选区激光熔融(SLM)18Ni300模具钢成型性能及热处理制度的研究

STUDY OF MECHANICAL PROPERTIES AND HEAT TREATMENT OF THE 18Ni300 MARAGING STEEL MANUFACTURED BY SELECTIVE LASER MELTING

郭文锋

11649138

硕士

机械工程

朱强

2018-06-08

2018-07-03

哈尔滨工业大学

深圳

金属 3D 打印技术可实现金属的自由制造，为随形冷却模具的制造带来新的契机，选区激光熔融技术（SLM）具有成型精度高、性能良好的优势，成为 3D 打印模具制造首选的快速成形技术。马氏体时效钢兼具超高强度和优良的塑性，广泛应用于模具制造业以及军工企业等精密加工产业，目前马氏体钢成为 SLM 制造模具的主要应用，其中 18Ni300 马氏体时效钢已成功实现 SLM 成形，是用于增材制造技术的第一代钢种。基于以上应用背景，本文以具有高强高韧的 18Ni300 模具钢作为研究模具 SLM 成型的材料，并分别对 SLM 18Ni300 的粉末特性、成型工艺以及对应的热处理机制进行系统研究，主要完成工作如下：（1）首先对 SLM 18Ni300 模具钢粉末特征进行表征分析，对价格上具有绝对优势的国内威拉里新材料科技有限公司（VILORY）提供的 18Ni300 粉末和 EOS公司 SLM 专用 18Ni300 粉末进行对比分析，探究了两种粉末的化学组成、颗粒形貌、粒径、堆积密度、流动性和物相组成等粉末特征，得出关于 SLM 专用粉末特 性的一些结论。电镜下两种粉末颗粒大部分是圆整的，夹杂一些破碎、结节状等不规则颗粒，但国内 VILORY 粉末球形度测量值略优于 EOS，且表面较光滑。粒径结果显示，国内 VILORY 粉末和 EOS 粉末颗粒尺寸分别在 10~80μm 和 5~80μm之间，90%的粉末尺寸均小于 55μm。两种粉末粒径分布曲线（PSD）呈高斯分布，其中 EOS 粉末的粒径分布曲线跨度值更大，更符合多组元粒径的搭配，而存在一定比例的超细粉（<10μm），能够填充大颗粒间的孔隙，粉末综合特性测试结果表明其粉末松装密度和振实密度更高，两种粉末的流动性能均为良好。X 射线衍射图谱结果表明，两种粉末都是以马氏体（α 相）为主，粉末中无任何杂质相。综合结果表明，国内 VILORY 粉末在颗粒形貌、粒径及综合性能特征上可与 EOS 公司的 SLM 18Ni300 专用粉末相媲美，满足 SLM 成型要求。 （2）研究 SLM 18Ni300 成型工艺，在激光功率 180W，扫描速度 600mm/s，扫描间距 0.105mm，铺粉厚度 30µm，线型扫描模式的工艺参数下， EOS 18Ni300模具钢粉末的 SLM 成型件致密性可达 99%，成型工艺稳定，组织致密均匀，力学性能优异，抗拉强度和屈服强度分别是 1152MPa 和 1020MPa，但二维形貌和三维CT 扫描图显示仍存在较少的孔隙和微裂纹缺陷。原始打印件的 XRD 图谱表明其物相是作为时效基体的单相固溶体 α 相，得出 SLM 18Ni300 可免去高温的固溶处理，直接进行时效强化。（3）研究不同热处理方式下的 SLM 18Ni300 成型件的力学性能和组织，得出热处理优化机制。直接进行时效热处理后的 SLM 18Ni300 的时效行为与传统锻件经固溶+时效后的结果一致，表明经 SLM 成型的 18Ni300 模具钢不需要复杂的固溶热处理。不同时效热处理后的行为显示，随着时效温度的提高，硬度峰值到达时间越短，但达到的硬度值也越小，在 460ºC 下的时效强化效果持续时间长，并 在 12h 达到强度的峰值（抗拉强度为2116MPa，屈服强度为 2051MPa），强化的原因是在时效中先后从马氏体基体弥散析出的金属间化合物 Ni3(Mo,Ti)和 Fe2Mo，发挥沉淀硬化作用。当时效温度高于 500ºC，部分马氏体会逆转变为奥氏体，逆变奥氏体作为一种软化相可改善 18Ni300 的塑性，同时会降低强度，但对塑性的改善高于强度下降。分析不同时效状态下，拉伸件的断裂机制，结果表明原始打印状态下的 18Ni300 模具钢表现出微孔聚集型的韧性断裂，经过 460ºC 下保温 12h 的时效处理后，其断裂形式表现为准解理断裂，而在较高温度 540ºC 下保温 2h，因为逆转变奥氏体的生成，改善了其塑性和韧性，断裂机理为韧性断裂。得出 SLM 18Ni300 模具钢可通过在较高温（500ºC~540ºC）保温较短时间（1~3h），使模具钢处于过时效的初期阶段，既能保证强化沉淀相的析出，保证强度，又能产生一定量的逆转变奥氏体，改善模具钢的塑性，达到理想的强韧化效果。

Additive Manufacturing (AM), the layer-by layer building-up of parts, has lately become an option for serial productions. The technology is applicable to manufacture dies with “ideal” design without construction of manufacturing reality, bringing a new possibility for the manufacture of a die with conformal cooling channels. Among AM techniques, selective laser melting (SLM) seems to be the most versatile process, capable to produce functional components from materials having good mechanical properties. The maraging steel 18Ni300, which combines high strength and excellent toughness, is widely used for tooling of injection moulding and die casting industries. Based on the above application background, in this study, the powder characteristics for SLM were investigated, and the microstructure and properties of SLM parts were assessed. At last, heat treatment behaviour of the 18Ni300 maraging steel manufactured by SLM is studied to improve mechanical properties.Different batches of commercially available 18Ni300 powder made by gas atomization were characterized with respect to particle size distribution, morphology, chemical composition, packing density and flowability. Domestic powders from VILORY has great advantage in price compared to SLM power from EOS, the experimental results indicate that the domestic powers have greater regular morphology with better sphericity by statistical measurements. The particle size distribution curve (PSD) of EOS powder has a large span value, which means a wider size distribution as a result of higher fractions in coarse and fine particles respectively. Therefore, powder grades tend to exhibit high packing densities. And chemical compositions of the different powders ware analysed. The element Ni in VILORY powder is lower than the EOS powder.The optimized parameters of SLM 18Ni300 have been achieved. The laser power is 180W, laser scan speed is 600 mm/s, laser scan space is 0.105mm, laser thickness is30μm, atmosphere is N2. It is concluded that the SLM process of the 18Ni300 maraging steel is stable, and dense parts with excellent mechanical properties were obtained, but some pores and micro crack defects were still founded. Microstructural analysis reveals fine cellular and dendritic structure in the original SLM maraging steel.This work studied evolution of microstructure and properties during heat treatment, by X-ray diffraction, scanning electron（SEM）and transmission electron microscopy (TEM). Upon aging, precipitation of nanoscale intermetallic compounds occurs, which in turn, results in remarkable improvements in yield and ultimate tensile strengths, while substantially reduces ductility and fracture toughness. When aging temperature is over 500 ºC, The formed reverted austenite (γ-Fe) fraction increases, while hardness decreases, with increase of aging time, more apparently at aging temperatures of higher than 540ºC. Meanwhile, precipitation also involves effects of heat treatment on mechanical properties. Mechanisms of the combined effects of softening by formation of reverted austenite (γ-Fe) and age hardening induced by precipitation are discussed in this dissertation. Finally, an optimal heat treatment procedure for SLM-processed 18Ni300 is proposed.

3D打印 选区激光熔融（SLM） 马氏体时效钢 粉末特性 力学性能 热处理

Metal additive manufacturing Selective Laser Melting Maraging steel Powder characteristics Mechanical properties Heat treatment

中文

联合培养

南方科技大学

郭文锋. 选区激光熔融(SLM)18Ni300模具钢成型性能及热处理制度的研究[D]. 深圳. 哈尔滨工业大学,2018.