Chinese Journal of Lasers, Volume. 51, Issue 10, 1002316(2024)

Effects of Laser Power on Molten Pool Morphology, Microstructure, and Mechanical Properties of Al2O3‐ZrO2 Eutectic Ceramics Shaped by Laser Powder Bed Fusion

Zhiwei Xiong, Kai Zhang*, Tingting Liu**, and Wenhe Liao
Author Affiliations
  • School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China
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    Objective

    Molten-grown Al2O3-ZrO2 eutectic ceramics exhibit outstanding high-temperature performance and have potential technological applications in aviation, aerospace, and nuclear engineering. Laser powder bed fusion (LPBF) technology has several advantages, including high design flexibility, low production costs, and short delivery cycles, making it promising in the direct manufacture of complex Al2O3-ZrO2 eutectic ceramic components with dense microstructures. However, the transient interaction between ceramic powder and lasers still requires clarification, and insufficient ceramic toughness poses significant challenges to the direct fabrication of ceramics via LPBF. Consequently, it is imperative to investigate the real-time morphology evolution of molten pools and establish a correlation among the “material-process-microstructure-mechanical properties” to lay the groundwork for producing high-performance ceramic materials. This paper presents an innovative study based on real-time high-speed imaging systems and three-dimensional confocal microscope reconstruction of surface information in single deposition tracks. Morphological characteristics such as molten pool length and single deposition track width are statistically analyzed. The results indicate that increasing laser power leads to longer molten pools and wider single deposition tracks. To improve the correlation among the material-process-microstructure-mechanical properties of Al2O3-ZrO2 eutectic ceramics, characterization and testing methods, including X-ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and microhardness measurements, are employed to investigate the surface quality, relative density, phase composition, microstructure, and mechanical property evolution of LPBF-produced Al2O3-ZrO2 eutectic ceramics under different laser powers. The findings demonstrate a gradual reduction in m-ZrO2 with increasing laser power owing to the inhibition of the martensitic phase transformation during rapid LPBF cooling. Additionally, the cellular grain size of the specimens exhibits an increasing trend with higher laser power, accompanied by a decrease in grain boundary density. Consequently, the measured microhardness and fracture toughness exhibit a decreasing trend.

    Methods

    This experiment is conducted on a self-developed LBPF device. The process parameters are set as follows: a scanning speed of 100 mm/s, laser power ranging from 60 W to 200 W (with interval of 20 W), scanning pitch of 100 μm, and layer thickness of 50 μm. Single deposition tracks of Al2O3-ZrO2 eutectic ceramics are printed under different laser powers, and a high-speed camera is employed to capture the process. Using a zigzag scanning strategy, eutectic ceramic samples with dimensions of 10 mm×10 mm×1 mm are fabricated to investigate phase formation, surface quality, microstructure, and mechanical properties. A high-speed camera with a sampling frequency of 83333 Hz is used to observe the laser forming process. To ensure image quality, an 808 nm light source is utilized for supplementary illumination, which is installed with an 808 nm filter and an 8× magnification lens group in the optical path. An X-ray diffractometer is employed to detect the phase and crystal structure information of the prepared samples. The field scanning electron microscope is used to observe the microstructure and morphology of the sample surfaces. Phase analysis is conducted using an energy-dispersive X-ray spectrometer (EDXS) system, and hardness and fracture toughness are measured using the Vickers hardness testing method.

    Results and Discussions

    According to Fig.3(e), the length of the molten pool and width of the single deposition channel increase consistently with increasing laser power. Additionally, when the laser power initially increases from 60 W to 120 W, the width exhibits significant growth, and the length increases slowly. However, as the laser power increases from 140 W to 200 W, the growth trends of the length and width are reversed. When fabricating Al2O3-ZrO2 eutectic ceramic bulks using LPBF, a low power leads to warping and deformation (Fig.4), whereas a high power results in molten track shift (Fig.5). Furthermore, the surface roughness and porosity decrease initially and then increase with increasing laser power (Figs.5 and 6). Therefore, selecting an appropriate laser power is advantageous for enhancing both the stability and quality of samples. As shown in Fig.7, the preservation of the substable martensitic phase t-ZrO2 is observed at a high laser power. This can be attributed to the limited conversion time during LPBF quenching, which hinders the energy exchange and spatial dimensions necessary for phase transition within the short period of solidification and crystallization. The microstructure of Al2O3-ZrO2 eutectic ceramics consists of cellular-like units (Fig.8). Based on ImageJ measurements and statistics, the grain size of these units increases with increasing laser power (Fig.9), and according to Hall-Petch theory, low-energy grain refinement generates numerous grain boundaries within the sample. These boundaries effectively impede dislocation movement when external loads are applied, resulting in enhanced sample hardness and fracture toughness (Fig.10).

    Conclusions

    Combining the images taken by the high-speed camera with the characterization of the samples, the following conclusions are drawn.

    1) With increasing laser power, both the length of the molten pool and width of the deposition track exhibit an increasing trend. At higher laser powers, the trailing phenomenon of the molten pool becomes more prominent.

    2) The surface roughness and porosity of the Al2O3-ZrO2 eutectic ceramics initially decrease and then increase with increasing laser power. The deterioration in surface quality at higher laser powers can be attributed to molten pool offset and the occurrence of large-sized pores.

    3) The main phases of the Al2O3-ZrO2 eutectic ceramics include α-Al2O3 and metastable m-ZrO2, with t-ZrO2 as a substable phase. As laser power increases, the content of m-ZrO2 gradually decreases, and the preservation of metastable t-ZrO2 at room temperature is due to the limited energy exchange and spatial dimensions required for phase transformation within the extremely short solidification time.

    4) The microstructure of LPBF-produced Al2O3-ZrO2 eutectic ceramics exhibits a cellular-like structure, with the size increasing with laser power.

    5) Among the tested laser powers, samples fabricated under P=60 W demonstrate the optimal microhardness (17.19 GPa) and fracture toughness (6.67 MPa·m1/2).

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    Zhiwei Xiong, Kai Zhang, Tingting Liu, Wenhe Liao. Effects of Laser Power on Molten Pool Morphology, Microstructure, and Mechanical Properties of Al2O3‐ZrO2 Eutectic Ceramics Shaped by Laser Powder Bed Fusion[J]. Chinese Journal of Lasers, 2024, 51(10): 1002316

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    Paper Information

    Category: Laser Additive Manufacturing

    Received: Dec. 1, 2023

    Accepted: Jan. 10, 2024

    Published Online: Apr. 17, 2024

    The Author Email: Zhang Kai (zhangkai@njust.edu.cn), Liu Tingting (liutingting@mail.njust.edu.cn)

    DOI:10.3788/CJL231464

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