Due to the unique characteristics of good corrosion resistance, high hardness and strength, good toughness, and excellent formability, precipitation hardening stainless steel (PHSS) has become a top candidate for advanced industrial applications such as aerospace, shipbuilding, automotive, high-end tools and biomedical applications.
In order to promote the scientific research, precise forming and industrial applications of a new generation of high-quality, difficult-to-process, multi-functional molds, Institute of New Materials of Guangdong Academy of Sciences has devoted to exploring the potential of SLM PHSS, such as seeking process window, investigating tailored microstructure and hence achieving customized high properties. Service life and reliability of advanced tools and molds are closely related to their processing technology, post-processing methods and corresponding mechanical properties. Thus, Institute of New Materials of Guangdong Academy of Sciences, together with Université de Technologie de Belfort-Montbéliard (UTBM) and Université de Technologie de Troyes (UTT), first reported and confirmed the precipitation hardening behavior of NiAl nanoprecipitates in SLM CX steel. The relationship between the nanoprecipitates – microstructure – properties of SLM CX steel in different heat-treated states was also established.
The nanoprecipitates distribution, microstructural evolution, and mechanical properties of SLM CX stainless steels in the as-built and heat-treated conditions were systematically studied using scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The XRD spectrum revealed that solution treatment resulted in the formation of a complete martensite phase, and a reverted austenite (γ’) phase formed after aging treatment. The TEM analysis indicated that numerous dislocations and nanoprecipitates were dispersed within the martensite matrix for both the as-built and aged samples. The rod-like NiAl precipitates with a size range of 3-25 nm for the as-built samples and 7-30 nm for the solution-aged samples were determined through high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS). Furthermore, the microhardness of the SLM CX stainless steel parts was found to significantly improve from 350 HV0.2 in the as-built state to 510 HV0.2 in the solution-aged state. The ultimate tensile strength (UTS) of the SLM CX stainless steel parts also increased from 1,043 MPa in the as-built state to 1,601 MPa after solution-aging heat treatment.
Through a series of explorations on process model optimization, crystallographic orientation analysis and precipitation behavior, these work have laid a rich practical and theoretical basis for the further development and use of SLM technology to fabricate martensitic PHSS with high strength and toughness. Provide abundant supports for vigorously promoting the application of SLM martensitic PHSSs in high-end tools and molds, etc.
Related research results have been published in Journal of Materials Science & Technology (Chinese Academy of Sciences Q1, Top, IF=6.155), Materials Science and Engineering: A (Chinese Academy of Sciences Q1, Top, IF=4.652), Journal of Materials Science (Chinese Academy of Sciences Q2, Top, IF=3.553).
These studies were financial supported by Natural Science Foundation of Guangdong Province, Science and Technology Development Project of Guangdong Academy of Sciences.
Fig. 1: (a) Schematic diagram of the heat transfer process of the molten pool in the SLM fabrication; (b) the enlarged view of (a) and the macro appearance of the SLM single track of CX steel (XY plane and XZ plane, respectively).
Fig. 2: EBSD analysis of the SLM CX samples under η=245 J/m: (a) IPF-Z of the cross-sections in XY plane; (b) grain size distribution in XY plane; (c) IPF-Y of the cross-sections in XZ plane; (d) grain size distribution in XZ plane.
Fig. 3：(a) TEM observation of SLM CX as-built steel sample with corresponding SAED pattern; (b) zoom-in photo of a; (c) HRTEM image of precipitate and amorphous-nanocrystalline composite corresponding to the region A marked in b; (d) Impact force-displacement curves of SLM CX as-built steel sample; (d) 3D CAD model of a conformal cooling mold; (e) A cross section of the corresponding mold in d.