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Open AccessArticle

Effect of Thermal Cycle on Microstructure Evolution and Mechanical Properties of Selective Laser Melted Low-Alloy Steel

1
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
2
National Key Laboratory for Remanufacturing, Army Academy of Armored Forces, Beijing 100072, China
3
Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China
*
Authors to whom correspondence should be addressed.
Materials 2019, 12(21), 3625; https://doi.org/10.3390/ma12213625
Received: 7 October 2019 / Revised: 22 October 2019 / Accepted: 28 October 2019 / Published: 4 November 2019
Low-alloy steel samples were successfully fabricated by selective laser melting (SLM). The evolution of the microstructure and the mechanical properties were investigated with different values of the energy area density (EAD). The results revealed that the initial solidification microstructures of the single tracks with different EADs were all martensite. However, the microstructures of bulk samples under different EADs were not martensite and differed significantly even from one another. When EAD increased from 47 to 142 J/mm2, the mixed lower bainite and martensite austenite microstructure changed to granular bainite; further, the morphology of bainite ferrite gradually changed from lath to multilateral. Moreover, with the increase of EAD, the grain size was remarkably reduced because of the increasing austenitizing periods and temperature during thermal cycling. The average grain size was 1.56 µm, 3.98 µm, and 6.31 µm with EADs of 142 J/mm2, 71 J/mm2, and 47 J/mm2, respectively. Yield strength and tensile strength of the SLM low-alloy steel increased with the increase in EAD; these values were significantly more than those of the alloys prepared by traditional methods. The microstructure of the SLM low-alloy steel samples is not uniform, and the inhomogeneity becomes more significant as EAD decreases. Simultaneously, when EAD decreases, the fracture mechanism changes from ductile to a mixture of ductile and brittle fracture; this is in contrast to the samples prepared by traditional methods. This study also found a stress concentration mechanism around large pores during plastic deformation that resulted in a brittle fracture. This indicates that large-sized pores significantly degrade the mechanical properties of the specimens. View Full-Text
Keywords: selective laser melting; thermal cycle; microstructure evolution; mechanical property; low-alloy steel selective laser melting; thermal cycle; microstructure evolution; mechanical property; low-alloy steel
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Kang, X.; Dong, S.; Wang, H.; Yan, S.; Liu, X.; Ren, H. Effect of Thermal Cycle on Microstructure Evolution and Mechanical Properties of Selective Laser Melted Low-Alloy Steel. Materials 2019, 12, 3625.

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