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Effect of Solution Treatment on Microstructure and Grain Boundary Distribution of 316l Stainless Steel Formed by Selective Laser Melting (SLM)
P. Zhu, J-G. Zhao, L. Lu, X-Y. Liu, Y-Z. Liu, J-L. Shang and B. Sun

Selective laser melting (SLM) was used to form 316L stainless steel. Furthermore, as-built samples were subjected to two solution treatments at 1050℃ for 2 hours followed by air cooling, and 1150℃ for 2 hours followed by water cooling. Microstructural characteristics and grain boundary character distribution (GBCD) are determined and tested by optical metallography, electron backscatter diffraction (EBSD), etc. The results showed that the as-built specimens consist of columnar crystal with the molten pool boundaries. After heat treatment, the molten pool boundaries and the dendrites disappears, the austenite grains grow, and the grain is mainly composed of columnar crystals that grow parallel to the direction of heat flow after HT at 1050℃, and there is an obvious texture in the <101> direction. After solution treatment at 1150℃, the grain is mainly composed of equiaxed grains, and the texture is no longer obvious. The small-angle grain boundaries at 0 to 10° after 1050℃ solution treatment account for about 80% of the total grain boundaries, Σ3 CSL is the most in low-ΣCSL. Compared with the solution treatment at 1050℃, the grain growth was obvious at 1150℃, the content of low-ΣCSL increased obviously, the grain boundary energy of low-ΣCSL was lower than that of the random boundary, the corrosion resistance was enhanced. The small-angle grain boundary of the sample cross-section decreases, the large-angle grain boundary increases and the twin crystals appear, in which the increase of the large-angle grain boundary is the most obvious with the difference of the misorientation angle of 60°, which is mainly <111>/60° twin boundaries, which belongs to the Σ3 CSL boundaries.

Keywords: Fibre laser, 316L stainless steel, selective laser melting (SLM), austenite, solution treatment, microstructure, grain boundary engineering

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