Microstructures and properties of porous TiAl-based intermetallics prepared by freeze-casting

Preparation of porous Ti Al-based intermetallics with aligned and elongated pores by freeze-casting was investigated. Engineering Ti-43 Al-9V-1Y powder(D50=50 μm), carboxymethyl cellulose, and guar gum were used to prepare the aqueous-based slurries for freeze-casting. Results showed that the porous Ti Al was obtained by using a freezing temperature of -5 ℃ and the pore structure was tailored by varying the particle content of slurry. The total porosity reduced from 81% to 62% and the aligned pore width dropped from approximately 500 to around 270 μm, with increasing the particle content from 10 to 30 vol.%. Furthermore, the compressive strength along the aligned pores increased from 16 to 120 MPa with the reduction of porosity. The effective thermal conductivities of porous Ti Al were lower than 1.81 W/(m·K) and showed anisotropic property with respect to the pore orientation.

Lightweight broadband microwave absorbing metamaterial with CB-ABS composites fabricated by 3D printing

The self-similarity,high geometric symmetry and spatial utilization properties of fractal structures provide new methods for the development of absorbing metamaterials.In this paper,the microwave absorption properties of the gradient dendritic fractal metamaterial structure(GDFMs)based on carbon black and acrylonitrile-butadiene-styrene composites were investigated.The optimal metamaterial structure has an effective absorption in the frequency range of 4.5-40 GHz.The rotational-symmetry GDFMs leads to the polarization independence,and the GDFMs exhibits a wide-angle absorption performance for both TE and TM waves.It is expected that the proposed GDFMs has good application prospects in electromagnetic wave absorption.

Intraocular pressure control of a novel glaucoma drainage device-in vitro and in vivo studies

AIM：To evaluate the intraocular pressure（IOP）control of an artificial trabeculum drainage system（ATDS）,a newly designed glaucoma drainage device,and postoperative complications in normal rabbit eyes.METHODS：Pressure drops in air and fluid of 30 ATDS were measured after being connected to a closed manometric system.Twenty of them were then chosen and implanted randomly into the eyes of 20 rabbits.Postoperative slitlamp,gonioscopic examination and IOP measurements were recorded periodically.Ultrasound biomicroscopy and B-scan ultrasonography were also used to observe the complications.Eyes were enucleated on day 60.RESULTS：Pressure drops of 4.6-9.4 mm Hg were obtained at physiological aqueous flow rates in the tests in vitro.The average postoperative IOP of the experimental eyes（11.6-12.8 mm Hg）was lower than the controls significantly（P〈0.05）at each time point.Complications of hemorrhage（n=1）,cellulosic exudation（two cases）and local iris congestion（two cases）were observed.The lumina of the devices were devoid of obstructions in all specimens examined and a thin fibrous capsule was found around the endplate.CONCLUSION：ATDS reduce IOP effectively.However,further studies on the structure are needed to reduce complications.

Effect of mould baffle technology on stray grain formation in single crystal blades by integral fabrication based on 3D printing

Stray grains,the most serious casting defect,mainly occur in the platform because of the abrupt transition of the cross-section in the directional solidification of superalloy single-crystal blades.A new mould baffle technology based on 3D printing and gelcasting is proposed herein to reduce the formation of stray grains in the platform.The influence of the proposed mould baffle technology on the temperature field in the platform during solidification was investigated by simulation and experiment.The numerical simulation results indicate that the proposed mould baffle technology can effectively hinder the radiation and heat dissipation at the platform extremities,and therefore,reduce undercooling in the platform and the formation of stray grains during directional solidification.Casting trials of a hollow turbine blade were conducted using CMSX-4 superalloy.The trial results demonstrate the potential of the proposed approach for manufacturing single-crystal superalloy blades.

High-accuracy calibration for multi-laser powder bed fusion via in situ detection and parameter identification

Multi-laser powder bed fusion(ML-PBF)adopts multiple laser-scanner systems to increase the build envelope and build speed,but its calibration is an iterative and time-consuming process.In particular,multiple large-scale scan fields have a complex distortion in the overlap area,challenging the calibration process.In this study,owing to the enormous workload and alignment problems in the calibration of multiple scan fields,a novel calibration system is designed in this study to realize in situ auto-detection of numerous laser spots in the build chamber to ensure high efficiency and accuracy.Moreover,because the detectable area could not cover the entire build area and the detection data still contained errors,a virtual laser-scanner system was established by identifying the assembly defects and galvo nonlinearities of the ML-PBF system from the detection data,which served as the system's controller to improve calibration accuracy.The multi-field alignment error was less than 0.012%,which could avoid the intersection and separation of scan paths in multi-laser scanning and therefore meet the requirements for high-precision ML-PBF.Finally,the reliability of the method was verified theoretically using principal component analysis.

Real-time process control of powder bed fusion by monitoring dynamic temperature field

This study aims to optimize the uniformity of the temperature field during sintering to improve part performance.A temperature-field monitoring system is established based on an infrared thermal imager and the temperature field data obtained during the sintering of a part can be measured in real time.The relationship among the sintering temperature field,sintering process parameters,and part performance is established experimentally.Subsequently,a temperature field monitoring and analysis system is constructed,and various sintering temperature-field control strategies are established for various part sizes.Finally,a dynamic control strategy for controlling the temperature field during sintering is proposed,experimentally validated,and fully integrated into a developed powder bed fusion(PBF)equipment.For eight-shaped standard parts,the range of sintering temperature field is optimized from 44.1℃to 19.7℃,whereas the tensile strength of the parts increased by 15.4%.For large-size H parts,localized over burning is eliminated and the final quality of the part is optimized.This strategy is critical for the optimization of the PBF process for large-sized parts,in particular in the large-sized die manufacturing industry,which offers promise in the optimization of part performance.

Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing

The precision casting method based on aluminabased ceramic cores is one of the main techniques used to manufacture hollow turbine blades.Additive manufacturing(AM)technology provides an alternate solution to fabricating ceramic cores quickly and precisely.As the complexity of the structure increases and the strength of the material improves,the leaching process of the cores becomes more complicated.This study proposes a compound pore-forming method to increase the porosity of ceramic cores by adding a preformed-pore agent and materials that convert to easy-to-corrode phases.The preformed-pore agents(e.g.,carbon fibers)can be burned off during sintering to form pores before the leaching,and the easy-to-corrode phases(e.g.,CaCO3,SiO2,^-A12O3)can be leached firstly to form pores during the leaching process.The pores formed in the aforementioned two stages increase the contact area of the cores and leaching solution,thus improving the leaching rate.In the current study,the additive amount of the preformed-pore agent was optimized,and the effect of the easy-to-corrode phases on the comprehensive properties of the cores was then compared.Based on this,the corresponding model was established.