Improvement of densification and microstructure of ASTM A131 EH36 steel samples additively manufactured via selective laser melting with varying laser scanning speed and hatch spacing

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2019-02, Vol.746, p.300-313
• Main Authors: Wang, Jingjing, Wu, Wen Jin, Jing, Wei, Tan, Xipeng, Bi, Gui Jun, Tor, Shu Beng, Leong, Kah Fai, Chua, Chee Kai, Liu, Erjia
• Format: Article
• Language: English
• Subjects: Additive manufacturing; ASTM A131 (EH36) steel; Cellular structure; Chemical precipitation; Columnar structure; Cooling rate; Dendritic structure; Densification; Density; Dimpling; Dislocations; Elongated structure; Grain boundaries; Heat treating; High strength steels; Laser beam melting; Lasers; Martensite; Mechanical properties; Microhardness; Microstructure; Morphology; Scanning electron microscopy; Selective laser melting; Solid solutions; Tensile properties; Tensile strength; Thickness
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2019.01.019

ASTM A131 (EH36 grade) steel samples with a high density (>99.5%) were additively manufactured through a selected laser melting (SLM) process. The additive manufacturing (AM) process parameters were optimized through tuning scanning speed from 100 to 300 mm/s and hatch spacing from 0.08 to 0.13 mm with a layer thickness of about 50 µm using a laser power of 175 W under a chess board scanning strategy. By varying heat input mainly via lowering scanning speed, a near full density of the samples could be achieved. The processing at lower scanning speed was broadened to a larger window of hatch spacing, which improved the density of the printed samples above 99%. The microstructure of the samples under different scan speeds was subsequently studied and compared. Cellular-dendritic grains with a diameter of around 1 µm and acicular grains of up to 2–3 µm were observed at different locations. A large plate-like martensite structure was found in coarse columnar grains (up to 20 µm). With lower scanning speed, the grains were coarser and more like a cellular structure, while with higher scanning speed the grains were finer and more like a cellular-dendritic structure. However, different morphologies of martensite grains were found mainly in cellular, columnar and lath shapes, which probably resulted from different transformation mechanisms and were controlled by cooling rate. Electron back scattered diffraction analysis showed that the prior elongated columnar grains with the martensitic plate-like structure delineated the columnar grain boundaries. The micro-hardness and tensile properties of the selected samples were correlated to their microstructures. The resultant mechanical properties were attributed to a combined effect of martensitic structure and possible precipitation, solid solution and dislocation strengthening for the EH36 steel samples built via SLM. The fractured surfaces, examined with scanning electron microscopy, showed mostly a dimple type of failure but with a low elongation of up to about 6%. The microsegregation behavior of the samples was believed to accompany this rapid solidification process and the degree of element escaping from the lattice was attributed to the cooling rate.