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Effect of constant temperature substrate on microstructure and hardness of Al2O3-based eutectic ceramics
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摘要:
Al2O3-YAG共晶陶瓷材料以其出色的高温强度、抗氧化性、高温结构稳定性成为航空航天领域高温合金的理想替代材料。本文采用激光近净成形技术，分别在普通基底、水冷恒温基底上进行了Al2O3-YAG共晶陶瓷薄壁件成形实验，得到了不同基底上成形的薄壁样件，比较了两者的微观组织及显微硬度差异。结果表明，采用普通基底成形的薄壁件微观组织呈三维网状结构，平均共晶间距为0.96 μm；采用水冷恒温基底后，薄壁顶部微观组织形貌呈晶团结构，薄壁底部呈枝状晶结构，微观组织逆热流方向生长特性明显，平均共晶间距减小至0.21 μm；和普通基底上成形的Al2O3-YAG共晶陶瓷薄壁件显微硬度相比，使用水冷恒温基底成形的Al2O3-YAG共晶陶瓷薄壁件硬度提高约10%。
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关键词:
- 激光近净成形 /
- Al2O3-YAG共晶陶瓷 /
- 水冷恒温基底 /
- 微观组织 /
- 显微硬度
Abstract:Al2O3-YAG eutectic ceramic has become an ideal alternative to high-temperature alloys because of its excellent high temperature strength, high oxidation resistance and high temperature structural stability. The technology of laser engineered net shaping was used to prepare the Al2O3-YAG eutectic ceramic thin-wall samples. These samples were prepared on a common substrate and a water-cooled constant temperature substrate respectively. Their microstructure and microhardness were compared. The results show that the microstructure of thin-wall sample prepared on the common substrate is three-dimensional network structure with an average eutectic spacing of 0.96 μm. And the microstructure of top part of the sample prepared on the water-cooled constant temperature substrate is colony structure, while the microstructure of bottom part is dendrite structure which grows in the reverse direction of the heat flow. The average eutectic spacing of samples prepared on the water-cooled substrate has reduced to 0.21 μm. Compared with the microhardness of two kinds of thin-wall samples prepared on the different substrates, it is found that the microhardness of the thin–wall samples prepared on the water-cooled constant temperature substrate is increased by about 10%.
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Al2O3-YAG eutectic ceramic has been considered as one of the most potential alternatives to current traditional superalloys applied in the aerospace field due to its extremely excellent high temperature mechanical properties, such as high temperature strength, oxidation resistance and high temperature structure stability. Al2O3-YAG eutectic ceramic thin-wall samples were prepared by laser engineered net shaping using Al2O3 and Y2O3 powders as raw materials. During the process of the laser engineered net shaping, the heat transfers to substrate and the heat accumulation is serious. According to this characteristic, a water-cooled constant temperature substrate was designed to change the temperature gradient and the cooling condition in the forming process. Al2O3-YAG eutectic ceramic thin-wall samples forming experiments were carried out on a common substrate and a water-cooled constant temperature substrate with the same laser power, scanning velocity, powder feeding rate and Z increment. The common substrate is TC4 substrate, while the water-cooled constant temperature substrate is composed of aluminum alloy cooling block, TC4 substrate, and plastic hose. The thermal plastic ensures the good contact between the TC4 substrate and aluminum alloy cooling block, and the heat accumulated on the TC4 substrate is rapidly dissipated through the aluminum alloy cooling block in which the constant temperature cooling water of 28 ℃ circulates. The Al2O3-YAG thin-wall samples prepared on different substrates were obtained, and their macro-morphology, microstructure and microhardness were compared. The results show that the water-cooled constant temperature substrate has a significant effect on the microstructure and eutectic spacing of Al2O3-YAG eutectic ceramic. The microstructure of thin-wall sample prepared on the common substrate is three-dimensional network structure with an average eutectic spacing of 0.96 μm. And the microstructure of top part of the sample prepared on the water-cooled constant temperature substrate is colony structure, while the microstructure of bottom part is dendrite structure which grows in the reverse direction of the heat flow. The average eutectic spacing of samples prepared on the water-cooled substrate has reduced to 0.21 μm. It is concluded that the morphological change of microstructures is mainly related to the degree of supercooling. The decrease of eutectic spacing is mainly due to the fact that the solidification rate is inversely proportional to the eutectic spacing. The applications of water-cooled constant temperature substrate can effectively improve the solidification rate, so the eutectic spacing decreases with the increase of the solidification rate. Comparing the microhardness of thin-wall samples prepared on the different substrates, the microhardness of the thin–wall sample prepared on the water-cooled constant temperature substrate is increased by about 10% due to the increase of the solidification rate and temperature gradient, the decrease of the eutectic spacing, grain refinement and the interaction between eutectic phase and cracks.
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图 1 实验设备和方法. (a)激光近净成形系统. (b)激光扫描路径.
Figure 1. Experimental equipment and methods. (a) Laser engineered net shaping (LENS) system. (b) Path diagram of laser scanning.
图 3 不同基底成形的薄壁顶端. (a)普通基底. (b)水冷恒温基底.
Figure 3. The thin-wall sample prepared on the different substrates. (a) Common substrate. (b) Water-cooled constant temperature substrate.
图 4 不同成形基底上成形的薄壁A、B区域SEM照片. (a)普通基底，A区域. (b)普通基底，B区域. (c)水冷恒温基底，A区域. (d)水冷恒温基底，B区域.
Figure 4. SEM patterns of different part of thin-wall sample under different substrates. (a) Common substrate, A region. (b) Common substrate, B region. (c) Water-cooled constant temperature substrate, A region. (d) Water-cooled constant temperature substrate, B region.
图 5 (a) 水冷恒温基底成形薄壁样件底部维氏硬度压痕. (b)显微硬度对比.
Figure 5. (a) Indentation of Vickers hardness. (b) The microhardness of samples.
表 1 激光近净成形工艺参数.
Table 1. Process parameters of laser engineered net shaping.
Laser power P/W Scanning velocity V/(mm/min) Powder feeding rate Q/(g/min) Interlayer lifting capacity △Z/mm 350 340 1.65 0.21 
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