选区熔化3D打印铜的研究进展

Research Progress on Selective Melting 3D Printing of Copper

选区熔化3D打印是按照零件三维数字模型的分层切片离散数据,以聚焦激光束或电子束等为热源逐层扫描熔化粉末,然后逐层凝固累积制造零件的新方法。这一方法无需模具直接实现形状复杂零件的成形,极大缩短了单件小批量复杂零件的生产周期,为实现航空航天等领域复杂零件的制造和结构优化提供了新途径。铜具有优良的导热性能,是制造航天发动机燃烧室冷却壁和喷嘴等复杂结构的理想材料,3D打印铜的研究和应用有重要意义。然而,铜对常见的波长>1060 nm的激光能量吸收率低,热导率高,3D打印铜成形难度大,工艺控制困难,研究应用落后于其他常见金属材料。在分析国内外铜3D打印研究进展的基础上,总结了激光选区熔化和电子束选区熔化3D打印铜的成形工艺特点、组织性能及其存在的问题。结果表明,提高铜对激光吸收率是激光选区熔化3D打印铜领域的主要研究热点,这是提高成形件致密度、控制微观组织、改善成形件性能的基础;电子束选区熔化克服了铜对激光吸收率低的问题,大大提高了成形件的致密度,但成形件精度较低,相关工艺还需进一步优化。

The selective melting 3D printing was an additive manufacturing method that scaned and melted powder layer by layer according to the sliced discrete data of the 3D digital model of the part.It used a focused laser beam or electron beam as heat source to scanning the powder,and the powder scanned by the heat source melted firstly,and then solidified and accumulated to form the manufactured parts layer by layer.This method could manufacture complex and precise metal parts directly that were difficult to form by traditional processing technology.The selective melting 3D printing had the advantages of low cost,short cycle time,high recycling rate of material and rapid forming,which provided a new way for manufacturing,structural optimization and light weighting of complex parts in aerospace and other fields.Copper had excellent thermal conductivity and was an ideal material for manufacturing complex structures such as cooling stave of combustion chamber and nozzles of aerospace engines.And it was meaningful to realize the additive manufacturing of copper and the control of microstructure and properties.However,the high thermal conductivity and the low energy absorption to the laser above 1060 nm were the technical difficulties of selective melted copper.These difficulties led to far behind development of research and application of selective melted copper compared with other common metallic materials such as steel and titanium.In this paper,based on the introduction of the characteristics of selective laser melting and selective electron beam melting,we summarized the research progress of copper of 3D printing.The process characteristics,surface quality,micro-structure and properties,main problems of selective laser melting and selective electron beam melting of copper were concluded.After that,the paper promoted the future directions of researches and application in the field of selective melted copper.The results showed that during the selective laser melting,the preheating temperature of copper powder was low,and the good powder flowability resulted easy post-processing with smooth surface and high dimensional accuracy of the parts.However,the high reflectivity of copper to laser brought challenges to selective laser melting of copper.In the early days,due to limitation of laser apparatus,the laser used in the selective laser melting system had defects of low beam quality,low power and poor stability,which made it difficult to achieve direct melting and forming of copper.Only the indirect sintering method,selective laser sinter,could be used to forming copper parts for that time.In this method,the alloy powder with low melting point or high laser absorption was added into copper powder as binder.And during the forming process,the binder melted to liquid phase to fill the pores between copper particles and solidified to continuous solid phase,and finally achieve sintering densification.The addition of the binder affected the thermal and mechanical properties of the parts.With the development of technology,the apparatus of high power,high beam quality,and high stability fiber laser were used to further enhance the densities of copper parts.However,since the reflected of copper to laser above 1060 nm was more than 90%,there were still lots of work needed to be done.Due to the extreme low laser absorption of copper in the selective laser melting,it was difficult to completely melt the copper powder when the laser power was low.And it caused defects such as un-melted particles,holes and cracks in the copper parts,which greatly reduced the quality and performance of the parts.The mechanical properties of copper parts by selective laser melting were still much lower than that of the parts formed by casting or forging.The relative densities of copper parts could be improved by increasing the laser power,but the high reflection of copper for the laser could lead to damage of the apparatus.The fundamental way to solve this problem was to improve the laser absorption of copper.The short wavelength laser was helpful to increase the absorption of laser,but there were still difficulties for manufacturing short wavelength lasers.In addition,due to the excellent thermal conductivity of copper,the boundary area of the copper parts appeared partial melting of the powder,which affects the dimensional accuracy of the parts.The selective electron beam melting used the electron beam as the heat source in a vacuum environment,which had the advantages of high energy utilization rate,low reflection,high power,fast scanning speed,and no pollution.The selective electron beam melting could effectively solve the problem of high reflectivity of copper to laser,and realized the effective melting of copper powder and obtain the copper parts of high density,proposing a good development prospect.However,the powder bed of copper needed to be preheated and pre-sintered to prevent"blowing powder"during the selective electron beam melting process.It made serious powder adhesion on the surface of parts,and it was difficult to form complex structure parts and clean the surface of the part.These limitations restricted the application of the complex structure parts forming by selective electron beam melting.Although the relative densities of copper specimens obtained by selective electron beam melting were much higher than that of the specimens by selective laser melting,the tensile mechanical properties of the copper specimens obtained by selective electron beam melting were anisotropic,and the quality of interlayer fusion was still the key problem that affected the performance of the specimens.And the process needed to be further optimized for selective electron beam melting.