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316L stainless steel fabricated by selective laser melting (SLM) has a wide range application in complex structural parts servicing at cryogenic temperature due to its good corrosion resistance and extremely low ductile-brittle transition temperature. In present work, the as-built and heat-treated microstructure and cryogenic mechanical properties of the SLM-fabricated 316L stainless steel were investigated. Due to the extremely high cooling rate in SLM process, the SLM-fabricated 316L stainless steel has unique microstructure, consisting of the heterogeneous columnar grain structure with a weak texture and intragranular cellular structure attaching with high dislocation network. The cryogenic yield, ultimate tensile strength, and elongation of as-built horizontal specimens are 840 MPa, 1510 MPa and 35%, those of as-built vertical specimens are 785 MPa, 1400 MPa and 37%. After annealing treatment, the yield strength decreases and the elongation increases, but the ultimate strength changes little. The high ultimate strength and the low yield ratio of SLM-fabricated 316L stainless steel is attributed to the martensite transformation during cryogenic deformation owing to its low stacking fault energy at low temperature. By comparing the martensite content of specimens with different strains and tensile directions, it was found that the grain texture is the dominant factor for the anisotropy of strength and ductility of specimens. The cellular structure is also in favor of the generation of shear band to increase the nucleation sites of α′ martensite, and finally promoting the occurrence of martensite transformation, but the promoting effect of cellular substructure on martensite transformation is limited compared with the influence of the grain texture.

The microstructures and mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) are evaluated in as-built, direct aging (DA), and homogenization + aging (HA) conditions on considering the effect of applied loading direction. The results show that the microstructure of as-built and DA specimens consist of the intracrystalline fine degenerated dendritic structures (0.6–1 μm in spacing) with a high density of dislocations (1013–1014 m−2). Fine dendritic structures disappear after HA treatment, and the average geometrically necessary dislocation (GND) density decreases to 1012 m−2. A quantitative analysis of strengthening mechanisms is established by taking into consideration the grain boundary, solid solution, precipitation and dislocation strengthening. For the as-built and DA specimens, the contribution to the yield strength by dislocation strengthening is ~85 MPa in vertical deposition and ~170 MPa in horizontal deposition, respectively, and the Laves phases provide approximately 100–110 MPa. The contribution of precipitation strengthening to the strength increment in the DA and HA specimens is 590–600 and 830–850 MPa, respectively. The precipitation strengthening is mainly contributed by shearing mechanisms among which coherency strengthening plays dominant effect rather than order strengthening. For as-built, DA, and HA regimes, the specimens always exhibit higher strength along the horizontal direction than that along the vertical direction. The anisotropy of yield strength in the as-built and DA specimens is mainly attributed to the difference in dislocation density and effective grain size, and the anisotropy of yield strength in the HA specimens is mainly caused by the difference in effective grain size and Taylor factor.

Optimizing the microstructure to enhance the high temperature stress rupture properties of Inconel 718 (IN718) alloy fabricated via selective laser melting (SLM) is an urgent issue. In this study, SLM-built IN718 specimens were heat-treated with hot isostatic pressing, solution and aging treatment, and their microstructure and high temperature stress rupture properties were investigated, focusing on the influence of grain structure and precipitates. The results show that the coarsening of γ″ precipitate with the aging time at 720 °C follows the Lifshitz-Slyozov-Wagner theory. As the γ″ precipitate size increases, the rupture life gradually increases, area reduction gradually decreases, and the uniform elongation first increases and then decreases. Furthermore, a wide distribution of grain size results in the fine grains regions with much higher geometrically necessary dislocations compared with coarse grain regions during deformation. Hence, the micro-cracks preferentially initiate and propagate along fine-grain boundary. The main deformation mechanisms of specimens aged at 720 °C for 4 h (HSA4) and 8 h (HSA8) with the small γ″ precipitate are dislocation slipping and micro-twinning mode during stress rupture test, while those of specimen aged at 720 °C for 32 h (HSA32) with relatively large γ″ precipitate are dislocation slipping and isolated faulting mode. The HSA8 specimen with the best comprehensive stress rupture properties has the average life of 130 h and uniform elongation of 7.5%, meeting the standard specification for wrought IN718. Finally, some potential strategies for further improving the stress rupture properties of SLM-built IN718 were proposed by comparing to forged-IN718.

The influence of post-heat-treatment on the resulting microstructure and room-temperature fracture toughness ( K I C ) of Inconel 718 fabricated by laser solid forming (LSF), a kind of laser based directed-energy-deposition additive manufacturing technologies, is investigated. Detailed microstructure characterization was performed on as-fabricated and heat-treated samples using direct aging (DA), solution treatment plus aging (STA), homogenization plus STA (HSTA), respectively. The results indicate that as-fabricated sample mainly consists of γ columnar dendrites and a small quantity of (γ+Laves) eutectics in the interdendritic areas. After DA post-heat-treatment, the nonuniform γ′′/γ′ precipitates exist around Laves phases. After STA post-heat-treatment, the short-acicular δ-phases precipitate around/in the Laves phases, the micro-segregation reduces and the distribution of γ′′/γ′ precipitates in the dendrite arm is almost homogeneous. After HSTA post-heat-treatment, Laves phase mostly disappears, micro-segregation completely removes and the γ′′/γ′ precipitates (~30 nm) are distributed in bimodal recrystallized grains. The results of K I C testing indicate that as-fabricated sample possesses the lowest K I C mainly due to its lowest elastic modulus and yield strength. However, the K I C of DA sample (~89.8 MPa m ) is close to that of as-fabricated sample, mainly due to its low ductility and strain-hardening exponent. But, the K I C of STA and HSTA samples increases by ~55.8% and 90.8% compared with as-fabricated sample respectively, mainly because of the better performance in strength–plasticity of HSTA sample (~1114 MPa, ~26.1%). Particularly, the K I C of HSTA sample with bimodal grains is ~60 MPa m higher than the lower limit of wrought IN718 (AMS 5662) and reaches ~164.1 MPa m . There exists significant difference in the K I C fractographs of as-fabricated and heat-treated samples. Overall, this research illustrates that an appropriate post-heat-treatment possesses obvious toughening effect on the LSF Inconel 718 alloy.