Research ArticleFormation process and mechanical properties in selective laser melted multi-principal-element alloys
Introduction
Selective laser melting (SLM) is a new additive manufacturing (AM) technology, which manufactures materials by tracing the two-dimensional (2D) cross sections of a three-dimension (3D) model layer by layer. Using the high-power laser beam as the energy source, many alloys, such as stainless steels, titanium alloys, and refractory alloys, can be manufactured using SLM [[1], [2], [3]]. Now, SLM has a broad use in the aerospace, biology, and deep sea. SLM is developed from selective laser sintering (SLS) with a much better layer-layer adhesion by remelting the alloy powders [4,5]. The SLM processing parameters, such as the laser power, laser-spot size, scan speed, hatch spacing, hatch style, and layer thickness, have adjustability in a wide range, which has a significant impact on the quality and microstructure of parts [6,7]. Thus, SLM is regarded as an effective means to achieve the target properties of materials. For example, the high laser power and low scanning speed produce high hardness due to the good melting pool quality [5]. The improved surface accuracy and the reduced porosity boost the fatigue performance of the material by adjusting the SLM parameters [6]. By controlling the SLM parameters, 316L SS with a 99% density can be manufactured [8].
Recently, the SLMed multi-principal-element alloys (MPEAs) show high strength, good ductility, and great hardness mechanical properties [9], [10], [11], [12], [13]. For instance, the SLMed CoCrFeNiMn MPEA with an almost fully dense structure endows an excellent combination of strength and ductility due to the hierarchical microstructures, such as dislocations, cellular structures, and columnar grains [9], [10], [11]. In addition, the synergistic effects of σ phase and nanotwin significantly improve the mechanical properties of the SLMed CoCrFeNiMn [14]. The SLMed FeCoCrNiC MPEAs with fine microstructures and full density have a high yield stress of 656 MPa and a splendid tensile strength of 797 MPa [15]. Meanwhile, the SLM technology combined with the well-controlled annealing effectively reduces the dislocation density and adjusts the cellular structure of MPEA, which is conducive to improving the hydrogen embrittlement resistance [16]. In addition, the SLMed FeCrNi MPEA shows the effective combination of the corrosion and mechanical properties due to the Cr segregation along the dislocation cell boundary [17].
In order to explore the dynamic microstructure evolution during the SLM process, the molecular dynamics (MD) simulation can be used as a powerful auxiliary tool to reveal the underlying mechanisms on the nanoscale. The MD simualtion provides the nanoscale microstructure evolution with a sharp temperature change at several microseconds, but this result can hardly be measured by the traditional experiment [18]. For example, the MD simulation shows the dynamic nucleation and the grain growth at the molten pool boundary [19]. The cooling rate effect on the mechanical behavior of the AlCoCrCuFeNi MPEA is investigated by MD simulation [20].
Although the mechanical properties of the SLMed MPEAs have been investigated [21], [22], [23], their real-time microstructure evolution and influence mechanisms during SLM progress are rarely revealed at the nanoscale. To solve this issue, we fabricated the sheet sample of SLMed FeCrNi MPEA, and systematically analyzed the microstructures. Meanwhile, based on the results of experimental characterization, the evolution of dynamic microstructure during SLM process was investigated at the nanoscale using an atomic simulation, and then the relationship between the microstructure and mechanical properties was further studied. In the present work, the experiment combined with MD simulation offers an intuitive perspective for studying the local forming process of the SLMed FeCrNi.
Section snippets
Experimental methods
The near-spherical FeCrNi MPEA powders with an average diameter of 17.9 µm were prepared by the high purity Ar gas atomization. By using the inductively coupled plasma mass spectroscopy (ICP-MS) and instrumental gas analysis (IGA), the element composition of the powder was measured, which contains Fe32.39Cr35.12Ni31.58 (at.%), and less than 0.1% C and O. The FS271M machine (Farsoon, Inc, China) was used to execute the SLM process under Ar gas atomization, and the sheet samples with dimensions
Experimental characterization and analysis
Fig. 3 shows the microstructures of SLMed FeCrNi in detail. The SEM micrograph of FeCrNi MPEA exhibits the melt pool morphology and characteristics of grain nucleation (Fig. 3(a)). The obvious columnar grain is observed at the boundary of melt pool. This reflects the inheritance of crystal orientation across the cladding layer, which is consistent with the previous studies [17,34]. Fig. 3(b, c) shows the grain filled by a large number of the cellular structures. The solidification condition
Microstructure-based strength model
Based on the MD simulation results and microstructure characteristics, in this section, a microstructure-based physical model is established to calculate the contribution of microstructure to strength. This model can be considered as a nano-layered structure. The thickness of the layer is much less than 50 nm, and the strength caused by the interface cannot be calculated according to the traditional Hall-Petch relationship [50]. Therefore, considering the interface itself and the interaction
Conclusions
In the work, we study the formation process and mechanical properties of the SLMed FeCrNi MPEA using experiment and atomic simulation. The columnar crystal grown across the molten pool and a large number of cellular structures within the grain are observed in the SLMed FeCrNi. By characterizing the distribution of elements at different scales, the elements are evenly distributed on the micron scale, while there is an obvious Cr cluster on the nanoscale. The atomic simulation shows that the
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 52020105013, 51871092, and 11902113), the Natural Science Foundation of Hunan Province (Nos. 2019JJ50068 and 2021JJ40032), and the Changsha Municipal Natural Science Foundation (No. kq2014126). Peter K. Liaw very much appreciates the support from the National Science Foundation (Nos. DMR-1611180 and 1809640).
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