激光功率对激光粉末床熔融成形Al_(2)O_(3)-ZrO_(2)共晶陶瓷熔池形态、微观组织和力学性能影响的研究

Effects of Laser Power on Molten Pool Morphology,Microstructure,and Mechanical Properties of Al_(2)O_(3)-ZrO_(2) Eutectic Ceramics Shaped by Laser Powder Bed Fusion

熔融生长的Al_(2)O_(3)-ZrO_(2)共晶陶瓷具有优异的高温性能。采用激光粉末床熔融(LPBF)直接制备Al_(2)O_(3)-ZrO_(2)共晶陶瓷,研究了不同激光功率下的单道形貌特征及块体表面质量、物相组成、微观组织结构的演变规律和力学性能。结果表明,激光功率的提升将增加熔池的长度和单沉积道的宽度。Al_(2)O_(3)-ZrO_(2)共晶陶瓷的表面粗糙度(Ra)和气孔率均随着激光功率的增加先降低后升高。在没有添加Y2O_(3)等稳定剂的条件下,Al_(2)O_(3)-ZrO_(2)共晶陶瓷的物相主要包括α-Al_(2)O_(3)、m-ZrO_(2)和亚稳相t-ZrO_(2)。随着激光功率的增加,m-ZrO_(2)逐渐减少,这是由于LPBF的快速冷却过程抑制了马氏体相变。样件的晶粒尺寸随着激光功率的增加呈增大趋势,晶界密度减小,因此测量的显微硬度和断裂韧性呈现下降的趋势。当激光功率为60 W时,得到硬度为Hv=17.19 GPa和断裂韧性为KIC=6.67 MPa·m1/2的最优力学性能样品。

Objective Moltengrown Al_(2)O_(3)-ZrO_(2) eutectic ceramics exhibit outstanding hightemperature 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 Al_(2)O_(3)-ZrO_(2) 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 realtime morphology evolution of molten pools and establish a correlation among the“material process microstructure mechanical properties”to lay the groundwork for producing highperformance ceramic materials.This paper presents an innovative study based on realtime highspeed imaging systems and threedimensional 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 Al_(2)O_(3)-ZrO_(2) eutectic ceramics,characterization and testing methods,including Xray 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 LPBFproduced Al_(2)O_(3)-ZrO_(2) eutectic ceramics under different laser powers.The findings demonstrate a gradual reduction in mZrO_(2) 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 selfdeveloped 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 100μm,and layer thickness of 50μm.Single deposition tracks of Al_(2)O_(3)-ZrO_(2) eutectic ceramics are printed under different laser powers,and a highspeed 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 highspeed 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 Xray 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 energydispersive Xray 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 Al_(2)O_(3)-ZrO_(2) 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 tZrO_(2) 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 Al_(2)O_(3)-ZrO_(2) eutectic ceramics consists of cellularlike 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 HallPetch theory,lowenergy 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 highspeedcamera 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 Al_(2)O_(3)-ZrO_(2) 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 largesizedpores.3) The main phases of the Al_(2)O_(3)-ZrO_(2) eutectic ceramics include α -Al_(2)O_(3) and metastable mZrO_(2),with tZrO_(2)as a substable phase. As laser power increases, the content of mZrO_(2)gradually decreases, and the preservation of metastable tZrO_(2)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 LPBFproducedAl_(2)O_(3)-ZrO_(2) eutectic ceramics exhibits a cellularlikestructure, 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).