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Metal components produced via selective laser melting (SLM) additive manufacturing (AM) can offer comparable and sometimes superior mechanical properties to those of bulk materials. ...Selection of the appropriate process parameters (e.g. laser power, build direction, scan hatch spacing) plays a fundamental role in determining final properties. For this reason, microstructure, defect formation and mechanical properties of AISI 316L components are investigated in this paper according to the process parameters used for their fabrication. A first experimental campaign establishes process parameters guaranteeing a density greater than 98%. Samples for microstructural and mechanical characterization are then produced based on these results, varying laser power from 100W to 150W, hatch space from 0.05mm to 0.07mm and orientation from 45° to 90°. A MYSINT100 SLM machine with laser power up to 150W and spot diameter of 50μm is employed for all experiments. The presented results establish a correlation between the process parameters and the resulting microstructure and mechanical properties of SLM 316L specimens.
The effect of processing parameters on the mechanical properties of AISI 304L stainless steel components fabricated using laser-based directed energy deposition additive manufacturing (AM) was ...investigated. Two walls were fabricated, with high linear heat inputs of 271 and 377 J/mm, to determine the effect of processing parameters on microstructure and mechanical properties of 304L made by AM. Uniaxial tension tests were performed on samples extracted from the walls in longitudinal and transverse directions. The yield strength, ultimate tensile strength, and ductility, were higher in the lower linear heat input wall compared to the higher linear heat input wall. The ductility in the longitudinal direction was less than that in the transverse direction, while there was no clear anisotropy in strength. A grain growth model adapted from welding was used to interpret and predict the grain sizes in the walls as a function of processing parameters and position. A Hall–Petch relationship was used to explain the effect of local grain size and morphology on the location- and direction-dependent yield strengths in each wall. The ultimate tensile strengths and elongations of the material made by AM were less than those of annealed 304L plate since a microstructural phase transformation from austenite to martensite, which provides a mechanism for significant macroscopic strain hardening, occurred in the annealed material, but not the material made by AM. Chemical analysis showed that walls made by AM had higher nitrogen content, which stabilizes the austenitic phase, than the annealed plate.
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The austenitic stainless steel 316L was fabricated by gas metal arc additive manufacturing (GMA-AM) and its microstructure and room temperature tensile properties were investigated. Results show that ...in the GMA-AM 316L plate, a large number of well-aligned austenitic dendrites vertically orient, forming large columnar grains in the middle and some dendrites bent toward the plate surfaces, forming small columnar grains near the edges. The microstructure of GMA-AM 316L consists of δ, γ and σ phases. After one layer was deposited, the δ phase exhibited reticular morphology within austenitic dendrites. The δ phase redissolved in austenite with the intermetallic σ phases forming at γ/δ interfaces under the thermal cycles influence of subsequent three deposition layers. And under the thermal influence after the fourth layers, both δ and σ phases turned into fine vermicular morphologies within austenitic dendrites. The tensile properties of GMA-AM 316L steel are comparable to wrought 316L and exceed the industry requirements for 316L. Its fracture type is ductile fracture due to the obvious fracture surface dimples. The microcracks initiate at the interior of σ phases and grow into large cracks leading to materials failure.
The mechanical and corrosion properties of gas metal arc additive manufacturing (GMA-AM) 316L could be optimized by modifying the volume fractions of sigma (σ) and delta-ferrite (δ) phases through ...heat treatment. Results show that the heat treatment at 1000°C to 1200°C for one hour will not obvious influence the morphology of grains in steel but largely influence the contents of σ and δ phases. The heat treatment at 1000°C effectively increases the amount of σ phase in steel, causing both increase of UTS and YS but decrease of El and RA. The heat treatment at 1100°C to 1200°C completely eliminates σ phase, leading to the decrease of UTS and YS but increase of El and RA. The σ phase has better strengthening effect than δ phase, but which may degrade ductility and increase the possibility for cracks generation in steel. Meanwhile, limiting the number of both σ and δ phases through heat treatment can improve the corrosion resistance of steel. And σ phase appears more detrimental impact on degradation the corrosion resistance of steel than δ phase.
Microbiologically influenced corrosion (MIC) of S32654 (654SMO) super austenitic stainless steel (SASS) by acid producing bacterium (APB), Acidithiobacillus caldus SM-1, a strain of sulfur-oxidizing ...bacteria (SOB) used in biohydrometallurgy field, was investigated using electrochemical measurements and surface characterizations during a 14-day immersion test. The results indicated that S32654 SASS was susceptible to MIC by APB, and A. caldus SM-1 was capable of producing an aggressive acidic environment underneath the biofilm, resulting in the dissolution of the passive film and severe pitting attacks against S32654 SASS, which is commonly regarded as a corrosion resistant material.
•The MIC behavior of S32654 SASS in the presence of Acidithiobacillus caldus SM-1 was investigated.•The sulfuric acid produced by A. caldus SM-1 led to a severe MIC attack against S32654 SASS.•The biogenic H2S may result in the formation of MoS2 under the biocatalysis.
The connectivity of high energy random boundaries was investigated on the basis of the fractal analyses of grain boundary microstructures in SUS316L stainless steel, to prove the usefulness of a ...refined approach to grain boundary engineering (GBE) for more precise prediction and control of intergranular corrosion in polycrystalline materials. It was found that the maximum connectivity for random boundary network, termed the maximum random boundary connectivity (MRBC) had a fractal nature in the studied specimens of SUS316L stainless steel. The fractal dimension of MRBC tended to decrease with decreasing fraction of random boundaries, or in other words with increasing fraction of low-energy low-Σ coincidence site lattice (CSL) boundaries. The lower coefficient of variation of grain size distribution suggesting a more homogeneous grain structure, was found to result in the lower fractal dimension of MRBC for the specimens with a similar grain boundary character distribution (GBCD). The optimum grain boundary microstructure for enhanced intergranular corrosion resistance in the SUS316L stainless steel was discussed based on the results from the fractal analyses of MRBC for different grain boundary microstructures.
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The effects of grain refinement on the quasi-static compressive behavior of AISI 321 austenitic stainless steel (ASS) were studied. The effect of strain on the final microstructure after compressive ...deformation was also investigated. The compression tests on steel specimens were conducted at a strain rate of 4.2 × 10−3 s−1. Ultrafine-grained (UFG) specimen with the grain size of 0.24 μm exhibits an excellent combination of high yield strength (∼1 GPa) and good strain hardenability. Meanwhile, the coarse-grained (CG) specimen with the grain size of 37 μm exhibits yield strength of ∼0.2 GPa. At 0.53 true strain, UFG and CG specimens exhibit compressive strengths of 5.95 and 4.80 GPa, respectively. The Hall-Petch relation constants, σo, and K, for the AISI 321 ASS were estimated to be 128 MPa and 478 MPa μm−0.5, respectively. The strain hardening behavior of both UFG and CG specimens occur in three distinctive stages. CG specimen exhibits higher strain hardening rate than the UFG specimen up to a critical true strain of 0.4, above which strain hardening rate in UFG becomes greater. X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) techniques were used for microstructural analyses to understand the underlying mechanisms behind the strain hardening behavior. Texture evolution during deformation, orientation relationship between phases and the sequence of martensitic phase transformation were also studied and are discussed in this paper. Visco-plastic self-consistent (VPSC) modeling was employed to decipher the role of deformation mechanisms in macroscopic stress-strain response and also in texture evolution during uniaxial compression loading.
•Effect of grain refinement on compressive behavior of AISI 321 was studied.•Yield strength in UFG (0.24 μm) and CG (37 μm) samples are ∼1 and ∼0.2 GPa, resp.•CG exhibits higher hardening rate than UFG up to a critical true strain of 0.4•Strain-induced transformation follows both FCC γ .→ BCC αʹ and FCC γ → HCP ɛ→ BCC αʹ•VPSC results deviate from experiment in CG and UFG at strains of 0.23 and 0.16, resp.
In this work, we examined the influence of different types of selective laser melting (SLM) devices on the microstructure and the associated material properties of austenitic 316L stainless steel. ...Specimens were built using powder from the same powder batch on four different SLM machines. For the specimen build-up, optimized parameter sets were used, as provided by the manufacturers for each individual SLM machine. The resulting microstructure was investigated by means of scanning electron microscopy, which revealed that the different samples possess similar microstructures. Differences between the microstructures were found in terms of porosity, which significantly influences the material properties. Additionally, the build-up direction of the specimens was found to have a strong influence on the mechanical properties. Thus, the defect density defines the material’s properties so that the ascertained characteristic values were used to determine a Weibull modulus for the corresponding values in dependence on the build-up direction. Based on these findings, characteristic averages of the mechanical properties were determined for the SLM-manufactured samples, which can subsequently be used as reference parameters for designing industrially manufactured components.
Microstructure evolution and mechanical properties of AISI 316 LN austenitic stainless steel (SS) after cryorolling with different strains were investigated by means of optical, scanning and ...transmission electron microscopy, X-ray diffractometer, microhardness tester, and tensile testing system. The deformation-induced martensite transition and the deformation microstructure occurred during cryorolling process were always composed of high-density dislocations, deformation twins, and deformation-induced martensites. Following the strain, the dislocation density in deformation microstructure approached saturation state and the volume fraction of deformation twins combined with deformation-induced martensites increased significantly. At the 70% strain, original austenite was transformed into martensite completely. Further increasing the strain to 90% would refine the martensitic lamellae to nanoscale. The deformation degree also led to remarkable increase of the strength and hardness of the cryorolled SS, and drastic reductions of the elongation. Due to the cryorolling, the tensile fracture morphology changed from typical ductile rupture to a mixture of quasi-cleavage and ductile fracture.
Acceptable weld formation in high nitrogen austenitic stainless steel (HNASS) was achieved when nitrogen gas (N2) was added to the Ar-based shielding gas. Although a relatively unstable process, in ...double-sided synchronous autogenous gas tungsten arc (GTA) welding (DSSAGW), added N2 enabled the arc plasma to shrink and accordingly heightened the arc voltage, leading to increased fusion areas of welds. Added N2 in the shielding gas was very beneficial for suppressing nitrogen loss in welded joints, with the nitrogen content in the weld zone (WZ) increased to 1.25% with pure N2 shielding gas, while the δ-ferrite content slightly decreased. With increased N2 in shielding gas, both the primary and secondary dendrite arm spacing in the WZ increased, while the equiaxed austenite in heat-affected zone (HAZ) varied rather little. The WZ microhardness increased with N2 addition to the shielding gas, indicating nitrogen’s strong solid solution strengthening effects. Based on these results, N2 was considered to be a promising shielding gas for HNASS welding in DSSAGW and the optimal N2 and Ar proportions judged to be below 2/8 (by vol.).
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