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摘要
Due to its high density and excellent radiation absorption，tungsten and its alloys are often used to prepare anti-scatter grids for absorbing scattered X-rays in computed tomography（CT）scans to improve imaging quality. Compared with traditional manufacturing，the pure tungsten anti-scatter grid prepared by selective laser melting（SLM）technology has the characteristics of high forming rigidity，thin wall thickness，high density，strong ability to absorb scattered radiation，and good shading degree and the research of SLM forming thin-walled tungsten parts gradually rises. At present，the research on SLM-formed tungsten thin-walled grid mainly focused on the thickness of the melt channel and its surface roughness，and had not explored its mechanical properties，the overall form-ability and forming accuracy of the anti-scatter grid. In order to explore the optimal process parameters and performance of SLM forming pure tungsten anti-scatter grid. In this paper，spherical pure tungsten powder was used to print tungsten solid and grid samples under different laser power and scanning speed conditions. The density of the solid sample was measured by the Archimedes drainage method，and the internal defects and microstructure of the melt channel were observed by optical microscope（OM）. Finally，the microhardness was measured under the condition of 1.96 N loading and 10 s loading time. The grid sample was used to observe the morphology of the melt channel and measure the thickness of the melt channel using a three-dimensional microscope with a super depth of field. The study found：（1）The increase of laser power and scanning speed would improve the powder stickiness of the melt channel，but increase the surface roughness of the melt channel. The spheroidization caused by the laser power above 225 W would seriously affect the surface roughness of the melt channel.（2）When the wall thickness of the melt channel was not less than the powder particle size，the limit width of the melt channel was mainly determined by the laser spot. Due to the high thermal conductivity of tungsten powder，the size of the melt channel was basically larger than the diameter of the laser spot. And its size would increase with the increase of laser power and decrease with the increase of scanning speed. Under the process conditions of 200 W-500 mm·s-1，the width of the melt channel was the closest to the preset value of 100 μm.（3）There were certain pores and cracks in the samples under different process parameters，which indicated that SLM process could not form fully dense tungsten samples. The increase of laser power would make the solid density increase first and then decrease，and the increase of scanning speed would make the density decrease，and finally，the maximum density of 98.36% was obtained at 375 W-500 mm·s-1.（4）The interface of the solid sample was mostly columnar grains in the forming direction，and many slender cracks woukd appear on the boundary of the grains. And the reduction of laser power and the increase of scanning speed would lead to grain refinement.（5）The microhardness of the solid sample at 375 W-500 mm·s-1 was up to HV 464. Compared with the tungsten samples prepared by traditional methods chemical vapor deposition（CVD）（HV 419）and spark plasma sintering（SPS）（HV 414），the pure tungsten samples prepared by SLM had higher hardness. When the laser power was constant，the compressive strength of the grating gradually decreased with the increase of the scanning speed. When the scanning speed was constant，the compressive strength of the grating increased with the increase of the laser power，When the scanning speed was 500 mm·s-1 and the laser power was 250 W，a sample with a maximum compressive strength of 172 MPa was obtained. In addition，the appearance of spheroidization and cracks caused the fracture during compression to always start from the overlap of the grid welds.（6）The solid part of the anti-scatter grid was formed at 375 W-500 mm·s-1，and the grid part of the anti-scatter grid was formed at 210 W-600 mm·s-1. The overall shape of the anti-scatter grid was good，and there were no obvious defects. After sandblasting，the sticky powder in the melt channel almost completely disappeared. And the measurement found that the dimensional deviation of a single melt channel could be effectively guaranteed in the range of ±10 μm，with good forming accuracy. ? 2023 Editorial Office of Chinese Journal of Rare Metals.
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