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Hanchao Liu, Fan Lu, Guangyi Ma, et al. Effect of constant temperature substrate on microstructure and hardness of Al2O3-based eutectic ceramics[J]. Opto-Electronic Engineering, 2017, 44(12): 1194-1199. doi: 10.3969/j.issn.1003-501X.2017.12.007
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Effect of constant temperature substrate on microstructure and hardness of Al2O3-based eutectic ceramics
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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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References
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Overview
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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Hanchao Liu, Fan Lu, Guangyi Ma, et al. Effect of constant temperature substrate on microstructure and hardness of Al2O3-based eutectic ceramics[J]. Opto-Electronic Engineering, 2017, 44(12): 1194-1199. doi: 10.3969/j.issn.1003-501X.2017.12.007
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Figure 1.
Experimental equipment and methods. (a) Laser engineered net shaping (LENS) system. (b) Path diagram of laser scanning.
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Figure 2.
Water-cooled constant temperature substrate.
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Figure 3.
The thin-wall sample prepared on the different substrates. (a) Common substrate. (b) Water-cooled constant temperature substrate.
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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.
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Figure 5.
(a) Indentation of Vickers hardness. (b) The microhardness of samples.
