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Additive manufacturing (AM) techniques have been widely used to fabricate structural components with complex geometries. Understanding AM materials under extreme environments is crucial for their implementation in various engineering sectors. In this study, the deformation behavior and irradiation damage of 316 L stainless steel (SS) fabricated by the direct energy deposition (DED) process are investigated. The fabrication-induced nanopores with an average diameter of 200 nm exhibit a core-shell structure with a local tensile strain. The precession electron diffraction (PED) reveals that austenite-to-martensite phase transformation preferentially occurs around the nanopores under tension test at room temperature. Proton irradiation experiments performed at 360 °C to a fluence of 1.09 × 1019 cm−2 and 5.42 × 1019 cm−2 show that the DED fabricated 316 L SS exhibits a stronger void-swelling resistance and lower dislocation loop density than its wrought counterpart. AM-induced features, such as nanopores and sub-grain boundaries, could serve as defect sinks to absorb irradiation-induced defects. The design of microstructure using AM processes opens up new avenues for the development of irradiation tolerant materials for nuclear applications.

Additive Manufacturing (AM) processes enable the reduction of manufacturing time, material waste, and allows for the creation of complex structures. However, anisotropic mechanical behaviour is frequently observed in additively manufactured parts, and it is directly linked to the component's grain structure characteristics, which itself is dependent on the process parameters. The formation of grain structure in 316 L stainless steel fusion lines is investigated in this paper, combining experimental results and numerical simulations. Experimentally, fusion lines are built on a 316 L substrate, using an instrumented LPBF process. The high-speed camera recordings combined with the characterization of the samples enables capturing of melt-pool sizes and grain characteristics. The numerical modelling is based on a three-dimensional “CAFE” model, coupling Cellular Automata and Finite Element models to predict grain formation. The thermal model is defined and calibrated using the experiments. The experimental and numerical grain characteristics are compared. Numerical results are discussed with regards to the growth models and the process parameters. The growth model defined here is compared to existing models and is well fitted to capture grain formation in single-track configurations. Finally, the average grain size and aspect ratio of the grains increase with an increase of the process' linear energy.

Relationships between microstructures and hardening nature of laser powder bed fused (L-PBF) 316 L stainless steel have been studied. Using integrated experimental efforts and calculations, the evolution of microstructure entities such as dislocation density, organization, cellular structure and recrystallization behaviors were characterized as a function of heat treatments. Furthermore, the evolution of dislocation-type, namely the geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), and their impacts on the hardness variation during annealing treatments for L-PBF alloy were experimentally investigated. The GND and SSD densities were statistically measured utilizing the Hough-based EBSD method and Taylor's hardening model. With the progress of recovery, the GNDs migrate from cellular walls to more energetically-favourable regions, resulting in the higher concentration of GNDs along subgrain boundaries. The SSD density decreases faster than the GND density during heat treatments, because the SSD density is more sensitive to the release of thermal distortions formed in printing. In all annealing conditions, the dislocations contribute to more than 50% of the hardness, and over 85.8% of the total dislocations are GNDs, while changes of other strengthening mechanism contributions are negligible, which draws a conclusion that the hardness of the present L-PBF alloy is governed predominantly by GNDs.

A 10 mm 316 L stainless steel plate was joined using multi-layer laser-MIG hybrid welding. Results showed that unmixed zone and “real” fusion zone (FZ) existed near the fusion line. Frequently met dendritic ferrite (δ) and newly encountered spherical nanoscale particles trapped in austenite (γ) were only observed in the latter region. Chemically, these particles turned out a SiO2 phase. From base metal to FZ, δ morphology was altered from originally linear to relatively diversely dendritic. Meantime, γ exhibited a converse manner where previously randomly oriented equiaxed was replaced by coarse, columnar and textured. Accordingly, a comparably high Schmidt factor in γ phase was obtained in FZ. Moreover, δ exhibited a relatively higher stress level than γ which presented a decrease in residual stress with grain coarsening in heat affected zone. In FZ, δ fraction displayed a dramatical increase with special BCC-FCC orientation relations established. Finally, tensile test and fracture appearance implied that FZ was exactly the weakest link of the whole welded joint in spite of a qualified strength achieved. In summary, lack in Σ3, largely increased δ fraction, high Schmidt factor in γ and especially being rich in SiO2 particles collectively led to the micro-void coalescence fracture in FZ.

Detailed understanding of the complex melt pool physics plays a vital role in predicting optimal processing regimes in laser powder bed fusion additive manufacturing. In this work, we use high framerate video recording of Selective Laser Melting (SLM) to provide useful insight on the laser-powder interaction and melt pool evolution of 316 L powder layers, while also serving as a novel instrument to quantify cooling rates of the melt pool. The experiment was performed using two powder types – one gas- and one water-atomized – to further clarify how morphological and chemical differences between these two feedstock materials influence the laser melting process. Finally, experimentally determined cooling rates are compared with values obtained through computer simulation, and the relationship between cooling rate and grain cell size is compared with data previously published in the literature. [ABSTRACT FROM AUTHOR]

In the present work, a new technique for creating uniform distributed space holder is used for producing porous 316 L stainless steel foam. Uniform pore distribution with Face-Centered Cubic (FCC) packing is created by a novel method using spherical carbamide as a space holder. The specimens have maximum porosity about 71.5% after sintering at 1200 °C for 60 min in N 2 atmosphere. The microstructure of the metal foams, cell size and shape was studied using optical microscopy and Clemex Vision PE commercial image-analyzer software. The compression test was used to evaluate the mechanical properties of the foam samples. The results showed that uniform pore distribution can improve elastic properties and compressive strength. [ABSTRACT FROM AUTHOR]