,合金的热导率与热膨胀系数均下降。水冷铜模亚快速凝固工艺制备的Al-11Si-5Ni共晶合金的热导率相比普通凝固工艺制备的合金略有下降,但经过热处理后热导率大幅提升；水冷铜模亚快速凝固工艺制备的Al-22Si-10Ni和Al-33Si-15Ni过共晶合金的热导率相比普通凝固工艺制备的显著提升,经过热处理后热导率进一步提升。水冷铜模亚快速凝固复合热处理工艺制备的Al-22Si-10Ni合金具有优异的综合性能,其室温热导率为129.9 W·m−1·K−1,100 ℃热膨胀系数为13.8×10−6 K−1,25~100 ℃平均热膨胀系数为12.9×10−6 K−1。
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图 1 不同工艺制备Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金的显微组织
Figure 1. Microstructures of Al-11Si-5Ni alloy (a–c), Al-22Si-10Ni alloy (d–f) and Al-33Si-15Ni alloy (g–i) prepared by different processes: (a, d, g) normal solidification; (b, e, h) water-cooled copper mold sub-rapid solidification and (c, f, i) water-cooled copper mold sub-rapid solidification combined with heat treatment
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图 2 水冷铜模亚快速凝固和水冷铜模亚快速凝固复合热处理工艺制备不同合金的SEM形貌
Figure 2. SEM morphology of different alloys prepared by water-cooled copper mold sub-rapid solidification (a, c) and water-cooled copper mold sub-rapid solidification combined with heat treatment (b, d): (a–b) Al-11Si-5Ni alloy and (c–d) Al-22Si-10Ni alloy
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图 3 水冷铜模亚快速凝固复合热处理工艺制备的3种合金的XRD谱
Figure 3. XRD patterns of three kinds of alloys prepared by water-cooled copper mold sub-rapid solidification combined with heat treatment
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图 4 普通凝固条件下Al-11Si-5Ni合金、Al-22Si-10Ni合金和Al-33Si-15Ni合金的凝固曲线
Figure 4. Solidification curves of Al-11Si-5Ni alloy (a), Al-22Si-10Ni alloy (b) and Al-33Si-15Ni alloy (c) under normal solidification condition
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