激光选区熔化纯钨防散射栅格的工艺及性能研究

Selective Laser Melting Process and Performance of Pure Tungsten Anti-Scatter Grid

为探究激光选区熔化(SLM)成形纯钨防散射栅格的最佳工艺参数,研究了不同工艺参数对于栅格试样的表面粗糙度、熔道厚度、压缩力学性能以及钨实体试样致密度、微观组织的影响规律。研究发现,栅格试样的表面粗糙度会随着激光功率和扫描速度的增加而增加,过高的激光功率容易产生球化现象。此外,激光功率的增加以及扫描速度的减小都会使得熔道的厚度尺寸增加,在200 W激光功率以及500 mm·s^(-1)扫描速度工艺条件下熔道厚度最为接近100μm的预设值。压缩测试结果表明,纯钨薄壁栅格件的抗压强度会随着激光功率的增加以及扫描速度的减小而增加,且试样最大抗压强度达到了172 MPa。实体试样的致密度会随着激光扫描速度增加而减小,并且随着激光功率的增加先增大再减小,最终在375 W激光功率以及500 mm·s^(-1)扫描速度工艺条件下获得98.36%的最大致密度。其构建方向组织多为柱状晶粒,并且晶粒会随着激光功率的减小以及扫描速度的增加而细化。最后根据探究的工艺参数对栅格熔道形貌及厚度尺寸的影响规律,通过工艺优化,在210 W-600 mm·s^(-1)以及375 W-500 mm·s^(-1)条件下分别成形了防散射栅格的栅格和实体部分,成功打印的栅格熔道形貌较好、且具有较高的成形精度((100±10)μm)。

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 manu-facturing,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 fo-cused 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 form-ing pure tungsten anti-scatter grid.In this paper,spherical pure tungsten powder was used to print tungsten solid and grid samples un-der 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 mi-crohardness was measured under the condition of 1.96 N loading and 10 s loading time.The grid sample was used to observe the mor-phology 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 af-fect 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 pow-der,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 100μm.(3)There were certain pores and cracks in the samples under different pro-cess 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 fi-nally,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 addi-tion,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 sin-gle melt channel could be effectively guaranteed in the range of±10μm,with good forming accuracy.