选区激光熔化成形零件支撑结构性能差异研究

Performance Differences of Support Structures for Parts Formed by Selective Laser Melting

利用选区激光熔化制造金属零件时,悬垂构件的可制造性和成形质量与添加的支撑结构密切相关。采用热弹塑性有限元分析方法,研究了不同支撑类型的热传导特性,并通过试验揭示了支撑类型对316L试件翘曲变形、表面形貌、显微组织及显微硬度的影响规律。结果表明:块状支撑的散热性能优于锥状支撑,将试件峰值温度降低了6.7%,温度振荡幅度降低了41.07%,而其较好的导热性能使试件下表面粗糙度降低了6.23%,同时悬垂区域金属组织中出现了细化晶粒(18.42μm)及脆而硬的网状Cr、Ni相,显微硬度提高到234 HV。锥状支撑将试件变形降低了1 mm,而试件底部较少的气孔缺陷使硬度波动降低了14 HV。分析认为,组合支撑中增加块状结构的比例有利于提高试件成形精度和力学性能,增加锥状结构的比例有利于减小试件翘曲变形程度。

Objective When metal components are manufactured by selective laser melting,a suspension structure cannot be directly formed in the unmelted powder layer.A suitable support structure must be added in time to dissipate the heat generated during the metal powder melting process and restrain the deformation of the overhanging characteristics.Therefore,the manufacturability and formation quality of the overhang structure are related to the added support structure.Various types of support structures,such as block,plane,tree,and cone supports,have been designed for overhanging structures,and considerable research has been conducted on support structure optimization,support strength,and easy removal.However,the type of performance that differs between support structures is not clear.In practice,this depends primarily on the experience of selecting a certain support structure.In this study,the effects of cone,block,and combination supports on the thermodynamics and formation quality of overhanging specimens are studied,and the selection principle of the support type is formulated;this can provide a technical reference for the selection of supporting structures.Methods First,the thermal elastic-plastic finite element method is used to simulate the additive manufacturing process of the cone,block,and combination support specimens.By quantitatively analyzing the temperature data at the end of the build process along vertical and horizontal paths,which are extracted from the model,the influence of the heat conduction characteristics of the different support structures on the temperature field of the specimen is revealed.Three types of supporting specimens made of 316L powder are prepared using the selective laser melting(SLM)method.The surface morphologies of the specimens after removing the support are analyzed using the laser scanning confocal microscope and scanning electron microscope.The effect of the supporting structure on the surface morphology is studied.The effects of the supporting structure on the microstructure and elemental distribution are studied using an optical microscope and electron probe.Finally,the microhardness of the formed specimens is measured using a microhardness tester to characterize their mechanical properties.Results and Discussions The geometry of the support structure affects the distribution of the temperature field in the overhanging specimen.The block support reduces the temperature gradient in the edge region(Fig.6),thus reducing the specimen warpage caused by thermal strain.The block support reduces the peak temperature by 6.7%,temperature gradient by 14.05%,and temperature oscillation amplitude by 41.07%of the specimen,indicating that the block support structure has excellent cooling performance,whereas a higher cooling rate helps the overhang structure to form fine grains with good microstructural properties.In addition,it is found that the combination support specimen has the smallest warpage(0.29 mm)and surface roughness(70.804μm)among the three types of specimens(Fig.12).The excellent support strength and heat dissipation performance of the combination support improve the formation accuracy of the specimen.The block and combination support specimens have refined grain and reticulated alloying element phases such as Cr and Ni in the sub-grains.The dense metal structure and the brittle and hard reticular phases increase the microhardness to 234 HV(Fig.17).However,the microhardness fluctuation of the block support specimen is the largest owing to the existence of many porosity defects,and its value is 22 HV.Therefore,a combination support structure with better comprehensive performance is preferred to improve the mechanical properties of the overhanging structure.Conclusions The support structure changes the thermomechanical evolution process of the overhang plate.The block support can significantly reduce the peak temperature,temperature oscillation amplitude,and edge temperature gradient of the specimen,concluding that the block support structure exhibits excellent cooling performance.The combination support can not only restrain the specimen's warpage by providing sufficient support strength but also improve the surface topography quality by reducing the powder clusters on the lower surface of the overhanging specimen.Therefore,choosing a combination support is conducive to the preparation of overhang structures with higher dimensional accuracy.The support structure affects the microstructure of the overhanging area of the specimen.The metal structure of the cone support specimen is coarse,uniform,and has fewer porosity defects that can be removed by setting a small machining allowance.Owing to the intense competition of metal grain growth in the block support specimen,the refined grains and the brittle and hard reticular phases formed in the overhang area significantly improve the microhardness.The combination support has the advantages of the other two types of support.Increasing the proportion of block structures in combination-type supports is beneficial to improve the mechanical properties of specimens,and increasing the proportion of cone structures is conducive to reduce material consumption.